Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L

[Ack-Ack]

Had a discussion with Odee about the flaps and dive flaps on the P-38L and told him I'd help clear up his confusion by posting the difference of the two.

In short, the Fowler Flaps used on the P-38 did not, nor was it intended to serve the same function of the dive flaps.  The Fowler Flaps, in layman's terms, extends the wing area by extending on rails or slots.



The dive flaps were intended to serve an entirely different purpose, and that was to aid the P-38 in recovering in a high speed dive.  Again, in layman's terms, it essentially changed the flow under the leading edge of the wing to prevent air turbulence over the elevators that caused "nose tuck" and make it easier for the pilot to recover from the high speed dive.  Basically, it helped resolve the compressibility issue with the P-38.



As you can see from the two different images, both systems were different and not used for the same purposes.  In addition, the Fowler Flaps couldn't be deployed at high speeds so it would have been useless in aiding the P-38 in recovering from a high speed dive or when it has entered into compressibility.

[Lusche]

 

[DaveBB]

Show an actual picture of the dive recovery flaps. It's amazing how small they are.

[oboe]

Here ya go:

[pembquist]

I thought the dive flaps moved the pitching moment of the wing forward so the plane would pitch up without elevator authority.

[oboe]

Yeah, I'm not sure how they would change airflow across the elevator when they are outboard of the booms and the elevtor is in between the booms?

[pembquist]

Also I recall that the first batch of recovery flaps was sent out on a c47 or such to England and got shot down by the RAF.

[Drano]

Oboe, iirc what was discovered was the airflow over and under the wings and past the cockpit nacelle came back together at about the location of the elevator. Depending on speed and alt it made the elevator feel like it was stuck in concrete. A bad thing if headed downhill! The flap interrupted/redirected the airflow to largely correct the problem. Being on the underside provided a bit of a pitch up action as well.

FYI there's an excellent video on YouTube with Jeff Ethell about the 38 where he does a walk around on a newly restored 38L describing the systems. It does show the flap and it deploying and retracting. You get a perfect idea of how it works. Sadly he was killed a couple of weeks after filming it crashing that plane on final.

Here's the link :

https://youtu.be/Y3nddCJbcdI

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[FLS]

Yeah, I'm not sure how they would change airflow across the elevator when they are outboard of the booms and the elevtor is in between the booms?

The dive recovery flaps change the center of pressure on the wing, they don't affect the elevator.

[Ack-Ack]

The dive recovery flaps change the center of pressure on the wing, they don't affect the elevator.

When the P-38 is in a compressibility state, the air flow becomes turbulent over the tail over the P-38, making it very difficult for the elevators to work.  Remember, compressibility is caused in the P-38 when the air hitting the leading edge of the wings breaks the mach barrier, which causes the turbulent air flow over the tail section.  The dive flap changes the air flow so it is no longer turbulent over the tail section of the plane and allowing the elevators to function with much more authority in addition to the slight pitch up from the dive flaps.

[FLS]

It isn't the air speed at the leading edge, it's the air moving faster over the top of the wing that causes the problem.

The airflow over a thick wing like the P-38 reaches compressibility at lower air speeds than a thin wing. The transonic flow moves the center of pressure on the wing back causing the mach tuck while the shock wave that forms moves back and locks the control surfaces. The P-38 is still subsonic when it compresses.

The dive recovery flaps disrupt the airflow and keep the center of pressure in the normal flight range.

[Drano]

Which is petty much what I said but thanks

Sent from my XT1585 using Tapatalk

[FLS]

The tail flutter caused by turbulence was fixed with the fillet where the wing meets the fuselage.

Even the NASM and EAA sites describe dive flaps as slowing the aircraft down to prevent compressibility. 

[oboe]

That was a great video link, Drano.  I hadn't seen that one before.  Sure was a tragedy about Jeff Ethell. 

Regarding the referred to NASM and EAA information, here it is:

https://airandspace.si.edu/collection-objects/lockheed-p-38j-10-lo-lightning

Seventeen months passed before engineers began to determine what caused the Lightning's nose to drop. They tested a scale model P-38 in the Ames Laboratory wind tunnel operated by the NACA (National Advisory Committee for Aeronautics) and found that shock waves formed when airflow over the wing leading edges reached transonic speeds. The nose drop and loss of control was never fully remedied but Lockheed installed dive recovery flaps under each wing in 1944. These devices slowed the P-38 enough to allow the pilot to maintain control when diving at high-speed.

https://www.eaa.org/en/eaa-museum/museum-collection/aircraft-collection-folder/lockheed-p-38l-lightning---n3800l

High-speed dives created turbulence and reduced elevator effectiveness, making it difficult or impossible to pull out of the dive – a phenomenon later understood as compressibility – the turbulence that builds up on a wing just before an airplane reaches the speed of sound. Lockheed engineers solved the problem with a speed brake that slowed the P-38’s dive and reduced the turbulence.

[FLS]

Now people will be quoting that when they complain the P-38 "dive brakes" don't work.

Jeff Ethel wrote books for the NASM, you'd think they and the EAA would know better. I assume the web content was written by less knowledgeable volunteers.