- Jul 28, 2003
First, for level flight - as you know, All changes in power necessitated a trim change for all high performance fighters. That is all about increase/same/decrease of lift at higher to lower Lift to offset Weight. CL is a function of angle of attack (pitch angle)... Also accompanied by rudder trim requirement. The P-47 and P-51 both tended to hunt during highspeed dives requiring rudder feed to dive in straight line. Dive tests were conducted at 1G.
Trim was only used when stick/elevator authority was lost in compressibility regime - but as discussed above, for the P-47 and P-38 the dominant reason for pitch down was a combination of loss of elevator authority AND a change in the aerodynamic center of the wing which caused the increased pitch down Moment coefficient. When elevator authority was lost during dives in a P-38 it was a combination of buffeting and loss of stable airflow over the elevator, but also movement of AC aft causing the pitch down. The theory and application of the dive flap was to delay the acceleration of airflow velocity over the wing into transonic shock wave and thereby increase resultant lift and delay movement of AC.
Hi Bill. Long time, no talk.
I am at Yanks Air Museum now and we have a P-38 that has the dive flaps installed on it. Ours has been modified with a Photo Recce nose. It was converted to F-5G at the factory.
Anyway, the dive flaps are fitted underwing, at somewhere near the 20 - 25% MAC point. They are about a yard long, give or take a bit, and have a split spanwise about 1/5 of the way forward from the trailing edge. Seems the front part hinges down at the leading edge and the rear part folds upward from the trailing edge of the dive flap. It appears as if the trailing edge stays flush with the wing surface, but I have never seen them deployed. Might be able to make that happen and get a pic. I'll check with the powers that be.
I am curious how you think this shape helps the pitch authority at high Mach numbers. The dive flaps is only on the underside of the wing, and I am really wondering how this help. other than maybe to force the shock wave at a point different from where it would normally form. Still, the shock wave diverges at a fairly constant angle from local flow, and is quite a way in front of the elevator. So, I am left wondering how it really helps, though I'm pretty sure it DOES help, if only from test reports.