poor lateral control on a slow tow?

On Jan 2, 2:49*am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote:



On Jan 1, 10:34=A0am, Doug Greenwell *wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3DA0from and aerodynamics standpoint (and I
am
abou=3D
t
as
far from and aerodynamicist as you can get) it should seem that
part
of the empirical data would suggest an experiment where you fly
a
glider equipped with and Angel of Attack meter at your typical
tow
speeds and record the AoA at various speeds. =3D3DA0Then fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to
the
extra roll inertia. =3DA0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =3DA0When he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3DA0If he's less than
equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change
in
local
incidence along the wing. =3DA0The overall lift may not change by
very
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root - putting
the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in
handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. =A0For example
=
a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02
x
Weight. =A0You don't need to increase lift to climb - you increase
thrust
to overcome the aft component of the weight, and the stick comes back
to
maintain speed ... at constant speed the increased power input comes
out
as increasing potential energy =3D increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. =A0

On your second point, if you are on tow anywhere sensible behind a tug
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine
a
cylinder with a diameter equal to the tug wing span extending back
from
the tug - that's the downwash region, and then in addition there's
an
upwash region extending perhaps another half-span out either side.-
Hide
=
quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie

it depends on your reference frame - lift and drag are perpendicular to
the direction of motion (relative to the air), which is inclined upwards -
so if you take 'aft' as relative to the glider flight path rather than
the earth, then there is an aft component of weight.- Hide quoted text -

- Show quoted text -

Yes, this is true......but to me it is better to keep the vectors
simple. If you apply a component along the line of the fuselage (aft
vector) then you have to add in the other component too. What
direction? Remember aft is parallel to the glider, not the flight
path of the glider.

We could in fact break any vector up into any number of
components......but eventually you have to combine them again.

To me, using the Earth (as horizontal and vertical reference) is
best. Then we can easily see the climb angle of the glider, the
direction of flight if you will, and the speed. We can also easily
see the angle of attack. With this reference we need to apply only 4
vectors (forces) lift, drag, weight, thrust. IF we use the glider
itself, longitudinal axis as reference, we right away have 8 vectors
to contend with. On tow, if we know any three forces, we can
calculate the forth. In gliding flight its only three forces (thrust
= 0) so its even easier.

Ultimately, if in "steady flight" there is in fact no force acting on
the glider......because the sum of all of the components = 0.

I like your explaination of climbing aircraft above. Another way to
look at it: (speed kept constant) IF thrust is greater than drag, the
aircraft will climb, IF thrust = drag, the aircraft will fly level
with the Earth. IF thrust is less than drag, the aircraft will
descend. If thrust = 0 the aircraft will descent at its L/D angle.
(assuming thrust is applied along the direction of flight)

Oh yeah...yet another factor from the earlier version of this
discussion. The force of the tow rope(thrust) does not necessarily
act through the glider's center of gravity. Neither does the drag
vector. This can cause a pitching moment, which will require elevator
input to counteract. Another factor that can give a different "feel"
on tow.


Cookie

On Jan 2, 6:01*am, Derek C wrote:
On Jan 1, 5:27*pm, Doug Greenwell wrote:





At 16:43 01 January 2011, Derek C wrote:

On Jan 1, 3:34=A0pm, Doug Greenwell *wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3DA0from and aerodynamics standpoint (and I
am
abou=3D
t
as
far from and aerodynamicist as you can get) it should seem that
part
of the empirical data would suggest an experiment where you fly
a
glider equipped with and Angel of Attack meter at your typical
tow
speeds and record the AoA at various speeds. =3D3DA0Then fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to
the
extra roll inertia. =3DA0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =3DA0When he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3DA0If he's less than
equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change
in
local
incidence along the wing. =3DA0The overall lift may not change by
very
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root - putting
the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in
handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. =A0For example
=
a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02
x
Weight. =A0You don't need to increase lift to climb - you increase
thrust
to overcome the aft component of the weight, and the stick comes back
to
maintain speed ... at constant speed the increased power input comes
out
as increasing potential energy =3D increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. =A0

