At 07:11 04 January 2011, Derek C wrote:
On Jan 3, 11:30=A0pm, ProfChrisReed wrote:
It seems to me that increased AoA must be a very large part of the
cause.

Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.

Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.

I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.

If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.

I had a high tow behind a 180 hp Piper Super Cub (a fairly slow tug)
to practice aerobatics in a K21 on Sunday. The nosehook on the K21 is
situated under the nose, just in front of the nosewheel, and I was
flying it solo. For the last thousand feet of the tow the airspeed
dropped to about 56 knots and I got the same symptoms as described
above. The normal free flight stalling speed for a K21 is only about
39 knots, and I had no problems in the early part of the tow when the
airspeed was 60+ knots. If the glider feels as though it is close to
the stall, then it probably IS close to the stall!

Derek C



yes, but the critical question is is it the same kind of stall? A
straight stall in a K21 is pretty benign, with no particular tendency to
drop a wing.