Don’t forget the USSR. A lot of these planes were sent as Lend-Lease to the Soviets (who apparently loved them, I guess it was that 37 mm cannon in the prop spinners that did the trick for them).
Web pages on the P-39 state that about 4500 were delivered to the Soviets. There was little or no high altitude aerial combat in the east so the P-39’s lack of supercharging and consequent lousy performance above about 10-12000 ft. didn’t matter. And the cannon made it a good anti-tank weapon.
Sam? I think you’ll find that the Packard V12 Merlin was pumping out well over 2,400 hp by 1943 or later. But probably only maximum throttle. Certainly, one thing’s for sure - all things being equal, an engine which is water cooled will produce greater horsepower per given displacement than an air cooled one. It’s one of the reasons that motorcycle engines all went water cooled in the mid 80’s - the greater heat dissipation capacity allows greater fuel/air input I’m told.
But as you and David Simmons both pointed out, the single greatest advantage to an inline V12 was frontal area - and obviously at 200+mph aerodynamic drag becomes a major issue.
Also, my research last night on the F4U Corsair indicated that it used a 13 foot 4 inch prop. I didn’t read anything about a 22 foot prop anywhere in the various sites I browsed through.
Another aspect about why the V12 aircraft were slippery and fast was a rather subtle aspect - namely, exhaust scavenging. You can see those 6 exhaust snouts per fuselage side and it’s obvious that the passing wind would have scavenged the exhaust far more efficiently than the radial engines. And as you know, good exhaust scavenging makes for either better horsepower, or better fuel efficiency, or both.
I believe that the Merlin V12 used gearing to regulate the final prop RPM - I very much doubt that the prop spun any faster than the Corsair. Still, it’s worth doing the research on.
My favourite ever story regarding a P-47 Thunderbolt was some US Airman returning from a sortie in Germany and he was chased all the way home by a ME262 which had run out of it’s 30 mm cannon, but still had a wing full of ammo. The German pilot just followed and strafed, followed and strafed - from every possible angle - and such was the toughness of the pilot’s cockpit armour and the plane in general that the German pilot finally emptied his ammo and the Thunderbolt kept flying.
Somewhat bemused and out of ammo, the German pilot flew alongside of the American and caught his attention, and then saluted. Smiles were exchanged and the German pulled away to leave the American to fly home in safety.
From this page. In other words 49 % and change of the production went to the Soviets ! About the supercharging, I thought that it was only the export version (P-400) who didn’t have it. I seem to recall a book on Guadalcanal (don’t remember exactly which one) which specifically mentioned this point.
No, the p-39 pilots on Guadalcanal often had to serve as low alititude bait for Japanese fighters, because they couldn’t climb high or fast enough to intercept incoming attacks.
Martin Caidin has an excellent book about the first year of the Pacific war, when the Japanese, by and large, had complete air superiority wherever the Zero appeared. The book is called *The Ragged, Rugged Warriors".
Sure. But then, Horsepower on the big radials went up, too. I think there was a Corsair making 2,800 horsepower, if I recall correctly. The point is that the radial makes it at lower RPM.
But the limiting factor appears to have been Mach limits, and not frontal area. Note that the P-47 was faster than the Mustang. Also note that even today, one of the fastest planes is ‘Rare Bear’, a variant on the P-47. The difference is that the P-47 would suck more gas to go that fast. Which is one reason why the Mustang had such great range.
Hmm… Maybe I got that confused with another aircraft.
Yeah, but the Mustang also needed that draggy belly scoop for the radiator.
If it did, it would still have added more weight and complexity.
I wasn’t trying to start a, “Which aircraft is better” debate - certainly the water cooled engines had some major advantages of their own. It’s all about design tradeoffs, like almost everything else in aviation.
I suspect that you are recalling the incident that Robert S. Johson narrated in is book Thunderbolt!. However, the plane was a FW-190 rather earlier in the war. The ME-262 had “only” four 30 mm nose cannons with no wing guns. It also lacked the range to chase an allied fighter for any distance. (Although with its speed and armament, it did not actually need to be able to go far; it could catch and destroy anything quickly, although a fighter in working order had several maneuvers available to avoid being shot down, even if it could not, in turn, catch the ME-262.)
