I've been wading through some old manuals as part of a private project looking at the Hurricane 1.
I understand MAP and CS props well enough on modern aeroplanes, but I'm struggling with how this maps to "Boost" as used on the Merlin engine. Can anybody explain what Boost actually is and is controlled - is it just an alternative term for MAP, or as I suspect something defined a bit differently?
At the same time, references to the early CS props on the Hurricane 1 / Merlin II combination refer to a "2 speed" prop, which suggests a CS prop with only two RPM settings - as opposed to the infinitely variable lever I'd see on a modern aeroplane. Is that correct, or is it more modern than I think, or even is it a 2-pitch (fine and coarse) prop such as is fitted on one or two modern motorgliders?
And if anybody knows the answers to those, I'm guessing that they might know the answer to my third question. At what point was the 2 blade fixed pitch prop finally withdrawn from the Hurricane fleet?
From someone's book - cannot remember which - early prop was 2-speed - coarse and fine - with a lever in the cockpit - I have a feeling it was DRSB who crashed an aircraft on take-off - only to have lever point at him like an accusing finger as it was still in "coarse" whereas it should have been in "fine" for take-off
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.
I am getting old. I mixed up lbs/inch2 with inches of mercury. LM I did train on a supercharged radial engined aircraft and also a supercharged radial engined helicopter. Same engine, different gauges.
Last edited by Fareastdriver; 17th Feb 2012 at 12:50.
So in a nutshell - Boost, normally expressed in in.Hg, is MAP minus (MAP at a nominal cruise power setting). So idle is something like -8 Boost, and combat power was around +10 boost, expressed in in.Hg.
It was used, as MAP is, for setting power in general preference to using RPM. The Hurricane 1 did not have a constant speed prop - the earliest models had a fixed pitch prop, the later models had a variable-fixed pitch prop with a choice of two or three pitch settings. So a pitch setting was made, then the powerplant was controlled on the throttle, primarily by reference to boost pressure, rather than RPM.
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.
we are getting close but not quite there yet, re-read my second paragraph.
"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."
The usual units for boost is psi and for MAP is In Hg. Think Fahrenheit and Celsius; two systems for measuring temperature but each with a different datum point and measured in units of different increments. Two different systems of measuring the same thing.