K13s, which by no stretch of the imagination can be called 'modern',
behave in exactly the same manner during a slow and low powered aerotow
behind a motorglider! They are not comfortable to fly much below 55 knots
on tow. With a conventional tug (Robin DR400/Piper Pawnee) we get about 60
to 65 knots, which gives you much better control in turbulent conditions.

When I fly flapped gliders I tend to use some amount of positive flap,
mostly to get a better view of the tug, but also because it feels more
controllable.

BTW we have been specifically warned against aerotowing in low tow as our
motogliders can run out of forward elevator if you do.

Derek Copeland


At 16:07 11 March 2009, Mike the Strike wrote:
Some gliders definitely will fly slower in free flight than on aero-
tow. The Discus 2 is a good example of this. I have run out of
elevator authority several times on tows slower than I'd like and
ended up slipping into low tow position or losing control altogether
and releasing.

I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

When dry, my Discus 2 stalls below 40 knots and will thermal happily
between 40 and 50. I cannot safely aero-tow at 50 knots and usually
call for at least 65.

This effect is most notable with high performance standard class ships
whose angle of incidence is more optimized for high-speed flight.
Flapped ships by and large do not suffer from this problem, nor do
most training ships or lower performance single-seaters.

We are not imagining this!

Mike