Constant speed or constant attitude?

On 14 Aug 2003 12:03:22 -0800, (Mark James Boyd)
wrote:

I also liked the comment to fly a certain airspeed.
Our local CFI at Avenal pointed out that if one hits
a gust up, the glider will speed up, because the
C.G. is forward of the center of lift, so the
nose drops.

This has nothing to do with CG position: F1A free flight competition
model gliders do this too - and they have a 55% CG and lifting stab
setup.

The explanation is easily seen if you draw a velocity vector triangle
for a glider flying in still air. Flying into rising air reduces the
vertical vector (sink speed minus rising air speed), which reduces the
AOA of the wing. A stable aircraft will react to this by pitching down
and increasing its airspeed. This is what you notice as the boot in
the bum as the glider surges forward on entering a thermal. The nose
drop is often quite noticeable as you point out.

The opposite effect happens as you fly out of the thermal or into sink
- the glider slows down and the nose rises. Again, a velocity vector
diagram shows what is happening - the increased AOA causes a stable
aircraft to correct by pitching up and shedding airspeed. The soggy
feeling as this happens is easy enough to notice, though for some
reason the pitch up is harder to see. Possibly its masked by the usual
reaction of stuffing the nose down to get more speed in the sink.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :