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

[JG53Frankyboy]

so, once upon a time, the developers decided what fuel is in the tanks of the different british fighters there should be no "boost control cut out" possibility in versions with 87octon fuel ?
or should there "only" a very fast engine damage if you engage the cut out and move your throttle to 110% than ?

[TomcatViP]

Originally Posted by *Buzzsaw*

+12 boost does not confer a lot of advantages over 10,000 ft.

The Spit II would be slightly slower than the Spit I, speeds at sea level were approx. 310mph.

Note that would mean the Spit would be faster than the 109 at sea level, but would fall behind it over approx. 15,000 ft when 1,4/1.45 ata is used by the 109. (my understanding is the 109 had a limitation of 1 minute at 1.4 ata) Since there are no charts showing speed/climb for the 109E at 1.4/1.45 ata, the developers are going to have to do an approximation as for the Spit II at +12. Of course, the increase in speed is not linearly related to the increase in horsepower, (990 PS for 1.3 ata vs 1100 PS for 1.4) so the 8% gain in horsepower will not result in a 8% gain in speed, as the drag factor multiplies very quickly and there is a lot of Hp required for each additional mph/kph. The 109 would pull away even when using only 1.3 ata at alts over approx. 21,000 ft.

Reports from the actual battle confirm the 109's preferred to stay over 20,000 ft and not engage on the deck. This fit with their escort mission, since the bombers were most often at between 16,000-20,000 ft.

Thanks to Mike Williams for his links from his Spitfire site:

http://www.spitfireperformance.com/spittest.html

+1

good post.

If I do remind well at alt ard 30kft the Spitfire IIa was supposed to be notably better. The early 1940 Cirkus were mostly flown at this alt.

[Viper2000]

Originally Posted by JG53Frankyboy

so, once upon a time, the developers decided what fuel is in the tanks of the different british fighters there should be no "boost control cut out" possibility in versions with 87octon fuel ?
or should there "only" a very fast engine damage if you engage the cut out and move your throttle to 110% than ?

Really "110% throttle" doesn't mean much in this context, because there was no gate on the throttle (gated throttles arrived later).

It's better to think about PLA (Power Lever Angle).

If we assume that PLA goes from 0-100, and that WOT operation of the Merlin III at sea level would produce +17 psi, that's 31.7 psi absolute.

Max rated boost for 87 octane = 6¼ psi = 20.95 psi absolute.

This would be about 66 PLA if we make the simplifying assumption that PLA maps directly to boost, and that 0 PLA maps to a stopped engine (ie nominal 0 manifold pressure).

Of course, reality is a bit more complex; the throttle valve is a flat plate which is turned through about 90º to go from idle to WOT. The minimum area therefore varies as the sine of the throttle valve angle. But the throttle can't close completely because otherwise this would just completely stop the engine. So actually 0 PLA will map to some non-zero absolute manifold pressure.

This all means that with the ABC cutout operated it's not immediately obvious that PLA would map directly to boost.

So far as I can piece together from the various imperfect sources available, the ABC for the Spitfire I at the end of the 87 octane era was set such that 100 PLA would give +6¼ psi boost, and some considerably lower PLA, perhaps as low as 66, would give this boost at sea level with the cutout operated.

If you climbed with the cutout operated at 3000 rpm then you'd reach 100 PLA at the FTH for +6¼ psi boost, which was about 15500'.

If you climbed at some lower rpm, say 2600, then the FTH would be lower, and so you'd reach 100 PLA sooner.

Later engines were designed for operation with gated throttles, and so there would be a good argument for thinking in terms of a PLA>100 or 110% throttle in 1C's lexicon.

Note that in the above discussion, PLA is deliberately left as a somewhat vague, dimensionless and therefore unit-less term.

The existence of the R.M.2.S.M. rating with a +8¼ psi takeoff rating on 100 octane fuel prior to the clearance of the +12 psi combat rating suggests that before the boost control cutout modifications were embodied, a margin was required to protect the pilot against accidental overboosting, which is easy to understand given that +8¼ psi would be about 72 PLA, whilst +12 psi would be about 84 PLA.

The combat rating remained +6¼ psi, presumably because in the heat of battle the micro-management required to keep boost within limits was considered disproportionate to the performance gain available, and it was therefore better for the pilot to rely on automation to protect his engine from immediate harm whilst he concentrated on the enemy.

The boost control cutout modification meant that 100 PLA with the cutout activated would deliver +12 psi boost up to the +12 psi FTH.

Note however that the Merlin XII's takeoff rating was +12½ psi with a +12 psi combat rating, so that engine at least must have been capable of slightly exceeding +12 psi at 100 PLA with the cutout activated.

I suspect that the cutout would have been set to give a maximum of perhaps 12.75-13 psi boost at sea level, 3000 rpm with 100 PLA, and then the pilot would have used slightly less than 100 PLA to get the official +12 psi rating, which would probably have gone out of the window in actual combat.

Whether this would have been the case with the Merlin III as well I don't know. It seems likely to me that in reality the mod would have been arranged so that 100 PLA would give slightly more than +12 psi boost in order to allow for engine deterioration, but this sort of thing is quite hard to trace through in a satisfactory manner.

