that is what I was taught that on my RAF engineering courses, it does produce thrust, the only one that does as far as I am aware and to quote the designer
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 its 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 didLee Attwood and the team at North Americanachieve 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 clever part of the Mustang cooling is not just in the intricately formed leading edge with its hand formed compound curves, but in the secondary section that comes after the air has entered the scoop. Nicknamed the ‘doghouse’ section, named after its 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
P51 Mustang Meredith Effect explained by Lee Atwood, designer
CONCLUSIONS
The additional thrust of the cooling system further increases the interest for piston engines propulsion
in aeronautical field. In fact, due to the Meredith effect, it has been possible to eliminate the radiator drag and to increase the total thrust of the power-pack. Therefore, Meredith ramjet improves the total available thrust and reduces fuel consumption. Both under-fuselage/under nacelle and in-wing configurations were analyzed
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