Best "Mustang" of em all? ...

[Mark Allen M]

XP-51J?
https://www.joebaugher.com/usaf_fighters/p51_12.html

[Mark Allen M]

In spring 1943 the Americans launched five programmes to improve the P-51 D performances.

For a weight saving, the airframe was extensively redesigned as XP-51F. To eliminate unnecessary structure, high-strength 75 ST aluminium and plastics were also employed. Principal changes included a much thinner cross-section wing, with straight leading edge, a lighter landing gear with small wheels and removal of the fuel tank behind the pilot.

To save drag, the radiator housing was redesigned with a smaller chin scoop.

The only added weight was due to the fitting of a longer bubble canopy that required one hydraulic device to move.

On February 1944 the XP-51F performing its first flight.

The prototype weighted 1,468 kg (3,229 lbs) less than the P-51 D-NA and was 47 kph (29 mph) faster powered by the same engine. To save weight the standard airscrew was replaced by one hollow-steel, three-bladed Aero Products propeller, the armament was reduced to four 0.50 cal machine guns with 250 rounds per gun and the oil cooler was replaced with a heat exchanger.

The XP-51 G was flown in 9 August 1944, powered by one 1,500 hp. Rolls-Royce Merlin 14 SM engine driving a five-bladed Rotol airscrew with 3.35 m (11 ft) of diameter. The British engine reaching 2,200 hp. emergency power burning 150 octane fuel, 130 per cent more power than the original engine.

The XP-51 G weighted 1,546 kg (3,401 lbs) less than the P-51 D-NA and was 57 kph (35.4 mph) faster.

Only two prototypes were built.

On 3 February, 1945 the P-51 H first flight tests were performed, powered by one Allison V-1650-9 engine, with 370 hp less than the V-1650-7 standard used by the P-51 D-NA. The P-51 H was 81 kph (50 mph) faster and weighted 370 kg (814 lbs) less, with the same armament.

Over 370 units were delivered to the USAAF prior the V-J Day, too late to see actual combat.

The fastest version of the line, named XP-51 J, was flown in 23 April, 1945 powered by one 1,500 hp. Allison V-1710-119 engine which offered over 1,720 hp. at 6,100 m (20,000 ft) altitude, with water injection and 150 octane fuel.

The prototype weighted 1,432 kg (3,150 lbs) less than the P-51 D-NA and was 87 kph (54 mph) faster.

To save drag the cooling systems were redesigned, with coolant and oil radiators in one ventral fairing. The carburettor intake was also moved to the ventral radiator scoop.

Only two prototypes were built.

On July 1944 the British launched a desperate attack against the new German cruise missiles V-1, to face the 'robot offensive' with their high performance interceptors Mustang Mk.III of RAF Squadrons 129, 306 (Polish) and 313 (Polish).

The aerodynamic drag of the V-1 airframe was higher than anticipated, due to low standards of manufacturing, decreasing from projected 900 km/h (559 mph) to the real 640 km/h (398 mph). Fortunately for the Allies this made the new missile susceptible to be intercepted by conventional fighters, but the game was dangerous.

They used to start the attack with a dive to gain speed, moving on to horizontal flight at 230 m behind the missile to be able to shoot it. The calculation of relative speeds was complicated, and some pilots died when firing from a too short distance.

It was necessary to modify the fighters to make them fast enough to intercept the V-1, the Mustang Mk.III changed their exhaust by those of the Spitfire engine that generated less drag, rear view mirrors and armour plates were also stripped. To reduce the drag on some airplanes, the camouflage paint was removed to gain some speed.

During the 'Operation Diver', between June 1944 and March 1945, sixteen squadrons of interceptors used the new aviation fuel ‘150 grade’ produced in the USA. Using trimethylpentane, that fuel had a higher octane-number, so allowed a higher compression and power output.

