On Sat, 09 Sep 2017 10:25:20 -0700, moshe.braner wrote:

On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie
wrote:
All aircraft entering a thermal will do this, some more noticeably than
others.

Reasoning: as the glider flies into the thermal it enters a rising air
mass and the further it moves into this air mass, the faster the
vertical movement becomes. The effect on the glider is that its
effective AOA is reduced by the air's vertical velocity ...

Seems the opposite to me. The rising air hits the wings from below, and
thus at a higher AOA.

If it was a body of air all rising at the same speed I'd agree, but in as
it enters a thermal the vertical velocity will be increasing as the
glider heads for the core, IOW its facing a continous disturbance which
has two components. One is the vertical velocity that the glider has
acquired from the air its just passed through. The second is the effect
of flying into air with a still higher vertical velocity.

("More so" in the sense of the strength of the resulting pitching
moments around the CG.) This is what positive stability means.

If it was in a parcel of air with the same vertical velocity I'd agree
with you, but it ain't: its flying onto steadily increasing vertical
velocity as it approaches the thermal core, so if things are as you
describe, you'd expect the glider to be pitching up. As that's not what
we see and feel, there must be something else acting on it, which I think
is the effect of flying into an increasing velocity gradient is to reduce
the effective AOA of the wing. Of course in reality it affects both wing
and tail, but its effect on the wing is a little bigger because the wing
gets to the higher velocity rising air before the tail does. Granted its
a small effect, but I think its enough to cause the nose to drop/tail to
rise.

It's not the trimmed speed it is seeking per se, it is the trimmed AOA.

Yes, you're right there. I should have made that clearer.

the stronger this pitching effect should be. Maybe
that's why I haven't noticed it in my gliders, since I have the CG near
the rear end of the range, i.e., weak stability.

Yes, that makes sense. My CG is pretty much in the middle of the
permitted range.

The force set up in the model will also be different: free flight gliders
normally have the CG at 52-55% of the wing chord, use a lifting section
on the tailplane and, yes, they are stable with a lifting tail. Almost
all designs operate with the wing at a Cl of around 1.2 and the tail at a
Cl of around 0.2. This puts the wing at an AOA of about 4 degrees more
than the tail, which keeps this layout stable with fast vertical upset
recovery. The tail is much bigger than on a sailplane - about 15-18% of
the wing area and, as a nice to have, most of the favoured tail sections
give minimum drag at a Cl of 0.2 - a nice bonus, since the tail is also
doing work against gravity as well as keeping the glider stable.



--
martin@ | Martin Gregorie
gregorie. | Essex, UK
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