motorgliders as towplanes

On 15 Mar, 13:15, Bob Cook wrote:

So it is possible to be in "good" tow position, and have the rope impart
a forward and downward pull (relative) to the glider! *

I'd expect high tow to give a downwards pull and low tow to give an
upwards one.

20 degrees still seems like a lot to me. *I guess it depends on the weight
of the rope, and the drag on the glider.

Seems a lot to me as well. What are these people towing with - chain?

Ian

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

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

At 21:00 15 March 2009, Nyal Williams wrote:
Help; what is "dimer" ?

A pair of vortices?

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!

At 19:58 15 March 2009, The Real Doctor wrote:

I'd expect high tow to give a downwards pull and low tow to give an
upwards one.

So, that implies that there is a position, on tow, where the rope would
neither impart an upward pull nor impart a downward pull.

This would certainly make the vector analysis simpler. Unfortunately, I
bet this position would be just about in the top of the wake somewhere.

I did go over to our tow plane yesterday, while it was sitting on the
ground, and I pulled on the loose end of the rope to see how hard a pull
would give how much sag. It took suprisingly little force to lift the
center of the rope off the ground. Figure my end of the rope was 4 feet
off the ground and the tow plane end was only 1 foot off the ground, I got
the center of the rope easily about 2 feet off the ground. Point being:
the angle with which the rope intersected with me was very small.

I also weighed a tow rope. Under 5 lbs, with two Schweizer rings, and an
adaptor with a tost ring. So I figure, in a "level" tow the weight of
the rope imparts only about 2 lbs down force to the glider. So you need
generate about two more lbs of lift to counter act this. Signifagant? i
don't think so.

A also paid attention to the rope on a couple of tows, and also noted that
although there is a noticable sag in the rope, it was really not all that
much.

I also noted, in the case of a 2-33 and a Blanik L-13 (the crap that I
usually have to fly) that BOTH of these gliders fly NOTICIBLY nose down on
tow! (which menas the rope is imparting a nose up force vector, even in
normal, tow position. )

Anyone who has flow a 2-33 knows that it takes CONSIDERABLE forward stick
force when on tow.

I have experienced the "too slow tow" phenomonon in the '33. (never
got a too-slow in the Blanik)

Again, my point being that I am still not convinced that anyone has
completly explained the phenomonon, that we all agree exists!

Cookie







forward and downward pull (relative) to the glider! =A0

I'd expect high tow to give a downwards pull and low tow to give an
upwards one.


Seems a lot to me as well. What are these people towing with - chain?

Ian

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.