What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.

John Galloway

At 21:42 15 August 2003, Jim wrote:
On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

On 15 Aug 2003 12:36:05 -0800,
(Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed

I thought a stable aircraft has the C.G. forward of
the center
of lift. If this is so, and this effect only happens
if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this
effect
still occur? If the C.G. is behind the center of lift
(my
understanding of 'unstable') does this occur?



My guess, and it sure is only a guess, is that the
changes in
the indicated airspeed as a result of the glider flying
into lift or
sink WOULD occur regardless of the stability or instability
of
the aircraft. I'm guessing this is so because I'm
also guessing
that THESE changes in the indicated airspeed are not
the
result of instaneous pitch changes in the aircraft's
attitude, but
rather are changes in dynamic and/or static pressure
directly
created by the changes in lift and sink themselves.

I suppose another way to say this is that the changes
in indicated
airspeed may be due to angle of attack changes that
are not due
to changes in the aircraft's attitude, but rather due
to changes to
the direction of the airflow (which are felt as changes
in lift and
sink.

I dunno. This is absolutely wonderful stuff, but it
leaves me
really wanting a wind tunnel so I could test these
things.



I think I only further muddled this by my saying 'actual
airspeed' may
not be changing. This is not at all the way to look
at things.
Indicated airspeed DOES change as a glider flies into
lift and sink.
Period. What I wanted to describe is a situation in
which the changes
in indicated airspeed are reflective of changes in
the airflow over
the glider created by the changed lift and sink, not
of accelerations
of the glider itself.

Phooey. This probably only made it worse. I know
what I want to say,
I just can't find the right way to say it.