On your second point, if you are on tow anywhere sensible behind a tug
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine
a
cylinder with a diameter equal to the tug wing span extending back
from
the tug - that's the downwash region, and then in addition there's
an
upwash region extending perhaps another half-span out either side.-
Hide
=
quoted text -

- Show quoted text -

So why did a K13 feel on the verge of a stall at 50 knots on tow? All
the classic symptoms of a stall were there, including mushy controls,
wallowing around and buffeting. If you got even slightly low it seemed
quite difficult to get back up to the normal position. Lack of
elevator effectiveness is yet another sympton of the stall!

Fortunately we have given up aerotowing with the Falke. It just seemed
like a good idea at the time because its flying speeds are more
closely matched to a glider; in theory anyway.

Derek C

good question - which suggests that something more complicated was going
on? *

Lack of elevator effectiveness is not really a symptom of stall as such
.. it's a symptom of low airspeed. *So for buffeting and mushy,
ineffective elevator to be happening at an indicated airspeed of 50-55
knots I'm wondering whether the tailplane was stalling rather than the
wing?

In this case you'd a tug with a wing span of a similar size to the glider
(14.5m to 16m), which would put the tug and glider tip vortices very close
together. *Two adjacent vortices of the same sign tend to wind up round
each other and merge quite quickly - if this happened with the two sets of
tip vortices it would generate an increased downwash near the tail and push
the local (negative) incidence past the stall angle.

I'd be the first to admit this is getting rather speculative - but these
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing *... a good student project for next
year!-
Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.
The elevator should still
be effective at 50 knots, so it's more likely that the wing is close
to the stall. The stall is only strictly related to the angle of
attack. During a aerotow climb the wing has to support an additional
weight component as well as drag, so the effective wing loading may
well be increased, requiring a greater angle of attack for a given
airspeed. Going 10 knots faster seems to cure the problem.

Derek C- Hide quoted text -

- Show quoted text -


'Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.'

HUH? Many cases possible where we could have full elevator and not
be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
All you need is heavy pilot (forward CG) and gentle stick back to the
stop. Glider will mush, but not stall. Elevator will not raise the
nose........wing does not have angle to stall.

On tow the only additional "weight component" would be a downward
component to the tow rope (thrust). Since the tension on the tow rope
is fairly low........it should not have a big effect, but there is
some effect.

But yeah, that extra 10 knots makes all the difference in the world.
(I remember occasionally getting a "slow tow" when flying a 2-32 with
three aboard..........what a handful!!!
Cookie

On Jan 2, 2:49*am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote:



On Jan 1, 10:34=A0am, Doug Greenwell *wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3DA0from and aerodynamics standpoint (and I
am
abou=3D
t
as
far from and aerodynamicist as you can get) it should seem that
part
of the empirical data would suggest an experiment where you fly
a
glider equipped with and Angel of Attack meter at your typical
tow
speeds and record the AoA at various speeds. =3D3DA0Then fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to
the
extra roll inertia. =3DA0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =3DA0When he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3DA0If he's less than
equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change
in
local
incidence along the wing. =3DA0The overall lift may not change by
very
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root - putting
the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in
handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. =A0For example
=
a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02
x
Weight. =A0You don't need to increase lift to climb - you increase
thrust
to overcome the aft component of the weight, and the stick comes back
to
maintain speed ... at constant speed the increased power input comes
out
as increasing potential energy =3D increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. =A0

On your second point, if you are on tow anywhere sensible behind a tug
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine
a
cylinder with a diameter equal to the tug wing span extending back
from
the tug - that's the downwash region, and then in addition there's
an
upwash region extending perhaps another half-span out either side.-
Hide
=
quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie

it depends on your reference frame - lift and drag are perpendicular to
the direction of motion (relative to the air), which is inclined upwards -
so if you take 'aft' as relative to the glider flight path rather than
the earth, then there is an aft component of weight.- Hide quoted text -

- Show quoted text -

Lift is perpendicular.......drag is parallel........

