North American P-51 Mustang

Design:  At the beginning of 1940, with the British aircraft industry attempting to cope with the demands of home defence, it became obvious to the British Government that aircraft would have to be purchased from abroad.  The British Direct Purchase Commission was established in the United States.  The best American fighter in operational service at that time was the Curtiss Tomahawk (P-40).  With the Curtiss production line fully occupied in meeting the demands of the US Army Air Corps the Purchasing Commission approached North American to build a variant of the P-40 under licence from Curtiss.  North American already had a broad outline for a fighter and submitted this design to the commission who had begun to have reservations about the suitability of the P-40 for the war in Europe.  The North American design was destined to become the P-51 ‘Mustang’. 

The Mustang was fitted with what is often referred to as a laminar flow wing, a concept originally developed by A.C. Robinson at NACA (National Advisory Committee for Aeronautics).  The wing essentially has a symmetrical aerofoil with the same curvature on its upper and lower surfaces with the leading edge being kept as thin as possible and the wing sections thickest point being placed as far to the rear as possible.  This layout allows the boundary layer airflow to adhere to the wing as far aft from the leading edge as possible with the result of a smoother airflow over the wing.  To achieve laminar flow the wing surface had to be highly finished.  Tests, towards the end of and after the war, showed that the drag on the Mustang wing was no less than that of a conventional wing of the same thickness and taper ratio.