North American's Lightweight Mustangs
P-51H (NA-126) / P-51M (NA-124) / P-51L (NA-129)
(Last Updated 5 May 2011)

Bibliography

MUSTANG DESIGNER: Edgar Schmued and the P-51 by Ray Wagner
Warbird Tech 5: North American P-51 Mustang by Frederick A. Johnsen
North American XP-51F, G, J Mustang by Joseph Baugher.
North American P-51H Mustang by Joseph Baugher.
Standard Aircraft Characteristics, F-51H Mustang, 22 March 1949

The fastest Mustangs to be built owed their existence to Mustang designer's Edgar Schmeud's exploration of weight issues with American aircraft; e.g. that they were too heavy. Upon being notified by the Air Force of this concern, Schmeud had North American's Field Service Department in England check with the British aircraft manufacturers, especially Supermarine to obtain detailed weight statements of their aircraft.

To Schmeud's surprise, no British company at the time had any detailed weight statement for their aircraft; they literally did not know how much specific parts of their aircraft weighed! To solve this problem, Schmeud had NAA's Field Service Department go out to where Spitfires were repaired and had them start to weigh all of the parts they could obtain, and soon Schmeud finally got a weight statement for the Spitfire.

Studying the statement, Schmeud finally learned why British aircraft turned out to be lighter than American aircraft built for the same roles and requirements.

1.) The Angle of Attack Load Factor in the USAAF was 12.0, while in the RAF it was only 11.0
2.) The Side-Load Factor on the engine in the USAAF was 2.0 g's, while the British did not have this requirement; due to side loading on the engine mount not being a real problem in normal flight.
3.) The Landing Gear Load Factor in the USAAF was 6.0 g's, but only 4.0 in the RAF.

Armed with this information, Schmeud began to design a lightweight Mustang built around RAF requirements, not USAAF requirements. This project resulted in the XP-51F and the XP-51G Mustang.

XP-51F, USAF Photo. Courtesy of www.mustangsmustangs.com

XP-51G, USAF Photo courtesy of US Air Force Museum

While the XP-51F resembled the earlier Mustangs externally; internally it was a virtually new aircraft structurally, with an empty weight 2,000 pounds less than a P-51D and carrying the same powerplant as the earlier P-51D, rated at 1,695 hp. The refinements in the -51F gave it a top speed 26 MPH higher than the P-51D, some 4,000 feet higher; and it first flew in February 1944.

The last two XP-51Fs were completed as XP-51Gs, fitted with Rolls-Royce Merlin 145Ms giving 1,910 hp. The extra 200~ hp given by the Merlin raised top speed to 472 MPH, with it's first flight occurring in August 1944.

Because the Merlin 145M was unsuitable at that point for mass production, it was decided to equip the production “Lightweight” Mustangs with the Packard V-1650-9 powerplant capable of producing 1,900 hp at 20,000 feet with water injection; mainly due to Packard's commitment towards large scale deliveries of it in late 1944.

Thus, the lightweight Mustang was ordered on a large scale in June 1944. Two thousand P-51Hs were scheduled to be produced at North American's Inglewood factory, with another 1,629 P-51Ms from North American's Dallas factory, the difference being that on the M models, the water injection capability present on the H was deleted.

P-51H, USAF Photo courtesy of US Air Force Historical Research Agency

By August 1945, a few squadrons in the USAAF were working up with their new mounts, when the war came to an end. With the cessation of hostilities, orders for the P-51H and M were canceled on a large scale; with final production by V-J day being 555 P-51Hs and one P-51M.

Additionally, the orders for all 1,700 P-51Ls – a product improved P-51H fitted with the Packard V-1650-11 producing 2,270 hp with water injection were canceled, with none being produced.

© 2011 – Ryan Crierie – Reproduction permitted with credit given.