Negative flaps for better low speed aileron control?

[[email protected]]

Hi Group
Will someone please explain why negative flaps supposedly
provides better aileron control. I know conventional wisdom
says that it does but WHY? It is not intuitively obvious at
least to me. Yes I have tried negative flaps at low speeds
both on the roll and braking but its effect as far as I could
judge was marginal and my thoughts were that it reminded me
of a placebo. So please direct me to the authorative articles
on the subject or if there is a simple explanation please
educate me. Thanks.
Dave

PS Also posted on the Stemme Owners Group where there is a
thread running on the use of negative flaps for better control.

[ttaylor at cc.usu.edu]

Dave,

Sorry I don't know the aerodynamics (was trained in fluid dynamics of
bioreactors, not aerodynamics) only that it made a world of difference
on both my Nimbus 2 and Ventus B. The wings will drop on both without
it, where with it I can usually come to a full stop before the wing
will drop. It is definitely not a placebo. Try it in an 8-10 knot
breeze. Point the plane into the wind and try both positions.

Tim

[[email protected]]

It's mainly useful on gliders that sit at a high angle of attack on the
ground. At low speeds, the wing is stalled, reducing aileron
effectiveness as they are in the detached airflow; selecting negative
flaps decreases the effective angle of attack of the wing and can
provide some aileron effectiveness at low speeds by unstalling the wing
earlier.

Same thing while slowing down - negative flaps will delay the wing
stalling and losing aileron effectiveness.

It also helps to get the tail up as soon as possible, although by the
time you can raise the tail the ailerons will probably be
working...what some gliders really need is an extended (and
retractable) tailwheel - start the roll with the tail level, lower the
tail as speed increases.

Just joking (kinda).

Kirk
66

[John Ferguson]

Going to negative on the ground run as you slow reduces
the angle of attack and stops the wing from stalling
on gliders that are prone to wing dropping.

Also reduces lift (AoA) and helps glider stick to the
ground, ie, no desire to keep flying as the wing can't
generate enough lift.

John

PS Some glider have ailerons that go negative (or do
not go as positive) as flaps are increased towards
landing, eg, Mosquito, ASH 25.

[Andrew Warbrick]

At 18:18 29 July 2005, wrote:

>It also helps to get the tail up as soon as possible,
>although by the
>time you can raise the tail the ailerons will probably
>be
>working...what some gliders really need is an extended
>(and
>retractable) tailwheel - start the roll with the tail
>level, lower the
>tail as speed increases.
>
>Just joking (kinda).
>
Ironically, the Slingsby Vega is the only glider I
know of with a retractable tailwheel and it's also
the only flapped glider I've ever seen with a severe
tendency to drop a wing on takeoff even in full negative
flap (caused by the huge main wheel).

[GM]

All of the following applies only to gliders, that have a Flap-Aileron
connection, i.e. changing the flap position changes the aileron
position as well.
I can confirm that the ground-runs with negative flaps make a
difference with my Libelle H301. The AoA is the angle between the chord
line and the relative wind. Try this: look at the wing from the tip
towards the fuselage with the stick centered and the flaps full
positive. Draw a mental line from the leading edge to the trailing edge
of the aileron. The airflow (relative wind) will be parallel to the
ground during the ground roll. Now put the flaps up full negative and
repeat the above. The line is now at a shallower angle since the
leading edge is still where it was before but the trailing edge is up.
The thing that really helps to gain roll control on take-off with the
Libelle is to get the tail up and 'flying' asap. That way, the wing is
at a small AoA and the ailerons are effective right away.

Uli Neumann

[Duane Eisenbeiss]

> wrote in message
oups.com...
> Hi Group
> Will someone please explain why negative flaps supposedly
> provides better aileron control. I know conventional wisdom
> says that it does but WHY? >

The flap/aileron interconnect has a lot to do with it.
With the help of another person;
Put the flaps in full negative then measure the max deflection of the
aileron (full up to full down).
Then put the flaps in full positive and again measure the max aileron
deflection.

With many sailplanes you will find that with negative flaps the interconnect
allows a greater range of aileron deflection.

Duane

[Chris Nicholas]

T o d d P a t t i s t wrote: Once you are on the ground, with your front
and back wheels

rolling (or skid/wheel, etc.) the angle of attack remains

unchanged. Stalling is a function of the AOA, not your

airspeed. Stall is only indirectly related to airspeed when

your aircraft weight is supported by lift. On the ground

your weight's supported by your wheel(s), so you can go very

slowly and not be stalled. (Unquote).

I'm open to correction, but I think Todd is only right in the
circumstances that the glider stays level laterally.

As soon as a wing starts to drop, and the other to rise, the dropping
wing has a higher AoA and the rising wing a lower one. On a nil wind
day, with the glider having reached say 10 knots during the take-off run
(faster than the wingtip runner can run), a tip dropping at even 1 knot
has an extra AoA of 6 degrees - may well be enough to stall it with
neutral aileron. Putting that aileron down (to try to lift the tip) just
increases the AoA even more into its stall - hence, ineffective.
Negative flaps, if ailerons move in sympathy with them, can be enough to
unstall that downgoing tip.

By the way, what Todd says about non-linear CL curve makes entire sense
to me, and Duane's point about geometry, and others' comments. I just
think all effects are in play, at least on some gliders.

Now, why does opening the airbrake (usually called spoilers in the USA
for some unknown reason) help on some gliders? And did it ever help on
gliders that really did have spoilers (UK-speak for things that spoil
lift but don't add much drag, unlike airbrakes [in UK-speak], which do
both in spades)?

Chris N.

=====================









__________________________________________________ _________
Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com

[[email protected]]

What Todd says about angle of attack on the ground makes sense and is
absoulutely correct - the problem is that we are working with wings
well below their stall speeds, with weight supported by the landing
gear, so "angle of attack" takes on a different meaning: at low speeds
and "high angle of attack", the airflow around the wing is probably
highly turbulent, and control surfaces will be inefficient. Reduce
that "angle of attack" and the control surface is working in
undisturbed (or less disturbed) air, and should be more effective.
Raise the tail, and turbulence should be at a minimum.

I would love to see some wind tunnel testing of, say, a Pik-20 wing at
low (below stalling) speeds. I would bet a high tow that at the normal
ground attitude, on the ground (i.e. a ground plane below the wing,
weight supported by the gear), at low speeds (zero to 20 knots?), the
airflow over the wing is probably really turbulent, and that setting
negative flaps/ailerons moves those control surfaces closer to
"undistrubed" flow, giving them a little more effectiveness.

Remeber, this is a short, transitory phase of the development of
airflow and lift over the wing - I wonder if much study of this
"pre-flight" regime has been done.

Then again, I may be full of sh*t!

I do know it works on a Pik-20b, and that my LS6 doesn't really care
where my flaps are during takeoff (but I use negative anyway, and go
full negative on rollout - gets the flaperons out of the rocks!).

Kirk
66

[5Z]

Chris Nicholas wrote:
> Now, why does opening the airbrake (usually called spoilers in the USA
> for some unknown reason) help on some gliders? And did it ever help on
> gliders that really did have spoilers (UK-speak for things that spoil
> lift but don't add much drag, unlike airbrakes [in UK-speak], which do
> both in spades)?

The best argument FOR this technique was presented in Soaring magazine
quite a few years ago.

At low speed the wing is producing some lift over the entire span.
This provides a damping effect to any rolling tendency. Imagine a
whole bunch of weak springs attached to the spar every few inches and
attached to the ceiling. The plane will resist a wing drop. Now,
remove the springs in the spoiler region and the glider will tip more
easily, as there are fewer springs working to keep it level.

So with the spoilers extended, the wing is easier to pick up as the
aileron has a little bit less force to fight.

Another argument is that on some wings, the turbulence caused by the
spoiler might help attach some flow to the aileron.

In any case, then next time you're sitting in the glider waiting for a
tow and there's a bit of a breeze on the nose, try some experiments.
Have someone hold your wing level, then do some slight rolls with and
without spoilers. I have found that there is a slight but noticeable
increase in response with the spoilers open.

-Tom

[David Smith]

I dont buy Chris's point about angle of attack changing as a wing drops =
on the ground, if the tail is on the ground there is no pitch change, =
atlthough it would be true banking into a turn at flying speed. =
Personaly I think the propwash and/or crosswind effect is much more =
important, given that the prop wash on most tugs affects the right =
wing, if that is tending to drop, it will lift in the propwash ( we use =
that affect in a downwing takeoff). Conversely a crosswind from the =
right and the propwash causing the left wing to drop can quickly get out =
of hand.=20


David Smith

[Eric Greenwell]

5Z wrote:
> Chris Nicholas wrote:
>
>>Now, why does opening the airbrake (usually called spoilers in the USA
>>for some unknown reason) help on some gliders? And did it ever help on
>>gliders that really did have spoilers (UK-speak for things that spoil
>>lift but don't add much drag, unlike airbrakes [in UK-speak], which do
>>both in spades)?
>
>
> The best argument FOR this technique was presented in Soaring magazine
> quite a few years ago.
>
> At low speed the wing is producing some lift over the entire span.
> This provides a damping effect to any rolling tendency. Imagine a
> whole bunch of weak springs attached to the spar every few inches and
> attached to the ceiling. The plane will resist a wing drop. Now,
> remove the springs in the spoiler region and the glider will tip more
> easily, as there are fewer springs working to keep it level.
>
> So with the spoilers extended, the wing is easier to pick up as the
> aileron has a little bit less force to fight.

The "spring" analogy doesn't sound right to me: it implies the wing is
attempting to hold the glider level, which is desirable. I suspect it is
the opposite: when a wing moves down a bit, it moves into slower moving
air nearer the ground, and loses some lift (and vice versa for the wing
going up a bit). This loss of lift tends to make that wing go down even
faster. Or if the wing is already down, it's definitely in slower moving
air compared to the up wing. The ailerons have to overcome this
disparity in lift and CG shift from the glider being tilted to the side.

So, with the spoilers out, the lift disparity is reduced (both wings
lose some lift), and it's easier for the ailerons to lift the wing off
the ground.

The above argument probably makes more sense when there is a wind. If
it's calm and the air motion is just the glider rolling on the ground
through the air, it might still make sense if the air near the ground is
more disturbed (somehow) the air several feet higher.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Eric Greenwell]

David Smith wrote:

> I dont buy Chris's point about angle of attack changing as a wing drops =
> on the ground, if the tail is on the ground there is no pitch change, =
> atlthough it would be true banking into a turn at flying speed. =
> Personaly I think the propwash and/or crosswind effect is much more =
> important, given that the prop wash on most tugs affects the right =
> wing, if that is tending to drop, it will lift in the propwash ( we use =
> that affect in a downwing takeoff). Conversely a crosswind from the =
> right and the propwash causing the left wing to drop can quickly get out =
> of hand.=20

The effects we're discussing also appear during landing in calm air, so
while crosswinds and tug wash affect the situation, they aren't
necessary for the wing dropping. A possile reason some people might
notice it as often on landing is they almost always have the spoilers
out (unlike takeoff).


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Don Johnstone]

I suspect there are many reasons why negative flap
and spoilers help roll response. In all cases when
there is no airflow over the wing unless the wings
are supported or they are very well balanced one will
drop. Airflow is required for the ailerons to have
any effect. As regards spoilers I suspect that their
use and resultant spoiling of lift results in the extremities
of the wing producing more lift than the inner parts
and of course the outer extrememities are where the
ailerons are resulting in the forces being greater
where there is more leverage. Even a very small airflow
will produce some lift and it is surprising just how
much even a gentle breeze can generate. Try weighing
your glider in a 5 knot breeze and then in the hangar
in still air, you may be amazed at the difference.

Negative flap will have the same sort of effect but
the premise that this is only the case where flaperons
as opposed to seperate flaps and aileron is in my experience
correct. The Kestrel 19, which has a separate land
flap was quite happy starting the roll if flying flap
which was also connected to the ailerons was left in
neutral and I found no problem with low speed roll
control. The ASW 17 is totally different and I find
requires full negative flap at the start of the take
off roll. An additional benefit is that the glider
will not leave the ground with negative flap connected
which means that the tailwheel can be firmly pegged
to the ground with full up elevator with no danger
of getting airborne helping to keep straight in a crosswind.

On a winch launch with the Kestrel one stage land flap
could be selected before takeoff, and the same is true
of the Janus A which has seperate flaps and ailerons,
not something I would care to do with the 17.

On landing the Kestrel full landing flap could be retained
for the landing roll with the flying flap in neutral,
with the 17 full negative is applied carefully after
touchdown. Sudden application of full negative can
cause both wingtips to strike the ground. I have never
experience a problem with wing drop on the landing
roll except in a significant crosswind. I suppose I
am really not sure why it works, I am only very glad
that it does.


