French coolant for the HS 12Y?

You are focusing too much on the inlet size.

The P-38 is supposed to have had the same radiators throughout it's life, it's ability to go from 1150hp engines to 1600hp engines (or higher in WEP) was handled, inpart, by moving the radiators further out form the boom center line and using larger ducts.
Also just about every airplane controlled the airflow through the radiator and duct by the use of the exit flap/s. close the exit flap and the air pressure builds up in the duct and more air approaching the inlet diverts around it as the high pressure "bubble" extends out past the inlet.

I would also note that many tests of the P-39 note that it's cooling either failed to meet US standards or just barely passed.

In response to jmcalli2 post#38,

Shortround6 gave what may be the entire answer to the P-38's large radiators in his post#39. Since the P-38, P-39, and P-40 engines were all designed around the 2/98 coolant mix, and originally had about the same power output, the basic cooling parameters would be the same except for the effective operating altitudes.

The only reason this might not be the entire answer is the different design concepts and understanding of the physics of cooling at the time. In other words, some designers/systems were more skilled/efficient than others. A well designed liquid cooled system for large high-power engines would, everything else being equal, be more compact (but probably heavier) than an air cooled system. And obviously, a less skilled designer might simply come up with a less efficient system.

As to the design concepts, the size related cooling effect of radiators is somewhat proportional to the overall frontal area x the depth of the cooling tube/fin array.
Everything else being equal, a deeper radiator can be more efficient at high air flow speed than a large frontal area radiator, and a radiator with a large frontal area will be more efficient at low air flow speed. But the larger the frontal area the more cross sectional drag in comparison to a deeper radiator, to a limit. There will be a balance between the volume available for installation, the overall drag of the radiator, the efficiency of the radiator, and the design speed of the aircraft.

In the time period we are talking about, much of the knowledge was based on trial and error for new designs, some were simply a carry over from older designs scaled to hopefully meet the new design requirements. The cooling system on the P-51 was one of the first designs to incorporate a mature understanding of the Meredith effect. (Interestingly enough, the cooling system on the Hurricane made use of the Meredith effect also, just not as efficiently.)

Sometimes the reason for a less efficient system might be as simple as avoiding patent infringement.

As for the oil coolers, most of the above answer applies also. The only additional significant factor I can think of is just how much the designers wanted to rely on the oil cooling system to also help cool the rest of the engine. The more cooling by the oil coolers desired, the larger the oil cooler radiators.

ThomasP said:

As for the oil coolers, most of the above answer applies also. The only additional significant factor I can think of is just how much the designers wanted to rely on the oil cooling system to also help cool the rest of the engine. The more cooling by the oil coolers desired, the larger the oil cooler radiators.

Click to expand...

Just to clarify, In some cases the size/requirements of the oil coolers and radiators were dictated by the engines. It didn't matter what the airframe designer wanted to do. The engine designer had already designed the engine to get rid of a certain percentage of the heat through the oil system and a certain percentage through the radiators in a liquid cooled engine.
The airframe designer fooled around with those requirements at his (or the pilots) peril.

Please note that even on the P-40 the Merlin powered versions used a different radiator and oil cooler set up than the Allison versions. I forget which way was which but if you try to swap the radiator/oil cooler assemblies between the engines you either wind up with low coolant temp and high oil temp or the other way around.

Hey tomo pauk,

I figured out where I went wrong in thinking that the P-38 used liquid cooling for the intercoolers. Apparently Allison had wanted to switch to 70/30 coolant mix fairly early in the war, but the USAAC did not allow it. I found a quote in my notes from a Allison mechanic who had been sent to England to look into some of the problems with the V-1710 set-up in the P-38. The quote was "We found that the cooling system for the engines was not able to keep up with the demand in actual service. We attributed part of the problem to the AAC's requirement of 2% water and 98% glycol. When you added the cooling problems with the turbocharger the result was engine damage due to multiple causes. The squadron mechanics were chasing their tales trying to figure it out."

I assumed that when he commented on the turbocharger cooling problems he was also talking about liquid cooling. DOH! Thank you for pointing out my mistake.

Hey elbmc1969,

In response to your original post.

I ran across a reference stating an intent to use a retractable radiator in concert with evaporative cooling in the early Merlin installations. The idea was to deploy the radiator for increased cooling at low speeds or when on the ground, then retract it when at higher speeds to reduce drag. I do not know how authoritative it is.

THE ROLLS ROYCE V12 R MERLIN ENGINE