A few thoughts.
Firstly, 'survived heavy damage' means, by definition, 'was hit'. If hit by ground fire, this would almost certainly be from the front. Radials offer a large target area. The ability to fly with a cylinder shot off is rather impressive, but if a liquid cooled engine, with it's smaller profile, would have avoided the damage completely it wouldn't have to worry about it's vulnerable cooling system in the first instance.
The placement of that cooling system is therefore a major concern to the designer. Buried in the wings, away from the point of aim, may offer some protection. Sticking it behind the propeller, where incoming fire would already have inflicted heavy damage on the fighter, is more fatalistic. Placing it deep in the fuselage, surrounded by armour, is almost certain to defend it from most ground fire.
The ability to armour the engine more effectively in a liquid cooled power plant cannot be ignored. The cost, in weight, must be considered, but liquid cooled engines tended to offer more power, compensating somewhat.
So the question can be answered as either 'the liquid cooled engine is delicate, therefore more vulnerable' or 'the air cooled engine is bigger, therefore more vulnerable'.
Some stats, to help out. Area/hp for Merlin 1200sq in/1500hp; R-1830 1809sq in/1200hp; Napier Sabre 1840sq in/2400hp; R-2800 2190sq in/2100hp.
Between July 25 and August 7, 1944 the 9th AF lost 80 aircraft, the majority Thunderbolts. Over the whole of those two months the 2nd TAF lost 151 Typhoons. The majority of losses to both were to ground fire. Typhoons and Thunderbolts were operating under very similar conditions, the Hawkers in direct support, the P-47s in close support, with considerable cross over in roles. During the Normandy operations the 9th AF flew 2891 sorties, the 2nd TAF 9896. The 9th AF claimed 2654 vehicles destroyed, the 2nd TAF 3597. The 2nd TAF claimed 257 of the 391 tanks destroyed by Allied aircraft, reflecting the slight difference in tasking between the two forces.
It's the closest example of air vs liquid cooled propulsion I can think of. Typhoon losses ran at about twice those of Thunderbolt equipped units, but they mounted three times the sorties. Just as well, since they made one third of the claims per sortie. It's possible to cut the figures to demonstrate various points- I fully expect Duncan and Lightning to do just that! - but I think they're close enough to refute claims of any superiority of either power plant.
I haven't mentioned air to air losses for a reason. There are a multitude of factors beyond the survivability of a single aircraft component involved in the loss of an aircraft. For example, late war Japanese fighters were much better protected than earlier models, but they suffered far greater attrition.
The figures above are largely derived from this pamphlet: http://www.ibiblio.o...DDay/index.html
I was going to use a Brit source, but then I remembered I may have let slip that I'm a bit of a Hawker fan! I'm pretty sure Richard Hallion holds that air cooled engines are tougher, amongst other things. Any primary documentation countering anything I've scrawled above will be gratefully received. Fire away, guys...
Edited by Flo, 21 March 2012 - 01:52 PM.