Best radiator cooler arrangement for Meredith duct?

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Staff Sergeant
Nov 3, 2022
Mansfield, Ohio, USA
Just wondering which would work better from a cooling standpoint and an aero standpoint. The P-51B/D had a small oil cooler ahead of the radiator/aftercooler assembly. The P-51H, XP-51 lightweights, and the XP-82/P-82B had only the radiator assembly in the duct, with the oil cooler housed in the nose sharing space with the supercharger intercooler at the rear of the engine bay. The Martin Baker MB5 had intercooler, radiator, and oil cooler grouped together in one assembly in the radiator duct.

Wondering which one works better from both cooling and aero efficiency. Interestingly, the Allison powered F-82s reverted back to the P-51D arrangement of a small oil cooler mounted in front of the radiator.
The more heat you dump into a Meredith duct, the better it works (aerodynamically). If you are going to arrange items within the duct sequentially, you would want to think in terms of heat load that you could remove with air at a given temperature...a small, hot oil cooler might not heat the air as much as a very large but slightly cooler radiator. You'd have to run the numbers to find the optimum arrangement.
I would guess one big cooling duct would be better, due to relatively less skin friction. To the extent the oil is hotter than the water-glycol coolant, might make sense to put that in the back part of the radiator matrix to produce the hottest possible exhaust and thus maximize the Meredith effect.
The reason I'm asking is that the XP-51 LWs, P-51H and Twin Mustangs did have a positive thrust vs drag gain, though all they had in the duct was the radiator (the XP-51s, P-51H and Merlin powered Twin Mustangs had the oil cooler integrated with the intercooler near the supercharger assembly). Oil usually runs (or can run) warmer than coolant, but I think the positive thrust gain was due more to the design of the ducting than what was placed in it.
Perhaps more a question of expediency rather than what is better theoretically? That is, to handle more powerful engines they needed a bigger radiator, and instead of redesigning the entire belly scoop they just moved the oil radiator elsewhere to make room for a bigger water radiator in the existing scoop?
Thing is that the LW Mustangs and the P-51H and F-82 had a redesigned scoop/duct work with the aim of improving efficiency. The Allison Mustangs got to within 5-10 percent of totally eliminating radiator drag (better than basically anyone else in 1942, let alone 1940), the first gen Merlin Mustangs (P-51B/D) broke even on cooling drag, while the LWs and the P-51H and the Merlin Twin Mustangs got about a 5-10 percent thrust to drag gain. Not sure about the Allison powered F-82s because though they used the same ducting design as the Merlin powered versions, the reverted to the P-51B/D arrangement of a small oil cooler in the duct ahead of the radiator.

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