..
*'Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.'

HUH? * Many cases possible where we could have full elevator and not
be stalled. *(I demonstrate this is 2-33 and grob 103 and ask-21.
All you need is heavy pilot (forward CG) and gentle stick back to the
stop. *Glider will mush, but not stall. *Elevator will not raise the
nose........wing does not have angle to stall.

..
whoa - depends on who's defining "stall". The FAA definition is indeed
that when the aircraft does not respond in the direction of the
control input that it's done. When you can no longer move the elevator
up, you're done. Nose doesn't respond in direction of aft stick
deflection, you're stalled. I don't remember exactly the way they
word it, but the result is that touch the elevator limit, that's it.
Slow entry rates result in higher stall speeds. Forward cg's give
higher stall speeds. Trim settings (on some configs) affect stall
speeds. Weight, etc., etc. The scene that seems the most insidious
is the slow entry rate. They sneak up on you, kind of like a slow tow.

On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. Maybe it all depends on what
you call high tow. I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

On Jan 2, 10:38*am, Andy wrote:
On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). *I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. *Maybe it all depends on what
you call high tow. *I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

On Sun, 02 Jan 2011 07:38:29 -0800, Andy wrote:

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here downward
would mean below the glider longitudinal axis, not necessarily below the
horizon). I'm quite sure that my ASW 28 being towed on the CG hook has
no downward force on the nose.

Hmmm, My Libelle glides at around 55 kts with the trim full forward so
should need its nose held down a bit when being towed at 60-65 kts on the
nose hook. Its possible that I am holding the nose down - all I can say
is that I'm not aware of doing so once I'm off the ground, stabilised
behind the tug and waiting for it to unstick, gain speed and start to
climb.

There is a noticeable catenary in the tow rope and, since that is a thin,
flexible rope the pull on the nose hook will be at the same angle as the
rope leaves the nose and not on the direct line between my nose-hook and
the rope attachment point on the tug. This probably puts the force line
above the glider CG and so is contributing a nose down moment.

FWIW I estimate that climbing at 600 fpm at 60 kts is a 5.67 degree climb
and that the tow rope tension is 37.62 kg for my glider (10 kg is drag
due to the glider and the rest is due to the glider hanging from the
rope).

However, I don't know rope weight or exact length or how to calculate the
sag in the rope and hence can't estimate the distance of the force line
above or below the CG.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Jan 1, 12:44*pm, Free Flight 107 wrote:
On Jan 1, 3:21*am, Doug Greenwell wrote: At 06:24 01 January 2011, Anne wrote:

I've certainly sparked some interest here - considering it's New Year
:-)- Hide quoted text -

And I mignt add this is a very fast moving discussion too! While I was
loging in 2 messages were posted..

Concerning the Tow Plane position while on tow, two of my CFIs have
said to position yourglider as if you were going to Machine Gun the
pilot of the Tow Plane. this is equivelent of aligning the horizontal
of the TP with a portion of his foweward fuslage, like the wheels on a
Pawnee.

Works great in all conditions I've come accross in 15 years flying 8
different types from 2-33 to Duo Discuss. Never been criticized for it
either in BFRs.

Wayne

Without a gunsight, how do you do that? ;^)

I don't understand why the high tow position is taught by reference to
the towplane or horizon, when what should be taught is how to find the
correct tow position (just above or below the wake, which is actually
the propwash). Simple - once safely airborne (usually before the
towplane), just ease down until you feel the towplanes turbulence,
then ease up a bit. THEN look at the towplane and pick whatever
convenient references you need to maintain this vertical alignment.
Any significant change in towplane speed will require a readjustment
of the tow position (normally only a factor if on an aerotow
retrieve).

Obviously, if you only tow behind the same towplane on every flight,
you will quickly learn where to position your glider. But if you have
a variety of towplanes, or are towing behind something different
(Agcat, Wilga, AN-2, whatever) for the first time, you can use this
process to find the correct position quickly.