At 17:00 30 July 2005, Eric Greenwell wrote:
>David Smith wrote:
>
>> I dont buy Chris's point about angle of attack changing
>>as a wing drops =
>> on the ground, if the tail is on the ground there
>>is no pitch change, =
>> atlthough it would be true banking into a turn at
>>flying speed. =
>> Personaly I think the propwash and/or crosswind effect
>>is much more =
>> important, given that the prop wash on most tugs
>>affects the right =
>> wing, if that is tending to drop, it will lift in
>>the propwash ( we use =
>> that affect in a downwing takeoff). Conversely a crosswind
>>from the =
>> right and the propwash causing the left wing to drop
>>can quickly get out =
>> of hand.=20
>
>The effects we're discussing also appear during landing
>in calm air, so
>while crosswinds and tug wash affect the situation,
>they aren't
>necessary for the wing dropping. A possile reason some
>people might
>notice it as often on landing is they almost always
>have the spoilers
>out (unlike takeoff).
>
>
>--
>Change 'netto' to 'net' to email me directly
>
>Eric Greenwell
>Washington State
>USA
>

[[email protected]]

writes:

> Will someone please explain why negative flaps supposedly provides
> better aileron control. I know conventional wisdom says that it does
> but WHY? It is not intuitively obvious at least to me. Yes I have
> tried negative flaps at low speeds both on the roll and braking but
> its effect as far as I could judge was marginal and my thoughts were
> that it reminded me of a placebo. So please direct me to the
> authorative articles on the subject or if there is a simple
> explanation please educate me. Thanks.

With -ve flap, the AoA of the wing centre is less, so less lift unless
you increase the pitch angle. Net result is increased lift from the
outer wing sections, where the ailerons are, and reduced lift in the
centre. Hence the lift distribution is wider and more effective at
damping out roll.


--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

[Eric Greenwell]

wrote:
> writes:
>
>
>> Will someone please explain why negative flaps supposedly provides
>>better aileron control.
snip
>
> With -ve flap, the AoA of the wing centre is less, so less lift unless
> you increase the pitch angle. Net result is increased lift from the
> outer wing sections, where the ailerons are,

I don't think the lift in the outer sections would increase; instead, it
ought to stay the same (for gliders that don't move the ailerons with
the flaps) or decrease (in most gliders the ailerons also rise when
negative flap is selected).

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[W.J. \(Bill\) Dean \(U.K.\).]

Aileron Authority & Flaps at Take-off and Landing.

Summary.

There are two types of launch.

Slow, such as aerotow, car and reverse pulley, when it is necessary to
control the glider at low speed, perhaps with a large crosswind component,
before the glider gains flying speed. It is necessary to start with the
controls set to give control at low speed, and perhaps to change the setting
as flying speed is gained.

Fast, such as winch and bungee (catapult), when the glider gains speed so
fast that it is not possible to change the control setting during the ground
run, and the glider should start with the control setting needed when first
airborne.

The problem.

We discovered the problem, the solution and the explanation at Lasham in the
early 70's when we started flying the Slingsby Kestrel 19.

At take-off we were in the two-point tail down pitch attitude.

In light winds on aerotow take-off, in neutral or thermal flap setting, and
especially when also cross-wind; we found that we had no lateral control at
the start of take-off. If a wing went down it stayed down. When the
airspeed was above about 30 knots we did have control, even if we were still
tail down.

The explanation.

Ailerons and lateral stability.

When we move an aileron down, we increase the Angle of Attack (AoA) at that
wingtip. This increases the lift at that wingtip PROVIDED the new, higher,
AoA is below the stalling AoA; the wing has lateral stability.

If the new, higher, AoA is above the stalling AoA the lift at the wingtip
will be reduced. The effect of moving the aileron down will be that the
wingtip goes down, the exact opposite of what the pilot intended. The wing
has lateral instability.

If the wingtip is at or above the stalling AoA with the aileron neutral, the
effect of moving the aileron down is immediate and marked, the wing goes
straight down.

Flying flap setting at takeoff.

The effect of moving the flaps (so far as affects flying the machine at
take-off) is to change the camber, i.e. as if we changed the angle at which
the wing is set on the fuselage. Since at take-off the glider is tail down
in the two-point attitude, this changes the AoA.

If the ailerons move with the flaps, then with the flaps down the neutral
aileron AoA will be higher than when the flaps are up, so we are more likely
to have lateral instability.

With the flap lever fully forward and the flaps fully up, we have the best
chance of lateral stability, the ailerons will work.

Change of stalling AofA with speed.

When we learn to fly, we are taught that the stalling AoA is the same at all
speeds, so that if we achieve the stall AoA at any speed, we will stall even
if the speed is high.

However, this is not true at very low speeds, due to Reynolds number
effects. At 30 knots the stalling AoA will be at the normal flying figure,
say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
degrees. This explains why we found in our Kestrel 19s (in neutral flap)
we had no control at 10 knots and full control at 30.

This change of stalling AoA with speed explains why we need full negative
flap to have aileron control on take-off at low speeds, but can still have
full control with thermal flap setting at 30 knots.

The solution.

Aerotow.

Start the take-off run with the flap lever fully forward, flaps fully up
(fully negative). If you have a separate landing flap control (e.g.
Kestrel) this flap should also be up.

If using a C of G hook, it may be wise to start by holding the wheel brake
on to ensure that there is no overrun, this may mean taking up slack with
the air-brakes out; warn the launch point crew first!

If not holding the wheel brake, or as soon as you have let it off and locked
the air-brakes, the left hand should be touching but not holding the
release.

When you are sure you have full control and will not have to release, move
your left hand to the flap lever.

As the speed builds, move the flap lever back to the position you intend to
use when flying. If you start to lose aileron control, move the flap lever
forward again at once, because you moved it back too soon.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Cable launching.

For car or reverse pulley launching, use the same method as for aerotow.

For winching, start with the setting you need once airborne. If the winch
and its driver behave as they should, the glider will not have time to drop
a wing, and you will not have time to move the flap lever. Use the same
method for bungee launching.

For winching with a Kestrel, use the half landing flap position (if fitted)
for launching, and neutral flying flap position; this setting will be
correct for an immediate landing after a low launch failure.

If it goes wrong.

If the wing goes down release at once. Do not hang on to see if you can
get the wing up.

If the glider does start to groundloop, it will happen so quickly that the
glider will be broken before you can release. If there is any appreciable
speed or wind, the groundloop will turn into a cartwheel, which will hurt
the pilot as well as the glider.

Remember, all the time the launch continues, energy is going into the
glider. If you lose control, this energy has to go somewhere.

Unflapped gliders.

Some unflapped gliders are very close to tip stalling (lateral instability)
at the start of the ground run. There are two strategies to try.

Stick forward.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Airbrakes.

Start with the airbrakes open. This gives better lateral control; I don't
know why, but it seems to. If you want to start with the wheel brake on,
and it is worked by the air brake lever, you are going to have to do this
anyway. Clearly the tug pilot must be warned, and anyone at the launch
point who may give a stop signal must be told.

Use of rudder.

A sharp application of rudder makes the glider roll as well as yaw. This
can be used as a last resort if the wings are not responding to aileron;
this will put you out of line with the tug, but this can be sorted out when
you have aileron control. Obviously, this cannot be used if it runs you or
the tug off the runway or otherwise into trouble.

Landing.

Putting the flaps up after landing achieves two things. It dumps lift,
making it less likely that bumpy ground or a gust will put you in the air
again when you thought you had landed. It improves aileron control when
you are moving slowly, but this is less important than when taking off
because you are losing energy and speed not gaining it, and you can use the
wheel brake. You have to let go of the air brake to move the flaps, if you
have near flying speed they may close and cause you to take off again before
you get the flaps up; consider raising the tail to reduce angle of attack
until the flaps are up.

In a Kestrel it is the flying flaps which should go up, to increase aileron
authority.

Beware of using the wheel brake unless you are dead straight, if you are
turning or drifting it may provoke a ground loop.

Flight manuals.

In general one should always read and obey the flight manual. However the
Kestrel manual was written before we knew much of the above, and does not
reflect current knowledge and practice. There may be other types to which
this applies. Use full negative (fully up) flap for starting aerotow
take-offs!

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.

>
> > wrote in message
> oups.com...
>
> Hi Group
> Will someone please explain why negative flaps supposedly
> provides better aileron control. I know conventional wisdom
> says that it does but WHY? It is not intuitively obvious at
> least to me. Yes I have tried negative flaps at low speeds
> both on the roll and braking but its effect as far as I could
> judge was marginal and my thoughts were that it reminded me
> of a placebo. So please direct me to the authorative articles
> on the subject or if there is a simple explanation please
> educate me. Thanks.
> Dave
>
> PS Also posted on the Stemme Owners Group where there is a
> thread running on the use of negative flaps for better control.
>

[Geoff Vincent]

Hi Bill,

Your comments are spot on. I'm a PIK 20B jockey and always use full
-ve flap for take-off, whether I have a wing runner or it's a
wing-down launch. Landings are treated similarly, moving to full -ve
flap as soon as the mainwheel is on the ground to ensure maximum
aileron authority.

Regards,

Geoff Vincent
Grampians Soaring Club
Australia


On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
> wrote:

>Aileron Authority & Flaps at Take-off and Landing.
>
>Summary.
>
>There are two types of launch.
>
>Slow, such as aerotow, car and reverse pulley, when it is necessary to
>control the glider at low speed, perhaps with a large crosswind component,
>before the glider gains flying speed. It is necessary to start with the
>controls set to give control at low speed, and perhaps to change the setting
>as flying speed is gained.
>
>Fast, such as winch and bungee (catapult), when the glider gains speed so
>fast that it is not possible to change the control setting during the ground
>run, and the glider should start with the control setting needed when first
>airborne.
>
>The problem.
>
>We discovered the problem, the solution and the explanation at Lasham in the
>early 70's when we started flying the Slingsby Kestrel 19.
>
>At take-off we were in the two-point tail down pitch attitude.
>
>In light winds on aerotow take-off, in neutral or thermal flap setting, and
>especially when also cross-wind; we found that we had no lateral control at
>the start of take-off. If a wing went down it stayed down. When the
>airspeed was above about 30 knots we did have control, even if we were still
>tail down.
>
>The explanation.
>
>Ailerons and lateral stability.
>
>When we move an aileron down, we increase the Angle of Attack (AoA) at that
>wingtip. This increases the lift at that wingtip PROVIDED the new, higher,
>AoA is below the stalling AoA; the wing has lateral stability.
>
>If the new, higher, AoA is above the stalling AoA the lift at the wingtip
>will be reduced. The effect of moving the aileron down will be that the
>wingtip goes down, the exact opposite of what the pilot intended. The wing
>has lateral instability.
>
>If the wingtip is at or above the stalling AoA with the aileron neutral, the
>effect of moving the aileron down is immediate and marked, the wing goes
>straight down.
>
>Flying flap setting at takeoff.
>
>The effect of moving the flaps (so far as affects flying the machine at
>take-off) is to change the camber, i.e. as if we changed the angle at which
>the wing is set on the fuselage. Since at take-off the glider is tail down
>in the two-point attitude, this changes the AoA.
>
>If the ailerons move with the flaps, then with the flaps down the neutral
>aileron AoA will be higher than when the flaps are up, so we are more likely
>to have lateral instability.
>
>With the flap lever fully forward and the flaps fully up, we have the best
>chance of lateral stability, the ailerons will work.
>
>Change of stalling AofA with speed.
>
>When we learn to fly, we are taught that the stalling AoA is the same at all
>speeds, so that if we achieve the stall AoA at any speed, we will stall even
>if the speed is high.
>
>However, this is not true at very low speeds, due to Reynolds number
>effects. At 30 knots the stalling AoA will be at the normal flying figure,
>say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
>degrees. This explains why we found in our Kestrel 19s (in neutral flap)
>we had no control at 10 knots and full control at 30.
>
>This change of stalling AoA with speed explains why we need full negative
>flap to have aileron control on take-off at low speeds, but can still have
>full control with thermal flap setting at 30 knots.
>
>The solution.
>
>Aerotow.
>
>Start the take-off run with the flap lever fully forward, flaps fully up
>(fully negative). If you have a separate landing flap control (e.g.
>Kestrel) this flap should also be up.
>
>If using a C of G hook, it may be wise to start by holding the wheel brake
>on to ensure that there is no overrun, this may mean taking up slack with
>the air-brakes out; warn the launch point crew first!
>
>If not holding the wheel brake, or as soon as you have let it off and locked
>the air-brakes, the left hand should be touching but not holding the
>release.
>
>When you are sure you have full control and will not have to release, move
>your left hand to the flap lever.
>
>As the speed builds, move the flap lever back to the position you intend to
>use when flying. If you start to lose aileron control, move the flap lever
>forward again at once, because you moved it back too soon.
>
>Start with the stick fully forward. Obviously, if you get the tail up, the
>angle of attack is lower, and also the effect of gusts is reduced. Lower
>the tail to the normal take-off attitude when the speed is high enough for
>good aileron control.
>
>Cable launching.
>
>For car or reverse pulley launching, use the same method as for aerotow.
>
>For winching, start with the setting you need once airborne. If the winch
>and its driver behave as they should, the glider will not have time to drop
>a wing, and you will not have time to move the flap lever. Use the same
>method for bungee launching.
>
>For winching with a Kestrel, use the half landing flap position (if fitted)
>for launching, and neutral flying flap position; this setting will be
>correct for an immediate landing after a low launch failure.
>
>If it goes wrong.
>
>If the wing goes down release at once. Do not hang on to see if you can
>get the wing up.
>
>If the glider does start to groundloop, it will happen so quickly that the
>glider will be broken before you can release. If there is any appreciable
>speed or wind, the groundloop will turn into a cartwheel, which will hurt
>the pilot as well as the glider.
>
>Remember, all the time the launch continues, energy is going into the
>glider. If you lose control, this energy has to go somewhere.
>
>Unflapped gliders.
>
>Some unflapped gliders are very close to tip stalling (lateral instability)
>at the start of the ground run. There are two strategies to try.
>
>Stick forward.
>
>Start with the stick fully forward. Obviously, if you get the tail up, the
>angle of attack is lower, and also the effect of gusts is reduced. Lower
>the tail to the normal take-off attitude when the speed is high enough for
>good aileron control.
>
>Airbrakes.
>
>Start with the airbrakes open. This gives better lateral control; I don't
>know why, but it seems to. If you want to start with the wheel brake on,
>and it is worked by the air brake lever, you are going to have to do this
>anyway. Clearly the tug pilot must be warned, and anyone at the launch
>point who may give a stop signal must be told.
>
>Use of rudder.
>
>A sharp application of rudder makes the glider roll as well as yaw. This
>can be used as a last resort if the wings are not responding to aileron;
>this will put you out of line with the tug, but this can be sorted out when
>you have aileron control. Obviously, this cannot be used if it runs you or
>the tug off the runway or otherwise into trouble.
>
>Landing.
>
>Putting the flaps up after landing achieves two things. It dumps lift,
>making it less likely that bumpy ground or a gust will put you in the air
>again when you thought you had landed. It improves aileron control when
>you are moving slowly, but this is less important than when taking off
>because you are losing energy and speed not gaining it, and you can use the
>wheel brake. You have to let go of the air brake to move the flaps, if you
>have near flying speed they may close and cause you to take off again before
>you get the flaps up; consider raising the tail to reduce angle of attack
>until the flaps are up.
>
>In a Kestrel it is the flying flaps which should go up, to increase aileron
>authority.
>
>Beware of using the wheel brake unless you are dead straight, if you are
>turning or drifting it may provoke a ground loop.
>
>Flight manuals.
>
>In general one should always read and obey the flight manual. However the
>Kestrel manual was written before we knew much of the above, and does not
>reflect current knowledge and practice. There may be other types to which
>this applies. Use full negative (fully up) flap for starting aerotow
>take-offs!
>
>W.J. (Bill) Dean (U.K.).
>Remove "ic" to reply.
>
>>
>> > wrote in message
>> oups.com...
>>
>> Hi Group
>> Will someone please explain why negative flaps supposedly
>> provides better aileron control. I know conventional wisdom
>> says that it does but WHY? It is not intuitively obvious at
>> least to me. Yes I have tried negative flaps at low speeds
>> both on the roll and braking but its effect as far as I could
>> judge was marginal and my thoughts were that it reminded me
>> of a placebo. So please direct me to the authorative articles
>> on the subject or if there is a simple explanation please
>> educate me. Thanks.
>> Dave
>>
>> PS Also posted on the Stemme Owners Group where there is a
>> thread running on the use of negative flaps for better control.
>>
>
>
>
>
>

