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.

[klem]

Originally Posted by Viper2000

.....................

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:
...................
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.

Thanks for that.

My concern was that in three cases the boost didn't change at all (Spit1s and Hurricane DH 5-20) and the Rotol Hurricane boost barely moved.

Did I miss somewhere that these are 87 Octane and boost isn't effective?

[fruitbat]

Originally Posted by klem

Thanks for that.


Did I miss somewhere that these are 87 Octane and boost isn't effective?

only plane that appears to be 100 octane is the spitII, all the rest 87 octane

[Viper2000]

Merlin marks II, III, IV and V were covered by the R.M.1.S. rating, which in its final state was:

Combat power = 1310 bhp/3000 rpm/9000' at +12 psi

or

1440 bhp/3000 rpm/5500' at +16 psi (Sea Hurricane only)

Takeoff power = 880 bhp/3000 rpm/SL at +6¼ psi.

There was also an R.M.2.S. rating which allowed increased takeoff power for 100 octane fuel before the +12 psi combat rating for R.M.1.S was introduced:

Combat power = 1000 bhp/3000 rpm/15500' +6¼ psi. (provisional rating, not confirmed by type test)

Takeoff power = 1000 bhp/3000 rpm/SL at +8¼ psi.

[This rating is a bit strange; my suspicion is that the +8¼ psi may actually be the 87 octane combat rating with boost cutout engaged, because otherwise if everything was rated at +6¼ psi boost then there wouldn't have been any purpose in fitting the cutout before 100 octane fuel was used...]

Cruise and climb were the same for both ratings:

Cruise power = 2600 rpm +4½ psi
Climb power = 2600 rpm +6½ psi

Power isn't specified for either of these ratings; you'd get what you got!

The Merlin XII had a higher supercharger gear ratio (9.089 vs 8.58, and it therefore achieved a higher FTH (but less power due to the increased supercharger drive requirement). It was covered by the R.M.3.S. rating:

Combat power = 1280 bhp/3000 rpm/10500' at +12 psi

Takeoff power = 1175 bhp/3000 rpm/SL at +12½ psi.

Cruise power = 2650 rpm +7 psi
Climb power = 2850 rpm +9 psi

Data from The Merlin in Perspective - the Combat Years, by Alec Harvey-Bailey, 4th edition, Rolls-Royce Heritage Trust.