The genesis of the North American Aviation (NAA) P-51H Mustang can actually be traced back to a series of lightweight P-51-derived development aircraft designated XP-51F, XP-51G, and XP-51J. In 1943 NAA engineers traveled to the UK to collaborate with Supermarine and to incorporate some of the design principles used in the famous Spitfire into a newly-contracted lightweight Mustang.
The P-51H was not powered by the same sweet-sounding Rolls-Royce or Packard Merlin engine as used in the iconic P-51D. The latest version of the Merlin was the V-1650-9, still a liquid-cooled V-12 but now with automatic boost control and a water injection system. The engine cowling was designed even tighter, resulting in reduced frontal area, which reduced drag.
The Aeroproducts A542-B2 propeller spun by the uprated Merlin engine was often simply dubbed the “H prop.” Spanning 11 feet and one inch, the four-bladed fat-paddled prop was actually lighter than the Aeroproducts A542S mill used to propel the P-51K. The paddle blades were wider overall and nearly uniform in that width all the way out the rounded tips.
NAA essentially went back to the drawing board with the P-51H. Perform a visual comparison of the P-51H to the P-51D and the aft fuselage shape, the main landing gear doors and the tailwheel location, the reduced vertical stabilizer fillet, and the engine cowling jump right out. However, NAA saved weight in nearly every component used in the P-51H. Parts commonality between the two variants was negligible. The P-51H was actually slightly less than 600 pounds lighter than the P-51D.
How? Some of the over-designed and over-built P-51D components were redesigned for use in the P-51H. Some weight was saved in the narrower fuselage structure, and more was saved by integrating the engine mounts into the engine cradle. The P-51H fuselage was actually longer than the P-51D’s by about a foot. The first production P-51Hs did not come with the taller vertical stabilizer added to keep yaw under control, but they were retro-fitted with them and production aircraft received them after the first 20 examples off the production line.
Mechanically the P-51H also incorporated disk brakes, relocated engine oil cooler and cockpit/canopy, redesigned ammunition doors for the wing-mounted guns, and a smaller 55 gallon capacity fuselage-mounted fuel tank that also helped with yaw control. The tailwheel was located further aft on the P-51H and many examples were equipped with dual dorsal antennae. The alloy used for the outer fuselage skins was thinner than that used on the P-51D. Another visual cue was the shape of the radiator scoop inlet- it was vertical on the P-51H as opposed to angled on the P-51D.
Though the P-51H was first flown during February of 1945 and it was in production before the end of the war, the P-51H never saw combat. A few squadrons in the Pacific received P-51Hs and were preparing for operational missions but the type never saw operational use in combat. Production was slacked off by the end of the war and stopped entirely in November of 1945 after 555 of the over 3,600 P-51Hs ordered had rolled off the line at NAA Inglewood. Had the war dragged on, NAA Dallas would have produced the P-51H under a P-51L designation similar to the way NAA produced the similar P-51B and P-51C in Inglewood and Dallas respectively.
During August of 1945 the US Navy borrowed another Mustang for carrier suitability trials. During Project Seahorse the previous tailhook-equipped P-51D (44-14017 and designated ETF-51D) was found to lack low speed and high angle of attack directional control- a potentially fatal flaw for aircraft intended for carrier-based operations. When the Navy tried out a P-51H (44-64420) with larger empennage surfaces and increased wingspan, they discovered that the taller vertical stabilizer on the P-51H resolved the directional control issues. However, the war was all but won at that point so there was no further development of a carrier-based Mustang.
In 1948 the designation of the P-51H was changed to F-51H. During 1952 and 1953, the 104th Fighter-Bomber Squadron of the Maryland Air National Guard (ANG) formed an aerial demonstration team called the Guardian Angels. These Guard pilots performed their four-ship shows flying F-51H Mustangs. The Guardian Angels were Team Lead Captain John F. R. Scott Jr., First Lieutenant William Marriott flying right wing, First Lieutenant Malcolm Henry flying left wing, and Captain Jesse D Mitchell Jr. flying the slot position.
Many of the F-51Hs that were completed by war’s end served with Air National Guard squadrons during the late 1940s. When Korea happened the combat-proven F-51Ds and F-51Ks made their way to Korea because they perceived to be stouter against ground fire. During the early 1950s jet-powered fighters became the new standard, forcing the retirement and wholesale replacement of propeller driven fighters. The last F-51H Mustangs were retired from ANG units in 1957. Of course the P-51H was used as the basis for the P-82 Twin Mustang too…but that’s another story.