How to thermal useful info

Originally Posted by Don Stackhouse

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Thermals near the ground tend to be small in diameter, and fairly weak, with very tight cores, although this one was a bit extreme in those regards. In particular, it requires some extremely tight turns, which requires a lightly-loaded plane and some special piloting techniques.

When the diameter of the turn is small, the airflow past the plane is very significantly curved, If the airflow is perfectly aligned with the fuselage at the wing, back at the tail the "relative wind" could be blowing upward (because of the bank angle) and inward towards the center of the turn by as much as 10-15 degrees! This tries to put the nose down, and yaw the plane towards the outside of the turn. If it's a 2-channel rudder-elevator model you need some yaw in that direction anyway, to get enough extra angle of attack at the inside wing tip to compensate for the lower airspeed there, and hold bank angle more or less constant. If the designer was really clever with juggling things like dihedral schedules, tail moment arm, tail design and wing airfoils, the yawing effect of the curvature of the airflow could be just enough to compensate for the effect of airspeed at the inside wing tip. Otherwise, you could need to apply some rudder, although it could be "top" or "bottom" rudder depending on the specifics.

However, you WILL need to apply some "up" elevator, typically a LOT of up elevator in a max-effort turn. How much is "a LOT"? In the example I described above, about twice as much as it takes to stall the plane in level flight!

The catch is that if there is a gust or some other disturbance that momentarily widens out the turn (thereby reducing the curvature of the flow), you have to IMMEDIATELY back off on the "up" elevator or the plane will stall. Once the disturbance has passed, you can ease the plane back into the turn, including the up elevator setting. It's a bit like playing a fish on the end of a line.
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