OK I checked my copy of The Ragged, Rugged Warriors (you’re right it is a good book), as well as Richard B. Frank’s Guadalcanal and in both cases they don’t mention the fact that the P-400 was lacking a supercharger because it was an export model, although they do mention that it lacked high-pressure oxygen because it was fitted according to British spec. I must have gotten the two confused, sorry, it’s been a few years since I read these books. I did remember there was a difference in the export version, just the wrong difference. And BTW, on 'Canal, they were flying P-400s, not P-39s. Small difference, I agree, but still one.
All engines, except marine engines that circulate sea or lake water, are ultimately air-cooled. The only difference that I can see between direct air and water moderated air cooling is that the latter results in a more uniform temperature throughout the engine. And I don’t see that as any world beater of a difference.
And, come to think of it, marine engines are also air cooled. Their heat is dumped into the water which heats that up which means more heat from the water is put into the atmosphere which raised its temperature causing an increase in radiation into space - we hope.
Well… There is a difference in drag requirements, because engineers have a lot more options for getting cooling air into a water-cooled engine. NACA ducts, etc. With a radial, it’s pretty much “Push a lot of air past those big fins on the pistons”. I would guess that for an equivalent horsepower, a well designed water cooled engine would require less cooling drag than a radial.
But notice that once the war engine, liquid cooled aircraft engines pretty much vanished, while the radials soldiered on, making it onto production aircraft well into the 1970’s (and even a handful today).
I’m sure there are a few liquid cooled aircraft engines in use, especially in the homebuilt field where auto conversions are popular. But radials stayed in use in a big way. For instance, the Douglas Skyraider used a radial in Vietnam.
This line of thinking just made me realize that I made a mistake in saying that “Rare Bear” was a P-47. It was a Bearcat, hence the name. Don’t know how I got that confused. Still a radial engine powered racer, though. Bearcats are one of the most competitive airframe designs on the unlimited racing circuit.
Well, yes and no. The VW Beetle with its flat, opposed four gets sufficient air without any more frontal area than cars with radiators. Modern light plane engines don’t have an excessive frontal area. It’s the radial layout that increases the frontal area. I suppose an inline V12, or flat 12, aircooled engine could be built. After all, the question basically is heat transfer to the air and I’m not sure that the transfer via radiator fins is any better that transfer through direct cylinder fins. I suppose it depends upon the area of fin that you can get exposed to the air in the two cases. The air cooled radial does run hot. Normal cylinder head temperature of the R-2800 on the Martin B-26 was 300[sup]o[/sup]C. Water temperature in auto engines these days runs about 85[sup]o[/sup]C. I have no idea what equivalent cylinder head temperature that gives. Probably close though because the coolant circulates through the head and the conduction to the coolant is probably pretty good.
P-400’s were Aircobras completed according to British spec’s and intended for use by “colonial” troops, e.g., South Africa, ANZAC, etc., in outlying theaters. The USAAF got them in early 1942 when some of those units intended for export were taken over as an interim measure until American production of first-line models could catch up. See, for example, this photo or here. So your original statement is close to correct.
A radial engine lends itself more to simple air cooling than an inline. With a radial the back cylinders usually have only one other row of cylinders between then and the cool air coming into the cowl and never more than three. With a long inline or V engine the back cylinders might tend to overhead. There have been air cooled V-12 engines but AFAIK only used in Franklin automobiles and not aircraft.
The VW engine has a tall fan shroud that would add a lot of frontal area if it was used in an aircaft application. The shroud also contains an oil cooler which is a significant part of the engine’s cooling capacity.
Also FWIW the Rolls Royce Merlin engine was only built under license by Packard. The original P-51A and A-37 used an Allison engine which was much less powerful. It’s easy to tell one from a P-51B/C because of the three bladed prop and flared exhaust stacks of the Allison engine.
As for the “draggy” scoop of the P-51 it wasn’t a bug, it was a feature. 
Engine heat added to the air flowing through actually added thrust.
http://www.boeing.com/news/feature/atwood/atwood3.html
I guess the original question has been answered so side issues won’t hurt much now. Although my guess as to why the US Navy had radial engines is that the manufacturers with whom it dealt all used radials, knew how to design with them, and had existing designs that could be updated. Inertia in other words.