[sandiego89]

"Best" is often a word that causes problems. Surely it was fast, and had a great climb ability, but without the fuselage tank, it likely lacked range, one of the useful traits of the Mustang. Sounds like a real hot rod. I recall that a few pilots were not as keen on the handing of the H (was it Yeager?) feeling more comfortable with the D.

It is great looking, the three bladed prop on a bubble canopy P-51 sure gives a unique look.

The pic of Zero EB-200 (#3372) was just too good not to sneak in! Thanks for posting!

[mike furline]

https://ww2aircraft.net/forum/threads/x ... ook.63124/

https://www.secretprojects.co.uk/thread ... pes.12426/

[Randy Wilson]

Looks like Herman's Zero.

Randy

[Saville]

Maybe better performance than the D but - and this is strictly my opinion - it sure uglied the airplane up. I like the looks of the D much better.

[Larry Kraus]

I took a look at the XP-51J Flight Manual. It had an odd fuel system with the right main tank holding 105 gallons and the right holding 75 gallons it also mentions two 75 gallon aux tanks. The description of the fuel system is somewhat confusing.

IMG_1540 by tanker622001, on Flickr

IMG_1538 by tanker622001, on Flickr

IMG_1537 by tanker622001, on Flickr

The airplane only had four .50 cal machine guns with 250 rounds per gun

IMG_1535 by tanker622001, on Flickr

The oil system diagram mentions an auxiliary supercharger and shows a large aftercooler ahead of the engine oil tank

IMG_1539 by tanker622001, on Flickr

IMG_1541 by tanker622001, on Flickr

This is the power plant chart from the manual


IMG_1536 by tanker622001, on Flickr

[JohnB]

I have always thought it looked a bit sub-scale to other Mustangs even though they're not.
If one is familiar to the "D", the lightweight models just look a bit "off".
Very close, but when looking at them, there is an unidentifiable something that's different...like a poor model kit or bad illustration (like mine, I find the Mustang a difficult aircraft to draw).

[JohnTerrell]

Unlike the XP-51 through P-51D production, in which every new variant was a natural development from the previous model, the lightweights (XP-51F, G and J) and P-51H were an entirely new concept and design altogether. When you realize that the P-51D and P-51H (and of course XP-51F, G and J) were as different as the P-39 vs. P-63, it's quite unbelievable that they still carried the P-51 designation.

The lightweight Mustangs came about after the USAAF wanted to know why their fighters were so much heavier than their British counterparts, and why, for instance, that a comparable variant of Spitfire could out climb the Mustang in 2/3 the time, or why the Fw-190 had twice the roll rate of the Mustang. To get to the root of the issue, Edgar Schmued spent a couple months in Britain, in early 1943, and met with British designers to compare design and production techniques, which included inspections of various European fighter aircraft and visits to de Havilland and to Rolls-Royce (Rolls-Royce was eager to show-off their latest developments, including the 2,200-hp RM.14.SM engine). In Britain, Schmued noted a few factors that were allowing the British fighters to be built so much lighter than the Mustang. In the US, they were designing aircraft with a high angle of attack load factor of 12G's, where as in Britain the requirement was 11G's. In the US, there was a side load factor requirement on the engine mount of 2G's, where as in Britain they didn't have that all. And in the US they had a landing gear load factor of 6G's, where as it was 4G's in Britain. Using the British load factors, and placing importance on weight-savings, 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 (the design would have of course been influenced by the experience/knowledge that North American Aviation had established by mid-1943).

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 itself, 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 in order to smooth-out the airflow/improve the aerodynamics - North American already knew then that there was an aerodynamic deficiency with the bubble canopy design and cut-down spine of the P-51D. The radiator scoop was also stretched to improve the airflow/aerodynamics. The fuselage was also designed to have a wider side profile to gain back the side-area that was lost with 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.

