On 12/31/2010 3:40 AM, Doug wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25 years
off, people keep asking me hard questions. One that has come up recently is
why a heavy glider on tow feels horrible, but thermalling in the same glider
at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug wing wake
(tip vortices generating a downwash inboard, upwash outboard) changing the
lift distribution on the glider wing - with an increased angle of attack out
at the tips reducing aileron effectiveness. There's possibly an interesting
academic research project here, but it's always best to get a reality check
first ...

The wake behind a climbing towplane should be well below a glider in the
high tow position. How can it lift it's weight and the glider, if the
wake isn't descending? Recall one of the exercises a student does while
learning to tow is to start in the high tow position, then move straight
down until the wake turbulence is felt. With the usual 150' to 200'
rope, it's way below the high tow position. So, I don't think it's wake
turbulence, IF the glider is in the high tow position.

I think a big part of the answer is the pilot's perception of
"attitude": the glider has an additional attitude (relative to the
horizon) over it's normal angle of attack because the glider and tug are
ascending at about a 6 degree angle. T His is perceived by the pilot as
a very nose high attitude, and makes him feel uncomfortable; that, along
with reduced control response, makes him feel it's worse than it really
is. Usually, this happens close to the ground, making the perception
even worse.

The unusually nose-high attitude can keep the glider pilot from using
enough up elevator, with the consequence that he does sink down into the
wake. That will make the situation actually worse, not just perceptually
worse. But, it's because he is far from the high tow position, not just
because the speed is slower than normal.

This analysis obviously assumes a high tow as the normal situation, and
may not apply to the situation where low tow is the norm.

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