On Dec 31, 6:19*pm, bildan wrote:
On Dec 31, 4:40*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 ...

Is poor handling at low speed on tow a common experience? *I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Doug Greenwell

I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. *Overuse of
ailerons is very common and it makes aero tow 'wobbly'. *If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right height
relative to the tug. *Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. *I haven't noticed any tendency for them to yaw a glider
towards a tugs wing tip.- Hide quoted text -

- Show quoted text -

There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being asked
to climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

Derek C