P38 Performance

[Randy1]

There is no flow separation in front of the shock wave.  So, if you want a flap to increase lift of the wing (to make up for the fact that everything behind the shock wave is producing no lift), you need to put it in the forward portion of the wing rather than the stalled-out rear of the wing.

Putting it outboard of the nacelles means it will generate less potential turbulence or other effects to airflow for the elevator.

The P38 dive flap, is a closer cousin to split flaps and therefore produces no significant lift as would a typical wing flap that alters the shape of the airfoil.  What it does do, I believe is, create, or make larger the bottom wing shock wave to counter the top wing shock wave.

"Bridgland  concludes that a flap area of about 0.01 of the wing area is generally required, if the flap is well inboard. If flap is so far outboard, as to have little effect on the downwash at the tailplane (or in the case of tailless aircraft), the flap areas will then need to besome three or four times this amount. "

[Brooke]

The P38 dive flap, is a closer cousin to split flaps and therefore produces no significant lift

Split flaps do produce lift.  Here (graph on the left) is an example of C_L vs. alpha for a wing with and without split flaps deployed.

[FLS]

If you want to compare the dive flap to a split flap you have to consider it's position under the wing. It's not a trailing edge flap so it masks some of the wing behind it. This may offset some of the lift it creates.

[Randy1]

Here is a good quote Brooke and FLS.  I think this pins it down.  It is all about the downwash angle.  It is so interesting that the people working on propellers new of the upcoming problem and Stark had written a paper foretelling of the upcoming compressibility issue on wings since he had encountered that problems on props.

http://history.nasa.gov/SP-4219/Chapter3.html

The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly. This is a graphic example of how, in the early days of high-speed flight, the NACA compressibility research was found to be vital as real airplanes began to sneak up on Mach one

[FLS]

Randy did you read the study I linked? It's the wind tunnel test on dive flaps on what loooks like a P-38 wing.

[Ack-Ack]

Randy did you read the study I linked? It's the wind tunnel test on dive flaps on what loooks like a P-38 wing.

It was the P-38 that was used in that wind tunnel test, there is another wartime NACA report from testing in the larger 16ft wind tunnel.

ack-ack

[GScholz]

And according to that document the drag of the wing increased by up to 59%.

[colmbo]

And according to that document the drag of the wing increased by up to 59%.

Was that increase related only to the dive flap?  Could some of that drag increase be because of shock wave, etc?

[Randy1]

Randy did you read the study I linked? It's the wind tunnel test on dive flaps on what loooks like a P-38 wing.

FLS, yes I did.  One of the best articles I have seen.  And a great find.
I also have a quote from the test pilot doing the test that Megaloden sent me.  

When I googled boundary layer in a reply to the other thread I found the same article and read it then as I remembered your reply post and link.

Having two threads is a bit confusing.  

[Brooke]

If you want to compare the dive flap to a split flap you have to consider it's position under the wing. It's not a trailing edge flap so it masks some of the wing behind it. This may offset some of the lift it creates.

The portion of the wing behind it, once compressibility sets in, is stalled out, negating lift there.

[Randy1]

Was that increase related only to the dive flap?  Could some of that drag increase be because of shock wave, etc?

Yes shock wave.  This makes me wonder in all compression related planes if the acceleration is reduced when Mach drag comes into play.  HTC is pretty sophisticated and may well include mach drag.

[GScholz]

Was that increase related only to the dive flap?  Could some of that drag increase be because of shock wave, etc?

It was worded like this: "At a Mach number of 0.850 for an angle of attack of 4 degrees, the dive-recovery flap represents an increase of approximately 59% of the wing alone-drag."

It's on page 17.

[FLS]

The portion of the wing behind it, once compressibility sets in, is stalled out, negating lift there.

There is a question about the effect of the dive flaps at subsonic  speeds. I wonder if at certain speeds the added and reduced lift cancel out.  That would explain anecdotes that the dive flaps don't have much effect unless compressing.

[Brooke]

There is a question about the effect of the dive flaps at subsonic  speeds. I wonder if at certain speeds the added and reduced lift cancel out.  That would explain anecdotes that the dive flaps don't have much effect unless compressing.

I suspect that deploying them increases lift for the forward portion of the wing.  In compressibility, the lift of the after portion of the wing is zero, so that isn't reduced, so the net is an increase in lift.  If the wing isn't in compressibility, deploying the dive-recovery flaps might reduce lift in the after portion of the wing (maybe because flow on underside after the recover flaps is disturbed) -- not sure -- but if so, then as you say increase in lift for forward portion could be cancelled out by decrease in lift for after portion.

[Randy1]

Stark is the designer of the flap. 


"Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly."

Note the problem in this case is separation on the top wing changes the down wash angle.  The bottom flap counters that by producing its own separation.