Alternate Approach to the P-38 Compressibility Problem (1 Viewer)

[drgondog]

Logically, there is no reason why having the P-38's engine turn inwards would cause the aircraft to be unable to lift off, but thath happened. So, tey changed the engines to outward-turning, and the issue "went away." Doesn't seem to be a logivcal explanation for that one, either, but the solution is real and well-documented.

Perhaps this is just another really odd thing about the P-38.

Stranger things have happened ...

Actually Greg, with tricycle landing gear, the inward turning prop created 'downflow' over the center section of the wing - which reduced relative angle of attack. Required abnormally high takeoff speeds to get required CL.

The P-82 as a tail dragger had the opposite problem with outward turning prop creating an upflow over center section and an artificially high AoA on takeoff which stalled the center section.

 

[drgondog]

I have seen some very questionable videos about WW2 aircraft on Youtube but I watched one tonight that seems to have some new information.

We have all heard about the P-38's compressibility problem and how the "dive flaps" ultimately solved it, but according to the video a different approach was developed in the fall of 1943. A P-38 pilot dove after a German fighter and got into compressibility. Luftwaffe pilots had learned to dive quickly away when bounced by P-38's at altitudes of 18,000 ft and above but to use a shallow angle to trick the P-38's int entering the compressibility regime. Desperate to do something to regain control of the airplane, the P-38 pilot tried shutting down his Right engine. To his amazement he found that he regained control and that he could actually speed up, to over 500 mph, and catch the German fighter he was diving after, and shot it down. Then he restarted the engine at 12,000 ft and went home.

Back home he reported on this approach and was told he was nuts. Then they found out it worked. Over the objections of the maintenance personnel, who were appalled by shutting down an engine in dive and then restarting it, a procedure for the technique was developed and published in March 1944. The Germans eventually found out about it from a captured P-38 pilot and revised their tactics accordingly. It was still used in the ETO until the end of the war.

Never heard about this before!

I don't believe a word of the account.

Cutting one engine on a P-38 in a dive would have two specific and immediate results - 1.) major reduction in thrust with accompanying increase drag due to the prop, creating major and dangerous yaw input, 2.) major reduction in local airspeed over dead engine inboard wing section.

IMO the result would be to slow down enough to transition the P-38 below critical mach - and restore both elevator authority and 'normal' pitch moment of the wings. Pilot would still be jamming the rudder to reduce dangerous yaw condition.

In the interim, the German fighter, already faster in a dive, is Not slowing up and waving bye-bye.

 

[MIflyer]

One of the dumber things about the video is how it it talks about the elevator being locked in place by the shock waves. That was more or less the thinking in 1942. In reality the approach to mach 1 at altitude led to an aft movement of the pressure on the wing, leading to the elevator being not locked but not effective enough. The "dive flaps" moved the center of pressure forward. Seeing a modern day "documentary" that uses that kind of "locked elevator" explanation is like hearing a medical doctor of today blaming evil spirits for disease.

If you were transported back to 1942 and tried to explain to Lockheed what was really going on I am not sure they would believe you or even understand you. So little was known about transonic flow then. It took the Bell X-1 to discover the importance of an adjustable horizontal stabilizer, even though such things had been used for decades back in the biplane era.

 

[Aeroweanie]

Here is an interesting perspective: the P-38 dove at a Mach number of about M=0.68 (314 KCAS at 20,000 feet). There are the results of a CFD analysis of the P-38 at this condition. The blue surface are the outer boundaries of the regions of supersonic flow. There is no supersonic flow on the horizontal tail and thus no shock waves on the tail. There is a lot of separated flow coming off the cockpit pod (final image) which immerses the horizontal tail, reducing dynamic pressure there.

 

[drgondog]

Here is an interesting perspective: the P-38 dove at a Mach number of about M=0.68 (314 KCAS at 20,000 feet). There are the results of a CFD analysis of the P-38 at this condition. The blue surface are the outer boundaries of the regions of supersonic flow. There is no supersonic flow on the horizontal tail and thus no shock waves on the tail. There is a lot of separated flow coming off the cockpit pod (final image) which immerses the horizontal tail, reducing dynamic pressure there.
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Dave - this is a fantastic visual, excellent display of the venturi effect between each engine nacelle and cockpit pod, particularly at the wing root. Seems to demonstrate the associated loss of pressure distribution aft of the shockwave causing reduction in CL.

 

[tomo pauk]

Here is an interesting perspective: the P-38 dove at a Mach number of about M=0.68 (314 KCAS at 20,000 feet). There are the results of a CFD analysis of the P-38 at this condition. The blue surface are the outer boundaries of the regions of supersonic flow. There is no supersonic flow on the horizontal tail and thus no shock waves on the tail. There is a lot of separated flow coming off the cockpit pod (final image) which immerses the horizontal tail, reducing dynamic pressure there.

Thank you
A question: is it easy to put through the program the NACA P-38 mods tested, like the extension of the chord by 20% of the par of the wing that sits between the pod and booms? The NACA report: link.

 

[special ed]

As I have always suspected, the P-38 Swordfish (with a cleaner canopy) would not have had as much problems as the stock versions.

 

[drgondog]

As I have always suspected, the P-38 Swordfish (with a cleaner canopy) would not have had as much problems as the stock versions.

The canopy isn't the primary issue. Acceleration of airflow over the inboard wing between engine nacelle and the center fuselage is the cause of early Mcrit compared to the wing sections outboard.

 

[Aeroweanie]

Dave - this is a fantastic visual, excellent display of the venturi effect between each engine nacelle and cockpit pod, particularly at the wing root. Seems to demonstrate the associated loss of pressure distribution aft of the shockwave causing reduction in CL.

The image below shows how the pressure distribution on the thick, forward loaded, inboard wing section interacts with the pressure distribution created by the canopy. The loss in total pressure aft of the shock is caused by shock-induced separation.

You can really see the total pressure loss in this iso-total pressure loss picture. Besides the shock wave, the design has the pod contracting as the wing surface contracts towards the trailing edge. Having two surfaces falling away from one another is a recipe for separated flow.