Meredtith's

Effect

It is not just more than a few years since we can see cars with engines as powerful as 1000 hp or more on our streets to be more precise, since 2005 and the introduction of Bugatti Veyron with about 1200 brake horse power. If you Google around, you will find that the engine has 10 radiators to keep it cool and discharge the rest of about equivalent 1000 hp of excess heat.

But the 16 cylinder engine of Veyron is not the most powerful mass produced piston engine. The average power of most of fighter planes of WWII era were well beyond that 1000 hp of Veyron and you can even find one gigantic 36 cylinder engine ­ XR­7755­ with a total power of about 5000 hp.

In mid and late 1930s finding the answer for an efficient cooling system was one of the biggest challenges for the designers. Other than the engine’s weight, there were two major issues: when you are sitting in a Veyron, usually nobody shoots at you that could rupture your radiator and force you to pull over. Keep in mind that you cannot pull over in the middle of sky anyhow. The second one was the issue of drag. To make sure that the engine will not overheat you need to pass a considerable amount of air through it, which means producing a considerable amount of air friction or drag. This drag affects your range, acceleration and top speed in situations when having a 20 km per hours more speed could be a matter of life and death. If you cannot believe it, imagine a Nazi pilot at your six with two 0.5 cal machine guns coming for you. Different people came up with different ideas: “it’s stick to the old air ­cooled system and forget about that cursed radiator”, but it meant a larger exposed area and subsequently more drag and vulnerability. Moreover they could not be sure that the new more powerful engines won’t overheat (e.g. BMW. 139 and P­47 Thunderbolt engines). Some other tried placing the radiators under the wings to have sufficient airflow but it was an easy target.

In 1935 F.W. Meredtith a British engineer who was working at the Royal Aircraft Establishment pointed out that the wasted heat of a piston engine can be used to produce thrust. The idea was to put the radiator in a duct and to add the radiator’s heat to the flowing air, raise its temperature, make it expand and go faster out of the duct and produce some forward pushing force; similar to what later became the jet engines or to be more precise the ram ­jet engines. This approach not only could provide a better protection for the radiator, but also theoretically could produce some thrust that can compensate for the drag and even surpass it.

British tried it right away on Hawker Hurricane in 1936 and later embedded the idea into the design of Super marine Spitfire. The early results were so contradictory and unpredictable that even after 70 years you can find people who are discussing if the Meredtith’s effect is/was real or not. The problem was not the idea but the details: finding the right shape for the duct and the right duct inlet and the outlet areas for varying speeds, altitudes, power levels and somehow makes it work.

It took about four more years before the Meredtith’s effect could be placed in real proven use. In 1940 WWII was burning through the Europe. The United States was not yet in the war; but was trying her best to assist through equipments the last country standing against the Nazis’ war machine and fighter planes were in the highest demand. 

The only American fighter that could come close to the required standard of European theatre was Curtiss P­40 while its production plants were running at full capacity. An agreement was made with the North American company to produce it under license. A few days after the 40 boxes of P­40’s blueprints received by the NA’s design team led by Edgar Schmued, he turned to his team and said: “This won’t work, we are going to design our own airplane, a better one”. That is how the legend of P­51 Mustang begins.

Though Schmued employed the best conventional knowledge of the time, he incorporated as many new technologies as possible into the design of P­51 and one of them was the Meredtith’s effect. The task fell on the shoulders of the their thity five year old engineer Lee Atwood. To cut the long story short, Atwood did the job by collecting the data sitting in a full scale Mustang in side a running wind tunnel. To have an idea about this, picture yourself sitting in an aircraft restrained within a confined space, upwind of a 5000hp fan that moves the air around 500 MPH or look at the photos here.You can bet he did not share this with his life insurance company and nowadays if you suggest something like that, the best thing that some very caring person at H&S would do to you will be sending you for a mandatory counselling service at your own expenses. Atwood used the data to design a zero ­drag cooling system with an automated varying inlet/outlet areas. Atwood’s original data still can be found in Sherlock’s book the “Propeller Dynamics: Qualitative Fundamentals”.

P­51 top speed of nearly 700 KPH surpassed most of the fighters of its own generation. It became the first fighter plane that had enough operational range to escort the defenseless heavy bombers deep into the enemy territories and on their way back. In a few words, to some extent it changed the course of war and the Meredith’s effect is yet a subject of research as late as this 2007 paper.

By the way, if you are still wondering why they had to put 10 radiators in Veyron, here is the story: the designers at the Bugatti studio came up with the most artistic design for the car without consulting the engineers about how much air intake it might need. Then they turned to the engineers and asked: Can you make it go by 250 MPH or more with this fancy shape? You know the answer for that. As engineers we have retracted the "sorry, it cannot be done " phrase from our vocabulary a long time ago.