Many US instructors seem to only teach HOW to do something without
going into WHY it is done. As a result, there are a lot of "shortcuts"
being taught, and a lot of poorly trained pilots, IMHO. A result of
not having a standardized curriculum, a la BGA, perhaps?

Kirk
66

-

Yes, this is true......but to me it is better to keep the vectors
simple. If you apply a component along the line of the fuselage (aft
vector) then you have to add in the other component too. What
direction? Remember aft is parallel to the glider, not the flight
path of the glider.

We could in fact break any vector up into any number of
components......but eventually you have to combine them again.

To me, using the Earth (as horizontal and vertical reference) is
best. Then we can easily see the climb angle of the glider, the
direction of flight if you will, and the speed. We can also easily
see the angle of attack. With this reference we need to apply only 4
vectors (forces) lift, drag, weight, thrust. IF we use the glider
itself, longitudinal axis as reference, we right away have 8 vectors
to contend with. On tow, if we know any three forces, we can
calculate the forth. In gliding flight its only three forces (thrust
=3D 0) so its even easier.

Ultimately, if in "steady flight" there is in fact no force acting on
the glider......because the sum of all of the components =3D 0.

I like your explaination of climbing aircraft above. Another way to
look at it: (speed kept constant) IF thrust is greater than drag, the
aircraft will climb, IF thrust =3D drag, the aircraft will fly level
with the Earth. IF thrust is less than drag, the aircraft will
descend. If thrust =3D 0 the aircraft will descent at its L/D angle.
(assuming thrust is applied along the direction of flight)

Oh yeah...yet another factor from the earlier version of this
discussion. The force of the tow rope(thrust) does not necessarily
act through the glider's center of gravity. Neither does the drag
vector. This can cause a pitching moment, which will require elevator
input to counteract. Another factor that can give a different "feel"
on tow.


Cookie


There are a multiplicity of possible axes systems that are used in flight
dynamics - which one you use (and what you call it) depends on where you
are (US, UK, rest of world), and which one makes the sums simpler!

The equations of motion for climb and descent in free-flight are exactly
the same - just some terms disappear, or change sign. On tow is another
matter, since the tow rope angle introduces an additional force, and a
constraint on the motion.

DLR, the German aero reseach instititute, did do some work in 1999 on the
longitudinal dynamics of aerotowing, looking at the effect of downwash,
rope forces and hook position on pitch stability ... at least I think they
did - I have the report, but need to get it translated! Doesn't look to
contain anything concrete on lateral stability though.

At 13:20 02 January 2011, wrote:
On Jan 2, 6:01=A0am, Derek C wrote:
On Jan 1, 5:27=A0pm, Doug Greenwell wrote:





At 16:43 01 January 2011, Derek C wrote:

On Jan 1, 3:34=3DA0pm, Doug Greenwell =A0wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3D3DA0am, Doug Greenwell =3DA0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3D3DA0pm, Todd =3D3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3D3DA0from and aerodynamics standpoint
(a=
nd I
am
abou=3D3D
t
as
far from and aerodynamicist as you can get) it should seem
tha=
t
part
of the empirical data would suggest an experiment where you
fl=
y
a
glider equipped with and Angel of Attack meter at your
typical
tow
speeds and record the AoA at various speeds. =3D3D3DA0Then
fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no
difference
in
AoA.

I've flown some pretty heavy high performance gliders behind
som=
e
pretty bad tow pilots - one of them stalled the tug with me
on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due
to
the
extra roll inertia. =3D3DA0A pilot trying to hold a precise
posi=
tion
behind
a tug needs and expects crisp aileron response. =3D3DA0When
he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3D3DA0If he's less
than
equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the
change
in
local
incidence along the wing. =3D3DA0The overall lift may not
change
=
by
very
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root -
putting
the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a
factor,
b=
ut
since
the same factor applies to maintaining bank angle in a
thermallin=
g
turn
I
don't see how it can account for a significant difference in
handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined
Falke
a=
s
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+
knots
with a normal tug), but K13s with a stalling speed of 36 knots
felt
very unhappy behind it, especially two up. In a conventional
powere=
d
aircraft you pull the nose up (to increase the angle of attack
and
produce more lift) and increase power to climb, the extra power
bei=
ng
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so
low
=
(
10deg) that the lift remains pretty well equal to weight. =3DA0For
exa=
mple
=3D
a
10deg climb angle at 60 kts corresponds to an impressive climb
rate
=
of
10.5kts - but that would only give Lift =3D3D Weight/cos(10deg)
=3D3=
D 1.02
x
Weight. =3DA0You don't need to increase lift to climb - you
increase
thrust
to overcome the aft component of the weight, and the stick comes
bac=
k
to
maintain speed ... at constant speed the increased power input
comes
out
as increasing potential energy =3D3D increasing height.

