poor lateral control on a slow tow?

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)

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

level.....0 degrees climb.. Cosine 0 = 1 so lift =100% glider
weight

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

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

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

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

Cookie

On Jan 5, 12:00*am, "
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 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)

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

level.....0 degrees climb.. *Cosine 0 = 1 * *so lift =100% glider
weight

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

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

90 degree climb * Cosine 90 = o * so lift would be zero.

If we keep the airspeed constant, the drag shoud be constant....so the
only variables are lift and thrust. * 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

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 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

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.

On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote:

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.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.


The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.


Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likeise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.
Voila, meet the the conditions for poor alieron efficiency (high AoA!)
and tip stall.


The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.
Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).


One interesting fact:
When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with
a back-swept wing), they encountered a nose-down momentum after
lift-off that could not be recovered and usually lead to a crash
immediately after lift-off.

Explanation:
The downwash of the tow plane (Robin Remorqeur) hit the inner part of
the wing, decreasing its AoA (and lift) and therefore shifting the
center of lift backwards due to the sweepback.

Increasing the length of the tow rope helped.



Greetings from a snowy Germany
Andreas

On Jan 5, 6:52*pm, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C

wrote:
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.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.

The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.

Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likeise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.
Voila, meet the the conditions for poor alieron efficiency (high AoA!)
and tip stall.

The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.
Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or *running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).

One interesting fact:
When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with
a back-swept wing), they encountered a nose-down momentum after
lift-off that could not be recovered and usually lead to a crash
immediately after lift-off.

Explanation:
The downwash of the tow plane (Robin Remorqeur) hit the inner part of
the wing, decreasing its AoA (and lift) and therefore shifting the
center of lift backwards due to the sweepback.

Increasing the length of the tow rope helped.

Greetings from a snowy Germany
Andreas

The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

I have not been towed by a Dimona or Katana, but they seem to be a bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Greetings from (now) snow free England,

Derek C

I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.

On Wed, 5 Jan 2011 15:09:39 -0800 (PST), Derek C
wrote:

Hi Derek,


The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

Well... 50 kts is pretty slow...

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

Clear case: Low aspect ratio, wing loading twice of the ASK-13. How
much above the stall speed of the Cub? 10 kts at maximum? Scary...

I guess you had a word with the tow pilot afterwards.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

Yes, the typical situation for a very slow aerotow.


I have not been towed by a Dimona or Katana, but they seem to be a bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Well, I guess we both agree that this problem only manifests itself at
the low-speed area of the envelope, don't we?

The general consensus here in Germany (as well as my own experience)
is that an aerotow behind a motorglider is *much* easier to control
despite the fact that it takes place at 110-115 kph (60-63 kts)
instead of the 130-140 kph (70-75 kts) that are typical for Morane MS
893 and Robin Remorqeur.
Of course similar wing loadings result in similar reactions to gusts,
which helps to follow the tow plane.


Regards
Andreas

On 1/5/2011 10:52 AM, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote:

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.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.


The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.


Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

(big snip)

Andreas' posting was the clearest description for me of the wake effect.
I'd love to see "3-D" perspective view of the wake behind a towplane, as
I doubt I'm visualizing it well.

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