North American P-51 Mustang

The fighter aircraft and designer that won the war for the allies, the story of the North American P-51 Mustang and Edgar Schmued

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Edgar Schmued

Firstly, please accept my apologies for riding roughshod and abbreviating some historical facts, I was conscious of keeping the article fairly short and for it not to become tedious.

Up to the outbreak of war in 1939 the RAF had relied upon British built fighter aircraft such as the Rolls Royce Merlin powered Hurricanes and Spitfires for their operational squadrons, these were good aircraft and offered better performance and speed than their American counterparts. The RAF did use American aircraft though, advanced trainers and bombers for instance. France however had a need for more fighter aircraft and was busy buying Curtiss aircraft from the US, although in true French tradition they had surrendered before most of these aircraft on order were even completed.
Spring 1940 arrived and the RAF discovered that they needed far more aircraft than local production could ever satisfy and America was the obvious supplier. Nothing was available that would match the speed and manoeuvrability of the Spitfire, designed as a short range interceptor and a superlative dogfighter, but they had their eyes on the defence of the colonies threatened by axis forces. It was felt that aircraft such as the Curtiss P-40 (of Flying Tigers fame) with its greater range would be able to take on Japanese aircraft likely to attack Singapore and the Italian aircraft around Egypt. The RAF hoped to have 1000 of these aircraft by 1941, a tall order for Curtiss and to solve the problem it was decided that a new kid on the block, an aircraft company called North American Aviation would also license build these aircraft in its own factory.

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Edgar Schmued, born in 1899 near Zweibrücken

Edgar Schmued was born in 1899 in a small German town near Zweibrücken, his father was a dentist and to find work moved near to Berlin. Thanks to his father he developed an intense appetite for all things technical and was forever bringing technical books home from the library. Counterintuitively his work at school suffered badly because he was learning how paper was made or how spinning machines worked. His love for aviation developed at the age of eight when he spotted a Wright aeroplane in the sky over Berlin, and he knew instantly that this would be his future life. Sadly Edgar did not graduate at school, but with the endless assistance of his father, who besides being a dentist had also invented a few things, continued to self-educate himself to become an engineer, although an engineer without formal training or qualifications. After leaving school Edgar became an apprentice in a local engine factory where the owner soon spotted his talent and gave him a project to build an engine from scratch. He became a developer there and registered five patents in his name.

The economic situation in Germany in 1925 wasn’t good so together with his eldest brother he emigrated to Brazil. There he found work as a mechanic at a General Motors garage and by chance met an executive and ended up working for GM Brazil and from there it was suggested he leave for the US to continue with GM. More good fortune arrived and GM had decided to move into the aviation business and formed a new company called North American Aviation, so at last Edgar had the opening that he had been waiting for. At NAA Edgar designed gun turrets, other parts of airframes and designed a basic trainer aircraft, the NA-35. Again Edgar’s talents had been spotted and he was now the chief designer. One day the NAA boss ‘’Dutch’’ Kindelberger walked into his office with the news that their factory was to start Curtis P-40 production for the RAF, “Edgar, do you want us to build P-40’s in our factory?” ‘’Well Dutch, don’t let’s build an obsolete fighter for the RAF, let’s build a new one, we can design a better one and build a better one’’ Edgar replied. That was what Dutch wanted to hear, so accordingly, Edgar was given two weeks to produce drawings and estimations of performance for a new fighter for Dutch to take to London for a scheduled visit to the RAF. It had to be fast, accommodate a man of 5’10” and carry cannons, crucially it must also meet design requirements of the US air force and cost no more than $40,000 as lend lease hadn’t yet kicked in. However Dutch returned from London with no contract, but Edgar and NAA carried on with development regardless and eventually the top dogs in the RAF who were delighted by their NAA Harvard trainer aircraft relented and asked for an initial 400 aircraft.

Thus on April 11th 1940, the day after the US president, Franklin Roosevelt gave the go ahead, Sir Henry Self the director of the Anglo/French purchasing commission, an Englishman, gave the go ahead for production of the greatest American and allied fighter of the war. NAA pulled out all the stops, burned the midnight oil and Edgar and team, armed with only books of formula and slide rules produced the first prototype that took to the sky on October 26th which was a truly remarkable and unsurpassed achievement within such a limited time frame. Some months later RAF Squadron Leader Chilton was in the US conducting test flights with a view to it gaining British acceptance but was unimpressed by the delay to a flight to check out the firing of its guns over the Pacific Ocean as the Coast Guard were taking their time granting permission. Chilton voiced his anger and said ‘’what’s the matter with you chaps, in England we just fire into the countryside and you would be surprised how few people get killed.’’

