Wing twist. Advances in aerodynamics and improvements in powerplants gave higher maximum lift, higher speeds and progressively higher wing loads. The typical wing loading of World War Two fighters was about five times that of the earlier conflit. A cantilever wing, because it can have only limited spar depth, tends to be inherently flexible. Early cantilever wings, therefore, were made as thick as possible, particularly at the root, and had cantilever ratios (i.e. semispan to root thickness) of order of 10. Wings of this low cantilever ratio were relatively stiff in bending but their torsional (i.e. twisting) stiffness was another matter. By World War Two cantilever ratios had increased to over 15 and serious consideration had to be given to aeroelastic effects in design of fighter aircraft. Wing twisting in response to aileron deflection decreased the available rolling moment as a function of dynamic pressure (i.e. speed²), because as speed doubled the associated loads quadrupled. In extreme cases, aileron reversal occurs, where the control surface loads deflect the wing rather than the air. Under these conditions the rolling moment obtained is in the opposite direction to that commanded. It was vital to establish, for each aircraft, that its aileron reversal speed was beyond the maximum speed attainable. The problem of aileron reversal became a critical design problem for World War Two fighters. Rapid rolling at high speeds became a commom combat manoeuvre and any loss in roll response put a fighter at a serious disadvantage. The advantage does not necessarily lie with the fighter that can out-turn its opponent speed for speed but the one with the fastest rate of roll or, even better, rate of acceleration into the roll, so that it can get into the trun faster. The designers of the Supermarine Spitfire, Focke-Wulf Fw 190 and the North American P-51 sought higher and higher roll rates. This led them to shorten their wing spans, balance their ailerons for larger deflections at high speed and stiffen their wings. Despite the excellent manoeuvrability of the Spitfire, it was unable to match the superior roll ability of the Fw 190.

Reference: Fundamentals of Fighter Design. Author: Ray Whitford. Edition: Airlife, England.