Note performance of the 'dive flap' isn't just drop flap and not compress. Combat trim and the need to deploy the flap before you dive can greatly effect it's effect.
Combat trim? I hope you mean that you shouldn't use combat trim if you're flying a P-38, let alone using the dive flaps on the P-38L. Another important thing to keep in mind that improperly manually trimmed P-38Ls can also decrease the effectiveness of the dive flaps, which is why I always recommend trying to keep the elevator trim as close to approximate neutral as possible, with maybe a notch or two below neutral.
Another thing with the dive flaps on the P-38L is that they can also be used to aid in high speed maneuvering as high speed flaps.
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