As far as cooling back cylinders in a long aircooled engine, side air scoops and ducting would get air to all cylinders and wouldn’t be any more complex that a liquid cooling hookup to a radiator located way in the rear of the plane. And as far as the drag added by the side scoops, if engine heat added to the air flowing through the P-51 radiator added to thrust in that case then it would do the same for an air cooled engine and make up for the drag added by the side scoops.
And as for the alleged superiority of the P-51 spoken of in the cite, the 51 wasn’t all that superior. It was an excellent fighter but an awful lot of them were also shot down. As has been pointed out, its main value to the 8th AF was that it could carry good performance in defending bombers over a long range and make deep, daylight penetrations of defended air space sustainable.
Not neccessarily. Airflow immediately behind the propellor isn’t the same as the smoother, high pressure flow beneath the wings.
My favorite plane is the P-51 but I don’t believe in the “my plane is superior to your plane” debate as it’s too one dimensional.
On a side note I saw the demise of one of the rare Goodyear built F2G racers, Super Corsair at the '94 Phoenix air races. I got a good look at the monster 28 cylinder R-4360 engine in the pits. It’s so long compared to ordinary radials they call it the corncob. Unfortunately the big engine caught fire during a race and pilot Kevin Eldridge had to bail out. IIRC he broke a leg hitting the tail but was otherwise okay as as the plane went down in flames.
I’m not sure that the R-4360 would have been a success in routine operations. Reliable simplicity seems to me to be a lot more valuable than the ultimate in performance in operational situations. It looks like as airplanes got bigger the reciprocating engine was reaching its limit. Designers were starting to have to put 6 engines on and think about using 8. More powerful engines that would be suitable for aircraft use didn’t seem practical.
Another side note. As I recall, the Wright engines on the B-29 were a continual problem because of overheating. Overheating would be a real headache, and Wright engines were enough of a headache without that.
With respect kind sir, the greater advantage that a water cooled engine has over an air cooled engine is somewhat dependant upon the law of relative temperature differences. As you noted earlier David, the heads on the B-26 engine hovered at 300C after a long flight - and this in turn develops the fuel air mix to expand demonstrably due to latent heat within the inlet stage.
With a water cooled engine, a surrounding water jacket can be maintained at a much lower temperature than the much less efficient air cooled fins. As such, the ability for the engine to run at a lower temperature is provided because the water pumping can be regulated to run off through a really huge radiator which is very efficiently designed to dissipate heat in THAT part of the cooling process.
The goal being ultimately to keep the heads, and inlet manifold as cool as possible so as to shove as much air and fuel as possible into those giant cylinder bores. That’s really where water cooling earns it’s stripes - keeping the heads and inlet manifolds much cooler - and that’s a part of the engine which has traditionally not been easy to cool via air cooling. Secondarily to that is keeping the block cool too.
However, I’ll happily concede that the Americans have always erred on the side of big capacity laziness in engines compared to highly stressed smaller engines. Ergo, if weight wasn’t too much of an issue, then why not make a 3000 cubic inch radial engine which develops the same horspower as a 2000 cubic inch water cooled engine?
And that’s really the issue here it seems to me. In the early days of 1939 and 1940, the original Spitfire was an amazingly light plane - I’m pretty sure about 6,000 pounds dry. As a result, it had great rates of climb and great speed etc. But it was fragile - no doubts.
The Americans later showed that a big capacity air cooled radial developed the same horsepower but in a much more reliable and lazy manner. We’re seeing differing philosophys here to getting the same job done it seems to me.
Ah yes. You’re taking me back over 50 years and I now remember a phrase called “volumetric efficiency” which is what I think you are talking about. Keep the intakes cool and it helps a lot in power output for a given size as I remember it. That’s one reason why good superchargers have intercoolers.
Or am I remembering wrongly?
In any case, thanks for the little lesson and reminder.
While the Navy never adopted the Airacobra, they did look at (and then dismiss) a tail dragger version that Bell submitted, the XFL-1. You can see a photo here:
http://www.daveswarbirds.com/usplanes/aircraft/aibonita.htm