On Sun, 17 Aug 2003 21:34:19 +0100, Martin Gregorie
wrote:

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.


Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.


On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.


If like most sailplanes the longitudinal stability isn't excessive
this effect may not be all that large but yes the glider will tend to
align itself with the local airflow but this effect only lasts for a
short time. This can also be masked by mass balancing or lack thereof
in the elevator control system, not just the elevator and how hard you
are holding on to the stick. In any case you are probably going to try
to hold the attitude constant unless you are cruising at high speed
and decide to take the thermal.
I seem to remember the SB13 flying wing had a problem with this when
first flown at forward C of G. The low moment of inertia in pitch
resulted in unpleasant characteristics as the glider flew through lift
and sink.

Mike Borgelt