An excerpt from the P-38L's pilot manual was posted in the aircraft forum:
"The dive recovery flaps which are installed under the wings between the booms and the ailerons, restore the lift to this portion of the wing and thus cause the uncontrollable nose heaviness to occur at a higher speed. The flaps also add some drag to the airplane, which in conjunction with the higher allowable dive speed, permits safe dives at a much steeper diving angle. The dive recovery flaps should be extended before starting the dive or immediately after the dive has started before a buffeting speed has been reached. If the airplane is buffeting before the dive recovery flaps are extended, the buffeting will momentarily increase and then diminish. With these flaps extended, the nose heaviness is definitely reduced but the diving speed should never be allowed to exceed the placard by more than 15 or 20 mph. With the dive recovery flaps extended before entering the dive, angles of dive up to 45° may be safely accomplished. Without dive recovery flaps extended, the maximum angle for extended dives is 15°."
Note that the flaps do not allow for an increase in dive speed of more than "15 or 20 mph" while allowing an increase in dive angle of 30 degrees (from 15 degrees without flaps to 45 degrees with flaps). That takes drag... and a lot of it.
Not 90 degree dive bomber speed brake drag of course, but 45 degree dive bomber speed brake drag...
So... All the data needed is right there. With the dive flaps extended the P-38L should be able to safely dive at 45 degrees, but the top dive speed should only be 15 to 20 mph more than in a 15 degree dive without dive flaps.
