motorgliders as towplanes

Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power
of this some years back -
http://www.nasa.gov/audience/forstud...ry/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

Hmm, I'm of the understanding that we use Pawnees because they are so
lightly wingloaded (relative to other tugs) and have such good power/
weight ratios when not full of bug juice and spray gear. I thought
this is also what allows them to happily fly too slow for our tastes
as well. While a Pawnee is perfectly content tugging at 55mph, I'm
not.


The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I disagree, I think we are flying well clear of this phenomenon, at
least in high tow. To me, the video you link illustrates just how far
below and behind the a/c this is taking place. The C-5 flies over, and
well past, and they even skip some time in the footage before the
vorticies reach down where the smokers are, which really shows the
downward trailing shape of this effect. It does not compute that you
would be subject to this effect if you were level with it and not
overly close behind (Isn't there a ratio of minimum rope length to
wingspan that is used as a rule of thumb?)

I saw a picture using smoke trails that demonstrates the scale and power
of this some years back -http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html

Neat pic, (very neat actually...) but tells little in the way of what
is happening to a glider on normal tow. A side shot, and one well
clear of the ground would show us what we are actually looking for for
this discussion, but this plane is in ground effect and the shot is
from behind. All bets are off when the disturbed air can't escape
below the flightpath where it wants to go... (and we are thus stuck
flying in this disturbance, which I don't recall as being very
disturbing either) Pretty pic, but somewhat useless as evidence for
this debate, or at least for what I am describing, which is not the
mechanics of towing while in ground effect but rather why gliders feel
like they are on the verge of a stall while on tow despite being well
above normal stall speeds.

There is a more impressive video athttp://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.

I think you are misinterpreting the photo/video and drawing incorrect
conclusions form them. Intuition (dangerous in aerodynamics, I
know..) tells me that if your vortex dimer was indeed striking the
glider as you suggest, the downward moving air would actually serve to
effectively decrease AoA since it would be striking the top of the
wing and not the bottom. I still think we fly pretty clean air, above
or below the really affected air, and are only suffering the
butterffly effect of this phenomenon when not actually inside or right
on the edge of the wake.

Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Affected? Sure. But my money is STILL on the pitching up of the nose
due to the rope's pull as being the primary cause of the sensation of
being on the verge of a stall while on tow, since the pull of the rope
is causing the glider to be drug through the air at an unnaturally
high AoA for any given airspeed, while at the same time drastically
reducing elevator effectiveness from reduced airflow and the fact the
nose is tethered. This increased AoA also greatly affects aileron
performance as well, since they too are operating at higher AoA's for
any given speed.

As mentioned elsewhere here, flaps most certainly help this effect
too, by pitching your nose back down some and thus reducing your AoA
for whatever given speed the tug is pulling at.

Unflapped,
-Paul

PS. the propwash to wingwash ratio should be pretty easy to figure
out. The main wing has to support the entire a/c (couple thousand
pounds) while the little spinning wing only needs to provide thrust.
(couple hundred pounds?) Which do YOU think is dominating the scene?
Unless there are tugs out there approaching 1:1 (thrust:weight) the
main wing is the main show, and hence the main contributor of
disturbed air. Ian, please do your power off tug test and please
post to youtube! Be sure to box the wake too though, because it
IS most certainly there, prop or not.

On 15 Mar, 17:18, sisu1a wrote:

Hmm, I'm of the understanding that we use Pawnees because they are so
lightly wingloaded (relative to other tugs) and have such good power/
weight ratios when not full of bug juice and spray gear. I thought
this is also what allows them to happily fly too slow for our tastes
as well. While a Pawnee is perfectly content tugging at 55mph, I'm
not.

The vortex strength is inversely proportional to the airspeed. For a
free vortex, the lift per metre is the vorticity times the free stream
velocity. For an aircraft, the vortex strength is therefore
approximately weight / (airspeed x span).

