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

At the moment, we've got early Spitfires & Hurricanes with combat ratings of +6¼ psi boost at 3000 rpm, and a Spitfire II with a +12 psi combat rating.

We already know that the use of +12 psi required various modifications to the engine, as well as the obvious switch to 100 octane fuel, and that one of these mods was associated with the boost control cutout:

http://www.wwiiaircraftperformance.org/ap1590b.jpg

I was left somewhat unsure as to what would happen if you operated the boost control cutout and firewalled the throttle prior to the incorporation of the boost control cutout mod; I just found out:

http://www.wwiiaircraftperformance.o...bs-14nov39.jpg

So it turns out that if the engine hasn't been modified to give +12 psi, operating the boost control cutout at sea level should deliver +17 psi boost. Clearly on 87 octane fuel this would lead to detonation which would destroy the engine in short order. Equally, if the aeroplane was operating on 100 octane fuel then the engine would almost certainly have been modified for +12 psi operation because otherwise it would just be a waste of fuel.

I therefore conclude that the most likely reason for providing a boost control cutout originally was fear that failure of the automatic boost control unit would excessively close the throttle, effectively causing engine failure, and so the cutout was provided in order to guard against this possibility rather than to provide some additional combat power, and that its later use for this purpose was essentially a hack, rather like the modification of the Dh 2 pitch prop for constant speed.

This would also explain the fact that the Pilot's Notes for the Spitfire make no mention of operating the boost control cutout to attain any kind of combat power on 87 octane fuel; the 87 octane combat rating is simply +6¼ psi boost and 3000 rpm, which is reached with the automatic boost control engaged.

Therefore, operating the boost control cutout for combat power should be a 100 octane fuel only procedure; on aircraft not modified for +12 psi combat power it should almost certainly be an emergency procedure for use in case of ABC failure, carrying with it the risk of over-boosting the engine, causing detonation and rapid failure.

The figures I have from SpitIIA pilots notes are

Max climb 2850rpm @ +9 (+7)
Max rich 2650 @ +7 (+5)
Max lean 2650 @ +4 (2.5)
Combat 3000 @ +12 (7) 5 mins max

100 octane fuel, figures in brackets 87 octane

This thread is about Spitfire I and Hurricane I aeroplanes whose engines had not been modified for +12 boost.

[The Spitfire II has a different engine (Merlin XII) and was designed for +12 psi from the start; therefore its boost control cutout would have incorporated the modification and so operating the cutout would give +12 psi; doing this on 87 octane fuel would be a Bad Idea, and you'd be able to get to the 87 octane boost limit with normal throttle movement.]

In any case, the point is that operating the boost control cutout and advancing the throttle to the stop only gives usable combat power if you're using 100 octane fuel.

If you're using 87 octane then it breaks the engine; it'll just do it somewhat faster (+17 boost instead of +12, and a generally weaker engine) if the engine in question is an unmodified Merlin III than a modified Merlin III or a Merlin XII.

AP 1590B Merlin II and III Aero-Engines

Your conclusion the Spit II is modelled with +12 boost is incorrect.

The original tests show just under 290 mph at sea level with 87 octane and 8.8 boost levels. Best alt figures are over 350 mph also at 8.8 boost. Those are actually higher than the results achieved for the game Spit II.

http://www.spitfireperformance.com/p7280speed.gif

The Spit II manual shows max boost using 100 octane is +12.5 at 3000 rpm. See page 13 of the manual:

http://www.zenoswarbirdvideos.com/Im...pit2Manual.pdf

For an actual 100 octane, +12 boost test, we unfortunately don't have one for the Spit II, but the Spit I shows sea level speeds of 314 mph, and best alt of 359 mph at 13,000 ft.

http://www.spitfireperformance.com/s...-rae-12lbs.jpg

+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

I took a Spit II up using the cross country flight in Quick missions yesterday and could only get 8lbs boost even with the boost cutout pulled and the throttle full forward. Changing the mixture made no difference. This was from an altitude of 0 - 20,000 with CEM on. I know the boost gauge only goes upto 8lbs so I used the full screen (CTRL+F1) instruments and this only showed 8lbs boost as well.