The US army air force and the RAF took delivery of their P-51’s, now known as Mustangs and off they went into battle. The early aircraft were powered by an Allison V12 liquid cooled supercharged engine that displaced 1710 cubic inches. (28 litres) It was a fine engine and met US Army air force requirements when conceived, gave the Mustang a good top speed that would leave a Spitfire behind at low level but it had an Achilles heel; It was optimised for low level operation because it was never envisaged that aircraft would be flying so high in combat. (remember that the biplane was still in use with air forces) Its single stage supercharger would be running out of puff at around 18,000’ which was a real issue as fighting the Luftwaffe above this height would be just about impossible.

There were still two pieces required to complete to jigsaw:

The speed and performance of the Spitfire had been noted in the US and it was correctly concluded that this was largely due to its Rolls Royce Merlin engine which had recently just been upgraded in response to new Luftwaffe fighters like the Focke-Wulf Fw 190 which outclassed the earlier marks in many respects. The new Merlin now sported a two stage supercharger that also had two gears, the high gear designed for high altitude operations with the gear change managed automatically by an aneroid device and finally a liquid cooled intercooler, or ‘’aftercooler’’ as it was more correctly called back then. Rather technical I know, but for the non mechanically minded it allowed the engine to make far more power and crucially, maintain good power right up to 30-40,000.’ Curtiss in the US wanted this engine for a new project, the XP-60 so the Packard Motor Car Company started to produce license built Merlins in Detroit. It was known as the Packard V-1650.

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Packard Merlin, rear view showing the intercooler at top rear and two stage supercharger below and at the bottom, partially obscured by the wooden crate is the Bendix Stromberg pressure injection carburettor

Meanwhile back in the UK, Rolls Royce test pilot Ron Harker had suggested that the Merlin should be fitted to the P-51, he was backed up by a company director, Hives, who also saw the Merlin P-51 as the best bet. Accordingly, Rolls Royce were allocated 5 Mustangs to be re-engine with Merlins and the first one took to the sky on October 13th 1942, the test pilots were hard at work with the converted airframes and soon had them worked up to a top speed of 433mph at 22,000’. After various bugs had been ironed out, some rather serious, NAA rushed into production with the new aircraft, the P-51B, a machine than could take on all Luftwaffe fighter variants at all altitudes.

So why was the Mustang so extraordinarily versatile and why was its performance so good? Firstly its diminutive Packard built Merlin that displaced a mere 27 litres had been worked on by Rolls Royce and would now produce over 1700bhp for a short period if the pilot selected war emergency power which pushed the P-51 on to a top speed of 440mph at 22,000’ and a ceiling of 42,000’. (higher that your Easyjet Airbus) It could also dive at a speed of 505mph and climb at 3,500 feet/minute. This engine was also very frugal on petrol consumption and would burn maybe just 50% of the fuel needed by the more traditional larger radial engines. The Spitfire could not touch those figures with the same engine so why should that be? Well firstly the wing used on the P-51 had a new aerofoil shape (cross-section) that enabled a laminar airflow over much of its upper surface, conventional aerofoil sections manage a laminar flow over perhaps only the first 1/3 of the upper surface and from there to the trailing edge the airflow would become turbulent and as we know, turbulence equals drag. The laminar flow was achieved very simply by keeping the maximum wing thickness for longer before tapering it off more sharply towards the rear of the wing. As ever, there is no such thing as a free lunch and such a wing with its high efficiency and low drag was not as good at providing lift which limited the P-51’s rate of turn which meant it was never as an accomplished dogfighter as a Spitfire and also it had rather challenging characteristics when approaching the stall which would arrive with little warning and would be fairly violent. This made for tricky handling that could require a sharp response from the pilot especially if the ground was close. It is interesting to note that the only bit of the aircraft tested in a wind tunnel was the wing and Edgar had developed another wing of a more standard aerofoil shape in case the laminar flow design didn’t perform correctly. Additionally, the engine cooling arrangement was ground breaking. Large aero engines have to dump spectacular amounts of heat at around 30% of the fuel’s energy content, otherwise overheating and seizure of moving components would occur quickly. The Mustang was no exception with arrangements needed to cool the engine coolant (glycol or antifreeze), the engine lubricant (oil) and the intercooler coolant. The three large radiators needed to do this job have to hang in the slip stream and thus create lots of unwanted drag. Edgar had the brilliant solution by arranging all these radiators under the belly of the aircraft, fed by one large air scoop and by shaping the ducting in a clever way, he was able to take advantage of another phenomenon known as the ‘’Meredith effect.’’ This utilised the heat energy from the coolers to produce an additional jet thrust from the duct’s exit. (Meredith was a Brit) Thus at a stroke the normal considerable cooling system drag was reduced to almost nothing.

