poor lateral control on a slow tow?

At 09:25 05 January 2011, Derek C wrote:
On Jan 5, 12:00=A0am, "
wrote:
So, in between level flight and vertical flight, there must be a
region
where the wing lift is less than in level flight, right? I'm saying
there is a continuous reduction in the lift the wing must provide as
th=
e
climb angle increases.

Only two months till March flying starts...gotta solve this problem
while we still have time!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us"
to
email me)

Yeah....you got it......the lift is the cosine of the climb angle
times the weight.........

level.....0 degrees climb.. =A0Cosine 0 =3D 1 =A0 =A0so lift =3D100%
glid=
er
weight

5 degree climb (reasonable tow climb angle) =A0 Cosine 5 =3D .996 =A0
=A0=
so
lift =3D 99.6% of glider's weight

45 degree climb (unlikely but just for demonstration) =A0 cosine 45
=3D
.
707 =A0so lift would be only 71% of glider's weight

90 degree climb =A0 Cosine 90 =3D o =A0 so lift would be zero.

If we keep the airspeed constant, the drag shoud be constant....so the
only variables are lift and thrust. =A0 as the thrust vector gets
bigger, the direction of flgith gets steeper climb, and the lift
vector gets smaller.

Cookie

So according to you, pulling a load up a 10 degree slope should
require less energy than pulling it on the flat! Anybody who has ever
ridden a bicycle can tell you that is not the case!

For a glider on tow, the combined vector of Lift and Thrust (provided
by the tug) has to equal the combined vector of weight plus drag. As
the glider is not rigidly connected to the tug, the extra lift has to
come from its wings (at least at moderate climb angles). For a given
airspeed this can only be done by increasing the angle of attack.
Hence you are closer to the stalling angle.

I am not sure that this is the correct explanation, but it seems to
fit the observed facts.

Derek C


There are two components to the energy required in this case - (1) the
energy required to overcome friction (which will indeed be slightly less,
because of the reduced reaction force perpendicular to the slope), (2) the
energy required to lift the load up a given height

(NB this assumes that you are pulling the load at a constant speed -
otherwise we would have to take kinetic energy into account as well)

(1) can be reduced to (near) zero by reducing friction - using rollers for
example, or in your alternative example of a bicycle - the equivalent
effect in a glider on tow is reducing drag by careful streamlining or
increased aspect ratio.

(2) is fixed, and independent of speed or slope angle - raising any object
a given height requires a fixed amount of energy (= mass*acceleration due
to gravity*height change).

Both components of the energy input are provided by you pulling the load
up the slope.

A glider on tow is exactly the same. The wing lift corresponds to the
reaction force between the surface and the load. The drag corresponds to
the friction force between the surface and the load. The tug corresponds
to you pulling the load - and is doing all the work against friction and
gravity. The lift/reaction force does no work - all it does is stop the
load sinking into the ground or the glider falling further and further
below the tug.

Imagine a perfect glider with no drag* on tow (= pulling a load up the
slope with no friction, or a perfect bicycle) ... what happens if you
release the rope (or stop pedalling)? If the wing lift were responsible
for the climb rate then you would carry on climbing until you ran out of
atmosphere (or hill)

* fortunately not currently available in the shops, since it would ruin
the sport!

On Wed, 05 Jan 2011 09:04:12 +0000, Doug Greenwell wrote:

At 02:48 05 January 2011, bildan wrote:
On Jan 4, 7:13=A0pm, AGL wrote:

Has anyone tried some flaps in an integrated flap machine (which
reduces stall speed) to see if the wallowing goes away?

With every flapped glider I've flown, negative flap improves aileron
response fairly dramatically. Positive flap does lower the stall speed
a little.

I've flown a 20 meter Nimbus 2C ballasted to 11 lbs/sq ft wing loading
behind a tug pilot accustomed to towing 2-33's. The speed was low
enough to need +1 flap but it didn't wallow. The tug pilot turned off
his radio when he got tired of me yelling for more speed than what he
"knew" was right.



Sorry if this is an obvious question (never flown a flapped glider), but
with an integrated flap system what is the relative movement of the
ailerons and flaps? Presumably the ailerons don't move at all for
negative settings?

