Mustang Design Philosophy

Arguably the finest fighter aircraft of World War II.
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ClipperLuna
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Mustang Design Philosophy

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I’ve been wondering something about the thinking that went into designing the Mustang. I used to assume it was designed to be an interceptor like the Spitfire or Bf109, and that its role as a long-range escort fighter was something it kind-of fell into. I thought this because, AFAIK, the P-40 was an interceptor and the P-40 was what the RAF wanted when they first approached North American. However, I heard something in a video made during the war (youtube link below) that suggests it was designed as a long-range escort fighter from the beginning. Is that true?

Here’s the link. The comment suggesting it was always meant to be a long-range escort fighter is about 2 minutes in: https://www.youtube.com/watch?v=caJtGXMdxGM

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Skycat
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Re: Mustang Design Philosophy

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Interesting question. I took the lazy route and looked on Wikipedia. The article suggsets NAA's goal was to supply the British a fighter similar to the P-40, and the resulting design just happened to fit the US Army's need for a long range bomber escort.
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Skycat
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Re: Mustang Design Philosophy

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Here's a good article about the development of long range bomber escort. In a nutshell it says that in 1941, and after two years of war, the British doubted that long range fighter escorts were practical or even possible. It's a long read but worthwhile.
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ClipperLuna
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Re: Mustang Design Philosophy

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Very good article, thanks for sharing!

Something else in there hints at an answer to another question I've had: why was this plane so heavy compared to other fighters of its day? Regarding the P-51's genesis, "In 1940, North American Aviation was asked to propose a fighter aircraft in response to a British requirement for a reconnaissance and ground attack fighter bomber. . . . It was [Schmued's] intention to build an exceptionally clean aircraft showing superior aerodynamic characteristics."

This suggests that in tackling the British requirement, Schumed's primary focus was making the plane fast and stable rather than making it light-weight. Does that seem right to anyone else?

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Skycat
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Re: Mustang Design Philosophy

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"Heavy for fighters of its day" would need to be qualified. Generally speaking, design philosophy evolved through the 1930s as nations moved away from open cockpits and fabric surfaces. Air forces were incorporating lessons learned from international air competitions but also conflicts around the world like the Spanish Civil War. Before World War II most theorists believed that bombers would rule the air, and that fighters would be little useful for anything other than reconnaissance and intercepting bombers. Lightweight aircraft could be seen as an advantage because they burned less fuel, and carrying minimal fuel was advantageous because it kept the aircraft light. The Hawker Hurricane Mk. I with a mean weight of 6,440 lbs, range of 585 miles, and a fuel capacity of 97 Imperial gallons somewhat exemplifies military aviation requirements in 1935. (Germany's Messerschmitt Bf 109D was the standard Bf 109 in service before 1938. Loaded weight: 5,340 lbs; range approximately 405 miles. By 1940, the Bf 109E was in service and Bf 109F was in development.)

Looking at how the U.S. Army's single engine pursuit fighters compared circa 1940:

P-39D "Airacobra": On September 13, 1940, 394 P-39Ds were ordered. The P-39D differed from the P-39C primarily in having four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun, plus the 37-mm cannon (with increased ammunition capacity of 30 rounds). Bulletproof windshield panels were added, and some armor protection for the pilot was provided. Self-sealing fuel tanks were introduced, which reduced internal fuel capacity from 141.5 Imp. gall. to 100 Imp. gall. This internal fuel could be supplemented by a 72.4 Imp. gall drop tank carried on a strongpoint fitted underneath the fuselage. In place of the drop tank, a 300 lb or 600 pound bomb could be carried. The bulletproof windshield and armor protection added 245 pounds to the weight of the aircraft, causing the climb and altitude performance to suffer. The maximum speed at 15,000 feet dropped to 360 mph. Specification of Bell P-39D Airacobra: One 1150 hp Allison V-1710-35 twelve-cylinder liquid cooled engine. Performance: Maximum speed 309 mph at sea level, 335 mph at 5000 feet, 355 mph at 10,000 feet, 368 mph at 12,000 feet, and 360 mph at 15,000 feet. An altitude of 5000 feet could be reached in 1.9 minutes. It took 5.7 minutes to reach an altitude of 15,000 feet and 9.1 minutes to reach 20,000 feet. Service ceiling was 32,100 feet. Maximum range (clean) was 600 miles at 10,000 feet at 231 mph. Range with one 145.7 Imp gal drop tank was 1100 miles at 196 mph. Weights: 5462 pounds empty, 7500 pounds gross, and 8200 pounds maximum takeoff. Dimensions: Wingspan 34 feet 0 inches, length 30 feet 2 inches, height 11 feet 10 inches, and wing area 213 square feet. Armament: One 37-mm cannon in the nose with 30 rounds. Four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun. One 250 lb, 325-lb, or 500-lb bomb could be carried underneath the fuselage.

P-40D "Kittyhawk": The P-40D introduced 175 pounds of armor. The fuselage guns were deleted, and two 0.50-inch machine guns with new hydraulic chargers were installed in each wing. Shackles were added under the belly to accommodate a 51-gallon auxiliary fuel tank or a 500-pound bomb. Wing rack attachment points were provided for six 20-pound bombs. Gross weight of the D model was increased to 8670 pounds. The climb rate and ceiling consequently were poor and the P-40D was unpopular compared with the lighter, faster and more agile P-40B/C Tomahawks. The USAAF did not actually order the P-40D into production until September 1940, nearly 5 months after the RAF had ordered the equivalent Kittyhawk I. As it happened, only twenty-two P-40Ds were produced for the USAAF.