The empty XP-51F weighed-in at 5,635-lbs. - an empty P-51D, by comparison, weighs 2,000-lbs more. The initial contract for the P-51F was for five aircraft, to be fitted with the British-supplied RM.14.SM engine. However, due to the speed at which the aircraft was designed and built, the first three XP-51F's were completed with the easily available V-1650-3 Merlin. Bob Chilton, chief test pilot for North American, who flew every version of the Mustang, considered the XP-51F his favorite. Then, with the RM.14.SM engines eventually in hand, the remaining two airframes were completed as the XP-51G (requiring quite a bit of design differences to fit the engine, and the engine itself requiring special adjustments that took many trials to perfect). By the time North American Aviation received the engines and began flight testing the XP-51G, they were well into developing the P-51F design into the improved/heavier P-51H. Around the same time, NAA were promised to receive the new Packard-built Merlin V-1650-9 engine by the end of 1944, by which point they would have airframes, in the form of the P-51H, ready to accept them. The V-1650-9, with water injection, could attain just over 2,200-hp, the same as was advertised with the Rolls-Royce RM.14.SM engine. The P-51H started coming off the assembly line at Inglewood in the summer of 1945, alongside some of the last D-models Inglewood would produce (too late to see action in WWII).

Quite unique is the fact that there was never a prototype XP-51H, as it was reasoned that all of the details of the design had already been fully developed and tested with the XP-51F and G. The contract for the P-51H was signed as early as April 26, 1944, with an order placed for 2,400 H's. The first P-51H produced, 44-64160, began flight testing in early February 1945. With VJ-Day, and sweeping government contract cuts, only 370 P-51H's had been completed. However, through a government agreement, North American was allowed to use up all of the sub-assemblies that had already been built in order to complete and deliver more aircraft, until a total of 555 P-51H's were completed, with production ending in early November 1945.

Two examples of the XP-51J were built, in order to test the installation of the Allison V-1710-119 engine and its infinitely variable two-stage supercharger. From the firewall back, they were largely identical to the F and G. The first one flew in the spring of '45, and one was later provided to the Allison Division of General Motors for testing the Allison G6 engine used in the F-82E.

Despite all of the lightweight Mustangs either achieving or exceeding their design goals, even by the time the XP-51F first flew in February 1944 the USAAF had already changed its mind about its desire for performance over ruggedness. 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 (late model P-51D's had a load-carrying capability allowing them to carry a 1,000-lb bomb under each wing, where as the P-51H could only carry a 500-lb bomb on each wing). 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. They would of course also serve in several Air National Guard units in the late 40's and early 50's, with many based with ANG units in coastal States around the US to serve as quick interceptors for the potential threat of soviet bomber strikes.

[AG pilot]

It's amazing the XP-51F could be that much lighter.

[Saville]

Withthe two stage supercharger (was it also 2 speed?) did the Allison finally approach Merlin performance at all altitudes?

[Hooligan2]

Maybe better performance than the D but - and this is strictly my opinion - it sure uglied the airplane up. I like the looks of the D much better.

Agreed, looks like someone pumped too much air into a D and over-inflated it a bit.

[Lon Moer]

not a real one, but still fun to look at.....


This is the Whittington P-51H with a five blade wooden Rotol propeller.

[tjc333]

Amazing how even with significant weight savings the new planes showed only a tiny speed improvement over the original design. Shows how great the original design was. Maybe the rate of climb and ceiling were much improved but not likely given such a tiny cruise speed increase.

[sandiego89]

Amazing how even with significant weight savings the new planes showed only a tiny speed improvement over the original design. Shows how great the original design was. Maybe the rate of climb and ceiling were much improved but not likely given such a tiny cruise speed increase.

Sounds like a great "interceptor", a term that was not really in vogue at the time. Point based interceptors with excellent climb characteristics were perhaps more needed by defensive minded forces (battle of Britain, defense of Germany, Japan...) not the all purpose "fighter" that the US Forces often needed that combined range, fighter and ground attack ability. The US did finally see a need for fast climbing defensive aircraft to respond to the Kamikaze threat.