Am I doing something wrong or do I have 87 octane fuel in my QM Spitty II?

Very interesting, thank you.
Here is stated that the use of 100 octane fuel came at the end of february 1940.
http://www.spitfireperformance.com/spit1vrs109e.html
Anyway the Spit in Cod is not working probperly because at 10000ft it starts the engine stutters, but this should be its best performance altitude. :confused:

It's not "my conclusion"; I haven't tested it at all. It's just what I've picked up from the forums.

In any case, it's beside the point, and arguments about it (legitimate though the are, since the whole point of the Merlin XII was to increase the FTH at +12 psi boost) belong elsewhere.

This thread is about the correct behaviour of the boost control cutout in aeroplanes that haven't been modified for +12 psi boost, my contention being that operating the cutout with the power lever fully forward should produce approximately +17 psi boost leading to rapid engine damage, based upon the sources I have cited in the original post.

Therefore I conclude that the original purpose of the boost control cutout was to act as a safety feature to guard against failure of the ABC fully closing the throttle and effectively failing an otherwise functional engine, and that combat power for an 87 octane Merlin III would be reached simply by advancing the power lever to give +6¼ psi boost and 3000 rpm.

I read it as +17 psi boost would only occur in a unmodified boost control with 100 octane fuel. I understood it that 87 octane fuel had a maximum boost pressure of 6.25 psi on the Merlin II/III.

I thought the whole reason for the automatic boost control was to reduce the burden on the pilot who could set an engine regime using the throttle\propellor and mixture levers and the boost would remain at that setting rather than keep needing to be altered due to changes in altitude.

The physics of the supercharger and the ABC are unaffected by the fuel used.

The fuel only affects the boost pressure that can safely be used without detonation.

If the engine hasn't be modified for +12 operation then disabling the ABC and moving the power lever fully forward will fully open the engine throttle which will deliver +17 psi boost. This will break the engine; but it will break rather faster with 87 octane fuel than with 100 octane fuel.

If the ABC has had the cutout mod embodied, operation of the cutout will only produce a maximum of +12 psi boost, which will prevent detonation and thus avoid immediate engine damage, provided that 100 octane fuel is used. If you operated the cutout with 87 octane fuel in the tanks and the power lever fully forward then the engine would start making expensive noises in relatively short order.

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 ?

+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.

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.

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?

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.

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

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...

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.

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 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.

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.

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.

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 :)

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).

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.

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.

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

RAE speed data, complete with your 246 mph TAS in weak mixture:
http://www.spitfireperformance.com/s...-rae-12lbs.jpg

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.

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. ;)

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

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

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.

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.

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.

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:evil:

~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. ;)

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.)

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

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!

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

12 lb boost in the real world
 

Some real world results:

http://www.wwiiaircraftperformance.o...-l1717-cal.jpg

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.

8.5 hours at 12lb boost...
 

These are test results from 1939, showing that a Merlin III could be run for many hours at 12lb boost:

http://www.spitfireperformance.com/spit1-12lbs.jpg

In the above test a Hurricane was run at 4.25 and 12lb boost for 20min and 5min repeatedly and only terminated at 49.5 hours when a glycol leak developed, by which time the engine had been run for over 8 hours at 12lb boost.

Yeah I wonder why RR still state for the merlin XX perfs in 1941 the following (quote)

Max T.O 3000rpm + 12lb/sq.in boost (M.S only)
Max climb 2650rpm + 9lb
Max all out level 3000rpm + 9lb
Max cruising 2650rpm +7b

But certainly RR R&D dep didn't new all abt field's Merlin.

