Effect of boost control cutout prior to +12 psi boost modifications

[Redroach]

hmm could you please provide a short explanation for the not-so-engine-savvy on why you have concluded that currently, RAF engines are running on 87 octane fuel? I didn't get it so far.

However, I agree that if that conclusion is correct, the FM should be changed to 100 octane across-the-board, since I don't believe that 87 octane was in front-line use after may 10th 1940.
Maybe the Tiger Moth used up remnants, though

[Viper2000]

Because operation of the boost control cutout with 100 octane fuel should allow +12 psi boost at 3000 rpm, which should give about 1100 bhp at sea level, rising to 1310 bhp at 9000'.

At present, the Spitfire I can only achieve the 87 octane rating of +6¼ psi boost at 3000 rpm which should give 880 bhp at sea level, rising to 1000 bhp at 15500'.

As I explain in this thread, operating the cutout should produce roughly +17 psi boost with 87 octane if the power lever is fully advanced, leading to rapid engine failure. At present, operating the cutout doesn't do much unless you're flying a Spitfire II.

The Spitfire II can get about +8 psi with the cutout operated, which would give 1000 bhp at sea level. However, it gets the sort of performance I would expect from a Merlin XII at +12 psi boost.

It's been a while since I tested the Hurricane. However, since it also has the Merlin III, logically the Hurricane I and Spitfire I should exhibit identical engine behaviour in all respects other than coolant temperatures (since they have different radiators).

[Redroach]

I still don't get what the problem is, sorry. Your argument about the Boost cut-out reads somehow self-fulfilling to me... you don't drill the little hole and you get 17psi, therefore, detonation will occur quickly with 87 oct. fuel. However, I've yet to see an engine detonate in any condition and moreover, why assume that the boost cut-out is modelled without the pressure bleeding hole? Strange. Furthermore, the way I read it, 6 1/4 psi is connected with 100 oct. fuel (Spit I pilot notes mention "100oct. only for operational units, 87 oct. fuel for 'other units'")
Soooo... I've yet to test the boost cut-out but in case everyone fails to reach 12 psi with boost cut-out engaged, wouldn't it be the right side of occam's razor if you just assume the boost cut-out is bugged, instead of assuming that every plane runs on 87 oct.?
I've just tested the spit I in level flight and tried to go all-out without engaging emergency power. With the radiator as closed as I felt comfortable with (3 'ticks' before closed, iirc), I went along just shy of 250mph at sea-level. Which I feel is pretty spot-on with the diagram someone posted earlier (it says 246mph on sea level, though that's on lean mixture, apparently, and I flew on fat mixture). Since I further assume that this diagram was made considering 100oct. fuel (it just makes no sense otherwise), my bet would be on 100oct. fuel and perhaps a non-working (buggy?) emergency power.

[Viper2000]

Originally Posted by Redroach

I still don't get what the problem is, sorry. Your argument about the Boost cut-out reads somehow self-fulfilling to me... you don't drill the little hole and you get 17psi, therefore, detonation will occur quickly with 87 oct. fuel. However, I've yet to see an engine detonate in any condition and moreover, why assume that the boost cut-out is modelled without the pressure bleeding hole? Strange.

Did you read read the documents I linked to in the first post?

The boost control cutout mod is part of the process of clearing the engine for +12 psi combat rating.

If you're running on 87 octane and therefore are not cleared to use +12, why would you go to the trouble of modifying the ABC?

As for not seeing detonation, we don't get a message about it. That doesn't mean that it isn't being modelled implicitly or explicitly as the root cause of engine failures.

In any case, that's another debate. The point which I am trying to make is that the effect of operating the cutout is not correctly simulated at present, and that the evidence suggests that the effect of the cutout should change depending upon whether or not the engine has been modified for the +12 combat rating. Since we currently appear to have 87 octane fuel and +6¼ combat power in the Spitfire I, we might as well have the model finished so that it behaves in accordance with the data.

Then (after what I'm guessing would be a minimal amount of work) we should move towards getting the correct engine ratings for the Battle, ie those associated with 100 octane fuel rather than 87 octane fuel.

Originally Posted by Redroach

Furthermore, the way I read it, 6 1/4 psi is connected with 100 oct. fuel (Spit I pilot notes mention "100oct. only for operational units, 87 oct. fuel for 'other units'")

The combat concessions for 100 octane fuel are in section (vii) on the next page. Otherwise the ratings given are for 87 octane.

Originally Posted by Redroach

Soooo... I've yet to test the boost cut-out but in case everyone fails to reach 12 psi with boost cut-out engaged, wouldn't it be the right side of occam's razor if you just assume the boost cut-out is bugged, instead of assuming that every plane runs on 87 oct.?
I've just tested the spit I in level flight and tried to go all-out without engaging emergency power. With the radiator as closed as I felt comfortable with (3 'ticks' before closed, iirc), I went along just shy of 250mph at sea-level. Which I feel is pretty spot-on with the diagram someone posted earlier (it says 246mph on sea level, though that's on lean mixture, apparently, and I flew on fat mixture). Since I further assume that this diagram was made considering 100oct. fuel (it just makes no sense otherwise), my bet would be on 100oct. fuel and perhaps a non-working (buggy?) emergency power.

RAE speed data, complete with your 246 mph TAS in weak mixture:


Note that +12 is substantially faster.

Indeed, +12 for Merlin III at sea level will be faster than +12 for Merlin XII at sea level since the latter has a higher supercharger gear ratio.

