At 23:35 31 December 2010, Eric Greenwell wrote:
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)


The wake does descend, although this is not necessary for a wing to
generate lift (otherwise wind tunnels would not work!) ... actually, the
downwash is a consequence of a reduction in lift and increase in (induced)
drag for a three-dimensional wing.

However, the turbulent prop wash also descends with it, so setting a tow
position on the basis of a reasonable distance above the prop wash would
automatically position you close to the tip vortices.

PS I've only ever come one other Greenwell outside the North East of
England ... any relation?