The P-51 is famous for its advanced-for-its-time aerodynamics, the prime example of which is the radiator design under the belly which utilizes the Meredith effect to, in effect, turn it into a mini jet engine. Now, the cylinder walls and cooling fins of air-cooled radials are much hotter than whatever cooling liquid that's used in the P-51's water cooler, so is it possible to produce a similar cowling for the air-cooled radials? Is there any example of such a cowling on WWII airplanes?


2 Answers 2


The Meredith effect is to recover radiator drag by using engine heat for thrust, just like a jet engine. It worked very well in the P-51 Mustang, actually adding a few knots of airspeed.

It also would work well with any source of heat, including a nuclear reactor, so yes it would work with aircooled engines.

The issue is drag from the radiator. Aircooled radials have much greater frontal drag, so it worked, just not as well. Efforts to reduce drag in the B-36 bomber resulted in roasted engines, particularly the rearward banks of the "corncob" R-4360 radial. Overheating is a major concern, as is drag. Engineers try to find a good balance.

  • $\begingroup$ the way I see it if we stay away from crazies like the Wasp Major the problem to solve is how to recover the pressure from the leeward half of the cylinder. since it can not be made like a golf ball some sort of guide vane must be present at the leeward side lest the turbulent wake forms before the second row of cylinders. $\endgroup$ Jan 23, 2019 at 14:51
  • $\begingroup$ Added to the turn to jets. Ton mile/gallon favors higher speed and less drag, even with slightly less efficient propulsion. Amazing that the freight train, with all cars following the first, is 4x more fuel efficient than my tractor trailer. The 747 is almost 2x more efficient than props, even though piston engines get better miles per gallon. $\endgroup$ Jan 23, 2019 at 15:43
  • $\begingroup$ The NACA cowling comes to mind $\endgroup$
    – Dohn Joe
    Apr 8, 2019 at 8:25

I had to go look up what the "Meredith Effect" was (and edited your question to include a link to Wikipedia).

As I was reading through the Wiki article, I found the answer to your question:

Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949. (Source)

So no, the Meredith Effect will not work on an air-cooled engine.

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    $\begingroup$ please read your own quote "does not benefit MOST". It does NOT say "does not enefit AT ALL". The Meredith Effect is not some inexplicable high science, an inlet does pressure recovery, the radiator adds heat to air, a nozzle reconverts pressure back to velocity. all I'm asking is whether someone had BOTHERED to design a cowling to make a REAL EFFORT at utilizing it rather than just be SATISFIED that the engine is cooled and not TOO MUCH DRAG is induced. $\endgroup$ Jan 23, 2019 at 14:36
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    $\begingroup$ @MeatballPrincess There is no need to yell, you can use markup in comments to denote emphasis $\endgroup$ Jan 23, 2019 at 14:52
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    $\begingroup$ @AEhere Honestly, boldface looks just as shouty as all-caps, to me. $\endgroup$ Jan 23, 2019 at 16:23
  • $\begingroup$ Your quote states it won’t work very well, but it unfortunately doesn’t state why. I think the other answer hits this nail on the head; the radiator flow exits nicely into the airstream, as it is aligned with the fuselage, and doesn’t have any obstacles in the exit flow. In comparison, the flow around the cylinder heads of a radial engine, aren’t smoothly aligned with any surface, and lots of other brackets, piping, and whatever on the outside of the engine create turbulence and cause a loss in the total pressure of the cooling flow, and hence the jet thrust. $\endgroup$
    – Penguin
    Jan 24, 2019 at 10:21

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