North American P-51 Mustang variant
North American P-51 Mustang NA-73X
The single Mustang prototype, ordered by the British Purchasing Commission and first flown on 26 October 1940, just 149 days into the contract.
North American P-51 Mustang sheet
- October 26, 1940
- First flight
- 1
- Units built
History
The NA-73X came out of a direct arrangement between the British Purchasing Commission and North American Aviation, with neither the US Army nor Wright Field involved. The design team, led by chief engineer Edgar Schmued, followed the best structural practice of the day and laid the aircraft out for mass production: a three-section semi-monocoque fuselage built entirely of 24S aluminium alloy, a form of Duralumin chosen to save weight, and an airframe split into five main assemblies — forward, centre and rear fuselage plus the two wing halves — each fitted with wiring and piping before being joined.
Two features set the design apart. The first was a wing using NAA/NACA 45-100 laminar-flow aerofoils, developed jointly with the NACA and tested at the University of Washington Kirsten Wind Tunnel against a wing built around five-digit NACA sections; the comparison showed the new aerofoils to be superior, generating low drag at high speed. The second was a rearward cooling installation that gathered the water and oil radiators into a single ducted assembly, cutting fuselage drag and its interference with the wing. Lacking a suitable tunnel of its own, North American tested the arrangement in the 3.0 m (10 ft) GALCIT tunnel at the California Institute of Technology; after much development it was found that the assembly exploited the Meredith effect, the heated air leaving the radiator with a slight amount of jet thrust. The fuselage was also among the first to be lofted mathematically using conic sections, which produced smooth, low-drag surfaces.
The prototype was rolled out on 9 September 1940, still without an engine, only 102 days after the order had been placed, and flew on 26 October, 149 days into the contract — an uncommonly short development period even in wartime. With test pilot Vance Breese at the controls it handled well and carried an impressive fuel load. The material consulted gives no measured dimensions or performance figures for this one-off aircraft, of which a single example was built.