[Ian Johnston]

On Fri, 29 Jul 2005 17:31:08 UTC, wrote:

: Will someone please explain why negative flaps supposedly
: provides better aileron control.

Here's a guess.

At low speeds on the ground, the wings are very near their stalling
AOA. If on wing does start going down, for whatever reason, the
additional vertical movement may cause it to increase its AOA beyond
stalling. Hence much reduced lift on that side, lift still being
produced on the other side, rolling moment, wing goes down faster. An
unstable situation.

By using negative flap you are effectively reducing the AOA of both
wings. Now a downward movement is less likely to lead to stall, and
more likely to lead to an increase (but sub-stall) AOA at the same
speed, so more lift and back up comes the wing. A stable situation.

This is all guesswork, since I don't fly flapped gliders. I'd expect
my explanation to work best on gliders with high wing loadings and
relatively high angles of attack sitting on the ground. Does that fit?

Ian
--

[Ian Johnston]

On Sat, 30 Jul 2005 00:05:03 UTC, "
> wrote:

: at low speeds
: and "high angle of attack", the airflow around the wing is probably
: highly turbulent

I doubt that. Turbulence is associated with high Reynolds number, and
that depends directly on speed. Low speed -> low Re -> less
turbulence.

Ian
--

[P. Corbett]

Geoff:
I also have a PIK-20B and have a question for you (or anyone in the group).

Given the PIK's high AOA when both wheels are on the ground, and given the
fact that going to full negative flap after touchdown will cause the tail
wheel to drop to the ground, do you attempt to keep the tail wheel up as
long as possible by applying forward stick when rolling up the flaps? As you
know, in a moderate headwind this is not likely an issue but in very light
wind or no wind, the abrupt loss of aileron control during the late stage of
rollout is irritating, especially if there is a crosswind component where a
mild groundloop is virtually guaranteed.

Paul
ZZ
"Geoff Vincent" > wrote in message
...
> Hi Bill,
>
> Your comments are spot on. I'm a PIK 20B jockey and always use full
> -ve flap for take-off, whether I have a wing runner or it's a
> wing-down launch. Landings are treated similarly, moving to full -ve
> flap as soon as the mainwheel is on the ground to ensure maximum
> aileron authority.
>
> Regards,
>
> Geoff Vincent
> Grampians Soaring Club
> Australia
>
>
> On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
> > wrote:
>
>>Aileron Authority & Flaps at Take-off and Landing.
>>
>>Summary.
>>
>>There are two types of launch.
>>
>>Slow, such as aerotow, car and reverse pulley, when it is necessary to
>>control the glider at low speed, perhaps with a large crosswind component,
>>before the glider gains flying speed. It is necessary to start with the
>>controls set to give control at low speed, and perhaps to change the
>>setting
>>as flying speed is gained.
>>
>>Fast, such as winch and bungee (catapult), when the glider gains speed so
>>fast that it is not possible to change the control setting during the
>>ground
>>run, and the glider should start with the control setting needed when
>>first
>>airborne.
>>
>>The problem.
>>
>>We discovered the problem, the solution and the explanation at Lasham in
>>the
>>early 70's when we started flying the Slingsby Kestrel 19.
>>
>>At take-off we were in the two-point tail down pitch attitude.
>>
>>In light winds on aerotow take-off, in neutral or thermal flap setting,
>>and
>>especially when also cross-wind; we found that we had no lateral control
>>at
>>the start of take-off. If a wing went down it stayed down. When the
>>airspeed was above about 30 knots we did have control, even if we were
>>still
>>tail down.
>>
>>The explanation.
>>
>>Ailerons and lateral stability.
>>
>>When we move an aileron down, we increase the Angle of Attack (AoA) at
>>that
>>wingtip. This increases the lift at that wingtip PROVIDED the new,
>>higher,
>>AoA is below the stalling AoA; the wing has lateral stability.
>>
>>If the new, higher, AoA is above the stalling AoA the lift at the wingtip
>>will be reduced. The effect of moving the aileron down will be that the
>>wingtip goes down, the exact opposite of what the pilot intended. The
>>wing
>>has lateral instability.
>>
>>If the wingtip is at or above the stalling AoA with the aileron neutral,
>>the
>>effect of moving the aileron down is immediate and marked, the wing goes
>>straight down.
>>
>>Flying flap setting at takeoff.
>>
>>The effect of moving the flaps (so far as affects flying the machine at
>>take-off) is to change the camber, i.e. as if we changed the angle at
>>which
>>the wing is set on the fuselage. Since at take-off the glider is tail
>>down
>>in the two-point attitude, this changes the AoA.
>>
>>If the ailerons move with the flaps, then with the flaps down the neutral
>>aileron AoA will be higher than when the flaps are up, so we are more
>>likely
>>to have lateral instability.
>>
>>With the flap lever fully forward and the flaps fully up, we have the best
>>chance of lateral stability, the ailerons will work.
>>
>>Change of stalling AofA with speed.
>>
>>When we learn to fly, we are taught that the stalling AoA is the same at
>>all
>>speeds, so that if we achieve the stall AoA at any speed, we will stall
>>even
>>if the speed is high.
>>
>>However, this is not true at very low speeds, due to Reynolds number
>>effects. At 30 knots the stalling AoA will be at the normal flying
>>figure,
>>say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
>>degrees. This explains why we found in our Kestrel 19s (in neutral flap)
>>we had no control at 10 knots and full control at 30.
>>
>>This change of stalling AoA with speed explains why we need full negative
>>flap to have aileron control on take-off at low speeds, but can still have
>>full control with thermal flap setting at 30 knots.
>>
>>The solution.
>>
>>Aerotow.
>>
>>Start the take-off run with the flap lever fully forward, flaps fully up
>>(fully negative). If you have a separate landing flap control (e.g.
>>Kestrel) this flap should also be up.
>>
>>If using a C of G hook, it may be wise to start by holding the wheel brake
>>on to ensure that there is no overrun, this may mean taking up slack with
>>the air-brakes out; warn the launch point crew first!
>>
>>If not holding the wheel brake, or as soon as you have let it off and
>>locked
>>the air-brakes, the left hand should be touching but not holding the
>>release.
>>
>>When you are sure you have full control and will not have to release, move
>>your left hand to the flap lever.
>>
>>As the speed builds, move the flap lever back to the position you intend
>>to
>>use when flying. If you start to lose aileron control, move the flap
>>lever
>>forward again at once, because you moved it back too soon.
>>
>>Start with the stick fully forward. Obviously, if you get the tail up,
>>the
>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>the tail to the normal take-off attitude when the speed is high enough for
>>good aileron control.
>>
>>Cable launching.
>>
>>For car or reverse pulley launching, use the same method as for aerotow.
>>
>>For winching, start with the setting you need once airborne. If the
>>winch
>>and its driver behave as they should, the glider will not have time to
>>drop
>>a wing, and you will not have time to move the flap lever. Use the same
>>method for bungee launching.
>>
>>For winching with a Kestrel, use the half landing flap position (if
>>fitted)
>>for launching, and neutral flying flap position; this setting will be
>>correct for an immediate landing after a low launch failure.
>>
>>If it goes wrong.
>>
>>If the wing goes down release at once. Do not hang on to see if you can
>>get the wing up.
>>
>>If the glider does start to groundloop, it will happen so quickly that the
>>glider will be broken before you can release. If there is any
>>appreciable
>>speed or wind, the groundloop will turn into a cartwheel, which will hurt
>>the pilot as well as the glider.
>>
>>Remember, all the time the launch continues, energy is going into the
>>glider. If you lose control, this energy has to go somewhere.
>>
>>Unflapped gliders.
>>
>>Some unflapped gliders are very close to tip stalling (lateral
>>instability)
>>at the start of the ground run. There are two strategies to try.
>>
>>Stick forward.
>>
>>Start with the stick fully forward. Obviously, if you get the tail up,
>>the
>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>the tail to the normal take-off attitude when the speed is high enough for
>>good aileron control.
>>
>>Airbrakes.
>>
>>Start with the airbrakes open. This gives better lateral control; I
>>don't
>>know why, but it seems to. If you want to start with the wheel brake on,
>>and it is worked by the air brake lever, you are going to have to do this
>>anyway. Clearly the tug pilot must be warned, and anyone at the launch
>>point who may give a stop signal must be told.
>>
>>Use of rudder.
>>
>>A sharp application of rudder makes the glider roll as well as yaw. This
>>can be used as a last resort if the wings are not responding to aileron;
>>this will put you out of line with the tug, but this can be sorted out
>>when
>>you have aileron control. Obviously, this cannot be used if it runs you
>>or
>>the tug off the runway or otherwise into trouble.
>>
>>Landing.
>>
>>Putting the flaps up after landing achieves two things. It dumps lift,
>>making it less likely that bumpy ground or a gust will put you in the air
>>again when you thought you had landed. It improves aileron control when
>>you are moving slowly, but this is less important than when taking off
>>because you are losing energy and speed not gaining it, and you can use
>>the
>>wheel brake. You have to let go of the air brake to move the flaps, if
>>you
>>have near flying speed they may close and cause you to take off again
>>before
>>you get the flaps up; consider raising the tail to reduce angle of attack
>>until the flaps are up.
>>
>>In a Kestrel it is the flying flaps which should go up, to increase
>>aileron
>>authority.
>>
>>Beware of using the wheel brake unless you are dead straight, if you are
>>turning or drifting it may provoke a ground loop.
>>
>>Flight manuals.
>>
>>In general one should always read and obey the flight manual. However
>>the
>>Kestrel manual was written before we knew much of the above, and does not
>>reflect current knowledge and practice. There may be other types to
>>which
>>this applies. Use full negative (fully up) flap for starting aerotow
>>take-offs!
>>
>>W.J. (Bill) Dean (U.K.).
>>Remove "ic" to reply.
>>
>>>
>>> > wrote in message
>>> oups.com...
>>>
>>> Hi Group
>>> Will someone please explain why negative flaps supposedly
>>> provides better aileron control. I know conventional wisdom
>>> says that it does but WHY? It is not intuitively obvious at
>>> least to me. Yes I have tried negative flaps at low speeds
>>> both on the roll and braking but its effect as far as I could
>>> judge was marginal and my thoughts were that it reminded me
>>> of a placebo. So please direct me to the authorative articles
>>> on the subject or if there is a simple explanation please
>>> educate me. Thanks.
>>> Dave
>>>
>>> PS Also posted on the Stemme Owners Group where there is a
>>> thread running on the use of negative flaps for better control.
>>>
>>
>>
>>
>>
>>
>

[[email protected]]

Ok, I'll buy that - makes sense. The explanation that the dropping
wing has a much higher momentary angle of attack also makes sense -
especially if your response is to bang on full aileron!

