It makes a great headline but it’s also the bold claim made by P51 Mustang designer. A few years ago now, in 1999 I was privileged to attend a talk by Lee Atwood, designer of the North American P51 Mustang. Despite being an elderly man at that point, he had traveled to the UK from his home in California to give a short talk to a group of aviation enthusiasts at the Yorkshire Air Museum, Elvington, North Yorkshire. The talk had the rather contentious title ‘I can built you a better airplane than the Spitfire XIX’. That was the claim that Atwood had made when he commenced design of the P51 in World War 2.
As the old man rose to speak in a small Nissen Hut in North Yorkshire, he looked a little tired and uncertain and I was momentarily concerned that this may be an embarassment. I need not have worried. As he moved past his first few sentences, he ceased to read his notes, as it all came flooding back and the great man spoke in a soft California accent and with a clarity of thought and presentation that had the audience enthralled.
He recounted his tales of the design concept of the P51 and on his ideas for aerodynamics at that time that led him to believe he could build a front line fighter with a higher top speed than the Spitfire MkIX. The key to the performance of the aircraft and it’s high top speed, he explained, was the air scoop that swung down below the fuselage to gulp great lungfulls of air. Nicknamed the doghouse, it makes the P51 instantly recognisable from any distance and has become a design icon of the second world war.
Yet what I was unaware of until hearing Lee Attwood’s presentation was that despite hanging down into the airflow like a basking shark, the whole assembly doesn’t add any significant drag to the airframe. In fact, at various speeds, it actually provides thrust. This thing wasn’t just designed to look stunning. It had a clarity of purpose that came from many hours wind tunnel testing theories which at that time represented the cutting edge of aerodynamics.
So how did Lee Attwood and the team at North American achieve this amazing feat? Any racing car designer will tell you that cooling an engine creates drag. The smaller the radiators, the less drag, more speed. Too small a radiator of course means that cooling becomes marginal. Many warbird fighters have very marginal cooling systems, unable to spend much time on the ground on hot days before overheating, just like a Formula One car on the grid.
The Spitfire Mk XIX has two radiators, one in each wing. They provide cooling, but Atwood’s calculations indicated that they also took away around 20 MPH from it’s top speed, the RAF themselves calculating around 13mph. Atwood wanted to create the holy grail of piston engined fighter designers – cooling without the penalty of drag. His calculations were based upon the Meredith Effect a phenomenon discovered in the 1930s whereby the aerodynamic drag produced by a cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced.
This theory was not secret and was used to some degree in both the Spitfire and the Bf109 designed. However, Lee Atwood took the concept further than anyone before him and concentrated on the restoration of cooling drag as a principle design component of the P51 Mustang. His design presentations to the RAF in 1940 resulted in the commissioning of the original Allison engined P51. While the US Air Force went on the be a major user of the Mustang, it was the Royal Air Force that set the Mustang production rolling.
The clever part of the Mustang cooling is not just in the intricately formed leading edge with it’s hand formed compound curves, but in the secondary section that comes after the air has entered the scoop. Nicknamed the ‘doghouse’ section, named after it’s shape resembling an upturned kennel, intricately shaped ramps and angles channel the air into a smaller and smaller space. As it’s forced into the smaller area, it’s forced rearwards, passing through what is effectively a choke, before being allowed to expand and pass through the radiator and oil cooler. The hot air then exits through a small flap, the size of which is continuously adjustable and creates the back pressure needed to achieve thrust. The difference in speed between the Spitfire XIX and the Mustang P51D is generally recorded as 405 vs 437 mph. Despite much discussion regarding laminar flow wings and fuselage fairings, Lee Atwood’s presentation, from the very man who designed the fighter, made it quite clear that it was the attention to detail and optimising the Meredith Effect that gave the P51 it’s high speed.
At the end of his talk, Lee spent time posing for photographs before he took a seat at what happened to be our table for a rest and a cool drink. He chatted about his experiences working for North American in the war and it was abundantly clear that his passion for aircraft design and aerodynamics was still burning as strongly as ever. I dropped into the conversation that his notes would make interesting reading and he duly made sure that several copies were made to hand out to anyone interested. Less than three months after he returned to California, J Leland Atwood passed away at the age of 94. I still have my copy of the great man’s handwritten and carefully typed notes from the evening, something that I treasure and will keep forever, together with my memories of the elderly man who’s eyes shone with all of the passion of a youngster as he talked to the audience that evening.
Images via Flickr / Robin Kearney / AirWolfHound / Victor XL231