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.

[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.

[Corsair9]

This is not a 100 octane thread, but FYI:

According to documents I have copies of from the PRO in Kent, at the end of June, 1940 the RAF had:

336,000 tons of 100 Octane
270,000 tons of 87 Octane

so, about 55% of aviation fuel stocks were 100 octane. In addition:

End of August 1940: 64% of stocks were 100 octane
End of Sept 1940: 64%
End of Oct 1940: 64%
End of Nov 1940: 63%

... with the total tonnage of both fuels increasing about 30,000 to 40,000 tons a month. All this right in the middle of the BoB. Not only were 100 octane stocks consumed through combat and wastage being replaced, stocks were increasing by 15,000 tons a month.

The reason so much 87 octane is consumed during the BoB is that all training activities used 87 (including Spitfires and Hurricanes used in training), as well as transports, etc. Also, I believe the RAF ground vehicles were all using 87 octane from the same aviation stocks.

Whenever Spits or Hurricanes modified for 100 octane were fueled with 87 octane, the ground crew inserted a locking pin in the boost cutout to prevent the throttle being pushed into the WEP zone. I think that came from a copy of a maintenance manual I have.

For some odd-ball reason all flight sim developers appear to ignore the +12 boost advantage that 100 octane fuel and the CS propeller gave both the Spitfire and Hurricane. I'm not sure why this is, but it is. It appears this is the same for this sim (I don't have a copy of it.)

[lane]

Originally Posted by Corsair9

For some odd-ball reason all flight sim developers appear to ignore the +12 boost advantage that 100 octane fuel and the CS propeller gave both the Spitfire and Hurricane. I'm not sure why this is, but it is. It appears this is the same for this sim (I don't have a copy of it.)

It’s been public knowledge since the publication of the March 28, 1940 issue of Flight Magazine, in the case of 100 octane and the May 23, 1940 issue of Flight, in the case of the constant speed propeller.

"The 100-octane fuel (which enables the Merlin to receive no less than 12 lb. boost in emergency) must have been an asset."

"A new airscrew specially designed for the fast single-engined fighters of the Royal Air Force, but which can, of course, be applied for civil purposes, is now in production at the factory of Rotol Airscrews, Ltd., and is also in service with the squadrons."

So flight sim developers haven’t much excuse for getting it wrong every time

Thanks for sharing the 100 octane info, very interesting!

Originally Posted by Viper2000

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.

<Chuckle></Chuckle>I still maintain we should have the right fuel modeled and model that correctly

[Seadog]

Some real world results:



There was no physical time limit on the use of 12lb boost, just a requirement to log the use and a suggestion that it be limited to 5 mins but some pilots used it for 30 mins. It did effect engine life but not severely, and later versions of the Merlin III on the Sea Hurricane were modded to allow 16lb boost and 1440hp, which was effective up to about 5500ft.

[TomcatViP]

Originally Posted by Viper2000

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.

I'm sry Vip but MerlinXX-> 100 oct fuel. 12lb allowed for T.O and used as emergency settings with extensive red taping (this is why you read traces of its use in logbooks).

Boost Curved toped at 9 - Maximum calibrating perf test run. Hurricane - Merlin XX.

I hve detailled the SHP/BHP to give reader a way to understand the origin of dispersiveness in Merlin data on the web