Merlin engine limits & failure behaviour

[Viper2000]

Originally Posted by 41Sqn_Banks

According to AP 2095 Pilot's Notes General:
[...]
So sounds like this behaviour is correct.

If you read further in AP 2095, it says:

"NOTE - In Merlin engines the throttle valve cannot open fully unless the throttle lever is moved forward of the setting which gives maximum weak boost at the lowest altitudes. Therefore, at higher altitudes, the throttle must be set forward - up to the climbing position if necessary - in order to get the maximum permissible, or obtainable, boost for economical cruising."
[page 37]

The Spitfire I notes state that maximum weak cruising is 2600 rpm, +2.5 psi.

Therefore, at +4 psi, the throttle valve should be able to open fully and the boost should not fall until the FTH for +4 psi at 2600 rpm is reached.

Forward movement of the throttle lever at this point should make no difference because the throttle valve should be fully open, and therefore the only way to increase power output would be to increase engine rpm (which naturally increases supercharger rpm and thus supercharger pressure ratio).

///

klem, I think the consensus is that the Spitfire IIa gives +12 psi boost when the cutout is operated, but since the gauge only reads to +8, you only see +8. However, I note with interest that the "no cockpit" gauge does read to +12, and yet this also only displayed +8 last I checked, so it's not obvious whether or not this is really a bug or a feature.

The Spitfire I notes say that operating the cutout with 100 octane fuel should give +12 psi as well.

I don't know exactly what is supposed to happen with 87 octane for several reasons:

#1 the maximum permissible boost should be less, but
#2 prior to the boost control cutout mod, operating the cutout at low altitude would give more than +12; I'm not sure how much, but if the boost control was literally disabled then WOT at sea level would give about +25 psi, which would consume the overhaul life of an early Merlin in about 15 minutes flat (which they actually did with N17 using special fuel when attempting to chase the world speed record; output was about 2500 bhp, FTH obviously 0).

It is implied in AP 2095 page 41 that the early cutout did indeed literally disable the ABC. I bet that must have been fun...

Really we need to try to find an old set of Notes for the Spitfire I from the 87 octane days to work out what the pre-mod boost control cutout was actually for, given that climbing boost and maximum boost are the same in the Notes I have which came with the CE.

The Hurricane basically had the same engine, as the Spitfire, and therefore I wouldn't expect to see different boost figures.

FTH for +12 was about 10000' at 3000 rpm for early Merlins, and therefore you should be able to get as much boost as you're allowed at 8000.