DEVELOPMENT test flying of a prototype aeroplane is playing an ever‐increasing role of importance in the ultimate design of high performance military aircraft. When a prototype aircraft is rolled out of the Experimental Shop and made ready for its initial test flight, it is far from being a finished product. The first flights invariably reveal numerous shortcomings in the aeroplane's performance, stability, control, and handling characteristics, many of which are of a limiting nature. That is to say they are sufficiently serious to prevent the pilot from achieving the estimated design performance of the aeroplane. The many months of intensive design effort on the part of engineers and shop personnel have set the stage for an all‐out development, modification, and flight test programme which frequently taxes their ingenuity and energy to a far greater degree than did the initial design work.