PPRuNe Forums - View Single Post - Boost (Hurricane 1 and Merlin II/III)
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Old 16th Feb 2012, 20:08
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3 Point
 
Join Date: Oct 2000
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Nope, zero boost isn't idle power, far from it. Zero boost is almost enough to tip a Spitfire or similar on it's nose while doing the run up to check mags. Zero boost would be a typical cruise power setting. Idle power would be off the scale, somewhere below -8 boost.

Zero boost is static atmospheric pressure; often referred to as "static boost" ie 29-30" MAP. They are both indications of the pressure in the inlet manifold and therefore are "the same" in a sense however, MAP uses absolute zero pressure as its datum and (usually) In Hg as the units, "boost" uses the prevailing atmospheric pressure as its datum and (usually) psi as its units.

Positive boost pressures are seen on a forced induction engine (Supercharged or turbocharged) and equate to MAP figures above 29"Hg.

Boost certainly serves a purpose as it allows you to set desired engine power at various RPMs (or with two speed VP (ie not CS) props). I have only known one aeroplane with a fixed pitch prop and a boost gauge. This was an engine which was supercharged to allow it to maintain sea level power output as the altitude increased; when you got high enough such that full throttle gave you zero boost that was effectively full throttle height; any further climb and you would start to lose power.
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