In 1999, the museum was honoured to host a guest presentation by the designer of the North American P51 Mustang, Lee Atwood. 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 gathering of enthusiastic people. His talk had the rather contentious title ‘I can built you a better airplane than the Spitfire XIX’. That was the brave claim that Atwood had made when he commenced design of the P51 in World War 2. At the time, the Spitfire was considered, quite rightly, to be the pinnacle of Allied interceptor fighter technology of the time.
Neill Watson was in the room that evening. “As the old man rose to speak in The Elvington Room, he looked a little tired and uncertain. For me, he was a legend of aerodynamics and fighter design and he had travelled a long way. 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.”
In his talk, Lee Atwood recounted his tales of the design concept of the P51 and on his thinking 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 hung down below the fuselage to gulp great lungfulls of air. Nicknamed the doghouse due to it’s upturned kennel shape, it makes the P51 Mustang instantly recognisable from any distance and has become a design icon of the second world war.
Yet what many were unaware of until hearing Lee Atwood’s presentation was that, despite hanging down into the airflow consuming large quantities of air, the whole assembly doesn’t add any significant drag to the airframe. In fact, at various speeds, it actually provides additional thrust. 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 designer of aircraft or racing cars 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 piston engined fighters, including the Spitfire, can have very marginal cooling systems, unable to spend much time on the ground on hot days before overheating.
The Spitfire Mk XIX has two radiators, one in each wing. They provide cooling, but Lee Atwood’s calculations indicated that they also took away around 20 MPH from it’s top speed. The RAF themselves were aware of this, thier calculations came to 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 designs. However, Lee Atwood took the concept further than anyone before him and concentrated on creating a zero speed loss due to 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.
In his presentation, Lee explained that 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 that evening, 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.
Neill Watson met him briefly afterwards. “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.
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.”
Lee also took the time to inspect Victor XL231, Lusty Lindy, while he was with us. Andre Tempest writes on XL231’s blog about how he was fortunate enough to guide him around the jet. “He went around every inch of the Victor with minute scrutiny and pointed out various air intakes and fairings. His comments went along the lines of; ” See these beautifull intakes in the nose? “Yes” I said. He replied with; “Very pretty but a total waste of time! Giving you a drag ratio of (whatever figure he said on the day!) Then he mentioned the oil cooler intakes under each engine; ” Well designed but useless aerodynamically”
Finally, he shook my hand and said; ” You’ve just shook the hand that shook the hand of Wilbur Wright”. What could I say to that??
Less than three months after he returned to California, J Leland Atwood passed away at the age of 94. His presentation to us that evening was in all probability his last speaking engagement.
Images via Flickr AirWolfHound / Victor XL231