It's not so much the effectiveness of the aircraft's flaps. Even based on stall speeds alone, the F4U's flaps generate an IMPRESSIVE amount of lift compared to other aircraft, so it's not surprising that they'd provide a significant benefit towards turn radius (that said, I personally try to stay out of situations where I need more than two notches).
The F4U flaps and their ingenious placement on the trailing edge is no doubt a very interesting design concept to consider, and it is entirely believeable that they may be
(up to some extent) more effective than some of its counterparts in various planes.
However, technically they are nothing but plain flaps, and this fact is clearly stated in even in
HTC's own Planes&Vehicles page, from which I quote;
The F4U-1D has a plain flap that is designed to also aid maneuvering at a setting of up to 20 degrees. This setting will provide an enhancement to the plane's turning capabilities at a low cost in additional drag. If you get caught in a turning fight, use of the maneuver setting is highly recommended. The maneuver flap setting can be used up to speeds of 230 MPH. For takeoff, 20 degrees of flaps is the normal setting. Maximum flap deflection is 50 degrees.
Ultimately, a flap is nothing but a mere aid.
There are many planes in AH with flaps which are techincally more efficient than those of the F4Us, and despite that there are clear limits to how it performs in relation to its basic spec.
For example, it has been long rumored throughout AH1 and AH2 that the P-38s can turn very well for its size. Actual turn radius testings, including some of my own, have proved that the magic of the P-38s lie in their inherent stability in knife fight situations due to the non-torque status, rather than the turn radius itself, which has proved to be nothing too impressive.
Both Mosq's and my own testings, confirm the P-38J turns the worst of the following 4 planes; Spit9, Bf109G-6, P-47D-11, P-38J: whether with or without flaps.
The actual testings on true physical capabilities of the P-38, is in great discrepancy with empirical experience of many people in which the P-38 seems to turn much better than it actually can. In effect, it proves what some people have been guessing for a long time.
Many people note highly of how the P-38 is equipped with a Fowler-type flap system which is often quoted to be most efficient. They usually attribute the excellent maneuvering characteristics of the P-38 to its flaps - which, quite contrary in reality, as proved by testings, does not actually help out the lumbering fifteen-thousand pound, twin-engined plane by much in turn maneuvering. The P-38, in game as it is, has its turn radius closest to the Fw190s than any other plane. (The Fw190s, ofcourse, being the worst turners in the game)
What does this tell us?
It tells us no matter how efficient a flap is, it's not going to make a big, heavy plane overcome a much lighter, nimble plane that is inherently designed to turn better. The P-38 utilizes its special status as a plane with neutered torque, which allows its pilots to excell in harsh maneuvering despite its unimpressive turning radius (even with the "efficient" Fowler flaps). So then, what's the F4U got, besides its flaps, that makes it physically turn so much better in actuallity?
Go to the AH Fighter comparisons page and try a comparison of the following 4 planes:
F4U-1, F6F-5, Spit9 and the FM-2.
Compare the turn radius performance of the 4 planes, and one immediately realizes something is truly weird with how the flaps benefit the F4U.
The worst turning aircraft in normal flight condition, suddenly becomes the 2nd best with full flaps, almost bordering on the performance levels of the FM-2. If the flaps alone can shake around turn radius performance like that, then it is logical to assume the F4U flaps are either providing too much lift, or providing too much stability -
because flaps alone, no matter how efficient the system is, will not be able to such drastically alter how a plane can turn. I don't think you can keep defending and justifying the current performance of the F4U with the "it's the flaps" reasoning any more. Even Widewing has some choice words to say about the F4U's turn performance being too extreme. He says the same things about the 109s, but 109s at least have the "slats" defense to go by, as well as being a very small and lightweight plane than compared to American iron.
What's the F4U got beside its flaps, that allows it to physically(not anecdotally) become such a well-turning plane, despite its large size and weight, and a powerful P&W engine full of dangerous torque forces, at such extremely low speeds during knife fighting?
Try another comparison in the fighter comparisons page, this time a F4U-1 against a Ki-84.
The Ki-84's also equipped with a highly efficient flap, much more than the Corsair. The two planes are both single-engined, and normal flight conditions show that the Ki-84 has a clear edge over the F4U in turning radius.
But then, in full flaps, the F4U plain smack outturns the Ki-84, a plane much lighter in weight, inherently better off in wingloading (as demonstrated by normal flight conditions), equipped with one of the most efficient flap systems in the game.
The Ki-84 is a lighter, better turning plane, with at least as efficient flap systems in place, and the much heavier F4U, with higher engine torque, higher wingloading, just outturns it because of the flaps?
Dude, how can this not be strange?
