Quote:
Originally Posted by 41Sqn_Banks
According to AP 2095 Pilot's Notes General:
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So sounds like this behaviour is correct.
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Presumably this is a function of the mod state of the engine/ABC combination. What date is your copy of AP2095? Also, what throttle opening are we talking about? +4 is quite a lot, and the passage you quote clearly implies that the ABC becomes double acting once the throttle is opened beyond a certain point; you could equally perform the test I suggest at say +6 and you'd get pretty much the same behaviour; I just picked +4 to make it obvious that the engine wasn't being stressed beyond its limits.
Quote:
Originally Posted by 41Sqn_Banks
AP 1590B Merlin II and III Aero-Engines:
[...]
So a Merlin that wasn't modified for +12 boost the boost control cut-out will disable the boost control. A modified engine however will not disable the boost control but increase the allowed maximum boost to +12.
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The mod was to drill holes in the valve such that the maximum boost comes out at +12; but the ABC isn't regulating in the normal way any more once the cutout is operated. AFAIK the eventual long term solution was to set up a gated throttle and keep the ABC in all the time, because drilling holes is a one shot deal which makes it very difficult to subsequently change the combat rating of the engine.
In any case, the fact remains that operation of the boost control cutout cannot increase the FTH of the engine. Once the throttle valve is open all the way, it cannot open further; there is no such thing as 110% throttle opening in a physical sense (though you can denote some arbitrary rating as 100% and then have some higher rating that you call 110% if you want, the fact remains that you cannot physically open a valve more than 100%).
Supercharger pressure ratio varies as rpm the square of its tipspeed. The FTH for +12 is given as 10500 feet or so. Ambient pressure at 10500 feet is 68.354 kPa. +12 psi is about 184.039 kPa. Supercharger pressure ratio is therefore about 2.69.
Meanwhile, at 17000 feet, ambient pressure is 52.752 kPa. Multiply by 2.69 and we get 147.282. Knock off the 101.325 kPa datum and we have +6.67 psi boost.
So I probably should have picked 10000' for the +12 FTH. You get the point though? The supercharger just sits there producing a pressure ratio of about 2.7 at 3000 engine rpm, and the ABC operates the throttle valve to control boost. The actual FTH you get is a function of the boost you ask for, but the FTH for any given boost level is set by the supercharger and ambient conditions, not the ABC.