Merlin engined Whirlwind

The p-38 never had leading edge radiators. It ducted the turbo-charging through a duct in the leading edge as a substitute for an intercooler. The coolant radiators were lumpy boxes in the booms. The un-turbo same rotation C-15 Allisons of the British lightnings were bad with any fuel.

In regards to the shortage of merlins, many were wasted on Defiants and such. Beaufighters were fitted with Merlins against a shortage of Hercules engines. and so on.

The Gloster twin-engined fighter had Bristol Taurus 1,050 hp engines for 360 mph, and was re-engined with Peregrine 885 hp for 330 mph. Taurus on Whirlwind, or dare I say, Hercules, would have been an improvement.
 
In regards to the shortage of merlins, many were wasted on Defiants and such. Beaufighters were fitted with Merlins against a shortage of Hercules engines. and so on.

The Gloster twin-engined fighter had Bristol Taurus 1,050 hp engines for 360 mph, and was re-engined with Peregrine 885 hp for 330 mph. Taurus on Whirlwind, or dare I say, Hercules, would have been an improvement.

Merlins were the bottleneck for production of fighters. See f.ex. McKinstry's "Spitfire" for more on that IIRC.

If I understand you correctly, you're making the argument that Merlins could be found, considering that a less successfull fighter such as the Defiant was built using them. I don't think that argument is valid, because;

a) the Defiant was available earlier then a RR Peregrine-engined Whirlwind (which is already delayed), let alone one with Merlins. The Peregrine-engined Whirlwind wasn't in operational service in meaningfull numbers untill late '40. OTOH the Defiant is up and running by March '40. Those few months would seem important if your country is hurrying to rearm for an approaching worldwar.

b) it's thanks to hindsight that we know that the Defiant wasn't exactly a warwinner. But before WWII broke out, there was much doubt about the value of fighters with wing-mounted guns. Even Churchill thought turreted fighters such as the Defiant were the way of the future!
Secondly, nobody could have foreseen the fall of France in 1940. The Defiant was designed to attack German unescorted bombers, which was logical considering the range from Germany itself to Britain.

c) there were 'only' about a thousand Defiant's built. Britain has about half a dozen types of strategic bombers of which at least a thousand were built, of which most people know pretty much zilch. Bottom point; 1000 Merlins isn't that much (although it would be a good start).

d) the Beaufighter MK II was fitted with Merlins, which wasn't in operational service in meaningfull numbers untill (May?) 1941. I'd assume the bottleneck of Merlins being a bit less by then. The Beaufighter getting Merlins also had something to do with production of the Hercules being even more of an issue then Merlins AFAIK.
I guess you could replace Beaufighters with Whirlwinds instead, but I doubt the Whirlwind can replace all the tasks the Beaufighter can do, although there are definitely area's where the Whirlwind excels over the Beaufighter.
Who's f.ex. going to do the over the water/long range navigation in the Whirlwind? In the RAF pilots aren't trained for that IIRC; that's the job of navigators. Where are you going to put the radar? etc
 
At what point did Packard built Merlin engines become available? That's what largely solved the engine bottle neck and allowed experiments such as putting Merlin engines in the American P-51B Mustang.
 
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