As for what would happen if the cutout was operated with 87 octane fuel, or with 100 octane fuel prior to the +12 psi modification, I would expect that a couple of psi beyond the cleared maximum boost, detonation would start. This would be worse at high ambient temperatures and/or low rpm.

Once detonation starts, I would expect failure to follow within a minute or two, depending upon severity.

Equally, I would imagine that if you operated the cutout and pushed to +8 on 87 octane at 3000 rpm for 30 seconds or something that you'd more than likely get away with it, though the engine life would be reduced.

Although there were engine mods in preparation for the use of 100 octane fuel, I get the impression that in 1940 they lagged behind the increases in load that the engine was suffering; one of the sources I cited talks about failure in just under 50 hours of 20 minutes off, 5 minutes on +12 operations, which is of course about 10 hours at +12. To put this into context, the theoretical life of a fighter Merlin in 1939 was 240 hours.

So very roughly, caning the poor thing at +12 has killed it in about 1/5th of its nominal life.

As such, I would expect that the engine would be less tolerant of overboosting beyond +12 on 100 octane than it would be of overboosting beyond +6¼ psi on 87 octane.

[JG53Frankyboy]

whatever, i think we should discuss how that would influence the game.....
and in game it works so of, if you have used the "boost cut out", you are able to get 110% throttle on the digital output info window. If not you can only get 100%

I am using a X52 throttle, and so i push my throttle "through the gate" (at least i have set the throttle control this way) .....

i belive, in the (far) future the Hurricane I and the Spitifre I both with two speed propellers are 'candidates' to be programmed with 87octane fuel. And im just intrested what that would mean for their CEM, should they have a "cut out" control at all ?!
or 'only' 100% throttle (again in game termilogie) as a maximum?

[lane]

The purpose of the boost control cut out is stated in AP 1564A Hurricane I Aeroplane, Merlin II engine, March 1939 (prior to adoption of Mod. No. Merlin/154 - Modified boost control cut-out valve).

Section I, para. 15: Emergency Control – Cut-out for automatic boost. – ... It is intended for use should the automatic boost control fail in flight or should it be necessary in an emergency to override the automatic control for increase of boost. (See attachment)

The effect was described by W/C R. C. Wilkinson who flew with 3 Squadron during the Battle of France.

The original Hurricanes fitted with two blade props had about 6.25 P.S.I. (pounds per square inch) and when the throttle was pushed “through the gate” to give full boost, it would put on 16 P.S.I. This was extremely hard on the engines so the regulator was modified by drilling a 1/32 inch hole in it, this would bleed off the excess pressure, so it would only put out 12 P.S.I.

When the three blade steel props were fitted to the Hurricanes and the throttle was pushed through the gate it would run about 2,600 R.P.M. with a 12 P.S.I.

Wilkie simply put a wooden match stick in the 1/32 inch hole thus his two blade wooden prop would run at 3,200 R.P.M. with 18 P.S.I. boost when it was pushed through the gate. He definitely had the fastest aircraft in the Squadron but he was extremely hard on engines. During the Battle for France Wilkie used nine aircraft in ten days, amazing what a match stick in a bypass line will do. However, it should be noted, Wilkie was always the last in the air as he needed more runway to build up speed.
From: Spitfire RCW, The Wartime Exploits of Wing Commander Royce Clifford Wilkinson (See attachment)

Spitfires were cleared for 100 octane in September 1938, the month following their introduction into operational service.


Memorandum dated 24th September 1938 from the Air Vice-Marshall, Assistant Chief of the Air Staff.

1. I am directed to inform you that in order to improve the take-off performance of Spitfire aircraft, the use of 100 octane fuel by squadrons equipped with this type has been approved.

2. Improvement in the take-off performance of Hurricane aircraft will be obtained by the adoption of C.P. airscrews but there will be a period of some months before this modification is introduced. In order, therefore, to obtain some immediate improvement in take-off performance, the use during this period of 100 octane for Hurricanes has been authorized by squadrons at those stations where you consider it to be necessary.

It seems apparent that in order to make use of 100 octane for take-off from certain fields (with early, not constant speed propellers) it would be necessary to operate the boost control cut-cut.

Since 100 octane was introduced in Fighter Command before the Battle of Britain and before the Battle of France for that matter, the whole subject of the effect of operating the boost control cut out with 87 octane fuel is rather irrelevant to a Battle of Britain simulation; or perhaps at best merely of academic interest.

[JG53Frankyboy]

Originally Posted by lane

......................................Since 100 octane was introduced in Fighter Command before the Battle of Britain and before the Battle of France for that matter, the whole subject of the effect of operating the boost control cut out with 87 octane fuel is rather irrelevant to a Battle of Britain simulation; or perhaps at best merely of academic interest.

as Hurricanes and Spitfires with two speed DH Propellers too i guess.......at least for the BoB time

[Viper2000]

Originally Posted by lane

Since 100 octane was introduced in Fighter Command before the Battle of Britain and before the Battle of France for that matter, the whole subject of the effect of operating the boost control cut out with 87 octane fuel is rather irrelevant to a Battle of Britain simulation; or perhaps at best merely of academic interest.

Since the sim currently models aeroplanes which are evidently running on 87 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.

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.

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...

[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.