On an ASW-20 flaps and ailerons move together so the trailing edge
remains straight with the stick central in the flying flap settings: +8
(thermal) through -9 (max negative flap). When stick is moved laterally
the flap deflects half as far as the aileron. In landing flap settings
the ailerons mover to -8 degrees - what the RC glider guys call 'crow
mode'. This reduces tip stalling tendencies and the handbook says this
also increases drag.


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

At 13:09 05 January 2011, Martin Gregorie wrote:
On Wed, 05 Jan 2011 09:04:12 +0000, Doug Greenwell wrote:

At 02:48 05 January 2011, bildan wrote:
On Jan 4, 7:13=A0pm, AGL wrote:

Has anyone tried some flaps in an integrated flap machine (which
reduces stall speed) to see if the wallowing goes away?

With every flapped glider I've flown, negative flap improves aileron
response fairly dramatically. Positive flap does lower the stall
speed
a little.

I've flown a 20 meter Nimbus 2C ballasted to 11 lbs/sq ft wing
loading
behind a tug pilot accustomed to towing 2-33's. The speed was low
enough to need +1 flap but it didn't wallow. The tug pilot turned
off
his radio when he got tired of me yelling for more speed than what he
"knew" was right.



Sorry if this is an obvious question (never flown a flapped glider),
but
with an integrated flap system what is the relative movement of the
ailerons and flaps? Presumably the ailerons don't move at all for
negative settings?

On an ASW-20 flaps and ailerons move together so the trailing edge
remains straight with the stick central in the flying flap settings: +8
(thermal) through -9 (max negative flap). When stick is moved laterally

the flap deflects half as far as the aileron. In landing flap settings
the ailerons mover to -8 degrees - what the RC glider guys call 'crow
mode'. This reduces tip stalling tendencies and the handbook says this
also increases drag.


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


Ok - so that would help in reducing stall speed slightly, but would not
help with the spanwise lift distribution.

Is the aileron/flap interconnect a standard arrangement, or are there
flapped gliders without it?

Eric has the right idea.

The forces acting on a glider towed upwards in steady climbing flight
are necessarily at equilibrium. The vectors lift + weight + drag +
thrust add up to zero. Otherwise, as some of the dafter ideas proposed
here imply, the glider will accelerate continuously toward outer
space.

Think of pulling a wheeled trolley with a rope up an inclined plane.
The steeper the incline, the greater is the tension in the rope until,
in the limit, with a vertical incline, the rope supports the entire
weight and the wheels none. At practical climb angles (less than 1:5)
the required wing lift should be slightly reduced from level flight by
the cosine of climb angle.

On that basis a glider on tow should stall at, or less than, the same
airspeed as straight and level flight off tow.

I think pilots who feel alarmed about stalling on tow may be misled by
cues from the nose high attitude combined with the need to maintain
position in turbulence behind the tug. Doesn't mean it feels good
though :-)

Ian Grant
(PEng and D2 driver)





On Jan 5, 2:15*am, Eric Greenwell wrote:
On 1/3/2011 11:51 PM, Darryl Ramm wrote:

On Jan 3, 8:54 pm, Eric *wrote:
Imagine an extreme tow, a 50 knot airspeed, but climbing at 35 knots (45
degree angle). The tow rope is providing 70% of the force holding the
glider in the air, so the wing needs to supply only 30% of the force.

Or imagine a really extreme, vertical tow: all the force required to
keep the glider moving steadily through the air is provided by the
towrope/towplane, and none by the wing.

I think you are trying to push this argument up an incline with a
rope. :-) But I'll take your points into consideration next time I'm
vertically towing behind a helicopter.

I'm serious! But, let me add this constraint to make the idea easier to
absorb: the glider pilot flies the tow so the rope is always parallel to
the fuselage.

In level flight, the rope pull equals the drag; the lift equals the
glider weight. Rope force vector and weight vector are at right angles.

In a 50 knot airspeed, 35 knot climb (45 degree angle of climb), the
rope vector and the glider weight vector are now at an obtuse angle, so
some of the rope force is supporting the glider.

