Dive Recovery Flaps on P-38

[Conslaw]

Did the dive recovery flaps on the P-38 slow add enough drag to keep the aircraft below the compressibility threshold, or did they raise the compressibility threshold?

 

[swampyankee]

I'm trying to search the NTRS for a definitive answer. So far, I just found this article. http://www.dtic.mil/get-tr-doc/pdf?AD=ADA604401, which is primarily a critique of the poor specs and poor communication between Lockheed and Allison.

The search continues.

 

[Thorlifter]

My VERY LIMITED understanding is the flaps disrupt the airflow over the control surfaces so compressibility does not occur. Looking forward to the input of more knowledgeable people.

 

[swampyankee]

My VERY LIMITED understanding is the flaps disrupt the airflow over the control surfaces so compressibility does not occur. Looking forward to the input of more knowledgeable people.

That's what the article I linked to said. I'm looking for something from NACA

 

[eagledad]

Swampyankee

Perhaps this document will answer your question


https://ntrs.nasa.gov/search.jsp?N=...y Albert L. Erickson&Ntx=mode matchallpartial

The above address will get you to the NACA server, and give you the listing for the report

WIND-TUNNEL INVESTIGATION OF DEVICES
FOR IMPROVING THE DIVING CHARACTERISTICS
OF AIRPLANES by Albert L Erickson.

Eagledad

 

[MIflyer]

The "dive flaps" on the P-38 are not anything like normal flaps. The are about mid-chord along the wing and come down to form a "V" shape and are designed to shift the center of lift. Compressibility effects shifted the center of lift from the forward to the aft portions of the wing, causing the aircraft to try to do an outside loop. The Dive Flaps shifted the center of lift back forward.

 

[drgondog]

Net - Dive flaps delayed onset compressibility by shifting the Center of Pressure aft. It did Not increase dive speed but it did lessen the onset nose tuck tendency as V~Mcr

 

[MIflyer]

Well, compressibility shifted the center of pressure of the airfoil aft - as transonic flow always does - and that brings the nose down, uncontrollably. The dive flaps shifted the center of pressure back forward so that the normal flight controls worked. They were not designed to either slow down the airplane (like Dive Brakes on dive bombers do) or magically make the airfoil more suitable for transonic speeds, is what I think you mean.

 

[drgondog]

Well, compressibility shifted the center of pressure of the airfoil aft - as transonic flow always does - and that brings the nose down, uncontrollably. The dive flaps shifted the center of pressure back forward so that the normal flight controls worked. They were not designed to either slow down the airplane (like Dive Brakes on dive bombers do) or magically make the airfoil more suitable for transonic speeds, is what I think you mean.

The first part of your statement is correct, but the dive flaps didn't move the CP 'back' , they Delayed the CP transition from near 1/4 chord to 1/2 chord for the 'classic' airfoil types before employment of low drag airfoils like the P-51.

The latter condition is what adversely changed the CMac to very strong 'Pitch down' moment beyond ability of pilot to recover from dive until density increased enough to drop the local velocity over wing below Mcr.

P-38 Pilots instructed to deploy dive flaps Before Mcr in dive.

 

[MIflyer]

Well, here is what Lockheed said about in 1944.

 

[drgondog]

Well, here is what Lockheed said about in 1944.

Lockheed said what I said. The Flap delayed the CP shift from moving aft.

 

[Aeroweanie]

Along these lines, does anyone have a copy of this report?
C. L., Johnson, "A Study of the Diving Characteristics of the Lockheed P-38 Airplane," Lockheed Aircraft Corporation Rep No 2646, May 8, 1942.

 

[drgondog]

Lockheed said what I said. The Flap delayed the CP shift from moving aft.

To add to the benefit of the flap - it provided an instant nose up pitching moment.

To Dave - I do not have either the P-38 dive test reports or the P-47 dive flap reports.

 

[Aeroweanie]

To add to the benefit of the flap - it provided an instant nose up pitching moment.

To Dave - I do not have either the P-38 dive test reports or the P-47 dive flap reports.

I agree on the benefit and am planning on proving it with CFD. My limited analysis, using the F8F, show this already.

Attached is what I was able to find in the Huntington Library Kelly Johnson collection.

 

[drgondog]

I agree on the benefit and am planning on proving it with CFD. My limited analysis, using the F8F, show this already.

Attached is what I was able to find in the Huntington Library Kelly Johnson collection.

Outstanding find. I learned something (again). I had always labored under the assumption that the NACA wind tunnel tests Recommended the filets. Here it is clear that Lockheed drew the correct conclusion, installed fuse/wing and boom/wing filets as well as change incidence of H.Stab Before sending to NACA.