PS. the propwash to wingwash ratio should be pretty easy to figure
out. The main wing has to support the entire a/c (couple thousand
pounds) while the little spinning wing only needs to provide thrust.
(couple hundred pounds?) *Which do YOU think is dominating the scene?

Each blade of a Pawnee propeller is about 1m long. Each Pawnee wing is
about 5m long. With the engine at 3000rpm, the tip velocity will be
about 300m/s, which is about 150kt. 200bhp (150kW) at 60kt (30m/s) is
5kN. The maximum takeoff weight of a Pawnee is about 12.5kN.

So ... 0.4 times the force, 0.2 times the span, 2.5 times the
airspeed ... the propeller vorticity will be around 80% of the wing
vorticity. Ball park.

Then you have to remember that the effects of the vortex shedding are
felt, mostly, in a cylinder of about twice the diameter of the span,
and that the air velocity is inversely proportional to the distance
from the vortex. The propeller's vortex street is going to be about 4m
across, the wing's about 20m across....

Finally, there is a danger of confusing two things here. The vortex
wake of a lifting surface is *not* the same as the turbulent wake.
It's bigger and lasts longer.

Ian

I dyslexed the tow rope length. It should have been 200 feet, not 120
feet, although they can vary between 190 and 230 (we use a longer rope
when launching off the dirt).

I have towed enough to know where the wake is, thank you very much!

Can I add another question, why does the adverse yaw at 60 on tow appear to
be more than 60 in free flight, many students learing to aerotow have
difficulty with the rudder.


At 00:27 16 March 2009, Mike the Strike wrote:
I dyslexed the tow rope length. It should have been 200 feet, not 120
feet, although they can vary between 190 and 230 (we use a longer rope
when launching off the dirt).

I have towed enough to know where the wake is, thank you very much!

I have not noticed any difference in adverse yaw on tow vs free flgiht. I
will pay attention to that and see.

On tow however, students often seem to try to "steer" with the stick
(only).

This of course leads to adverse yaw, which causes to student to further
try to steer with the stick adding further adverse yaw, etc. It can get
out of hand quickly.

In fact, on tow, "steering" should be done with (almost) only rudder! I
find that application of rudder will yeild bank angle as well. (dihedral
effect, advancing wing etc.)

I tell students to use the sitck in order to match the glider's wings to
the tow planes wings (bank angle) and use the rudder to "point" the
nose where desired. (They wrongly try to point the nose using the stick
and adverse yaw gets them every time)

I have noticed that while turning on tow, the stick is often to the
outside of the turn, preventing "overbanking" tendancy!.

I also tell students, use 80% rudder and 20% stick to demonstrate the need
for rudder on tow, and minimal stick forces.

Here is another one. In normal tow position, I notice that the stick has
to be slightly LEFT of center. The glider has a tendancy to want to roll
right. I beleive this is because to tug's wake has a rotation to it.
(even above the wake) Students tend to mechanically "center" the stick,
and the glider will roll right. Experienced pilots naturally put the
stick whereven necessary to achieve wings level. I often ask them if
they were holding left stick. Invariably they say , "I dunno"

Cookie









At 10:15 16 March 2009, Alan Garside wrote:
Can I add another question, why does the adverse yaw at 60 on tow appear
to
be more than 60 in free flight, many students learing to aerotow have
difficulty with the rudder.

On 16 Mar, 10:15, Alan Garside wrote:
Can I add another question, why does the adverse yaw at 60 on tow appear to
be more than 60 in free flight, many students learing to aerotow have
difficulty with the rudder.

Do you think it's an aerodynamic problem? I'd always thought, based on
my own experience, that it came from having a much clearer indication
of yaw (the tow rop) than normal and overcorrecting some PIOs into the
system.

But I hadn't really considered the aerodynamics. Hmmm.

Ian

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power

of this some years back -
http://www.nasa.gov/audience/forstud...ry/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power

of this some years back -
http://www.nasa.gov/audience/forstud...ry/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power

of this some years back -
http://www.nasa.gov/audience/forstud...ry/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul