On Aug 2, 3:35*am, Scott wrote:

As a noob to gliders (coming from powered airplanes), I always thought
that spoilers were "the bomb" * *One thing I like about spoilers over
flaps is that you can go from full spoilers to no spoilers instantly
without bad effects. *Can't do that with flaps...

You can't? That's odd, I've gone from flaps 90 to flaps minus 10, and
then back again on the approach. Many times. So this must be one of
those things that works in practice but not in theory.

Flaps decrease the stall speed. Airbrakes increase it. Flaps can also
be used to reflex the wing for more performance at high speed.
Airbrakes are a collection of hundreds of parts that you might use for
thirty seconds every flight. Flaps require seams back at the 83% chord
where the laminar flow has probably already tripped. Airbrakes
requires seams up at the 50% chord where it is likely they will trip
the boundary layer prematurely.

I've gone through the exercise of designing, fabricating, and
installing a complete airbrake system, and I am so looking forward to
someday making another glider without them.

The biggest downside to flaps is that they are hard to train for,
since there are few two-seaters that have them. And too many gliders
that do have them (I'm looking at you, SGS 1-35), don't have enough of
them, so it's too easy to float down the runway. Flaps that go to only
60 degrees aren't enough; it takes at least 85 to reliably kill the
float and get onto the ground with authority.

The secret to making flaps work is feed-forward. You increase or
decrease pressure on the stick at the same time as you change the flap
setting, in an open-loop fashion. Only then do you close the loop and
refine the pitch pressure based on feedback from the various
indicators (ASI reading, wind noise, deck angle, etc).

If you try to do it completely closed-loop, that is, by changing the
flaps, waiting for signals that your speed is changing, and then
adjusting the pitch in response to the signals, you can end up chasing
the signals right out of the envelope. That's bad, and too many
inexperienced people try it and decide that it can't be done at all.

Thanks, Bob K.