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B-17’s flying in high relative humidity with vapour forming in the propeller slipstream.

‘’The bomber will always get through’’ – former British PM, Stanley Baldwin

This statement was a simple assumption that if you send a wave of heavy bombers out to destroy a city then enough aircraft will survive to do the job irrespective of defences.

The US Air Force that would fight in Europe was known as the Eighth and was activated in January 1942 in Georgia and from there the HQ was relocated to High Wycombe. It started daylight bombing operations in mainland Europe flying from the UK in August ’42 working on Baldwin’s assumption. Despite British and German experience of daylight bombing and heavy losses the US thought that tightly packed formations of heavy 4 engine bombers such as the Flying Fortress with their large numbers of defensive guns would be able to deal with attacking fighters, one axis fighter against maybe 50 defensive guns in a small ‘’parcel’’ of sky perhaps. Initial bombing missions didn’t work at too badly for the Americans so in early ’43 the allies created the ‘’combined bomber offensive’’ which effectively created around the clock bombing, the RAF by night and the 8th Air Force by day. What happened next was that the Luftwaffe brought many of their fighters back from the Eastern front, slowly moved them further west and during the notorious Schweinfurt-Regensburg mission the US lost almost a third of the raiding force on one mission, 77 out of 291 bombers. At that time the German fighter pilot was highly skilled, very experienced (many aces from the Eastern front) and expert at his job. In addition they were highly aggressive with their tactics, frequently launching head-on attacks at the bomber formations, pressing home with these attacks until they were within a few feet of the bombers. Such tactics could cause the bomber stream to fragment so that defensive formations were lost and then the fighters armed with cannon and machine guns flying very fast, appearing as no more than a ‘’speck’’ to the bomber gunners until the last few closing seconds would just carve them up one after the other. It was effectively a turkey shoot.

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A typical briefing to a bomber crew before a dangerous mission: “Consider yourself dead. Some of you won’t come back from this. Some of you will, but you’ll be the lucky ones.” I’m afraid that the internet suggests there were no survivors here, typically ten crew members
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Here’s a rather more lucky crew that arrived home

The Eighth was left reeling after this mission and further long range missions were halted temporarily and it was clearly evident that Baldwin’s statement was erroneous. An escort fighter would have to be used to give the bombers a chance, but what small single engine fighter aircraft could fly with the bombers from the UK to perhaps Berlin, engage in combat and return? Such a mission could take six hours and for example, a Spitfire could manage perhaps 90 minutes and rather less if the pilot had needed to throttle the engine up due to combat. It is true to say that the US did have the P-38 Lightning twin engine fighter that was capable of extended missions, but it was in very short supply and moreover it had serious issues with its turbocharging arrangement and frequently suffered from serious failures. The P-51 was the obvious answer and became available to the 8th during the winter of 1943 in sufficient numbers. The aircraft was fast and manoeuvrable, heavily armed with 6 Browning 0.5’’ wing mounted machine guns and crucially it could fly a 6 or 7 hour mission. To achieve this it had fuel tanks mounted within the wing structure totalling 184 gallons and mounted behind the pilot and his armour plate was a fuselage tank that held a further 85 gallons giving a total of 269 gallons. Additionally two large drop tanks beneath the wings of 110 gallons each took the total capacity up to 489 gallons of fuel. Typical average fuel burn at typical altitudes and cruising speed was only 60 gallons an hour due to the exceedingly frugal Merlin, which gave it over 8 hours of endurance although obviously reserves had to be considered and additional fuel for likely combat. A six hour mission was thus comfortable, although the pilot would likely need assistance to get out having been strapped in, immobile for such a lengthy period.
Such a huge amount of fuel for a small aircraft was unprecedented and was not without problems. The fuselage rear tank when full moved the centre of gravity so far aft that the aircraft was very tricky to fly, requiring great concentration during take-off and constant attention when airborne as it was markedly unstable in pitch. It could not dogfight in this condition and abrupt control stick inputs from the pilot would likely cause structural failure, so this fuel had to be used first Management was the key, but if the drop tanks had also been jettisoned when full, as was required if enemy aircraft approached, some careful calculations had to be made before reaching the point of no return to England with only enemy territory and ice cold North sea between you and safety.

