I have found that whether the nose pitches UP or DOWN depends on how the
plane is trimmed -- near neutral stability or more pitch stable (nose heavy
trim). And the inertia of the system must also be considered in how the
plane responds to an upset.
All the explanations I've seen about nose-down pitch (and slight temporary
speed-up) upon thermal entry agree with my experience with NEAR-NEUTRALLY
STABLE trimmed planes.
Some of my planes that are trimmed to attain more pitch stability (am I
using the right term, or is it pitch damping?), in other words more nose
heavy, I have always observed the opposite happening. The upward draft of
the thermal, increasing the wings AoA, SLOWS the plane down due to the
increased drag (?) of the momentarily higher CL. The dynamic involved as
vertical acceleration of the plane occurs in the first moments after thermal
entry also comes into play, I believe. The pitch stability (damping?)
resists the nose rotating down as quickly and the reduction in airspeed
(while the inertia of the plane remains the same) lessens the correcting
forces of the stab.
I may have a bit of the theoretical stuff mis-applied, but the phenomena
I've observed on these airspeed/attitude changes is clear.
Any insights from y'all???
Dan deVries
DEVRIES DESIGN, Inc.
Product Design --+-- Patent/Litigation Graphics
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Dan deVries
290 - 68th St. SW
Grand Rapids, MI 49548
Office: 616-281-2271
Fax: 616-281-2294
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