At 21:47 31 December 2010, Martin Gregorie wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:


On Dec 31, 6:19Â*pm, bildan wrote:
On Dec 31, 4:40Â*am, "Doug" wrote:





As an aerodynamicist/flight dynamicist recently re-soloed after 25
years off, people keep asking me hard questions. Â*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. Â*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? Â*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. Â*Overuse
of
ailerons is very common and it makes aero tow 'wobbly'. Â*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. Â*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. Â*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.

If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will make

the glider look quite nose-high to its pilot.

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not what
its angle might be at the end of a tow rope.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |


The downwash angle doesn't change much past the tail, and a half to a
third of the tug AoA is a good first guess.

My modeling suggest that there does seem to be an overall reduction in the
glider wing lift (downwash over the centre wing having more of an effect
than upwash over the tips), so the glider requires another degree or two
in AoA - so feeling even more nose-up to the pilot!