One hard-core solution is to fill up to the gills with water and let
inertia keep the wings level until aerodynamic control is reached -
just don't move that stick off the forward & center position until you
have enough speed to keep the wings level!

Only partially joking - heavy wings do seem less susceptible to wing
drops, in my limited experience.

Kirk
66

[Bob Gibbons]

For what its worth, based in my 18 years and 2000+ hours in a PIK20B,
I usually landed with 45-60 deg of flap and held the ship off slightly
for a normal 2-point landing. I would then leave the flaps down for
the initial high-speed rollout (on both main and tail wheel) to aid in
decelleration. As the speed decreased, I would roll the flaps to full
up, which would firmly plant the tail wheel. Since the ship was
already rolling on both wheels from initial touchdown till flap
transition, there was no occasion to have the tail wheel "drop to the
ground" as you mention. In any significant crosswind, when I rolled
the flaps to full negative, I would also apply back elevator to help
keep the tail planted and assure directional stability.

Hope this helps.

Bob

On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
> wrote:

>Geoff:
>I also have a PIK-20B and have a question for you (or anyone in the group).
>
>Given the PIK's high AOA when both wheels are on the ground, and given the
>fact that going to full negative flap after touchdown will cause the tail
>wheel to drop to the ground, do you attempt to keep the tail wheel up as
>long as possible by applying forward stick when rolling up the flaps? As you
>know, in a moderate headwind this is not likely an issue but in very light
>wind or no wind, the abrupt loss of aileron control during the late stage of
>rollout is irritating, especially if there is a crosswind component where a
>mild groundloop is virtually guaranteed.
>
>Paul
>ZZ
>"Geoff Vincent" > wrote in message
...
>> Hi Bill,
>>
>> Your comments are spot on. I'm a PIK 20B jockey and always use full
>> -ve flap for take-off, whether I have a wing runner or it's a
>> wing-down launch. Landings are treated similarly, moving to full -ve
>> flap as soon as the mainwheel is on the ground to ensure maximum
>> aileron authority.
>>
>> Regards,
>>
>> Geoff Vincent
>> Grampians Soaring Club
>> Australia
>>
>>
>> On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
>> > wrote:
>>
>>>Aileron Authority & Flaps at Take-off and Landing.
>>>
>>>Summary.
>>>
>>>There are two types of launch.
>>>
>>>Slow, such as aerotow, car and reverse pulley, when it is necessary to
>>>control the glider at low speed, perhaps with a large crosswind component,
>>>before the glider gains flying speed. It is necessary to start with the
>>>controls set to give control at low speed, and perhaps to change the
>>>setting
>>>as flying speed is gained.
>>>
>>>Fast, such as winch and bungee (catapult), when the glider gains speed so
>>>fast that it is not possible to change the control setting during the
>>>ground
>>>run, and the glider should start with the control setting needed when
>>>first
>>>airborne.
>>>
>>>The problem.
>>>
>>>We discovered the problem, the solution and the explanation at Lasham in
>>>the
>>>early 70's when we started flying the Slingsby Kestrel 19.
>>>
>>>At take-off we were in the two-point tail down pitch attitude.
>>>
>>>In light winds on aerotow take-off, in neutral or thermal flap setting,
>>>and
>>>especially when also cross-wind; we found that we had no lateral control
>>>at
>>>the start of take-off. If a wing went down it stayed down. When the
>>>airspeed was above about 30 knots we did have control, even if we were
>>>still
>>>tail down.
>>>
>>>The explanation.
>>>
>>>Ailerons and lateral stability.
>>>
>>>When we move an aileron down, we increase the Angle of Attack (AoA) at
>>>that
>>>wingtip. This increases the lift at that wingtip PROVIDED the new,
>>>higher,
>>>AoA is below the stalling AoA; the wing has lateral stability.
>>>
>>>If the new, higher, AoA is above the stalling AoA the lift at the wingtip
>>>will be reduced. The effect of moving the aileron down will be that the
>>>wingtip goes down, the exact opposite of what the pilot intended. The
>>>wing
>>>has lateral instability.
>>>
>>>If the wingtip is at or above the stalling AoA with the aileron neutral,
>>>the
>>>effect of moving the aileron down is immediate and marked, the wing goes
>>>straight down.
>>>
>>>Flying flap setting at takeoff.
>>>
>>>The effect of moving the flaps (so far as affects flying the machine at
>>>take-off) is to change the camber, i.e. as if we changed the angle at
>>>which
>>>the wing is set on the fuselage. Since at take-off the glider is tail
>>>down
>>>in the two-point attitude, this changes the AoA.
>>>
>>>If the ailerons move with the flaps, then with the flaps down the neutral
>>>aileron AoA will be higher than when the flaps are up, so we are more
>>>likely
>>>to have lateral instability.
>>>
>>>With the flap lever fully forward and the flaps fully up, we have the best
>>>chance of lateral stability, the ailerons will work.
>>>
>>>Change of stalling AofA with speed.
>>>
>>>When we learn to fly, we are taught that the stalling AoA is the same at
>>>all
>>>speeds, so that if we achieve the stall AoA at any speed, we will stall
>>>even
>>>if the speed is high.
>>>
>>>However, this is not true at very low speeds, due to Reynolds number
>>>effects. At 30 knots the stalling AoA will be at the normal flying
>>>figure,
>>>say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
>>>degrees. This explains why we found in our Kestrel 19s (in neutral flap)
>>>we had no control at 10 knots and full control at 30.
>>>
>>>This change of stalling AoA with speed explains why we need full negative
>>>flap to have aileron control on take-off at low speeds, but can still have
>>>full control with thermal flap setting at 30 knots.
>>>
>>>The solution.
>>>
>>>Aerotow.
>>>
>>>Start the take-off run with the flap lever fully forward, flaps fully up
>>>(fully negative). If you have a separate landing flap control (e.g.
>>>Kestrel) this flap should also be up.
>>>
>>>If using a C of G hook, it may be wise to start by holding the wheel brake
>>>on to ensure that there is no overrun, this may mean taking up slack with
>>>the air-brakes out; warn the launch point crew first!
>>>
>>>If not holding the wheel brake, or as soon as you have let it off and
>>>locked
>>>the air-brakes, the left hand should be touching but not holding the
>>>release.
>>>
>>>When you are sure you have full control and will not have to release, move
>>>your left hand to the flap lever.
>>>
>>>As the speed builds, move the flap lever back to the position you intend
>>>to
>>>use when flying. If you start to lose aileron control, move the flap
>>>lever
>>>forward again at once, because you moved it back too soon.
>>>
>>>Start with the stick fully forward. Obviously, if you get the tail up,
>>>the
>>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>>the tail to the normal take-off attitude when the speed is high enough for
>>>good aileron control.
>>>
>>>Cable launching.
>>>
>>>For car or reverse pulley launching, use the same method as for aerotow.
>>>
>>>For winching, start with the setting you need once airborne. If the
>>>winch
>>>and its driver behave as they should, the glider will not have time to
>>>drop
>>>a wing, and you will not have time to move the flap lever. Use the same
>>>method for bungee launching.
>>>
>>>For winching with a Kestrel, use the half landing flap position (if
>>>fitted)
>>>for launching, and neutral flying flap position; this setting will be
>>>correct for an immediate landing after a low launch failure.
>>>
>>>If it goes wrong.
>>>
>>>If the wing goes down release at once. Do not hang on to see if you can
>>>get the wing up.
>>>
>>>If the glider does start to groundloop, it will happen so quickly that the
>>>glider will be broken before you can release. If there is any
>>>appreciable
>>>speed or wind, the groundloop will turn into a cartwheel, which will hurt
>>>the pilot as well as the glider.
>>>
>>>Remember, all the time the launch continues, energy is going into the
>>>glider. If you lose control, this energy has to go somewhere.
>>>
>>>Unflapped gliders.
>>>
>>>Some unflapped gliders are very close to tip stalling (lateral
>>>instability)
>>>at the start of the ground run. There are two strategies to try.
>>>
>>>Stick forward.
>>>
>>>Start with the stick fully forward. Obviously, if you get the tail up,
>>>the
>>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>>the tail to the normal take-off attitude when the speed is high enough for
>>>good aileron control.
>>>
>>>Airbrakes.
>>>
>>>Start with the airbrakes open. This gives better lateral control; I
>>>don't
>>>know why, but it seems to. If you want to start with the wheel brake on,
>>>and it is worked by the air brake lever, you are going to have to do this
>>>anyway. Clearly the tug pilot must be warned, and anyone at the launch
>>>point who may give a stop signal must be told.
>>>
>>>Use of rudder.
>>>
>>>A sharp application of rudder makes the glider roll as well as yaw. This
>>>can be used as a last resort if the wings are not responding to aileron;
>>>this will put you out of line with the tug, but this can be sorted out
>>>when
>>>you have aileron control. Obviously, this cannot be used if it runs you
>>>or
>>>the tug off the runway or otherwise into trouble.
>>>
>>>Landing.
>>>
>>>Putting the flaps up after landing achieves two things. It dumps lift,
>>>making it less likely that bumpy ground or a gust will put you in the air
>>>again when you thought you had landed. It improves aileron control when
>>>you are moving slowly, but this is less important than when taking off
>>>because you are losing energy and speed not gaining it, and you can use
>>>the
>>>wheel brake. You have to let go of the air brake to move the flaps, if
>>>you
>>>have near flying speed they may close and cause you to take off again
>>>before
>>>you get the flaps up; consider raising the tail to reduce angle of attack
>>>until the flaps are up.
>>>
>>>In a Kestrel it is the flying flaps which should go up, to increase
>>>aileron
>>>authority.
>>>
>>>Beware of using the wheel brake unless you are dead straight, if you are
>>>turning or drifting it may provoke a ground loop.
>>>
>>>Flight manuals.
>>>
>>>In general one should always read and obey the flight manual. However
>>>the
>>>Kestrel manual was written before we knew much of the above, and does not
>>>reflect current knowledge and practice. There may be other types to
>>>which
>>>this applies. Use full negative (fully up) flap for starting aerotow
>>>take-offs!
>>>
>>>W.J. (Bill) Dean (U.K.).
>>>Remove "ic" to reply.
>>>
>>>>
>>>> > wrote in message
>>>> oups.com...
>>>>
>>>> Hi Group
>>>> Will someone please explain why negative flaps supposedly
>>>> provides better aileron control. I know conventional wisdom
>>>> says that it does but WHY? It is not intuitively obvious at
>>>> least to me. Yes I have tried negative flaps at low speeds
>>>> both on the roll and braking but its effect as far as I could
>>>> judge was marginal and my thoughts were that it reminded me
>>>> of a placebo. So please direct me to the authorative articles
>>>> on the subject or if there is a simple explanation please
>>>> educate me. Thanks.
>>>> Dave
>>>>
>>>> PS Also posted on the Stemme Owners Group where there is a
>>>> thread running on the use of negative flaps for better control.
>>>>
>>>
>>>
>>>
>>>
>>>
>>
>
>

[Geoff Vincent]

Paul,

Personally I found it a bit difficult to consistently 2-point the PIK
20 due to its inherent tendency to lift off again at the slightest
bump. Now I fly it onto the main wheel and move flaps smoothly to
full -ve using a little forward stick to ensure a soft tailwheel
touch-down. Once both wheels are on the ground full back stick
ensures that they stay there and minimises the risk of cross-wind
drift and ground loops. I agree that in nil-wind conditions it is
common for a wing drop in the final couple of metres but this is never
a problem.

