motorgliders as towplanes

On Mar 11, 6:30*pm, Derek Copeland wrote:
Bruce,

So can you explain why the stalling speed definitely seems to increase
during an aerotow? Either the wing must be flying at a greater angle of
attack, i.e. producing more lift for a given airspeed, or the wing loading
must increase in some way.

As I said before, gliders that will quite happy fly at 40 knots in free
flight seem to need at least 50 knots on aerotow, even in smooth air.

I agree that this effect exists, and I have noticed it, even behind
fast powerful tugs. For example I usually fly a Janus, which is
recommended to tow with +6 flaps. You'd normally switch to zero flap
anywhere above about 50 knots and be thinking about -4 at normal
towing speed. But if you go to zero flap while towing it feels very
mushy.

I'm not convinced that the stall speed is *actually* increased. I've
never been game to find out.

The only thing I can think of is that even though you're not flying
through the prop wash, you're still in the air that the wings of the
towplane have imparted a downward velocity to. The downwardly flowing
air exists maybe 5m or so both above and below the path that the wing
took. I don't know how to calculate the actual downward velocity, or
whether it is significant, but it may amount to the glider effectively
flying through significant sink.


I agree that the accepted theory of flight says that in steady flight, the
vector of lift plus thrust must equal weight plus drag. I suppose that if
you had a tug powerful enough to produce enough thrust to more than equal
it's own weight plus the weight of the glider, then you could go
vertically up without the wings producing any lift.

Right. Try towing with a Huey :-)