North American P-51 Variants - Experimental Mustangs

Experimental Mustangs

In early 1944, the first P-51A-1-NA, 43-6003. was fitted and tested with a lightweight retractable ski kit replacing the wheels. This conversion was made in response to a perceived requirement for aircraft that would operate away from prepared airstrips. The main oleo leg fairings were retained, but the main wheel doors and tail wheel doors were removed for the tests. When the undercarriage was retracted, the main gear skis were housed in the space in the lower engine compartment made available by the removal of the fuselage .50 in (12.7 mm) Brownings from the P-51As. The entire installation added 390 lb (180 kg) to the aircraft weight and required that the operating pressure of the hydraulic system had to be increased from 1,000 psi (6,897 kPa) to 1,200 psi (8,276 kPa). Flight tests showed that ground handling was good, and the Mustang could take off and land in a field length of 1,000 ft (300 m); the maximum speed was 18 mph (29 km/h) lower, although it was thought that fairings over the retracted skis would compensate.

Concern over the USAAF's inability to escort B-29s all the way to mainland Japan resulted in the highly classified "Seahorse" project (NAA-133), an effort to "navalize" the P-51. On 15 November 1944, naval aviator (and later test pilot) Lieutenant Bob Elder, in a P-51D-5-NA 44-14017, started flight tests from the deck of the carrier Shangri-La. This Mustang had been fitted with an arrestor hook, which was attached to a reinforced bulkhead behind the tail wheel opening; the hook was housed in a streamlined position under the rudder fairing and could be released from the cockpit. The tests showed that the Mustang could be flown off the carrier deck without the aid of a catapult, using a flap setting of 20° down and 5° of up elevator. Landings were found to be easy, and, by allowing the tail wheel to contact the deck before the main gear, the aircraft could be stopped in a minimum distance. The project was canceled after U.S. Marines secured the Japanese island of Iwo Jima and its airfields, making it possible for standard P-51D models to accompany B-29s all the way to the Japanese home islands and back.

While North American were concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued. To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), FX858 and FX901, were fitted with different Merlin engine variants. The first of these, FX858, was fitted with a Merlin 100 by Rolls-Royce at Hucknall; this engine was similar to the RM 14 SM fitted to the XP-51G and was capable of generating 2,080 hp (1,550 kW) at 22,800 ft (7,000 m) using a boost pressure of +25 lbf/in2 (170 kPa; 80 inHg) in war emergency setting. With this engine, FX858 reached a maximum speed of 453 mph (729 km/h) at 18,000 ft (5,500 m), and this could be maintained to 25,000 ft (7,600 m). The climb rate was 4,160 ft/min (21.1 m/s) at 14,000 ft (4,300 m).

FX901 was fitted with a Merlin 113 (also used in the de Havilland Mosquito B. Mk 35). This engine was similar to the Merlin 100, but it was fitted with a supercharger rated for higher altitudes. FX901 was capable of 454 mph (730 km/h) at 30,000 ft (9,100 m) and 414 mph (666 km/h) at 40,000 ft (12,200 m).