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Old 17th Feb 2012, 16:16
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Lancman
 
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Boost pressure and manifold air pressure are two different names for exactly the same thing. American engine manufacturers favored MAP and the British favoured boost. Only the scaling on the gauges/gages differed. If you put a boost gauge and a MAP gage next to each other at sea level on an ISA standard day the boost gauge needle would point at 0 boost on the scale and the MAP needle to a shade below 30"Hg.

The early Merlins weren't fitted with constant speed propellors, they were fixed pitch. Later propellors could be selected to either of two fixed pitches any time that the engine was running. RPM varied as a function of engine power and TAS. If a take-off was attempted in coarse pitch the RPM, and thus engine power, would initially be kept low by the very high torque load on the engine but they would increase as the aircraft accelerated. If that Hurricane had had a much longer runway it would have got airborne eventually just as the Schneider Trophy seaplanes did. Incidentally, the Schneider Trophy racers, with their very coarse fixed pitch props, used to start their take-off runs at 90 degrees to their intended take-off direction just to allow for that torque.
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