At 08:57 04 January 2011, BruceGreeff wrote:
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


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


No - that's a pretty good summary. Specific excess power (excess
power/weight) is a standard parameter used to define military aircraft
performance, characterising the ability to change energy (height/speed) as
fast as possible.

Winch launching is very different from aerotow because the flight path is
curved and hence you are accelerating all the time, even after transition.