Effectiveness of the P-38

[MIflyer]

I've always thought the center of pressure moves aft when you go supersonic while the center of gravity doesn't move

That's why the F-106A had an automatic fuel transfer system, activated when you hit Mach 1.05, to change the CG.

 

[drgondog]

Hi Bill,

I've always thought the center of pressure moves aft when you go supersonic while the center of gravity doesn't move, thereby causing a nose-down pitching moment.

If the tail gets blanked by the shock wave, it becomes impossible to pull up until the airflow reattaches.

If you aren't supersonic, and you speed up, a stable aircraft will nose up in order to maintained trim airspeed.

Greg, I mistakenly believed the same cause and effect. What actually happens is that lift in a dive is Near zero to slighty negative at -2degrees AoA with occasional forward pressure on stick required to maintain sufficient negative lift to remain neutral in the dive. The Moment Coefficient for a NACA 2015 airfoil is slightly negative pitch down throughout the angle of attack range until the stall break where it goes severely negative. In normal flight slightly up elevator trim is required.

What I believe is that when the shock wave first occurs on the 23015 wing section, the pressure distribution and lift (near neutral) is reduced aft of the shock wave. As a result, the constant negative CM for all angle of attack generate the pitch down moment and the shock wave wake turbulence blanks the elevator and trim.

My conclusions are that a.) the flap generates an immediate Pitch up and positive angle of attack, b.) the associated lift distribution in front of the shock wave increases, giving positive lift to the wing/body, c.) begins the degradation of the airspeed (and shock wave) by increased lift and drag, d.) enables natural dive pullout before the elevator is unblanked, slowing the P-38 in the dive until the airflow dive speed reduces below Mcr due to increase in temperature as f(altitude).
We know Lift is reduced behind the shock wave as major separation occurs. Logically the lift/pressure distribution center would move forward - but even if not so, the CMac remains negative for the NACA 23015.

Go look at the 2315 and 23015 section data and see if you draw the same conclusions.

Or my brain has been irreparably addled by drink and Wolfhound slobber.

 

[GregP]

Wolfhound slobber is one of those mysterious liquids that can affect life, love, and sometime aerodynamics.

I think it is, overall, due to the FM Effect (Freaking Magic).

 

[SaparotRob]

I think it is, overall, due to the FM Effect (Freaking Magic).

Knew it. Folks, all those diagrams, charts and formulae are really just incantations. There is no science.

 

[GregP]

And, you have to hold your tongue in the right position for it to work.

Obviously, the person below is thinking ... as should be intuitively obvious to the casual observer ...

 

[Thumpalumpacus]

Or my brain has been irreparably addled by drink and Wolfhound slobber.

I'm a few Shiners out of a helluva birthday party. Not sure why you're slobbering around with wolves but hey, we got no judgement here, Bill. You do you.