On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
> wrote:

>Geoff:
>I also have a PIK-20B and have a question for you (or anyone in the group).
>
>Given the PIK's high AOA when both wheels are on the ground, and given the
>fact that going to full negative flap after touchdown will cause the tail
>wheel to drop to the ground, do you attempt to keep the tail wheel up as
>long as possible by applying forward stick when rolling up the flaps? As you
>know, in a moderate headwind this is not likely an issue but in very light
>wind or no wind, the abrupt loss of aileron control during the late stage of
>rollout is irritating, especially if there is a crosswind component where a
>mild groundloop is virtually guaranteed.
>
>Paul
>ZZ
>"Geoff Vincent" > wrote in message
...
>> Hi Bill,
>>
>> Your comments are spot on. I'm a PIK 20B jockey and always use full
>> -ve flap for take-off, whether I have a wing runner or it's a
>> wing-down launch. Landings are treated similarly, moving to full -ve
>> flap as soon as the mainwheel is on the ground to ensure maximum
>> aileron authority.
>>
>> Regards,
>>
>> Geoff Vincent
>> Grampians Soaring Club
>> Australia
>>
>>
>> On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
>> > wrote:
>>
>>>Aileron Authority & Flaps at Take-off and Landing.
>>>
>>>Summary.
>>>
>>>There are two types of launch.
>>>
>>>Slow, such as aerotow, car and reverse pulley, when it is necessary to
>>>control the glider at low speed, perhaps with a large crosswind component,
>>>before the glider gains flying speed. It is necessary to start with the
>>>controls set to give control at low speed, and perhaps to change the
>>>setting
>>>as flying speed is gained.
>>>
>>>Fast, such as winch and bungee (catapult), when the glider gains speed so
>>>fast that it is not possible to change the control setting during the
>>>ground
>>>run, and the glider should start with the control setting needed when
>>>first
>>>airborne.
>>>
>>>The problem.
>>>
>>>We discovered the problem, the solution and the explanation at Lasham in
>>>the
>>>early 70's when we started flying the Slingsby Kestrel 19.
>>>
>>>At take-off we were in the two-point tail down pitch attitude.
>>>
>>>In light winds on aerotow take-off, in neutral or thermal flap setting,
>>>and
>>>especially when also cross-wind; we found that we had no lateral control
>>>at
>>>the start of take-off. If a wing went down it stayed down. When the
>>>airspeed was above about 30 knots we did have control, even if we were
>>>still
>>>tail down.
>>>
>>>The explanation.
>>>
>>>Ailerons and lateral stability.
>>>
>>>When we move an aileron down, we increase the Angle of Attack (AoA) at
>>>that
>>>wingtip. This increases the lift at that wingtip PROVIDED the new,
>>>higher,
>>>AoA is below the stalling AoA; the wing has lateral stability.
>>>
>>>If the new, higher, AoA is above the stalling AoA the lift at the wingtip
>>>will be reduced. The effect of moving the aileron down will be that the
>>>wingtip goes down, the exact opposite of what the pilot intended. The
>>>wing
>>>has lateral instability.
>>>
>>>If the wingtip is at or above the stalling AoA with the aileron neutral,
>>>the
>>>effect of moving the aileron down is immediate and marked, the wing goes
>>>straight down.
>>>
>>>Flying flap setting at takeoff.
>>>
>>>The effect of moving the flaps (so far as affects flying the machine at
>>>take-off) is to change the camber, i.e. as if we changed the angle at
>>>which
>>>the wing is set on the fuselage. Since at take-off the glider is tail
>>>down
>>>in the two-point attitude, this changes the AoA.
>>>
>>>If the ailerons move with the flaps, then with the flaps down the neutral
>>>aileron AoA will be higher than when the flaps are up, so we are more
>>>likely
>>>to have lateral instability.
>>>
>>>With the flap lever fully forward and the flaps fully up, we have the best
>>>chance of lateral stability, the ailerons will work.
>>>
>>>Change of stalling AofA with speed.
>>>
>>>When we learn to fly, we are taught that the stalling AoA is the same at
>>>all
>>>speeds, so that if we achieve the stall AoA at any speed, we will stall
>>>even
>>>if the speed is high.
>>>
>>>However, this is not true at very low speeds, due to Reynolds number
>>>effects. At 30 knots the stalling AoA will be at the normal flying
>>>figure,
>>>say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
>>>degrees. This explains why we found in our Kestrel 19s (in neutral flap)
>>>we had no control at 10 knots and full control at 30.
>>>
>>>This change of stalling AoA with speed explains why we need full negative
>>>flap to have aileron control on take-off at low speeds, but can still have
>>>full control with thermal flap setting at 30 knots.
>>>
>>>The solution.
>>>
>>>Aerotow.
>>>
>>>Start the take-off run with the flap lever fully forward, flaps fully up
>>>(fully negative). If you have a separate landing flap control (e.g.
>>>Kestrel) this flap should also be up.
>>>
>>>If using a C of G hook, it may be wise to start by holding the wheel brake
>>>on to ensure that there is no overrun, this may mean taking up slack with
>>>the air-brakes out; warn the launch point crew first!
>>>
>>>If not holding the wheel brake, or as soon as you have let it off and
>>>locked
>>>the air-brakes, the left hand should be touching but not holding the
>>>release.
>>>
>>>When you are sure you have full control and will not have to release, move
>>>your left hand to the flap lever.
>>>
>>>As the speed builds, move the flap lever back to the position you intend
>>>to
>>>use when flying. If you start to lose aileron control, move the flap
>>>lever
>>>forward again at once, because you moved it back too soon.
>>>
>>>Start with the stick fully forward. Obviously, if you get the tail up,
>>>the
>>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>>the tail to the normal take-off attitude when the speed is high enough for
>>>good aileron control.
>>>
>>>Cable launching.
>>>
>>>For car or reverse pulley launching, use the same method as for aerotow.
>>>
>>>For winching, start with the setting you need once airborne. If the
>>>winch
>>>and its driver behave as they should, the glider will not have time to
>>>drop
>>>a wing, and you will not have time to move the flap lever. Use the same
>>>method for bungee launching.
>>>
>>>For winching with a Kestrel, use the half landing flap position (if
>>>fitted)
>>>for launching, and neutral flying flap position; this setting will be
>>>correct for an immediate landing after a low launch failure.
>>>
>>>If it goes wrong.
>>>
>>>If the wing goes down release at once. Do not hang on to see if you can
>>>get the wing up.
>>>
>>>If the glider does start to groundloop, it will happen so quickly that the
>>>glider will be broken before you can release. If there is any
>>>appreciable
>>>speed or wind, the groundloop will turn into a cartwheel, which will hurt
>>>the pilot as well as the glider.
>>>
>>>Remember, all the time the launch continues, energy is going into the
>>>glider. If you lose control, this energy has to go somewhere.
>>>
>>>Unflapped gliders.
>>>
>>>Some unflapped gliders are very close to tip stalling (lateral
>>>instability)
>>>at the start of the ground run. There are two strategies to try.
>>>
>>>Stick forward.
>>>
>>>Start with the stick fully forward. Obviously, if you get the tail up,
>>>the
>>>angle of attack is lower, and also the effect of gusts is reduced. Lower
>>>the tail to the normal take-off attitude when the speed is high enough for
>>>good aileron control.
>>>
>>>Airbrakes.
>>>
>>>Start with the airbrakes open. This gives better lateral control; I
>>>don't
>>>know why, but it seems to. If you want to start with the wheel brake on,
>>>and it is worked by the air brake lever, you are going to have to do this
>>>anyway. Clearly the tug pilot must be warned, and anyone at the launch
>>>point who may give a stop signal must be told.
>>>
>>>Use of rudder.
>>>
>>>A sharp application of rudder makes the glider roll as well as yaw. This
>>>can be used as a last resort if the wings are not responding to aileron;
>>>this will put you out of line with the tug, but this can be sorted out
>>>when
>>>you have aileron control. Obviously, this cannot be used if it runs you
>>>or
>>>the tug off the runway or otherwise into trouble.
>>>
>>>Landing.
>>>
>>>Putting the flaps up after landing achieves two things. It dumps lift,
>>>making it less likely that bumpy ground or a gust will put you in the air
>>>again when you thought you had landed. It improves aileron control when
>>>you are moving slowly, but this is less important than when taking off
>>>because you are losing energy and speed not gaining it, and you can use
>>>the
>>>wheel brake. You have to let go of the air brake to move the flaps, if
>>>you
>>>have near flying speed they may close and cause you to take off again
>>>before
>>>you get the flaps up; consider raising the tail to reduce angle of attack
>>>until the flaps are up.
>>>
>>>In a Kestrel it is the flying flaps which should go up, to increase
>>>aileron
>>>authority.
>>>
>>>Beware of using the wheel brake unless you are dead straight, if you are
>>>turning or drifting it may provoke a ground loop.
>>>
>>>Flight manuals.
>>>
>>>In general one should always read and obey the flight manual. However
>>>the
>>>Kestrel manual was written before we knew much of the above, and does not
>>>reflect current knowledge and practice. There may be other types to
>>>which
>>>this applies. Use full negative (fully up) flap for starting aerotow
>>>take-offs!
>>>
>>>W.J. (Bill) Dean (U.K.).
>>>Remove "ic" to reply.
>>>
>>>>
>>>> > wrote in message
>>>> oups.com...
>>>>
>>>> Hi Group
>>>> Will someone please explain why negative flaps supposedly
>>>> provides better aileron control. I know conventional wisdom
>>>> says that it does but WHY? It is not intuitively obvious at
>>>> least to me. Yes I have tried negative flaps at low speeds
>>>> both on the roll and braking but its effect as far as I could
>>>> judge was marginal and my thoughts were that it reminded me
>>>> of a placebo. So please direct me to the authorative articles
>>>> on the subject or if there is a simple explanation please
>>>> educate me. Thanks.
>>>> Dave
>>>>
>>>> PS Also posted on the Stemme Owners Group where there is a
>>>> thread running on the use of negative flaps for better control.
>>>>
>>>
>>>
>>>
>>>
>>>
>>
>

[Bob Caldwell]

On the subject of roll authority early on ground roll how about some comments about standard class ships starting ground roll with air brakes deployed? My ASW 28 definitely benefits from this procedure. I have heard various rational for the practice but am curious about other comments.

Thanks,

Bob

[5Z]

Read back in this thread for the effect of spoilers on aileron
authority at low speed.

-Tom

[For Example John Smith]

Sounds like this might be a lot like the Glasflugel Mosquito. If you do a 2
point landing any bump will have you airborne. If land tail first though,
that doesn't happen.

on topic.... full -ve at the beginning of the roll & move quickly to the
first position, then neutral for lift off. No idea why that works best. It
does, so I do it.