Stating it another way: we know the rope is pulling a lot harder, but
the glider is not accelerating, so what force is opposing the rope pull?
It can't be additional drag (glider is still going only 50 knots
airspeed); it can't be the lift (regardless of it's value), because
that's acting almost entirely perpendicularly to the rope. So, what
force is opposing all that extra rope pull? I say - it's the weight of
the glider (about 70% of the weight).

Another way to imagine the situation, using the helicopter to provide a
50 knot airspeed tow, rope always parallel to the glider fuselage:
* ** level flight, wing lift = weight of glider
* ** vertical flight, wing lift = 0 (or the glider won't have right rope
angle)

So, in between level flight and vertical flight, there must be a region
where the wing lift is less than in level flight, right? I'm saying
there is a continuous reduction in the lift the wing must provide as the
climb angle increases.

Only two months till March flying starts...gotta solve this problem
while we still have time!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On Jan 5, 10:33*am, Doug Greenwell wrote:
At 09:25 05 January 2011, Derek C wrote:



On Jan 5, 12:00=A0am, "
wrote:
So, in between level flight and vertical flight, there must be a
region
where the wing lift is less than in level flight, right? I'm saying
there is a continuous reduction in the lift the wing must provide as
th=
e
climb angle increases.

Only two months till March flying starts...gotta solve this problem
while we still have time!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us"
to
email me)

Yeah....you got it......the lift is the cosine of the climb angle
times the weight.........

level.....0 degrees climb.. =A0Cosine 0 =3D 1 =A0 =A0so lift =3D100%
glid=
er
weight

5 degree climb (reasonable tow climb angle) =A0 Cosine 5 =3D .996 =A0
=A0=
so
lift =3D 99.6% of glider's weight

45 degree climb (unlikely but just for demonstration) =A0 cosine 45
=3D
.
707 =A0so lift would be only 71% of glider's weight

90 degree climb =A0 Cosine 90 =3D o =A0 so lift would be zero.

If we keep the airspeed constant, the drag shoud be constant....so the
only variables are lift and thrust. =A0 as the thrust vector gets
bigger, the direction of flgith gets steeper climb, and the lift
vector gets smaller.

Cookie

So according to you, pulling a load up a 10 degree slope should
require less energy than pulling it on the flat! Anybody who has ever
ridden a bicycle can tell you that is not the case!

For a glider on tow, the combined vector of Lift and Thrust (provided
by the tug) has to equal the combined vector of weight plus drag. As
the glider is not rigidly connected to the tug, the extra lift has to
come from its wings (at least at moderate climb angles). For a given
airspeed this can only be done by increasing the angle of attack.
Hence you are closer to the stalling angle.

I am not sure that this is the correct explanation, but it seems to
fit the observed facts.

Derek C

There are two components to the energy required in this case - (1) the
energy required to overcome friction (which will indeed be slightly less,
because of the reduced reaction force perpendicular to the slope), (2) the
energy required to lift the load up a given height

(NB this assumes that you are pulling the load at a constant speed -
otherwise we would have to take kinetic energy into account as well)

(1) can be reduced to (near) zero by reducing friction - using rollers for
example, or in your alternative example of a bicycle - the equivalent
effect in a glider on tow is reducing drag by careful streamlining or
increased aspect ratio.

(2) is fixed, and independent of speed or slope angle - raising any object
a given height requires a fixed amount of energy (= mass*acceleration due
to gravity*height change). *

Both components of the energy input are provided by you pulling the load
up the slope.

A glider on tow is exactly the same. *The wing lift corresponds to the
reaction force between the surface and the load. *The drag corresponds to
the friction force between the surface and the load. *The tug corresponds
to you pulling the load - and is doing all the work against friction and
gravity. *The lift/reaction force does no work - all it does is stop the
load sinking into the ground or the glider falling further and further
below the tug.

Imagine a perfect glider with no drag* on tow (= pulling a load up the
slope with no friction, or a perfect bicycle) ... what happens if you
release the rope (or stop pedalling)? *If the wing lift were responsible
for the climb rate then you would carry on climbing until you ran out of
atmosphere (or hill)

* fortunately not currently available in the shops, since it would ruin
the sport! *- Hide quoted text -

- Show quoted text -

To take your points above in order:

1) Gliders, at least decent ones, are pretty low drag anyway.

