Chris

As I understand things - you are confusing climbing with vertical
acceleration.

While flying without vertical acceleration within the performance
available to gliders (I.e. constant vertical speed, modest climb angles,
tiny descent angles) the wing has to support pretty much one glider
weight (1g*mass). This is true whether you are gliding at 1:nn, towing
behind an anaemic cub at 9000" density altitude, or screaming skywards
behind a turbo Cmellak (Zlin 37). All that changes is the angle your
flight path makes relative to the ground. While the rate of climb may
seem significant it has nothing to do with AoA.
So - the flight path angle to any given frame of reference is largely
irrelevant to the AoA required. The AoA required is dependant on many
things - as I understand it these include -
- the Cl of the wing,
- relative velocity of the air over the airfoil,
- relative density of the air,
- the surface area of the wing
- the force it must support.

The force it must support will vary slightly depending on the vector of
force applied by the propulsive device. This could be an engine, a tow
plane or gravity. But it is only significant for one propulsive tool I
am aware of - winching does involve a short period of significant
vertical accelleration. In the transition from the initial climb to the
steep climb part of a winch launch the accelleration changes from ~1g to
about 2g. So the wing has to generate enough lift to generate the force
to change the flight vector - for a short while the AoA is high, close
to the ground. If it goes wrong here the prospects for a stall are good.
In the steep climb the angle described by the flight path relative to
the ground can easily reach 45 to 50 degrees, but the AoA on the wing
remains constant. It will be supporting a constant about 1g after the
transition.

The bending moment on the wing root is higher for reasons related to
where the winch vector is applied, and to the direction and magnitude of
that force, but this is not the load the wing must support. As the
glider has no significant vertical acceleration, the wing is
aerodynamically supporting a constant ~1g. (It must be a little higher
because of the added component of the winch force vector normal to the wing)

Of course - the angle that the flight path can make relative to the
ground is proportional to the excess power available - hence the low
rate of climb behind the cub, versus the extreme angle on a winch.

Aerodynamics guys - Am I confused?

Bruce

On 2011/01/04 1:30 AM, 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.


--
Bruce Greeff
T59D #1771 & Std Cirrus #57