"Geoff Vincent" > wrote in message
...
> Paul,
>
> Personally I found it a bit difficult to consistently 2-point the PIK
> 20 due to its inherent tendency to lift off again at the slightest
> bump. Now I fly it onto the main wheel and move flaps smoothly to
> full -ve using a little forward stick to ensure a soft tailwheel
> touch-down. Once both wheels are on the ground full back stick
> ensures that they stay there and minimises the risk of cross-wind
> drift and ground loops. I agree that in nil-wind conditions it is
> common for a wing drop in the final couple of metres but this is never
> a problem.
>
> On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
> > wrote:
>
> >Geoff:
> >I also have a PIK-20B and have a question for you (or anyone in the
group).
> >
> >Given the PIK's high AOA when both wheels are on the ground, and given
the
> >fact that going to full negative flap after touchdown will cause the tail
> >wheel to drop to the ground, do you attempt to keep the tail wheel up as
> >long as possible by applying forward stick when rolling up the flaps? As
you
> >know, in a moderate headwind this is not likely an issue but in very
light
> >wind or no wind, the abrupt loss of aileron control during the late stage
of
> >rollout is irritating, especially if there is a crosswind component where
a
> >mild groundloop is virtually guaranteed.
> >
> >Paul
> >ZZ
> >"Geoff Vincent" > wrote in message
> ...
> >> Hi Bill,
> >>
> >> Your comments are spot on. I'm a PIK 20B jockey and always use full
> >> -ve flap for take-off, whether I have a wing runner or it's a
> >> wing-down launch. Landings are treated similarly, moving to full -ve
> >> flap as soon as the mainwheel is on the ground to ensure maximum
> >> aileron authority.
> >>
> >> Regards,
> >>
> >> Geoff Vincent
> >> Grampians Soaring Club
> >> Australia
> >>
> >>
> >> On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
> >> > wrote:
> >>
> >>>Aileron Authority & Flaps at Take-off and Landing.
> >>>
> >>>Summary.
> >>>
> >>>There are two types of launch.
> >>>
> >>>Slow, such as aerotow, car and reverse pulley, when it is necessary to
> >>>control the glider at low speed, perhaps with a large crosswind
component,
> >>>before the glider gains flying speed. It is necessary to start with
the
> >>>controls set to give control at low speed, and perhaps to change the
> >>>setting
> >>>as flying speed is gained.
> >>>
> >>>Fast, such as winch and bungee (catapult), when the glider gains speed
so
> >>>fast that it is not possible to change the control setting during the
> >>>ground
> >>>run, and the glider should start with the control setting needed when
> >>>first
> >>>airborne.
> >>>
> >>>The problem.
> >>>
> >>>We discovered the problem, the solution and the explanation at Lasham
in
> >>>the
> >>>early 70's when we started flying the Slingsby Kestrel 19.
> >>>
> >>>At take-off we were in the two-point tail down pitch attitude.
> >>>
> >>>In light winds on aerotow take-off, in neutral or thermal flap setting,
> >>>and
> >>>especially when also cross-wind; we found that we had no lateral
control
> >>>at
> >>>the start of take-off. If a wing went down it stayed down. When
the
> >>>airspeed was above about 30 knots we did have control, even if we were
> >>>still
> >>>tail down.
> >>>
> >>>The explanation.
> >>>
> >>>Ailerons and lateral stability.
> >>>
> >>>When we move an aileron down, we increase the Angle of Attack (AoA) at
> >>>that
> >>>wingtip. This increases the lift at that wingtip PROVIDED the new,
> >>>higher,
> >>>AoA is below the stalling AoA; the wing has lateral stability.
> >>>
> >>>If the new, higher, AoA is above the stalling AoA the lift at the
wingtip
> >>>will be reduced. The effect of moving the aileron down will be that
the
> >>>wingtip goes down, the exact opposite of what the pilot intended. The
> >>>wing
> >>>has lateral instability.
> >>>
> >>>If the wingtip is at or above the stalling AoA with the aileron
neutral,
> >>>the
> >>>effect of moving the aileron down is immediate and marked, the wing
goes
> >>>straight down.
> >>>
> >>>Flying flap setting at takeoff.
> >>>
> >>>The effect of moving the flaps (so far as affects flying the machine at
> >>>take-off) is to change the camber, i.e. as if we changed the angle at
> >>>which
> >>>the wing is set on the fuselage. Since at take-off the glider is tail
> >>>down
> >>>in the two-point attitude, this changes the AoA.
> >>>
> >>>If the ailerons move with the flaps, then with the flaps down the
neutral
> >>>aileron AoA will be higher than when the flaps are up, so we are more
> >>>likely
> >>>to have lateral instability.
> >>>
> >>>With the flap lever fully forward and the flaps fully up, we have the
best
> >>>chance of lateral stability, the ailerons will work.
> >>>
> >>>Change of stalling AofA with speed.
> >>>
> >>>When we learn to fly, we are taught that the stalling AoA is the same
at
> >>>all
> >>>speeds, so that if we achieve the stall AoA at any speed, we will stall
> >>>even
> >>>if the speed is high.
> >>>
> >>>However, this is not true at very low speeds, due to Reynolds number
> >>>effects. At 30 knots the stalling AoA will be at the normal flying
> >>>figure,
> >>>say 18 degrees. But at 5 or 10 knots the stalling AoA will be about
10
> >>>degrees. This explains why we found in our Kestrel 19s (in neutral
flap)
> >>>we had no control at 10 knots and full control at 30.
> >>>
> >>>This change of stalling AoA with speed explains why we need full
negative
> >>>flap to have aileron control on take-off at low speeds, but can still
have
> >>>full control with thermal flap setting at 30 knots.
> >>>
> >>>The solution.
> >>>
> >>>Aerotow.
> >>>
> >>>Start the take-off run with the flap lever fully forward, flaps fully
up
> >>>(fully negative). If you have a separate landing flap control (e.g.
> >>>Kestrel) this flap should also be up.
> >>>
> >>>If using a C of G hook, it may be wise to start by holding the wheel
brake
> >>>on to ensure that there is no overrun, this may mean taking up slack
with
> >>>the air-brakes out; warn the launch point crew first!
> >>>
> >>>If not holding the wheel brake, or as soon as you have let it off and
> >>>locked
> >>>the air-brakes, the left hand should be touching but not holding the
> >>>release.
> >>>
> >>>When you are sure you have full control and will not have to release,
move
> >>>your left hand to the flap lever.
> >>>
> >>>As the speed builds, move the flap lever back to the position you
intend
> >>>to
> >>>use when flying. If you start to lose aileron control, move the flap
> >>>lever
> >>>forward again at once, because you moved it back too soon.
> >>>
> >>>Start with the stick fully forward. Obviously, if you get the tail
up,
> >>>the
> >>>angle of attack is lower, and also the effect of gusts is reduced.
Lower
> >>>the tail to the normal take-off attitude when the speed is high enough
for
> >>>good aileron control.
> >>>
> >>>Cable launching.
> >>>
> >>>For car or reverse pulley launching, use the same method as for
aerotow.
> >>>
> >>>For winching, start with the setting you need once airborne. If the
> >>>winch
> >>>and its driver behave as they should, the glider will not have time to
> >>>drop
> >>>a wing, and you will not have time to move the flap lever. Use the
same
> >>>method for bungee launching.
> >>>
> >>>For winching with a Kestrel, use the half landing flap position (if
> >>>fitted)
> >>>for launching, and neutral flying flap position; this setting will be
> >>>correct for an immediate landing after a low launch failure.
> >>>
> >>>If it goes wrong.
> >>>
> >>>If the wing goes down release at once. Do not hang on to see if you
can
> >>>get the wing up.
> >>>
> >>>If the glider does start to groundloop, it will happen so quickly that
the
> >>>glider will be broken before you can release. If there is any
> >>>appreciable
> >>>speed or wind, the groundloop will turn into a cartwheel, which will
hurt
> >>>the pilot as well as the glider.
> >>>
> >>>Remember, all the time the launch continues, energy is going into the
> >>>glider. If you lose control, this energy has to go somewhere.
> >>>
> >>>Unflapped gliders.
> >>>
> >>>Some unflapped gliders are very close to tip stalling (lateral
> >>>instability)
> >>>at the start of the ground run. There are two strategies to try.
> >>>
> >>>Stick forward.
> >>>
> >>>Start with the stick fully forward. Obviously, if you get the tail
up,
> >>>the
> >>>angle of attack is lower, and also the effect of gusts is reduced.
Lower
> >>>the tail to the normal take-off attitude when the speed is high enough
for
> >>>good aileron control.
> >>>
> >>>Airbrakes.
> >>>
> >>>Start with the airbrakes open. This gives better lateral control; I
> >>>don't
> >>>know why, but it seems to. If you want to start with the wheel brake
on,
> >>>and it is worked by the air brake lever, you are going to have to do
this
> >>>anyway. Clearly the tug pilot must be warned, and anyone at the
launch
> >>>point who may give a stop signal must be told.
> >>>
> >>>Use of rudder.
> >>>
> >>>A sharp application of rudder makes the glider roll as well as yaw.
This
> >>>can be used as a last resort if the wings are not responding to
aileron;
> >>>this will put you out of line with the tug, but this can be sorted out
> >>>when
> >>>you have aileron control. Obviously, this cannot be used if it runs
you
> >>>or
> >>>the tug off the runway or otherwise into trouble.
> >>>
> >>>Landing.
> >>>
> >>>Putting the flaps up after landing achieves two things. It dumps
lift,
> >>>making it less likely that bumpy ground or a gust will put you in the
air
> >>>again when you thought you had landed. It improves aileron control
when
> >>>you are moving slowly, but this is less important than when taking off
> >>>because you are losing energy and speed not gaining it, and you can use
> >>>the
> >>>wheel brake. You have to let go of the air brake to move the flaps,
if
> >>>you
> >>>have near flying speed they may close and cause you to take off again
> >>>before
> >>>you get the flaps up; consider raising the tail to reduce angle of
attack
> >>>until the flaps are up.
> >>>
> >>>In a Kestrel it is the flying flaps which should go up, to increase
> >>>aileron
> >>>authority.
> >>>
> >>>Beware of using the wheel brake unless you are dead straight, if you
are
> >>>turning or drifting it may provoke a ground loop.
> >>>
> >>>Flight manuals.
> >>>
> >>>In general one should always read and obey the flight manual. However
> >>>the
> >>>Kestrel manual was written before we knew much of the above, and does
not
> >>>reflect current knowledge and practice. There may be other types to
> >>>which
> >>>this applies. Use full negative (fully up) flap for starting aerotow
> >>>take-offs!
> >>>
> >>>W.J. (Bill) Dean (U.K.).
> >>>Remove "ic" to reply.
> >>>
> >>>>
> >>>> > wrote in message
> >>>> oups.com...
> >>>>
> >>>> Hi Group
> >>>> Will someone please explain why negative flaps supposedly
> >>>> provides better aileron control. I know conventional wisdom
> >>>> says that it does but WHY? It is not intuitively obvious at
> >>>> least to me. Yes I have tried negative flaps at low speeds
> >>>> both on the roll and braking but its effect as far as I could
> >>>> judge was marginal and my thoughts were that it reminded me
> >>>> of a placebo. So please direct me to the authorative articles
> >>>> on the subject or if there is a simple explanation please
> >>>> educate me. Thanks.
> >>>> Dave
> >>>>
> >>>> PS Also posted on the Stemme Owners Group where there is a
> >>>> thread running on the use of negative flaps for better control.
> >>>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >
>

[[email protected]]

Hi Gang
I have received an enormous number of responses to my original posting
the use of negative flaps to improve effective aileron control while on
the ground. I think I now have a reasonable understanding of the issues
involved and would like to present a summary of my understandings.
Probably the worst situation occurs with flaperons whereby the flaps
are combined with the ailerons from the wing tips to the root of the
wings. I own 2 gliders - a Stemme S10 VT and a DG800B that have these
flaperons and these 2 ships are difficult to handle in gusty conditions
both on the initial role and breaking on landing. Definitely not entry
level gliders!
So lets make sure we first agree on some of the fundermentals:

1) The angle of attack AOA of the wing is defined by the angle between
the cord of the wing and the effective direction of the flow of the air
that the wing is moving in.

2) The stall of a wing (or a plank of wood) occurs when the AOA reaches
about 17 degrees IRRESPECTIVE, to the first order, of the speed that
the wing is moving through the air whether it be 5mph or 150mph. For
almost all wings ever invented the stall AOA is between 16 and 19
degrees. This is a remarkable fact - not intuitively obvious. Onset of
stall is loosely defined as a dramatic drop of lift.

3) As the AOA of a wing is increased from zero lift also increases
somewhat linearly to an AOA of about 8 degrees. Then the rate of
increase of lift decreases for a further increase of AOA and lift
reaches a maximum at around 12 degrees (minimum sink). At AOAs greater
than 12 degrees lift then diminishes at an ever increasing rate so that
around 17 degrees lift is a small fraction of what is was at 12
degrees. Note that, AND THIS IS EXTREMELY IMPORTANT, except at minimum
sink there are 2 AOA values that will give the IDENTICAL value of lift.
I will show this to be the Archille's Heel for many of our low speed
control problems.

OK now a typical situation with a flaperon ship such as my Stemme on
initial role say with 5 degrees of flaps. The tail wheel is on the
ground and the Stemme, because of its high undercarriage, is pointing
its nose upward. The AOA of the wings are around 12 degrees near
minimum sink. A gust hits me from the side and a wing drops. I react by
full aileron usage and the wing I am trying to lift now has an
effective AOA of 16 or 17 degrees whereas the opposite wing has an AOA
of 7 or 8 degrees. Which wing has the highest lift? The stalled wing
or the one with the AOA of 8 degrees? THE UNSTALLED WING has the
highest lift! In other words the the aileron control has reversed
itself and I am aggravating the problem rather than solving the
problem. If I am unlucky the wing that the gust has hit will itself hit
the runway through my over reaction with the ailerons. What should I
have done? 2 things - the first started off with negative flaps and
secondly have been gentler on the aileron control. In so doing the AOAs
of both wings would have been less than 12 degrees (minimum sink) and
aileron control would be normal not reversed.

Lets recap for a moment. What I am saying is that, if the AOA is
around 12 degrees (minimum sink), and you use use full aileron
deflection, you have a good chance of reversing the operation of the
ailerons. On the ground that means loss of control and in the air the
very real possibility of a spin. All this because there are 2 values of
AOA that give the same value of lift. The only exception is exactly at
minimum sink.

OK What to do? Clearly if you have flaperons use negative flaps for
the initial roll until the tail comes up and then go to whatever the
book says (normally 5 or 8 degrees positive). On landing do what the
book says and then on braking go to full negative flaps. Avoid large
movements of the ailerons. Don't over react!

I would recommend for all gliders that you be aware of the effective
AOA of the wings with the tail wheel on the ground. Some gliders such
as the Mosquito, I am told, are in a stalled condition until the tail
is raised. Again small movements of the ailerons are called for and get
that tail wheel up as soon as possible. Know your glider.
Dave

PS It is claimed that spoilers improve low speed aileron control. Well
maybe. How about someone figuring out in a rational manner if this is
so. I would be real interested in the science if it were shown to be
true.

[Nyal Williams]

It seems to me that the various comments on this subject
are confused. If there is any kind of twist in the
wing - aerodynamic or otherwise, then each part of
the wing has its own angle of attack. Moving an aileron
changes the angle of attack only of that part of the
wing (plus whatever disturbance it creates for a short
distance inboard).

The original question was about what effect flaps had
on aileron efficiency. I presumed this to mean a wing
with no interconnection between flaps and ailerons
and definitely not a wing with flaperons. With such
a wing in mind, it appears to me that any effect the
flap setting would have on the aileron would be the
disturbance the flap causes at its outboard end and
across the inboard end of the aileron. I'd like to
read something about that. (I'm no aerodynamicist,
and have no mathematical skill.)



