At 12:49 04 January 2011, wrote:
On Jan 4, 7:23=A0am, Doug Greenwell wrote:
At 23:30 03 January 2011, 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.

In a steady climb (or descent) lift is very close to being equal to
weight, even taking account of tow rope inclination. =A0You don't
need
extra lift to climb, you need extra thrust to increase potential
energy.
(A pull-up or zoom climb is different, because in this case you are
trading speed for height).

The Pawnee is a significantly heavier aircraft than the Citabria, so
woul=
d
generate stronger tip vortices at a given tow speed, and hence have
more
effect on a glider behind it.- Hide quoted text -

- Show quoted text -

Yes, to be exact the lift, is the cosine of the (climb or descent)
angle x the glider's weight. This works out to 99 "point something"
%
for reasonable climb or descent angles.............that tiny
difference may or may not be significant depending on what we are
looking at. But it is reasonlble to say "lift =3D weight" or lift
"just about =3D weight"

Cookie


'very close' or 'just about' leaves me enough margin for the uncertain
(but small*) effect of the rope inclination/bow on wing lift.
Whether the effect of the moments due to the rope tension on
pitch/roll/yaw stability are significant (and how important the hook
position is) is a much much harder question, so I'll pass on that for the
time being!

*worst case scenario ? say L/D = 15:1 on tow, rope angled off downwards
by 20 deg gives additional lift force required approximately as
sin(20deg)/15 = +2.3%, which would correspond to a 1% change in stall
speed.