motorgliders as towplanes

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.