At 13:54 05 August 2005, T O D D P A T T I S T wrote:
' wrote:
>
>>Probably the worst situation occurs with flaperons
>>
>
>I'm not sure what you mean by 'worst situation.' My
>Ventus
>C has flaperons. It does have reduced aileron effectiveness
>at low speed when the flaps are positive or zero, but
>not so
>much that it's uncontrollable. I flew it that way
>for my
>first few flights. Putting the flaps in negative position
>improves aileron effectiveness significantly.
>
>> So lets make sure we first agree on some of the fundermentals:
>>
>>2) ... Onset of stall is loosely defined as a dramatic
>>drop of lift.
>
>Not really. Stall is at maximum lift, and lift drops
>off
>moderately after that. The big difference is that
>at AOA
>above stall, the lift decreases with AOA. When flying,
>this
>means that beyond stall, the nose drops, the wing descends,
>which increases the AOA even more, which reduces lift
>more,
>which decreases lift more, etc. This runaway decrease
>in
>lift is why so many think that stalling means that
>lift
>drops to zero or near zero at stall. It's actually
>at the
>maximum there and just beyond stall..
>
>On the ground, this effect is different, as the weight
>of
>the aircraft is not supported by the air, so it can't
>drop
>and thereby increase the AOA in the same way it drops
>in
>the air.
>
>>3) As the AOA of a wing is increased from zero lift
>>also increases
>>somewhat linearly to an AOA of about 8 degrees.
>
>Yes.
>
>>Then the rate of increase of lift decreases for a further
>>increase of AOA
>
>I agree - the 'rate of increase' decreases. This is
>the
>nonlinearity of the CL curve I discussed.
>
>>and lift reaches a maximum at around 12 degrees (minimum
>>sink).
>
>No. Lift is max at around 17 degrees - at the critical
>AOA
>(stall angle).
>
>>At AOAs greater
>>than 12 degrees lift then diminishes at an ever increasing
>>rate so that
>>around 17 degrees lift is a small fraction of what
>>is was at 12
>>degrees.
>
>This is wrong. Lift increases smoothly to its maximum
>up
>to 17 degrees. The rate of that increase varies, but
>it's a
>positive rate up to the critical angle and then the
>'rate
>of increase' is zero and it's about to turn negative.
>
>>Note that, AND THIS IS EXTREMELY IMPORTANT, except
>>at minimum
>>sink there are 2 AOA values that will give the IDENTICAL
>>value of lift.
>
>No. Lift is a function of airspeed and AOA (and air
>density,
>which we can ignore) There are an infinite number
>of AOA
>values that give the same lift. You tell me the AOA
>and
>lift you want, and I'll calculate the airspeed.
>
>>I will show this to be the Archille's Heel for many
>>of our low speed
>>control problems.
>>
>>OK now a typical situation with a flaperon ship such
>>as my Stemme on
>>initial role say with 5 degrees of flaps. The tail
>>wheel is on the
>>ground and the Stemme, because of its high undercarriage,
>>is pointing
>>its nose upward. The AOA of the wings are around 12
>>degrees near
>>minimum sink. A gust hits me from the side and a wing
>>drops. I react by
>>full aileron usage and the wing I am trying to lift
>>now has an
>>effective AOA of 16 or 17 degrees whereas the opposite
>>wing has an AOA
>>of 7 or 8 degrees. Which wing has the highest lift?
>>The stalled wing
>>or the one with the AOA of 8 degrees? THE UNSTALLED
>>WING has the
>>highest lift!
>
>No, the wing at 17 degrees has the highest lift. In
>fact,
>it will have the higher lift, even if it's stalled
>at 18
>degrees. ( I should mention that you can't just assume
>that
>the aileron changes the AOA of the wing. Lowering
>the
>aileron changes the camber of the wing, which produces
>a
>different airfoil having a different CL curve.
>
>>In other words the the aileron control has reversed
>>itself and I am aggravating the problem rather than
>>solving the
>>problem.
>
>No. Control reversal does not occur on the ground.
>
>
>>If I am unlucky the wing that the gust has hit will
>>itself hit
>>the runway through my over reaction with the ailerons.
>>What should I
>>have done? 2 things - the first started off with negative
>>flaps
>
>Yes.
>
>>and secondly have been gentler on the aileron control.
>>In so doing the AOAs
>>of both wings would have been less than 12 degrees
>>(minimum sink) and
>>aileron control would be normal not reversed.
>
>No, although the earliest possible response is best,
>using
>the least aileron required to do the job.
>
>> Lets recap for a moment. What I am saying is that,
>>if the AOA is
>>around 12 degrees (minimum sink), and you use use
>>full aileron
>>deflection, you have a good chance of reversing the
>>operation of the
>>ailerons.
>
>No.
>
>>On the ground that means loss of control
>
>No.
>
>>and in the air the very real possibility of a spin.
>>
>
>'In the air' is a much different condition.
>
>>All this because there are 2 values of
>>AOA that give the same value of lift. The only exception
>>is exactly at
>>minimum sink.
>
>No.
>
>> OK What to do? Clearly if you have flaperons use negative
>>flaps for
>>the initial roll until the tail comes up and then go
>>to whatever the
>>book says (normally 5 or 8 degrees positive). On landing
>>do what the
>>book says and then on braking go to full negative flaps.
>
>Agreed.
>
>> Avoid large
>>movements of the ailerons. Don't over react!
>
>You shouldn't need full aileron, and I agree overreaction
>is
>bad, but if you need full, then use it. It may not
>be
>enough, but don't expect more control from less aileron.
>You won't get it.
>
>
>
>T o d d P a t t i s t - 'WH' Ventus C
>(Remove DONTSPAMME from address to email reply.)
>

[Bob Whelan]

> The original question was about what effect flaps had
> on aileron efficiency. I presumed this to mean a wing
> with no interconnection between flaps and ailerons
> and definitely not a wing with flaperons.

I've flown a 15-meter glider with no *rolling* connection between flaps and
ailerons since 1981. By 'no rolling connection' I mean only ailerons impart
roll, regardless of flap position...the flaps lack capability for
differential movement. On this particular ship, assuming neutral
roll-stick, at zero and negative flap settings the entire trailing edge is
in-line (i.e. ailerons and flaps camber-track identically). During the
camber-changing portion of positive flap deflection, the flaps droop twice
as much as the ailerons. Once the ailerons have reached 'max droop' they
remain there while the flaps continue down to ~75-degrees for glide path
control.

From the pilot's perspective, when the ship is on the ground, at
low/early-in-takeoff-roll/post-landing-rollout speeds, the ailerons are
distinctly most effective with flaps negative, less so with flaps neutral
and worst with flaps positive. (Experimenting - ahem, 'roll playing!' -
with a stationary glider, whether flapped and/or spoilered, etc., in a
steady headwind is recommended for the curious. If 'it' happens - i.e.
altered aileron effectiveness - it must be possible.)

For the purposes of the following discussion, I'll take the paraphrased
question about 'what effect flaps had on aileron efficiency' to mean 'what
effect flap position has on perceived aileron effectiveness at
low-relative-wind speeds.'

Though there may be an exception or two out there (I know of none), in
general, negative flaps early in the roll make ailerons 'more effective.'
One way to view why this is so is inertially. In a given state (i.e.
un/partially/fully ballasted), any glider has its minimum roll inertia at
zero airspeed. Any lift created by air flowing past the wing effectively
increases roll inertia by 'stiffening' the glider in roll, meaning aileron
effect will be diminished from what it would be if (say) the air flowed ONLY
over the wings over the span of the ailerons. In other words, anything that
can be done to minimize lift produced from the aileron-less portion of the
wings is to the relative good of maximizing the ailerons' roll
effectiveness. I suspect this relative reduction in roll inertia is the
largest contributor to the 'improved aileron effect.' (Consider fully
ballasted roll inertial effects for example...assuming an equal and
nominally good wing run, which is more likely to drop a wing, a ballasted or
an unballasted otherwise identical glider? For skeptics who claim I'm
confusing aerodynamic effects with mass/inertial effects, F=ma.)

> With such
> a wing in mind, it appears to me that any effect the
> flap setting would have on the aileron would be the
> disturbance the flap causes at its outboard end and
> across the inboard end of the aileron. I'd like to
> read something about that. (I'm no aerodynamicist,
> and have no mathematical skill.)

Any break in the trailing edge between flap and aileron will indeed have
some 2nd-order (e.g. transverse airflow) aerodynamic effects beyond the
1st-order (i.e. 2-dimensional-flow-based) ones...exactly zero of which will
be distinguishable to the pilot in the cockpit. Additionally, Todd
Pattist's conception of what's happening when viewing things from the
perspective of the lift-curve vs. angle-of-attack is on-target...but again,
the extent a pilot in the cockpit actually *uses* these sorts of thoughts in
the heat of the moment is debatable. Prioritizing things, it seems to me
what matters to a (non ab-initio) PIC's perspective is: 1) s/he conceptually
grasps 'what's likely to happen when,' maximizing chances of remaining ahead
of the plane and doing the right things in a timely manner, prior to 2)
comprehending 'technically why'. If one's grasp of 'technically why' is
missing, incomplete or downright inaccurate at mathematical or scientific
levels, it matters not so long as it doesn't interfere with 'conceptual
reality,' in which case "No harm, no foul," applies.

For me, having some grasp of 'technically why' helps me remember 'conceptual
reality.' Ideally that grasp will also be accurate, of course!

Regards,
Bob W.

[5Z]

wrote:
> PS It is claimed that spoilers improve low speed aileron control. Well
> maybe. How about someone figuring out in a rational manner if this is
> so. I would be real interested in the science if it were shown to be
> true.

I posted a description if this earlier in the thread - go back and take
a look.

Bottom line is that if the full span of the wing is producing some
lift,the roll is damped. If the spoilers are opened the lift of that
portion of the wing is reduced and so the roll is less damped and the
feeble aileron inputs have more effect on keeping the wings where you
want them.

-Tom

[Ian]

On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:

> In all normal flight the maximum lift, i.e. stalling, AoA is constant.
> However, at the very low speeds met with in light winds at the start of
> take-off and the end of landings, the stalling AoA is very much less.
>
> Whereas in all normal flight conditions, from 1G "stalling speed" to Vne,
> the stalling AoA remains constant at a figure probably between 16 and 19
> degrees, at 5 or 10 mph it is around 10 degrees.

This is interesting, it explains a lot. I would like to read up some
more. Do you know of any references (perhaps on the net, or Reichman) that
describe this effect?

Also is there any "hysteresis" in this effect. In other words, if an
airfoil is "flying" at say 12 degrees angle of attack, and you reduce the
airspeed, to say 20 km/h, until it "stalls", not in the conventional
sense - in that it is not producing lift to support the aircraft mass at
1G, but because the airflow separates and the co-efficient of lift
deteriorates.

Will the same airfoil, at the same angle of attack "unstall" if the speed
is increased above 20 km/h, or will it require a higher speed, say 25
km/h before the airflow normalises and the expected co-efficient of lift
returns?

In my experience, in a glider with marginal aileron (or rudder) control,
in a hot & high cross wind take off, it is better to keep the controls
neutral until the glider has some airspeed before correcting for a wing
drop (or yaw). If you immediately apply full control deflection then wait
for the speed to build up, it seems to take longer before the controls
"unstall" and become effective enough to correct the situation. (Either
way the left hand is never far from the release...)

Thanks

Ian

[W.J. \(Bill\) Dean \(U.K.\).]

> "Ian" > wrote in message
> . ..
>
> On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:
>
>>
>> In all normal flight the maximum lift, i.e. stalling, AoA is constant.
>> However, at the very low speeds met with in light winds at the start of
>> take-off and the end of landings, the stalling AoA is very much less.
>>
>> Whereas in all normal flight conditions, from 1G "stalling speed" to Vne,
>> the stalling AoA remains constant at a figure probably between 16 and 19
>> degrees, at 5 or 10 mph it is around 10 degrees.
>>
>
> This is interesting, it explains a lot. I would like to read up some
> more. Do you know of any references (perhaps on the net, or Reichmann)
> that describe this effect?
>
> Also is there any "hysteresis" in this effect. In other words, if an
> airfoil is "flying" at say 12 degrees angle of attack, and you reduce the
> airspeed, to say 20 km/h, until it "stalls", not in the conventional
> sense - in that it is not producing lift to support the aircraft mass at
> 1G, but because the airflow separates and the co-efficient of lift
> deteriorates.
>
> Will the same airfoil, at the same angle of attack "unstall" if the speed
> is increased above 20 km/h, or will it require a higher speed, say 25
> km/h before the airflow normalises and the expected co-efficient of lift
> returns?
>
> In my experience, in a glider with marginal aileron (or rudder) control,
> in a hot & high cross wind take off, it is better to keep the controls
> neutral until the glider has some airspeed before correcting for a wing
> drop (or yaw). If you immediately apply full control deflection then
> wait for the speed to build up, it seems to take longer before the
> controls "unstall" and become effective enough to correct the situation.
> (Either way the left hand is never far from the release...)
>
> Thanks
>
> Ian
>

I do not know of any references.

