wuzak
Captain
Hey wuzak,
Sorry, I was not clear in my question. Moving the boost lever to +16 lbs (for example) at take-off, even if the system is not fitted with automatic boost, would (of course) maintain +16 lbs as long as the engine/supercharger rpms did not change and as long as the aircraft did not climb above FTH. Turning and such would not affect this.
If the engine does not have an automatic boost control system, the boost will fall as the aircraft climbs unless the pilot adjusts the throttle to maintain the boost.
The situation I was wondering about was for an engine/supercharger combination such as the Merlin III in the pre- and early-war period, I may be misremembering/misunderstanding what I have have read, but what I think I read in many of the pilot accounts was that they had to monitor their boost as they descended to prevent overboost. I got the impression that the early Merlins (for example) did not have any built in any bypass/vent ability for the compressed air coming from the supercharger. What I am thinking is that with the engine at Combat rating (say 3000 rpm and +6.75 lbs), if the aircraft descended to SL the effective boost could?/would? increase to somewhere around +19 lbs at SL if 3000 rpm was maintained and if the boost lever was not pulled back. So I am not referring to an overspeed condition, although the same problem could occur if I am thinking right. Yes/No? Clarification would be appreciated (from anyone) please.
Not sure if the early Merlins had automatic boost control.
Also, my understanding (which again could be wrong) is that pre- and early-war the engine/supercharger did not always have the ability to safely descend without the pilot pulling back on the boost in order to keep the IHP in the safe range. An example of this is the 2-speed Merlin XX in 1942, the pilot could set the boost to +16 lbs at altitudes above ~12,000 ft, but had to manually reduce the boost to a max of +14 lbs at lower altitudes in order to prevent detonation and/or overstressing the engine. I think the early 60 series engines had the same problem? Admittedly, this was due to the change in supercharger rpm, but still.
+16 was for FS (high gear) and +14 for MS (low gear). Not sure if there was a system which would restrict boost in low gear with the automatic boost controller.
Soon enough the 2 speed engines were rated the same in both gears.
I think maybe teh Merlin 61 and 63 had different boost allowance for MS and FS gears, but thereafter it was the same.
This may have been more to do with the increased power required to drive the supercharger at the same boost in FS, possibly too much for the drive shaft or gear sets.