Design Background of the P-51H

Lower weight plus greater power = higher performance
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Bomber_12th
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Design Background of the P-51H

Post by Bomber_12th »

First off, it's great to finally see this one released, and congratulations to all that were involved in putting it together/making it happen! With the release of the Aircraft Factory P-51H, I thought I would add a bit of information about how the design of the P-51H came about (as I've written about before).

The P-51H - the ultimate Mustang. Unlike the earlier production model Mustangs, the H was very much an outlier. While there was a clear developmental path from the NA-73/XP-51 through to the P-51D, with constant developmental changes/improvements over time, the P-51H was developed from a separate line of design with a separate line of thinking.

The P-51H was the direct result of the experimental lightweight Mustangs that North American began working on in early-mid 1943, which would include the XP-51F, XP-51G, and eventually the XP-51J. The lightweight Mustangs came about after the USAAF wanted to know why their fighter planes were so much heavier than British fighter planes, and why, for instance, that a comparable Mark of Spitfire could climb to a set altitude in 2/3 the time it would take the Mustang, or why the Fw-190 had twice the roll rate of the Mustang. They wanted to know if the design of the Mustang could be improved to match or better the performance of those other nation's fighters. Edgar Schmued of North American Aviation was sent to Britain at the beginning of 1943, for two months, to meet with British designers to compare design and production techniques (which included inspections of various European fighters and visits to de Havilland and to Rolls-Royce, where he was shown the latest Merlin engine developments).

In getting to the bottom of why the American aircraft designs were so much heavier than their British counterparts, Schmued would write that - "First, the high angle of attack-load factor was 12 in America, but only 11 in England. Second, there was a side-load factor on the engine mount in America of 2G, which the British didn't have at all. Then the landing gear load factors that we used were 6G's, but were only 4G's in England. We used all of these factors to help whittle out a good deal of weight in a new design."

Utilizing the British load factors, and placing importance on weight-savings, essentially an entirely new design was drawn-up in the form of the XP-51F, which was given the in-house designation of NA-105. Any likeness to the previous Mustang design has been noted as purely coincidental - obviously the design would be directly influenced by all of the fighter-design experience/knowledge that North American Aviation had established by mid-1943.

With this design, to save weight, North American did some new things, like making part of the engine cowl at the engine mount a structural member of the engine mount, and on the fuselage, aft of the firewall, they used .042 skins rather than .065 skins. Just about everything throughout the aircraft, including the main and tail gear, were newly-designed to save on weight. The lightweight Mustangs are also sometimes referred to as the 'plastic Mustangs', as there was a much larger use of phenolic resin (an early plastic) throughout the aircraft, such as using the material as backing to parts made of thinner aluminum. With the lightweights, a longer bubble canopy was designed, to better smooth-out the airflow/improve the aerodynamics (NAA noted that there was an aerodynamic deficiency with the D-model canopy design and cut-down spine). The radiator scoop was also made longer/more stretched out, to also improve the airflow/aerodynamics. Smaller main wheels and the newly designed landing gear also meant that the leading edge of the wings, at the root, no longer needed to be canted. The fuselage was also designed to be wider vertically (up/down), to gain back the side-area that was lost in the D-model. The position of the pilot, being raised up higher, and having the cowling sloping away from the windscreen, also allowed for better visibility from the cockpit - a trait that did not go unnoticed.

All of the importance placed on weight savings paid off, and the empty XP-51F weighed-in at 5,635-lbs. - an empty P-51D, by comparison, weighs 2,000-lbs more.

The XP-51G then followed, and based on the test results of the F and G, the production-variant P-51H was born. The P-51H was designed and developed to use the Merlin V-1650-9 engine, which featured water injection and could attain significantly greater power/performance over the previous -3 and -7 engines that were fitted to the P-51B/C/D. The P-51H started coming off the assembly line at Inglewood in mid-1945, alongside some of the last D-models Inglewood would produce. Unfortunately they were produced too late to see action in WWII.

All of the lightweight Mustangs achieved or exceeded their design goals. However, the USAAF would have a change of mind, even by the time the XP-51F first flew. By this point, they had grown accustom to the ruggedness of the many existing American fighters, and no longer liked the idea of an aircraft built to British load factors, nor one where structural strength was dropped in favor of increased performance. The lightweights were very strong aircraft, but just not as strong as the P-51D, and this would directly influence why the P-51H would also never see combat in the Korean War. The P-51H proved its worth as a test-lab in the immediate post-war years, with examples flown by the NACA and Grumman, in high-speed dives, testing new and radical model airfoil and aircraft design concepts mounted to the wings (in the dive reaching Mach .83, during the pull-out the air passing over the wings would be going super sonic - this is the point at which data would be recorded from the model airfoils and aircraft designs). They would of course also serve in several ANG units in the late 40's, and in the early 50's, taking the place of all of the D-models that were sent to Korea. Many of the P-51H's were based with ANG units in coastal States around the US, to serve as quick interceptors for the potential soviet bomber threat.
Last edited by Bomber_12th on 07 Sep 2015, 06:26, edited 1 time in total.
John Terrell

Bomber_12th
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Re: Design Background of the P-51H

Post by Bomber_12th »

I should add that, the P-51H design is so different to that of the other production model Mustangs - with nothing but the most basic/government-furnished parts in common - that it is almost unbelievable that it didn't receive a different contract designation (like the P-39/P-63).
John Terrell

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lucas
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Re: Design Background of the P-51H

Post by lucas »

Lot's of great information. Thank you, John!

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Scott - A2A
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Re: Design Background of the P-51H

Post by Scott - A2A »

Great stuff Bomber, thanks for posting.

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