With Mustangs becoming available in sufficient numbers the Americans were able to resume daylight bombing. Initially the Germans countered the new Mustang threat by formating in large numbers ahead of and above the bomber streams, attack in a single high speed diving pass and then make off quickly in a continued dive, the idea being to avoid a fight with the US Mustangs as much as reasonably possible. These tactics were pretty sound but the numbers of bombers downed wasn’t so great. The need to shoot down more bombers was crucial as they were being manufactured and flown across the Atlantic to England in ever increasing numbers. The Luftwaffe’s solution was to equip some fighters with heavier armament to attack the bombers decisively and keep the Mustangs away from these now cumbersome heavier machines with regular Messerschmitt 109’s. Again this was a perfectly sound theory but the logistics of the co-ordinated plan proved rather hard to achieve in practice.

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A typical P-51 formation of the day.

Standard escort practice would be for Mustangs to assemble and rendezvous with the bomber streams at a designated place and time and provide protection over hostile areas. A Mustang would perhaps cruise at around twice the speed of a Fortress, so normally operating in pairs of a leader and wingman they would fly with the bombers but constantly weave so as to match bomber progress. The fighters could slow down but that would put them at a tremendous disadvantage if ‘’bounced’’ by the enemy and so speed was always maintained. There also existed the possibility that prolonged running of the Merlin at low power settings would cause fouling of the sparkplugs from the lead additives in the fuel.

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Here we can see part of a bomber formation (Fortresses) and above one can see escorting Mustang fighters, operating in groups of two, leader and wingman, engaged in weaving so as not to overtake the bombers

The war in the sky was one of attrition and allied production of aircraft remained well ahead of the axis production. Germany was losing fighter pilots at a rate in excess of its ability to train new ones and tragically for them, the experienced ‘aces’ were being lost and the inexperienced replacements were no match for the Mustang. Several US pilots were becoming ‘’Aces,’’ household names like Bud Anderson and Chuck Yeager had amassed victories running into the teens.

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The Mustang’s bite. Six 0.5’’ Browning machine guns, three in each wing, staggered in a fore/aft direction to allow for the ammunition belt feeds.

As the war progressed in favour of the Allies, Mustangs would often run ahead of the bomber streams on the look-out for Axis fighters, trying to entice them into the air to engage or at the least have them run low on petrol and so have to return to base leaving the sky clear for the bombers. If this tactic didn’t work then Mustangs would attack German airfield and try and destroy aircraft on the ground, this was a far more hazardous operation than aerial combat as below about 3000’ near guarded airfields every light calibre weapon on the ground would be firing at you.

By the summer of 1944 the numerical superiority of the allies with their largely experienced pilots and Mustangs had crippled the enemy who were critically short of aircraft, fuel and experienced pilots. Hermann Göring, commander of the Luftwaffe was quoted as saying ‘’when I saw Mustangs over Berlin I knew the Jig was up.’’ And to prove that Germans do have a sense of humour, in 1945 the Wehrmacht joked: When we see a silver plane, it’s American. A black plane, it’s British. When we see no plane, it’s German.”

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More than 15,000 Mustangs were built of which almost 300 survive with around 170 remaining airworthy. Here’s a survivor based in Florida and another regular at Duxford, England

I’ll conclude with a couple of links to YouTube clips. The first is an interview with a chap now in his 90’s, a European theatre ace, Bud Anderson. He describes an encounter with a number of enemy fighters.

And lastly, two great friends are enjoying one another’s’ company seventy years after their work was done, a Spitfire and a Mustang.


© Cardinal Puff 2018