We discovered this effect when we first flew the Slingsby Kestrel 19 at
Lasham in the 1970s. We discovered that we had a low speed control problem
on take-off; there were many theories as to why, and as to what to do.
Then someone discovered that using negative flaps at the start of the ground
run "worked", so we all did it without understanding why it worked.

Later we were told it was a Reynolds number effect, which resulted in a
lower stall AoA at very low speeds; my memory is that it was Derek Piggott
(our Chief Flying Instructor) who told us, whether he worked it out himself
or whether he consulted others such as Frank Irving I have no idea.

I do not understand about Reynolds numbers, but I understand that they
change with density as well as with speed, so that the behaviour of a glider
at say 20,000 ft. at a 1 G stall may well be different to that at sea level
at a 1 G stall. Also, I understand that model builders find that an exact
model of a glider may fly differently from the real thing, because both the
wing chord and the speed are different and it flies at a very different
Reynolds number; I think that models cannot get as good an L over D as the
real thing.

I should be very surprised if there is any hysteresis effect as the speed
changes. If you are using aileron when this results in tip stalling and
lateral instability, you will have more to do when the aileron starts flying
properly; just as if you use aileron when stalling in flight it can trigger
a wing drop against the aileron moving down.

I don't know what you mean by " "stalls", not in the conventional sense - in
that it is not producing lift to support the aircraft mass at 1G,". The
wing is either stalled or it is not. If it is stalled then an increase in
AoA will produce a decrease in lift, and whether or not the lift at the
stalling AoA is more or less than the weight of the glider is irrelevant.

Those who cable launch must be trained and practiced at recovering from
launch failures. A recovery from a break when in a steep climb may involve
a push over to the recovery attitude during which the speed drops well below
the 1G stall speed as you go over the top; provided the AoA is kept well
below the stall AoA the glider is not stalled even though the wing is not
producing enough lift to support the glider.

I have no experience of "hot and high", the highest glider site in the UK is
the Long Mynd at about 1,400 ft. It may well be that there is a change in
behaviour at high density altitudes, either in quality or in relation to
indicated speeds; but I would expect the same principals to apply.

I think most glider handbooks give an indication of the maximum density
altitudes flown when test flying (by implication the height at which the
glider was tested to Vne).

I suspect that glider flying in the hot and high parts of the USA may reveal
things about a glider which the designers and test pilots did not know.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.

[Bill Daniels]

"W.J. (Bill) Dean (U.K.)." > wrote in message
...
> > "Ian" > wrote in message
> > . ..
> >
> > On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:

>
> Snip------
> Those who cable launch must be trained and practiced at recovering from
> launch failures. A recovery from a break when in a steep climb may
involve
> a push over to the recovery attitude during which the speed drops well
below
> the 1G stall speed as you go over the top; provided the AoA is kept well
> below the stall AoA the glider is not stalled even though the wing is not
> producing enough lift to support the glider.

Very true.

>
> I have no experience of "hot and high", the highest glider site in the UK
is
> the Long Mynd at about 1,400 ft. It may well be that there is a change
in
> behaviour at high density altitudes, either in quality or in relation to
> indicated speeds; but I would expect the same principals to apply.
>
> I think most glider handbooks give an indication of the maximum density
> altitudes flown when test flying (by implication the height at which the
> glider was tested to Vne).
>
> I suspect that glider flying in the hot and high parts of the USA may
reveal
> things about a glider which the designers and test pilots did not know.
>
> W.J. (Bill) Dean (U.K.).
> Remove "ic" to reply.
>
I have spent my whole flying career flying gliders and airplanes at very
high and hot western US conditions and have never experienced or heard of
any differences in handling or stall/spin characteristics due to high
density altitudes. The noticeable differences are the faster ground speed
at which the glider takes off and lands, the amount of runway required, the
much lower rate of climb on air tow and the higher cross country speeds
achievable in the high thin air. Of course, you need oxygen on almost every
flight.

Bill Daniels

[Ian]

On Sat, 06 Aug 2005 23:36:39 +0100, W.J. (Bill) Dean (U.K.). wrote:

> I have no experience of "hot and high", the highest glider site in the
> UK is the Long Mynd at about 1,400 ft. It may well be that there is a
> change in behaviour at high density altitudes, either in quality or in
> relation to indicated speeds; but I would expect the same principals to
> apply.

It's very simple, but very real. In a "hot & high" (low altitude
density) take off, you need a significantly faster ground speed in before
you gain control authority. However your average wing runner does not
run any faster. Net result there is more time for things to go wrong
between the time the wing runner lets go and the time the pilot has
control.

I used to fly off a narrow tar strip at 5200' altitude and 30 deg plus
temperatures common in summer. The prevailing wind was 60 degrees off the
runway, but fortunately not normally stronger than 10 knots. But when it
did blow, we discovered many gliders have marginal control characteristics
taking off in these conditions.

Lots of time was spend briefing wing runners. They had to run the
downwind wing (there is a natural tendency to hold the wing back a
little which helps keep the glider straight). A skilled wing runner would
start running with the wing held a little behind his back, then when he
reached full speed he would allow the wing to overtake but still kept it
under control until it was at full stretch in front, effectively add some
speed to their run. Some guys would hold the wing by the leading edge then
when they let go it would fly out from under their palm, allowing them to
prevent it lifting up (which the downwind wing tends to do) for a short
extra distance of the run.

We lined up the gliders pointing to the downwind side of the centerline,
so the weathercock effect would turn them straight. Tail wheels were
better than skids. If you had one, you held the stick back to keep it on
the ground (except if you had a V tail). C of G hooks were always more of
a problem than nose hooks. The standard class guys often used airbrakes.
Then off course the flap ships always started in negative flaps.

But most important of all was to be ready to release when things went
wrong. Sometimes it took two or three attempts to get a sensitive glider
airborne in marginal conditions. So long as the run off area was kept
clear and the pilot aborted at the first signs of things going wrong,
there was no danger of damage.

Regards

Ian

[Ian Johnston]

On Tue, 2 Aug 2005 20:01:36 UTC, wrote:

: One hard-core solution is to fill up to the gills with water and let
: inertia keep the wings level until aerodynamic control is reached -
: just don't move that stick off the forward & center position until you
: have enough speed to keep the wings level!

I believe that one of the main reasons for wing drops at launch is
wings being held level by tip runners. If the runner has to exert a
significant force - wind, non-central ailerons, combination - then as
soon as s/he lets go the glider will roll. If I'm holding the tip
before launch and, say, having to apply a significant up force, I let
it go down about half way. Normally the pilot's reflexes kick in with
some opposite aileron, the load comes off and I bring the wing up to
level again. If that doesn't work I tell the pilot what the problem
is.

Ian


--

[Eric Greenwell]

Ian Johnston wrote:

> If I'm holding the tip
> before launch and, say, having to apply a significant up force, I let
> it go down about half way. Normally the pilot's reflexes kick in with
> some opposite aileron, the load comes off and I bring the wing up to
> level again. If that doesn't work I tell the pilot what the problem
> is.

I instruct my wing runners to balance the wing, not hold it level. I've
found some wing runners don't really know what level looks like when
they are 50 to 60 feet from the other tip, and the wing is drooping
because of the water weight. Wind and slight control deflections play a
role, too, but if the wing is balanced, it won't drop. Being level is
generally not important.

If balanced is way off level, the pilot should notice something is
wrong, and he should determine the cause. It might be the ballast is not
equal, for example. I would rather not have the wing runner attempting
to "correct" the situation.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[dav]

Hi all,

1.I have a guess at the question of airbrakes improving low speed
response-they simply deflect extra air around their sides, increasing
the airflow over ailerons and wing root.

2.One point I must take issue with was in regards to Reynolds numbers.


Qoute:"I doubt that. Turbulence is associated with high Reynolds
number, and
that depends directly on speed. Low speed -> low Re -> less
turbulence."


At low RE the boundary layer is much more easily transitioned to
turbulent flow(In fact at RE=60,000 flow is considered critical and
boundary layer attachmnent will fail), therefore turbulence is much
more likely at low RE NOT high RE.

Interesting that no one else noticed that eh?

But as a modeller as well as fullsize pilot I have studied the effects
of low RE a lot more I guess.....

3.I fly a Hornet often, an aircraft with I believe the same fuselage as
the Mosquito, and it also shows the wing drop on ground run tendency,
due to high AOA in the two point position.

4. Small chord tips at low speed may well be sub RE60,000, not creating
ANY lift until RE60K is reached!(RE is proportional to speed, chord and
air density).

Regards & Safe flying to all

Dave Lawley


--
dav
------------------------------------------------------------------------
Posted via OziPilots Online [ http://www.OziPilotsOnline.com.au ]
- A website for Australian Pilots regardless of when, why, or what they fly -

[Eric Greenwell]

dav wrote:

> 1.I have a guess at the question of airbrakes improving low speed
> response-they simply deflect extra air around their sides, increasing
> the airflow over ailerons and wing root.

Looking at top views of gliders, it appears the spoiler is always well
inboard of the aileron, so I don't see how significant air can be
deflected to them. With my ASW 20, the improvement occurred whether the
airbrakes were open 10 mm or 50 mm, so I think the improvement is caused
by spoiling lift rather than any deflected flow.

Tufting the wing between the spoilers and ailerons would allow a direct
check of the change in airflow. This could be done in a 20 mph wind, so
the glider would not even have to be moving.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[5Z]

Once it starts moving, even slowly, the wing is producing lift. The
more lift is produced, the more difficult it is to cause a wing to
drop, as this lift distributed over the whole wingspan damps a tendency
to roll.

Eliminate this damping by opening the spoilers and now the feeble
aileron is able to cause a roll.

Try this experiment: On a day with a light wind, sit in the glider
while it's pointed into the wind. Starting from a wings level
position, start rocking the wings. With the spoilers open, you will
find that you are able to go steeper and make quicker reversals. If
the wind is very light, it may all you can do just to keep the wings
level.

-Tom

[Bob Gibbons]

On 19 Aug 2005 14:33:29 -0700, "5Z" > wrote:

>Once it starts moving, even slowly, the wing is producing lift. The
>more lift is produced, the more difficult it is to cause a wing to
>drop, as this lift distributed over the whole wingspan damps a tendency
>to roll.
>
>Eliminate this damping by opening the spoilers and now the feeble
>aileron is able to cause a roll.
>
>... text deleted ...
>
>-Tom
>

This topic of why opened spoilers assists low speed roll control
really is one of those questions that comes up again and again. From a
post 5 years ago on r.a.s.

Bob

==============================*=================== ===========


From: Bob Gibbons )
Subject: Re: Dropping a wing on takeoff
Newsgroups: rec.aviation.soaring
View: Complete Thread (63 articles) | Original Format
Date: 2000-11-02 21:18:53 PST


This topic, that is, why deploying spoilers seems to help roll control

at low speeds, has come up often in RAS during the
years. Unfortunately, we seldom seem to get responses from
knowledgeable aeronautical engineers as to the real cause for what is
generally acknowledged as a real effect.


Earlier today I asked Dick Johnson the cause of this effect. I will
try to summarize his reply, but I may miss some of the finer details,
so any aerodynamicists, feel free to correct me. BTW, for those who
are not familiar with Dick credentials, in addition to his long
history of contest successes, Dick is a professional aerodyamicist
with 50+ years of experience.


Dick's response was that the effect of increased roll effectivness
with spoiler deployment is real, and is caused by a 3-dimensional flow

effect initiated by the spoiler deployment disturbing the normal low
pressure area over the top of the wing in the area of the
spoilers. The breakup of the normal low pressure in the spoiler area
results in a lateral spanwise flow in the airfoil forward of the
ailerons. This lateral flow modifies the normal pressure distribution
on the airfoil forward of the ailerons and allows the airfoil in the
aileron section to operate at a higher angle of attack (without flow
separation, i.e., stalling) than would otherwise be possible, thus
allowing greater aileron effectivness during the takeoff roll.


The effect is more pronounced in standard class gliders since without
the benefit of flaps to modify the airfoil camber, standard class
airfoils are often set on the fuselage at a higher angle of attack
than the corresponding airfoil on a 15m flapped ship. Furthermore, 15m

ships, with interconnected ailerons, can decamber their airfoil
(select negative flap) and generate increased resistance to flow
seperation at the ailerons during the early ground roll.


Hope this helps, and, as I said, this is an restatement by a
non-professional, corrections by practicing aerodynamicists welcome.


Bob

================================================== ============

[Bruce Hoult]

In article >,
dav > wrote:

> 3.I fly a Hornet often, an aircraft with I believe the same fuselage as
> the Mosquito, and it also shows the wing drop on ground run tendency,
> due to high AOA in the two point position.

I've observed that the Club Libelle (similar to the Hornet) suffers from
wing drop, while the Std Libelle doesn't.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------