On Jan 3, 6:30*pm, 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.

Actaully, comparing climbing steeply say, 10:1 on tow, to gliding at
40:1, the lift vector is (a tiny bit) SMALLER during the tow!

During the 10:1 tow, lift would be 99.5% of the glider's weight, while
during a 40:1 glide, lift would be 99.97% of the glider's weight!
(the missing 0.5% on tow is made up by the thrust vector...the missing
0.03% in glide is made up by the drag vector.

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