2) Kinetic energy from the tug is being used to raise the mass of the
glider up against gravity, so that it gains potential energy. Once
that source of kinetic energy is removed (i.e. you pull off tow), the
mass will stop going up and will start to descend due to the force of
gravity acting downwards. To maintain forward momentum gliders have to
continually descend through the air in which they are flying.

Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

Derek C

On Fri, 31 Dec 2010 11:40:53 -0000, "Doug"
wrote:

Is poor handling at low speed on tow a common experience? I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Yes, it is common. I use to fly mainly at competitions, and among the
5-10 tow pilots, there's always at least one who, despite being
briefed by the towmaster, flies too slowly.

In my personal experience, it happened to me 3 times in a double
seater (Janus B and DuoDiscus). I don't remember any occurrence in my
single seater.
I can describe it as being unable to raise the nose. As the towplane
was flying below 100 km/h, I just couldn't match the climbing rate
with the glider, so I was more and more into the propwash. A gentle
pull up wouldn't work; pulling more hits the stop and the glider feels
like it's sinking.

I also cannot find an easy and believable explanation for this
phenomenon. I didn't recognize a lack of _lateral_ control, anyway.

aldo cernezzi

At 17:23 05 January 2011, Derek C wrote:
On Jan 5, 10:33=A0am, Doug Greenwell wrote:
At 09:25 05 January 2011, Derek C wrote:



On Jan 5, 12:00=3DA0am, "
wrote:
So, in between level flight and vertical flight, there must be a
region
where the wing lift is less than in level flight, right? I'm
saying
there is a continuous reduction in the lift the wing must provide
as
th=3D
e
climb angle increases.

Only two months till March flying starts...gotta solve this
problem
while we still have time!

--
Eric Greenwell - Washington State, USA (change ".netto" to
".us"
to
email me)

Yeah....you got it......the lift is the cosine of the climb angle
times the weight.........

level.....0 degrees climb.. =3DA0Cosine 0 =3D3D 1 =3DA0 =3DA0so
lift
=
=3D3D100%
glid=3D
er
weight

5 degree climb (reasonable tow climb angle) =3DA0 Cosine 5 =3D3D
996
=
=3DA0
=3DA0=3D
so
lift =3D3D 99.6% of glider's weight

45 degree climb (unlikely but just for demonstration) =3DA0 cosine
45
=3D3D
.
707 =3DA0so lift would be only 71% of glider's weight

90 degree climb =3DA0 Cosine 90 =3D3D o =3DA0 so lift would be
zero.

If we keep the airspeed constant, the drag shoud be constant....so
the
only variables are lift and thrust. =3DA0 as the thrust vector gets
bigger, the direction of flgith gets steeper climb, and the lift
vector gets smaller.

Cookie

So according to you, pulling a load up a 10 degree slope should
require less energy than pulling it on the flat! Anybody who has ever
ridden a bicycle can tell you that is not the case!

For a glider on tow, the combined vector of Lift and Thrust (provided
by the tug) has to equal the combined vector of weight plus drag. As
the glider is not rigidly connected to the tug, the extra lift has to
come from its wings (at least at moderate climb angles). For a given
airspeed this can only be done by increasing the angle of attack.
Hence you are closer to the stalling angle.

I am not sure that this is the correct explanation, but it seems to
fit the observed facts.

Derek C

There are two components to the energy required in this case - (1) the
energy required to overcome friction (which will indeed be slightly
less,
because of the reduced reaction force perpendicular to the slope), (2)
th=
e
energy required to lift the load up a given height

(NB this assumes that you are pulling the load at a constant speed -
otherwise we would have to take kinetic energy into account as well)

(1) can be reduced to (near) zero by reducing friction - using rollers
fo=
r
example, or in your alternative example of a bicycle - the equivalent
effect in a glider on tow is reducing drag by careful streamlining or
increased aspect ratio.

(2) is fixed, and independent of speed or slope angle - raising any
objec=
t
a given height requires a fixed amount of energy (=3D
mass*acceleration
d=
ue
to gravity*height change). =A0

Both components of the energy input are provided by you pulling the
load
up the slope.

