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

[lane]

Originally Posted by Viper2000

Since the sim currently models aeroplanes which are evidently running on 100 octane fuel (+6¼ psi boost flat out at 3000 rpm), it seems to me that it might as well do so as well as possible.

Hmm, typo? You must have meant “Since the sim currently models aeroplanes which are evidently running on 87 octane fuel (+6¼ psi boost flat out at 3000 rpm)” since that’s the only language that makes sense in that context. Personally I think modeling that for a Battle of Britain simulation is erroneous. Certainly the historic record is quite clear on this matter.

I agree that we also need realistic 100 octane engine performance, preferably both R.M.2.S. and the later R.M.1.S. with +12 psi combat rating if we are to accurately simulate the Battle.

There’s no point in modeling a Merlin with a R.M.S.2 rating since this was superseded by the use of combat ratings on the Merlin III – R.M.S.1 before the Battle of Britain started. It’s irrelevant, don’t bother. See The Merlin in Perspective and Merlin II and III – Use of + 12 lb./sq.in. Boost Pressure (20.3.40). I can’t say I’ve ever heard of Spitfire or Hurricanes equipped with Merlin II or III operating with R.M.S.2. rating. If they were cleared for this rating it must have been 1938-39? Why not model the Hurricane I and Spitfire I with the +12 boost they were cleared for and used, for this supposed Battle of Britain simulator?

Personally, for completeness, I'd also like to see the fixed pitch wooden propeller & early R.M.1.S. engine rating and early airframe mod states so that we can experience the Spitfire and Hurricane as they were when they entered Service, and so that we can also investigate what might have happened had the Munich Crisis precipitated war rather than appeasement; the Bf-109E-1 would probably be reasonable opposition; some earlier versions would probably also be appropriate, but might not be considered worth the development work required...

Personally I’d really like to see the Spitfire I and Hurricane I modeled in the configuration, with respect to engine power and appropriate propeller, which they used during the Battle of Britain. Currently they are not. I suspect that most people who buy this sim expect it to be a simulation of the Battle of Britain. Lately I’ve been thinking that the developers should focus on what they know best - i.e. the Eastern Front. The recent post by the developer is encouraging is this regard. I’m afraid that cultural and linguistic barriers prevent them from getting a Battle of Britain simulator right. Unfortunately, they have clearly got the aircraft modeling for Cliffs of Dover wrong.

[Viper2000]

Yes my post should have said 87 octane. I'll fix it.

It seems to me that since we've got a load of 87 octane engines we might as well have them modelled correctly, since the vast majority of the effort required has already been expended.

I agree that we should also have, as a priority, realistic aircraft correctly modelled for the Battle itself, with 100 octane fuel and +12 combat ratings.

I don't think that there is any particular barrier preventing this from happening.

There have probably been a few typographical errors made along the way. Given the major bugs that make the sim unplayable for quite a lot of people I think it's pretty easy to understand why getting accurate performance is taking a while; it's just not going to be at the top of the "to do" list.

The chances are that once the sim is patched to a stage where it runs smoothly for the majority of users who meet the minimum system requirements then we'll start to see a shift in emphasis towards aircraft performance and other "non-show-stopping" bugs.

Naturally, I'm somewhat disappointed with the fact that the sim doesn't work properly out of the box, but past experience with IL2 suggests that we've got more chance of seeing the problems fixed by 1c than we might have with other developers.

[Viper2000]

PS - I know that RM2S was superseded by the combat rating of RM1S, but the existence of the rating implies that something was done with it, at least for a while. It also provides some "fossilised" evidence of the old RM1S rating.

I think that it is worth investigating, because the Spitfire II currently indicates about +8 psi boost with the cutout operated. Although some people say that this is just due to the boost needle hitting the stop, it's notable that in no-cockpit view the boost gauge goes up to +12 but also only indicated about +8.

I therefore have a sneaking suspicion that the Spitfire II is erroneously modelled with an engine having the RM2S takeoff rating. Either that or they just picked +8 arbitrarily.

In any case, it's probably worthwhile tracing this stuff through because if they match the top speed with the wrong engine power then the errors will manifest themselves elsewhere in the envelope (eg zero lift drag or prop efficiency will be wrong).

If, as I suspect, we have a botched RM2S then again, as with the 87 octane RM1S, I think it's probably worthwhile to fix it and just add an extra Spitfire & Hurricane model to the list, because choices are nice to have, and also because this would provide additional opportunities to crosscheck the airframe drag characteristic.

[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.

[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!

[Viper2000]

Originally Posted by TomcatViP

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!

I don't understand why you have posted this here. It's the wrong engine from the wrong year fitted to the wrong aeroplane. You have also only posted a small part of the power curve (ie FS gear) and quote neither boost nor rpm. So what's the point?

If, as I suspect, you've posted a +9 curve then great. But why?

If the objective is to refute +12 psi for combat then RTFM because it's in the Pilot's Notes for the correct aeroplanes from the correct year fitted with the correct engines, as is evidenced in the original post.

The existence of a +9 curve does not preclude the existence of a +12 curve.

In any case however, the title of this thread is Effect of boost control cutout prior to +12 psi boost modifications. I wrote the OP because the engine instruments indicate that we've got 87 octane fuel, and I figure that if we're going to have the wrong fuel modelled, we might as well have it modelled correctly.

[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.