I'll try again.

Positive flap limiting speeds have two justifications:
1) To prevent damage to the flaps themselves or more likely to their hinges.
2) To stop the wings producing more lift than is good for them at high
speeds, remembering that lift increases with the square of the speed for a
given angle of attack, and that flaps increase the camber and therefore lift
produced by the wings.

Airbrakes (when open) reduce the amount of lift produced over the portion of
the wing they occupy and increase the drag. The rest of the wing (mostly
outboard of the brakes) has to produce more lift to support the weight of
the glider and hence the bending moment on the wingspar is increased. The
fact that some stressing is taking place in the region of the airbrakes is
often witnessed by the gel coat cracks that tend to appear in this area.
Please also re-read my account of my test flight to check the double paddle
airbrake mod!

There have been two N4D break-up accidents where the common factor is that
control has been lost in a thermal, either a spin followed by a spiral dive,
or just a straight spiral dive. It is likely that in both cases that
thermalling (ie positive) flap was selected and speed and g. increased very
rapidly. In at least one case the pilot tried opening the airbrakes, but the
glider still broke up.

We had a fatality in the UK involving an ASW20 that dived vertically at very
high speed into the ground after a similar thermalling upset. The flaps also
to some extent act as elevators and it was thought that the pilot was unable
to overcome the nose down pitch tendency by pulling back on the stick.

My main original point was that the first action in any sort of loss of
control situation in a flapped glider must be to select neutral or negative
flap. If you have to open the brakes, do so before Vne is reached.

Derek Copeland
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Bert Willing wrote:
you're mixing up things. Speed limits for positive flap settings have
nothing to do with g-load; they are to limit the forces on the flap.




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