Wow. And here I thought I understood it before.
Just one more thing. If I flew into a really big thermal--one that was
so big I didn't need to turn long enough for my stable glider to reach
an equilibrium again--the true airspeed of the glider (not necessarily
what I'm observing) would be higher than it was prior to hitting the
thermal. Is this right?
The stuff about the stab being more sensative to the change in effective
AOA is bunk then? Heck, I really thought that made sense.
-Ben
[EMAIL PROTECTED] wrote:
>
> 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
> RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
>"unsubscribe" requests to [EMAIL PROTECTED]
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