I'm somewhat disappointed with this graph really, because it's dated 1941 and has no data points for the pencil lines at +12 and +16. I know that +16 was eventually only used in the Sea Hurricane (for which engine life was not an issue) . However, it's better than a poke in the eye with a sharp stick.

The documentary evidence as to the use of +12 during the Battle is pretty solid, so the fact that the speed graph is less than perfect doesn't materially affect the argument.

[Kurfurst]

Originally Posted by Redroach

However, I agree that if that conclusion is correct, the FM should be changed to 100 octane across-the-board, since I don't believe that 87 octane was in front-line use after may 10th 1940.
Maybe the Tiger Moth used up remnants, though

Kinda hard to explain then why rougly 2/3s-3/4 of the avgas consumed during the Battle was of 87 octane, with the rest being 100 octane.. as per British avgas consumption figures during the Battle.

[IvanK]

Groan not this 87 Octane v 100 Octane conspiracy theory again !

[Viper2000]

The title of this thread is Effect of boost control cutout prior to +12 psi boost modifications, and I fear it's drifting...

[Redroach]

Yes, it may be drifting, but I'm still unconvinced. Why should they model the boost cutout without the little bleeding valves? It makes no sense whatsoever except for seeing the engine blow up.
I'm quite convinced that the RAF runs on 100oct fuel; however, the boost control cutout seems to be non-working in the Spit MKI. I just flew the MkIa (i think) and there, boost control works as it is described in the pilot notes. Well, I can't be 100% sure because the gauge ends at 8psi, but I'm convinced it provides the advertised 12 psi.
On a side note: I don't consider the fine pencil lines that seem to be erased out (perhaps he wanted to draw another diagram for a different plane at first) as relevant. The red line is max throttle and the blue line is "boost cutout engaged". It's named "all out", after all.

[Viper2000]

Originally Posted by Redroach

Yes, it may be drifting, but I'm still unconvinced. Why should they model the boost cutout without the little bleeding valves? It makes no sense whatsoever except for seeing the engine blow up.

Because we currently have 87 octane engine performance, so we might as well have an 87 octane boost cutout.

It is obviously silly to have a boost control cutout which doesn't do anything in the Spitfire I.

Originally Posted by Redroach

I'm quite convinced that the RAF runs on 100oct fuel; however, the boost control cutout seems to be non-working in the Spit MKI. I just flew the MkIa (i think) and there, boost control works as it is described in the pilot notes. Well, I can't be 100% sure because the gauge ends at 8psi, but I'm convinced it provides the advertised 12 psi.
On a side note: I don't consider the fine pencil lines that seem to be erased out (perhaps he wanted to draw another diagram for a different plane at first) as relevant. The red line is max throttle and the blue line is "boost cutout engaged". It's named "all out", after all.

This is like arguing with a brick wall. Maybe we have 100 octane fuel in the tanks, but what's the point beyond retarding the intellectual development of children through lead poisoning if we have 87 octane engine ratings?

The pilot's notes clearly state that the combat concessions allow +12 psi with 100 octane fuel.

If you test in no cockpit view you'll see that the virtual boost gauge goes up to +12, but you still only get about +8 indicated with the Spitfire II; therefore it really is +8 psi rather than simply the needle hitting the stop.

What has probably happened is that 1c have implemented the RM2S rating which was +8¼ psi boost, superseded by the use of the +12 combat concession appended to the RM1S rating, but have fudged the model such that we get +12 speeds for the Spitfire II.

However you look at the situation, fixes are needed.

[TomcatViP]

Originally Posted by Viper2000

Because operation of the boost control cutout with 100 octane fuel should allow +12 psi boost at 3000 rpm, which should give about 1100 bhp at sea level, rising to 1310 bhp at 9000'.

At present, the Spitfire I can only achieve the 87 octane rating of +6¼ psi boost at 3000 rpm which should give 880 bhp at sea level, rising to 1000 bhp at 15500'.

As I explain in this thread, operating the cutout should produce roughly +17 psi boost with 87 octane if the power lever is fully advanced, leading to rapid engine failure. At present, operating the cutout doesn't do much unless you're flying a Spitfire II.

The Spitfire II can get about +8 psi with the cutout operated, which would give 1000 bhp at sea level. However, it gets the sort of performance I would expect from a Merlin XII at +12 psi boost.

It's been a while since I tested the Hurricane. However, since it also has the Merlin III, logically the Hurricane I and Spitfire I should exhibit identical engine behaviour in all respects other than coolant temperatures (since they have different radiators).

Ok I dig out part of my database (in fact I took a short cut).

Merlin XX !!

ALT(ft) SHP BHP (diff correspond to the power used to drive the supercharger)
15K 1267 1048
20K 1298 1073
20K+ 1362 1126
25K 1162 960
30K 945 778
35K 700 568

What is funny is that I made this exat assumption based on value long stacked in my memory and was told I was wrong.
Same As the exhaust gaz power (remind that conv some years ago on WC forum ?)

I do repeat thus are Merlin XX -20 data 1941/RR !!

Max power boost is at 21k is 9lb and then decrease steeply at a 7lb/10Kft ratio.

As I made the assumption max boost (the famous 12lb is only for Take off - supposedly 1min max emergency power at low alt too)

I do repeat : I am not trying to be the guy with the right info or data but only want to help (if I ever could ?!!) this sim to be better to her ancestry, path that I am certain is off all the Ninja FMed planes.

Typical 1940 RAF pilot had guts, faith and anger against the Nazi war machine. Those are the only WunderWaffen they had in hands.

I'll stay tunned in case some needs more of this.... but I hve to say that I am a bit angry

~S!