Stability and Control characteristics of the Early Mark Spitfires

[JtD]

Originally Posted by Crumpp

Now let's see the instability.

Green:

You can find similar spots in turn time histories for nearly all WW2 aircraft tested this way. Or, you can pick another spot on the very same curve and will find different figures or pick the curves 16-18 and get different results again. If it was a plane quality, there'd be the same behaviour in every test they've done, but it isn't.

NACA evaluates the the behaviour:

"In turns at speeds high enough to prevent reaching maximum lift coefficient because of the excessive accelerations involved, the small static longitudinal stability of the Spitfire caused undue sensitivity of the normal acceleration to small movements of the stick. As shown by the time histories of high-speed turns (figs. 15 to 18), it was necessary for the pilot to pull back the stick and then ease it forward almost to its original position in order to enter a turn rapidly without overshooting the desired normal acceleration. Although this procedure appears to come naturally to a skillful pilot, flight records from other airplanes show that a turn may be entered rapidly and the desired normal acceleration may be held constant by a single rearward motion of the stick provided the static stability of an airplane is sufficiently large. By careful flying, the pilot was able to make smooth turns at high speed, as shown by figures 17 and 18. Ordinarily, however, small movements of the stick caused appreciable variations in the normal acceleration, as shown in figures 15 and 20."

So, what they are saying again is that there are large reactions to small stick travel. Not that the plane was unstable. The stick force gradient and the elevator angle gradient were both found to be positive, as I've said already.