At 18:19 31 December 2010, bildan wrote:
On Dec 31, 4:40=A0am, "Doug" wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25
years
off, people keep asking me hard questions. =A0One that has come up
recent=
ly is
why a heavy glider on tow feels horrible, but thermalling in the same
gli=
der
at lower speeds is fine? (see also Mike Fox's article on aerotowing
in
th=
e
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug
wing
w=
ake
(tip vortices generating a downwash inboard, upwash outboard) changing
th=
e
lift distribution on the glider wing - with an increased angle of
attack
=
out
at the tips reducing aileron effectiveness. =A0There's possibly an
intere=
sting
academic research project here, but it's always best to get a reality
che=
ck
first ...

Is poor handling at low speed on tow a common experience? =A0I'd
apprecia=
te
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.


Certainly my early aerotows were a bit wobbly, although they've settled
down now - I (re-)soloed on an aerotow, which I think is relatively
unusual in the UK, but most of my launches since then have been on the
winch.

The problem is a bit more than wobbliness though - there does seem to be a
common theme of experienced pilots having real control difficulties when
heavy and a bit slow on tow (not a situation I've been in myself ...
yet).

Part of the difficulty in analysing this is splitting out the effects of
the aerodynamic wake/wing/fin interaction(s) and the mechanical cable
dynamics.

It just occured to me that it might be interesting to do this by flying a
motor glider in formation with a tug - no cable, so only the aerodynamic
effects to contend with. There has been a lot of work done on
aerodynamics of flrmation flight and towing, but everything published has
been on aircraft with either the same span or with the lead aircraft much
larger than the trailing aircraft.