Quoted from "Trees" :"The old fighter planes had an entirely different set of problems to work with. The P 51 was liquid cooled where as most of the others were air cooled. The air cooled engines had cowl flaps to maintain cylinder heat temps in the "green" to prevent detonation (hot) or excessive fuel consumption (cold) These flaps were open for take off and landing and were mostly fully closed when at high altitude cruise. Opening the cowl vents too early or going full open at too high an altitude could cause rapid cooling and a bunch of warped heads, jugs etc. The P 51 radiator cooling air could be regulated also, but usually was not required. The unique way the aircraft was flown for the long range, high altitude bomber escort missions was developed by Charles Lindberg. Upon reaching cruise altitude and airspeed, they would lean the engine back until it quit, then slowly increased enrichment until it fired and then they set the throttle and mixture there. As fuel burned off, they would slowly climb, maintaing air speed and throttle/ mixture settings. Lindberg had to prove to all the old time aircraft engine mechanics that this procedure and super lean fuel mixtures did not damage the engines. His procedure increased the range of the aircraft by about 50% and were the only aircraft that could escort the bombers to Berlin and beyond."
You know, I knew that, someday, my being an extreme WWII fighter aircraft freak would come in useful (sort of).
Ok, some mis-information in the above...not intentionally incorrect, certainly, just not quite on-the-mark in some areas.
Most WWII fighters were liquid cooled, rather than the other way 'round. All USN fighters flown from CV's (aircraft carriers) were air cooled, of course (the Navy refused to use liquid cooled fighters aboard ship), but most Allied USAAF, RAF, etc. were water cooled, with some notable exceptions such as the Jug (P-47), and the Hawker Typhoon (RAF ship).
Secondly, Lindberg's foray into the South Pacific and his work done with a fighter group down there did, indeed, greatly benefit the US as far as fuel economy goes, but his ideas to lean out throttle mixtures (and other things) were met with resistance because of the lean mixtures themselves, which, as we all know, can cause an engine to run hotter, and allow detonation at a far lower power loading. This was the primary concern...that the lean mixtures, even at cruise settings, would be certain death to an engine (most of his testing was done on P38's, twin-Allison engined turbo-supercharged fighters) because the turbo boost would cause detonation at those leaner settings, burning holes in pistons and cooking valves. He proved this wasn't the case by flying his ship "leaned out" while others in his flight flew theirs in the conventional manner (not sure how many sorties he flew this way..I could probably look it up, but I'm too lazy...it was more than just a few, I'm certain) and afterwards, the engine was torn down and inspected, and found to be in perfect condition. This simple adjustment doubled the range of the P38, and did much toward helping us win the war in the Pacific....With so little land, and most sorties, by necessity, lasting as much as 8 hours or more, it was a miracle for those fighter pilots who fought in that theatre.
Cowling flaps on those ships that had them ( the P47, F4U Corsair) were generally opened only during METO power setting, or WEP, when the extra cooling ability was absolutely needed. A P47, for example, could increase turbo/supercharger boost enough to gain over a 33% horsepower increase, using water injection, and understandably it generated enormous amounts of heat. Also, the P51's cold air induction was often
used, in the heat of combat when high power (boost) settings were used. I've never read (or been told...I've inverviewed a number of WWII fighter pilots) about jugs being cracked because of overcooling, though that doesn't necessarily mean it isn't true...
If any of you guys want to know anything about WWII fighters, just ask...I probably know the answer, whether it's concerning combat tactics and maneuvers generally used by specific aircraft pilots, powerloading differences between aircraft, or even loadouts and ordnance capabilities for specific aircraft....or historical facts about engagements, fighters, etc. Alot of this information is extremely tough to find these days, and I'd be happy to pass it on if I know it