Quote:
Originally Posted by JG53Frankyboy
so, once upon a time, the developers decided what fuel is in the tanks of the different british fighters there should be no "boost control cut out" possibility in versions with 87octon fuel ?
or should there "only" a very fast engine damage if you engage the cut out and move your throttle to 110% than ?
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Really "110% throttle" doesn't mean much in this context, because there was no gate on the throttle (gated throttles arrived later).
It's better to think about PLA (Power Lever Angle).
If we assume that PLA goes from 0-100, and that WOT operation of the Merlin III at sea level would produce +17 psi, that's 31.7 psi absolute.
Max rated boost for 87 octane = 6¼ psi = 20.95 psi absolute.
This would be about 66 PLA if we make the simplifying assumption that PLA maps directly to boost, and that 0 PLA maps to a stopped engine (ie
nominal 0 manifold pressure).
Of course, reality is a bit more complex; the throttle valve is a flat plate which is turned through about 90º to go from idle to WOT. The minimum area therefore varies as the sine of the throttle valve angle. But the throttle can't close completely because otherwise this would just completely stop the engine. So actually 0 PLA will map to some non-zero absolute manifold pressure.
This all means that with the ABC cutout operated it's not immediately obvious that PLA would map directly to boost.
So far as I can piece together from the various imperfect sources available, the ABC for the Spitfire I at the end of the 87 octane era was set such that 100 PLA would give +6¼ psi boost, and some considerably lower PLA, perhaps as low as 66, would give this boost at sea level with the cutout operated.
If you climbed with the cutout operated at 3000 rpm then you'd reach 100 PLA at the FTH for +6¼ psi boost, which was about 15500'.
If you climbed at some lower rpm, say 2600, then the FTH would be lower, and so you'd reach 100 PLA sooner.
Later engines were designed for operation with gated throttles, and so there would be a good argument for thinking in terms of a PLA>100 or 110% throttle in 1C's lexicon.
Note that in the above discussion, PLA is
deliberately left as a somewhat vague, dimensionless and therefore unit-less term.
The existence of the R.M.2.S.M. rating with a +8¼ psi takeoff rating on 100 octane fuel prior to the clearance of the +12 psi combat rating suggests that before the boost control cutout modifications were embodied, a margin was required to protect the pilot against accidental overboosting, which is easy to understand given that +8¼ psi would be about 72 PLA, whilst +12 psi would be about 84 PLA.
The combat rating remained +6¼ psi, presumably because in the heat of battle the micro-management required to keep boost within limits was considered disproportionate to the performance gain available, and it was therefore better for the pilot to rely on automation to protect his engine from immediate harm whilst he concentrated on the enemy.
The boost control cutout modification meant that 100 PLA with the cutout activated would deliver +12 psi boost up to the +12 psi FTH.
Note however that the Merlin XII's takeoff rating was +12½ psi with a +12 psi combat rating, so that engine at least must have been capable of slightly exceeding +12 psi at 100 PLA with the cutout activated.
I suspect that the cutout would have been set to give a maximum of perhaps 12.75-13 psi boost at sea level, 3000 rpm with 100 PLA, and then the pilot would have used slightly less than 100 PLA to get the official +12 psi rating, which would probably have gone out of the window in actual combat.
Whether this would have been the case with the Merlin III as well I don't know. It seems likely to me that in reality the mod would have been arranged so that 100 PLA would give slightly more than +12 psi boost in order to allow for engine deterioration, but this sort of thing is quite hard to trace through in a satisfactory manner.
As for what would happen if the cutout was operated with 87 octane fuel, or with 100 octane fuel prior to the +12 psi modification, I would expect that a couple of psi beyond the cleared maximum boost, detonation would start. This would be worse at high ambient temperatures and/or low rpm.
Once detonation starts, I would expect failure to follow within a minute or two, depending upon severity.
Equally, I would imagine that if you operated the cutout and pushed to +8 on 87 octane at 3000 rpm for 30 seconds or something that you'd more than likely get away with it, though the engine life would be reduced.
Although there were engine mods in preparation for the use of 100 octane fuel, I get the impression that in 1940 they lagged behind the increases in load that the engine was suffering; one of the sources I cited talks about failure in just under 50 hours of 20 minutes off, 5 minutes on +12 operations, which is of course about 10 hours at +12. To put this into context, the theoretical life of a fighter Merlin in 1939 was 240 hours.
So very roughly, caning the poor thing at +12 has killed it in about 1/5th of its nominal life.
As such, I would expect that the engine would be less tolerant of overboosting beyond +12 on 100 octane than it would be of overboosting beyond +6¼ psi on 87 octane.