P-40E "Warhawk": The P-40E introduced a new engine, the Allison V-1710-39 of 1150 hp. This engine had originally been proposed for the experimental XP-46 fighter, but the USAAF had decided not to interrupt the P-40 production lines for a new type and decided instead to adapt the new engine to the existing P-40. Substitution of the modified P-40 for the experimental P-46 was proposed on June 10, 1940, and Curtiss agreed to adapt the basic P-40 to the new engine. The P-40E featured a new shorter nose design that was retained by all subsequent P-40s. The 1150 hp V-1710-39 engine had spur gear reduction that raised the thrust line by six inches, giving a completely different nose geometry. The overall length was reduced by six inches, the cross section of the fuselage was reduced, and the undercarriage was shortened. The radiator was increased in size and moved forward. An order dated February 18, 1941 increased the armament of the P-40 to six 0.5 inch machine guns in the wings, and subsequent aircraft equipped with this armament were designated P-40E. Maximum speed was 335 mph at 5000 feet, 345 mph at 10,000 feet, and 362 mph at 15,000 feet. Initial climb rate was 2100 feet per minute. An altitude of 20,000 feet could be attained in 11.5 minutes. Service ceiling was 29,000 feet. Maximum range was 650 miles (clean), 850 miles (with one 43 Imp gal drop tank), 1400 miles (with one 141.5 Imp gal drop tank). Weights were 6350 pounds empty, 8280 pounds normal loaded, and 9200 pounds maximum. Dimensions were wingspan 27 feet 4inches, length 31 feet 2 inches, height 10 feet 7 inches, and wing area 236 square feet. Armament was six 0.5 inch machine guns and up to 500 pounds of bombs.

P-47 "Thunderbolt": On June 12, 1940, Kartveli submitted his ideas to the USAAC. The USAAC was sufficiently impressed with the proposal that on September 6, 1940 ordered a prototype under the designation XP-47B. All work on the XP-47 and a lightweight derivative, the XP-47A, was cancelled. One week later, on September 13, 1940, 773 production examples of the new fighter were ordered by the USAAC, 171 to be delivered as P-47Bs and 602 as P-47Cs. The XP-47B prototype flew for the first time on May 6, 1941, only eight months after the order had been placed. The XP-47B was the largest single-engine fighter built up to that time. Its eighteen-cylinder XR-2800-21 radial engine offered 1960 hp at 25,800 feet, and gave it a maximum speed of 412 mph, 12 mph faster than Kartveli had projected. An altitude of 15,000 feet could be attained in five minutes. Empty and normal gross weights were 9189 pounds and 12,086 pounds respectively.

And it is here that the design of the P-51 begins in 1940...

To reasonably make a comparison we'd have to look at the earliest production variants, not a late war P-51D with increased fuel capacity. Mustang Mk.I delivered to the British was essentially the production version of the prototype NA-73X.

Mustang Mk. I: The Mustang Mk.I (NAA Model NA-73) was a single-place, single-engine fighter primarily of metal construction with fabric control surfaces. It was 32 feet, 3 inches (9.830 meters) long with a wingspan of 37 feet, 5/16-inches (11.373 meters) and height of 12 feet, 2½ inches (3.721 meters). The airplane’s empty weight was 6,280 pounds (2,849 kilograms) and loaded weight was 8,400 pounds (3,810 kilograms). The Mustang Mk.I was powered by a liquid-cooled, supercharged 1,710.597-cubic-inch-displacement (28.032 liter) Allison Engineering Company V-1710-F3R (V-1710-39) single overhead camshaft (SOHC) 60° V-12 engine with four valves per cylinder and a compression ratio of 6.65:1. This engine gave the Mustang Mk.I a maximum speed of 382 miles per hour (615 kilometers per hour) and cruise speed of 300 miles per hour (483 kilometers per hour). The service ceiling was 30,800 feet (9,388 meters) and range was 750 miles (1,207 kilometers).

Anyhow, if you compare Mustang Mk. I to the P-40E you can see the origins of the P-51 as a superior design for range and high altitude performance.
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ClipperLuna
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Re: Mustang Design Philosophy

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That's a lot of great info, Skycat, thanks for taking the time. You're right, when I said "fighters of it's day", for whatever reason I didn't think to compare it to other American designs, but except for the P-47 (kind of in a class of its own), it's right there with the rest. I'm almost surprised how close its weight is to the P-40E's.

I was thinking of European fighters from approximately that same time period. Here are some weights I collected for a few of them:
Spitfire IX: 5,090 empty, 7,400 gross
Bf 109G-6: 4,954 empty, 6,940 gross
Macchi C.205: 5,690 empty, 7,513 gross
Yak-9: 5,020 empty, 6,327 gross
Dewoitine D.520C.1: 4,689 empty, 5,902 gross (an earlier design than the rest but the closest French fighter I found).

Compared to these, an empty Mustang I has 590 pounds on the heaviest of the bunch, the Macchi C.205, and more than 1,000 lbs on either a Spit or a 109.

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Skycat
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Re: Mustang Design Philosophy

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ClipperLuna
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Re: Mustang Design Philosophy

Post by ClipperLuna »

Skycat wrote: 03 Mar 2020, 21:01 A discussion worth checking out: https://ww2aircraft.net/forum/threads/d ... st-1152749
Great find, just the info was I looking for! I feel like I understand this plane's backstory much better now.

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