I have done a great deal of flight testing of the P-39s. The one glaring issue has been flap function, or lack thereof. Of all fighters in the plane set, the P-39s have, by far, the most abysmal flaps. With a clean turn radius of 595 feet, the P-39Q gains very little when deploying full flaps, at 525 feet. That is just a 70 foot decrease in turn radius, with a huge hit in turn rate. Consider that the F4U-1 gains 250 feet, and the SpitIX with a simple split flap gains 200 feet. That simply didn't seem right, so I did some testing to verify that the drag increase using flaps was excessive.
I elected to test the P-39Q against the P-40E.
To begin, let's look at the coefficient of drag of the two.
P-39: .0217
P-40: .0242
Let's look at wing area:
P-39: 213.2 sq/ft
P-49: 236.0 sq/ft
Let's look at flat plate area:
P-39: 4.63 sq/ft
P-40: 5.71 sq/ft
So, the P-39 is much cleaner than the P-40 in terms of basic drag.
Let's examine the flaps of the two fighters. I'll provide flap area and full down angle. Both use simple split flaps.
P-39: 26.2 sq/ft @ 43 degrees
P-40: 33.1 sq/ft @ 45 degrees
So, the P-40 has greater flap area and when fully down, the flaps are at a more acute angle. Again, the P-39 flaps should generate less drag. The problem is that they don't. Indeed, they generate far greater drag than those of the P-40.
In level flight, with full flaps, the manifold pressure required to maintain a constant speed of 128 mph is:
P-39: 34" Hg
P-40: 27" Hg
Thus, we see that the P-39 requires considerably greater power to maintain the same speed as the P-40, yet it should have much lower drag, with or without flaps.
To verify this, I pulled off power to idle and pushed the nose over into a shallow dive. I established a dive angle that produced zero acceleration. IE: A constant speed. What I discovered is that the P-39 required a significantly greater dive angle to attain zero acceleration. See the images below.
My conclusion is that the P-39 suffers from excessive drag related to flap deployment.
Pyro, please have a look at the drag properties of the P-39's flaps.
Thanks,
Widewing