A glider on tow is exactly the same. =A0The wing lift corresponds to
the
reaction force between the surface and the load. =A0The drag
corresponds
=
to
the friction force between the surface and the load. =A0The tug
correspon=
ds
to you pulling the load - and is doing all the work against friction
and
gravity. =A0The lift/reaction force does no work - all it does is stop
th=
e
load sinking into the ground or the glider falling further and further
below the tug.

Imagine a perfect glider with no drag* on tow (=3D pulling a load up
the
slope with no friction, or a perfect bicycle) ... what happens if you
release the rope (or stop pedalling)? =A0If the wing lift were
responsibl=
e
for the climb rate then you would carry on climbing until you ran out
of
atmosphere (or hill)

* fortunately not currently available in the shops, since it would
ruin
the sport! =A0- Hide quoted text -

- Show quoted text -

To take your points above in order:

1) Gliders, at least decent ones, are pretty low drag anyway.

2) Kinetic energy from the tug is being used to raise the mass of the
glider up against gravity, so that it gains potential energy. Once
that source of kinetic energy is removed (i.e. you pull off tow), the
mass will stop going up and will start to descend due to the force of
gravity acting downwards. To maintain forward momentum gliders have to
continually descend through the air in which they are flying.

Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

Derek C


2) that's exactly the point! The energy from the tug (not its kinetic
energy, but the work done in pulling the tow rope) is being used to
increase the potential energy of the glider ... the glider wing lift is
not contributing to the increase in potential energy because it is
perpendicular to the direction of motion and hence does no work.

At 17:25 05 January 2011, cernauta wrote:
On Fri, 31 Dec 2010 11:40:53 -0000, "Doug"
wrote:

Is poor handling at low speed on tow a common experience? I'd
appreciate

any thoughts/comments/war stories ... particularly bad tug/glider/speed

combinations, incidents of wing drop during a tow etc etc?

Yes, it is common. I use to fly mainly at competitions, and among the
5-10 tow pilots, there's always at least one who, despite being
briefed by the towmaster, flies too slowly.

In my personal experience, it happened to me 3 times in a double
seater (Janus B and DuoDiscus). I don't remember any occurrence in my
single seater.
I can describe it as being unable to raise the nose. As the towplane
was flying below 100 km/h, I just couldn't match the climbing rate
with the glider, so I was more and more into the propwash. A gentle
pull up wouldn't work; pulling more hits the stop and the glider feels
like it's sinking.

I also cannot find an easy and believable explanation for this
phenomenon. I didn't recognize a lack of _lateral_ control, anyway.

aldo cernezzi


Interesting - most people are reporting problems with lateral control
(which seems to have a reasonably simple explanation), but running out of
nose-up pitch control also seems to occur ... and is harder to understand.


Did you notice any kind of change in elevator control force before you hit
the stops?
Did you experience this effect with any specific type of tug? Derek
Copeland has decribed a similar loss of elevator authority when towed by a
motor glider.

Doug

On 1/5/2011 7:11 AM, Doug Greenwell wrote:

Is the aileron/flap interconnect a standard arrangement...

Kinda-sorta, "Yes, but..."


or are there
flapped gliders without it?


....because the answer to this question is also (if unequivocally so), "Yes."
(I've owned 3.)

Regards,
Bob W.

P.S. Very e-e-enteresting discussion with (apparently ) real potential to
clarify some folks' understanding of things. I remain in the F = Ma camp!

At 17:59 05 January 2011, Bob Whelan wrote:
On 1/5/2011 7:11 AM, Doug Greenwell wrote:

Is the aileron/flap interconnect a standard arrangement...

Kinda-sorta, "Yes, but..."


or are there
flapped gliders without it?


....because the answer to this question is also (if unequivocally so),
"Yes."
(I've owned 3.)

Regards,
Bob W.

P.S. Very e-e-enteresting discussion with (apparently ) real potential
to
clarify some folks' understanding of things. I remain in the F = Ma
camp!


Thanks - so the question of whether flaps make things better or worse on
tow is also going to be 'it depends' ... :-)

It's certainly been unexpectedly interesting - having only recently
returned to gliding I'm having a load of fun improving my flying, but
also a load of fun trying to understand what is happening from an
academic/technical point of view. You can't argue with F=ma ... but
which 'F' and which 'a' and what direction they are in is another
matter!