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What was the average life expectancy of a R-2800 engine during 1941?
The vast majority of those engines were the "A" series and the most common model is the -5.
1850hp at take-off 2600rpm and 49in of boost. Military ratings of 1850hp/2,600rpm/2,700ft and 1,500hp/2,600rpm/14,000ft. "normal" (max continuous) 1500hp/2400rpm/7,500ft and 1450hp/2400rpm/13,000ft.
weight 2270-2300lbs and a frontal area of 14.8sq ft.
it might have 1280hp at 20,000ft no RAM.
Compare to your favorite engine of the second half of 1941?
What's the point? USAAF needed a good high altitude fighter in the ETO, I don't think anything with a single stage engine would've cut it. If the fighter is big enough for a two-stage R-2800 and all its accompanying hardware all you have is a redundant competitor to the P-47, and in the meantime it would've probably shared the fate of the P-40--durable and good for low to medium altitude and ground attack, not much else.
USAAF in 1941-mid 1943 did not wanted an high altitude fighter in the ETO, and the one they had was a problematic one. Single stage R-2800 has 20% more power than the single stage, 2 speed Merlin (I admit, more weight drag)
What's the point? USAAF needed a good high altitude fighter in the ETO, I don't think anything with a single stage engine would've cut it.
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Then, the USAAF in 1941-43 (and beyond, of course) needed a plane that would threat IJA/IJN opposition, and a Hayate-sized plane with R-2800 would've fit there just fine. Second area of interest is the MTO, not much of high flying duties there prior 1943, but plenty of under-20000 ft action. Just for such a fighter.
Ju-87D routinely carried 1,000 kg. Powered by 1,340 hp Jumo 211 V12 engine.
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You are not going to get a R-2800 into a Hayate sized airplane. The Homare engine was 2190 cu in (Wiki is wrong, figure the displacement yourself using the bore and stroke) but the engine was compact, about 6in smaller in diameter (smaller than an R-1830) with a frontal area of 11.8ft. even more importantly it was about 400lbs lighter than an R-2800-5. The R-2800 is about 21% heavier than the Homare.
You also have the US armament problem. A Hayate carried about 291KG worth of guns and ammo (gun accessories, ammo boxes and links not counted) A P-40E carried about 332 kg and a P-51D carried 385kg. Unless you use the R-2800 to carry just four guns you wind up with 30-90kg more in gun/ammo weight depending on length of firing time desired.
You need 12-20 more square feet of wing just for the dry weight of the engine and guns/ammo to keep the same wing loading of the Hayate let alone any additional structure needed for the larger, heavier engine and armament.
Hayate carried 697 liters (?) 184 US gallons of internal fuel. All but 217 liters in the wings. Bigger fuselage of the R-2800 plane may house fuel ( and no fuselage guns) displaced from wings by US wing gun installation.
Did the Japanese build to the same 12 "G" limit as the Americans?
The Grumman F8F-1 went 9,600lbs with 185 gallons of fuel, I don't think you are going to get much lighter than that. An R-2800 single stage can suck down 60-63 gallons of furl per hour at 1650-1700rpm and 33in MAP at 0-5000ft cruise. Max continuous can be as high 198GPH at 2400rpm and 42in MAP at 5000ft.
Hmm, what's your take about the P-51 sized plane that has 1-stage R-2800, compared with, say, historical P-51D? Maybe the wing area of some 250 sq ft? 6 guns, 350 rpg? I'm not trying to have a plane doing 430 mph, but 400 seem within reach?