I think a lot of people confuse the actions needed to initiate a
cli=
mb
with what is actually happening in a steady climb. =3DA0

On your second point, if you are on tow anywhere sensible behind
a
t=
ug
yo=3D
u
are in its wake and are being affected by the wing downwash.
=3DA0Wa=
ke
is
n=3D
ot
really a good word, since it seems to get confused with the much
mor=
e
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to
imagi=
ne
a
cylinder with a diameter equal to the tug wing span extending
back
from
the tug - that's the downwash region, and then in addition
there's
an
upwash region extending perhaps another half-span out either
side.-
Hide
=3D
quoted text -

- Show quoted text -

So why did a K13 feel on the verge of a stall at 50 knots on tow?
All
the classic symptoms of a stall were there, including mushy
controls,
wallowing around and buffeting. If you got even slightly low it
seemed
quite difficult to get back up to the normal position. Lack of
elevator effectiveness is yet another sympton of the stall!

Fortunately we have given up aerotowing with the Falke. It just
seemed
like a good idea at the time because its flying speeds are more
closely matched to a glider; in theory anyway.

Derek C

good question - which suggests that something more complicated was
goin=
g
on? =A0

Lack of elevator effectiveness is not really a symptom of stall as
such
.. it's a symptom of low airspeed. =A0So for buffeting and mushy,
ineffective elevator to be happening at an indicated airspeed of
50-55
knots I'm wondering whether the tailplane was stalling rather than
the
wing?

In this case you'd a tug with a wing span of a similar size to the
glid=
er
(14.5m to 16m), which would put the tug and glider tip vortices very
cl=
ose
together. =A0Two adjacent vortices of the same sign tend to wind up
rou=
nd
each other and merge quite quickly - if this happened with the two
sets=
of
tip vortices it would generate an increased downwash near the tail
and
=
push
the local (negative) incidence past the stall angle.

I'd be the first to admit this is getting rather speculative - but
thes=
e
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing =A0... a good student project
for
n=
ext
year!-
Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.
The elevator should still
be effective at 50 knots, so it's more likely that the wing is close
to the stall. The stall is only strictly related to the angle of
attack. During a aerotow climb the wing has to support an additional
weight component as well as drag, so the effective wing loading may
well be increased, requiring a greater angle of attack for a given
airspeed. Going 10 knots faster seems to cure the problem.

Derek C- Hide quoted text -

- Show quoted text -


'Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.'

HUH? Many cases possible where we could have full elevator and not
be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
All you need is heavy pilot (forward CG) and gentle stick back to the
stop. Glider will mush, but not stall. Elevator will not raise the
nose........wing does not have angle to stall.

On tow the only additional "weight component" would be a downward
component to the tow rope (thrust). Since the tension on the tow rope
is fairly low........it should not have a big effect, but there is
some effect.

But yeah, that extra 10 knots makes all the difference in the world.
(I remember occasionally getting a "slow tow" when flying a 2-32 with
three aboard..........what a handful!!!
Cookie



Depends on your definition of 'stall' - whether the nose drops or not
depends so much on the aircraft configuration, the aerofoil section and
the stall entry technique.

NASA did a 'deep-stall' test program in the early 80s with a modified
1-36 which was able to carry on pitching up to 70deg AoA ... look at the
incidence on this photo!
http://www.dfrc.nasa.gov/gallery/pho.../ECN-26845.jpg