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The fastest Mustang----XP51G, with a 2200HP engine, with rotel five blade wood propeller.
The XP-51G was a development aircraft that combined the light weight airframe developed for the XP-51F with an experimental Rolls Royce RM-14SM engine, capable of producing 2,000hp at 20,000 feet. The new aircraft achieved a top speed of 495 mph, and a climb rate of 5,000 feet per minute, well over 1,000 feet per minute faster than the P-51D. However, the new Rolls Royce engine was too complex and did not always produce its best power.
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Flight Tests on the North American
P-51D Airplane, AAF No. 44-15342
Summary
Clean Configuration (with bomb racks).
1. Maximum speed at critical altitudes
High Blower
War Emergency power (3000 RPM and 67") 26000' 442 MPH
Military power (3000 RPM and 61") 28000' 439 MPH
Normal Rated power (2700 RPM and 46") 29400' 420 MPH
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XP-51G max level speed=495 mph, P-51D=442 mph.
If everything being equal, XP-51G need (495/442)^2= 125% engine thrust of P51D. Obviously, 2200HP Rolls Royce RM-14SM is around 125% output of 3000 RPM and 67" stanard Merlin(1760HP at altitude).
Roughly, we can say Britain Rotel 5-balde wood propeller is as effective as those 3-4 blade CSP at speed envelop. Namely, around 80% efficiency at 495mph=800km/h TAS.
It seems that allied believed in wwii that 4 or 5 blade propellers are better than 3-blade when speed is high(>700km/h?).