Pitch-down in compressibility with the P-47

drgondog said:

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.

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GregP said:

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.

Any comment?

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drgondog said:

Yep, Often wrong but never uncertain.

The Dive flap served two purposes, 1.) retarded continued acceleration into the dive (as a brake), and 2.) hanged the CMac to slight 'pitch up' as the shock wave was forming. It did Not permit higher speed dives, only better pilot control while in the dive.

I'm going to have to dig but IIRC the primary function was to disrupt the flow on the bottom of the wing (at 1G, neutral AoA), giving the upper pressure distribution more authority, however diminished by the shock wave.

It still didn't solve compressibility effects in the fuslage to engine region,even after fillets were installed to reduce flutter.

Quotes
About the P-38 specifically, Johnson said:

"On returning to Burbank [from high-speed wind tunnel tests with NACA], we decided that if we could not solve compressibility, we could discover a way to slow the airplane to a speed where the effect no longer was a factor. The answer was external dive flaps, or brakes. Put in the right place, they would cause the nose to come up out of a dive and stop buffeting."

(From "Kelly - More than my Share of It All" by Clarence L. "Kelly" Johnson with Maggie Smith.)

By Hall L Hibbard
Vice-President and Chief Engineer, Lockheed Aircraft Corp.
"of the interesting new cases of P-38 pioneering is the use of recently added dive flaps to offset compressibility effect, which shifted center of lift from fore to aft portions of wing. Due to the unusually high speeds attained by the heavy P-38 in power dives, shifting of the center of lift caused loss of normal control above the "hydrodynamic" speeds — where air reacts much like water — with a resulting tendency of the plane to go into an outside loop. Since installation of the flaps this characteristic has been overcome."

The 0.65 Placard Vne still applied.

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GregP said:

Thanks, Bill, it neatly makes the nose rise without reinstating the airflow over the elevators. At some point, the shock wave dissipates and the airflow reattaches. Once you said it, now it makes sense.

I'll try to get the powers that be to let me open the P-38 and deploy the dive flaps for pics. If so, I'll start a new thread and post it if I can't find this again.

Again, thanks!

Edit: Frank Wright says that in about a month or two, give or take a bit, they are going to pull the P-38 out and run it up for the first time in some years. Who knows, maybe take it for a short flight (no promises). When they do that, I'll make sure to ask them to exercise the dive flaps and take pics, so everyone can see where they are and what they look like stowed and deployed. There has been enough discussion about these things over the last 15+ years or so that maybe it will be of interest. Can't speed it up ... they do it when it comes around on the priority list.

Cheers!

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GregP said:

Thanks, Bill!

I wasn't aware of the dive flaps on the P-47. As it happens, we have two of them.

One is a P-47D-23-RE (42.27480) that is being returned to flight status. We are working on the propeller now. The other is the XP-147M-1, later redesignated as a YP-47M. The SN is 42-27385 and it was one of four trial airframes modified to P-47M configuration, and spent some time in P-47M testing at Republic. In total, they only built 130 P-47Ms.

When I go in Tuesday, I'll look and see if either of them has dive flaps installed.

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