Guys, guys, guys........
It's a relatively simple explanation. I may be totally wrong (Manufacturing
work for 7year may have my AE schooling a bit rusty), but one of the initial
posts on this thread had this "phenomenon" explained.
Condition 1: Glider is flying along in a steady state, dead air situation.
Lets call this level flight, your preferred trim settings for the glider
(Best L/D or whatever). Assume an AoA of +1deg.
Condition 2: The glider enters lift/thermal/rising air. The glider is now
sees an airstream that has a larger vertical (upward) velocity compnent. Do
a simple vector analysis. A horizontal vector for the neutral, pre-thermal
airflow (which is relavtive to the glider's true airspeed), and a small
vertical vector for the"rising" airmass. Add these together and you will
notice that the airflow the glider now "sees" has resulted in an increased
angle of attack.
^ ___/--->
---------> + | = --/ (VERY crude sketch)
If the glider is trimmed to fly at +1deg AoA, and it now sees an airflow
which results in an AoA of +5, and if you glider is set up with positive
pitch stability, it will correct itself back to the trimmed +1deg AoA. This
stability correction shows up as a raising tailplane. Relative to your
ground based reference point, the glider appears to be flying at an attitude
that is -4 degrees off of steady flight. Why do wingtips rise as they brush
the side of a thermal, same deal. An added vertical component to the air
velocity, changes the effective AoA for the wingtip, and it produces a bit
more lift and causes the plane to bank.
Condition 3: You <accidentally> fly into sink or "falling" air, and the tail
drops and you get that sick mushy feeling in your gut. Reverse the above
vertical component of the airflow and see how it will affect your glider's
attitude.
One of the reasons you think the glider returns to a neutral air steady state
flying attitude once you've begun to work the thermal, is because the pilot
will instinctively play with the elevator once in the good air. By moving
that little tranny stick up and down, you correct the plane's attitude to
"normal", and actually you're changing the plane's 'trim' setting, and
telling it to fly at a higher AoA, producing more lift, and climbing out to a
speck. But, if you would just leave the tranny sticks alone, you would see
the tail rise as it enters and flies thru the thermal, then return to the
"normal" attitude once back in neutral air.
Just remember that your glider is ALWAYS decending relative to the airflow
that it sees, not counting when it's got powered assitance, tow line or tow
plane. :]
There is an excellent visual explanation of this in Dave Thornburgs video,
"Old Buzzard Goes Soaring", Bill's got a copy of it floating around if you'd
like to borrow it Rick.
Sorry if this got long, but I only use windvanes to tell me which slope to
drive to. And this windvane theory also doesn't readily explain why Zagi's
and Boomerangs, etc appear to get up "on step" when in good air, no true
tailplane to cause rotation.
Later,
Tom
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