>Because the dive test does NOT indicate >anything about pitch stability. >It ONLY indicates elevator trim/decalage >angle effect during increasing airspeed.
The second sentence is in fact one definition or at least one direct consequence of pitch stability. A pitch-stable aircraft must see a decrease in trimmed airspeed with up-elevator and vice versa. A pitch-unstable aircraft will do the opposite (if it's artificially sustained in trimmed flight via a gyro or whatever). To answer the original question... Any given real airplane does not have a fixed stability margin at all airspeeds. Normally the stability margin decreases with airspeed. There are typically three reasons for this: 1) Cm of low Reynolds number airfoils is not really constant. It typically becomes more negative (nose down) at small AoA. This looks and smells just like a reduced stability margin. 2) If the CG is below the wing's center of area, like in any poly glider, the CG will effectively move aft at lower AoA and reduce pitch stability. 3) Flexibility of the wing, tailboom, tail surfaces, and linkages tends to have the same effect as imparting down-elevator trim at increasing airspeeds (reducing AoA). Again, this looks and smells like a reduced stability margin. If the glider has slight positive stability in a slow glide, it can easily go unstable in a fast dive, resulting in a tuck-in. The dive test, if done correctly, is useful in that it represents a worst-case scenario where all three effects above gang up. - Mark RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