Baahhh those stupid engineers ... :evil:

The Merlin XX played only a small part in the BofB, but it was approved for 12lb boost during emergencies in Nov 1940:
http://www.wwiiaircraftperformance.o...xx-15nov40.jpg

and, since 12lb boost was available during TO right from its introduction, any pilot, at his discretion could use 12lb boost at any time or any altitude where it was possible, and there was no physical lockouts in the cockpit engine management system to prevent a pilot from doing this.

Lol

- The XX was a post BoB product hence as a fighter eng it shld hve beneficed of the former improvements
- If 12lb MS is indicated it EXCLUDE any use of this level of boost above s/c alt

Sry but your statement does not match any engineering rules and is contradicted by the submitted text.

Let me explain : Supercharging put strain on your eng (300HP at recovering s/c alt)
If at low alt a boost level is restricted it certainly can't be achieved at alt !!

To give an exemple, my car has a turbocharger at 1.4 bar of boost (nearly 1.4 atm)

I once had a friendly race against a motorbike in a mountainous road from 1K up to 3K meters. My boost is electronically regulated. My turbo s/c is capable of much higher boost value to be reliable in a series car. The higher we went the better was my engine against the motorbike naturally aspirated engine (same boost same HP for me- Lowering boost for him) up to the point that my 1.4t cars "matched" his much lighter vehicle. BUT the temp went to high as did the strain and (hopefully) the electronic module give it away before I blew the eng.

That's what you risk with super/turbo charging. The lower the outside pressure is, the more the strain on the engine (mechanical or thermal respectively) will be.

If a limit is settled at low alt, it means that it can't be superseeded at high alt otherwise the limit would be put at s/c alt (but as it was difficult to have accurate pressure measurement at alt in a moving ac at the time they might have choose to rely on a low alt limit).

Baaah forget it I know that I am certainly loosing time writing this :(

The Hurricane/Merlin XX was introduced to combat in August 1940, but not given the designation Hurricane II until Sept 1940.

The text specifically allows 12lb boost in M gear for short periods of time during combat and during TO. It states the the boost override will provide 12lb boost up to 8500 ft after which it will gradually decline (to 9lbs at about 13000 ft):


However, as I've stated there is no physical interlock preventing a pilot from using the boost override in S gear, and using the Boost Override in S gear will provide additional power up to about 19000ft. Yes, this is an overload on the engine and cooling system but every pilot flying a Hurricane II always had the option to engage the boost override at any altitude and in either M or S gear.

I think that getting near-unlimited use of war emergency power settings should only be allowed if

1) we are forced to fly with the same virtual aircraft, damage carrying over between missions and so on: i push it on one sortie, i do it on the next, i keep doing the same thing, it dies on me mid-channel on the 10th sortie or so...happy swimming back to Dover :-P

2) this also applies to multiplayer along with an incentive to keep our aircraft alive (the usual IL2 server conditions where losing all your aircraft means you lose the mission, along with public stat-tracking so that everyone on the server can see who's the points maniac that's depleting all the team's aircraft), instead of having everyone over-boosting and "recycling" them by deliberately crash-landing worn-out airframes

3) it applies to all aircraft, 109s running a constant 1.45 Ata for example

and then we have IL2:1946 all over again :-P

If COD is to have accurately reproduced flight and engine characteristics then the use of 12lb boost must be allowed with pilots only facing administrative penalties for doing so. The Merlin cooling and lubrication system could cope with 12lb boost and 3000rpm for long periods of time, but of course above 17750ft the Merlin III was no longer able to provide more than 6.25lb boost at 3000rpm.

If using quotes to backup your agenda at least make sure the plane in question is a Spitfire. The cooling system on a Wellington II was very different than that of a Spitfire Mk I, Hurricane, Defiant or any other plane with a Merlin. Finding one quote stating a Merlin could run at +10 lb boost for five hours does not mean they ALL could. Adequate cooling in the early Spitfires was a problem that was not overcome until the extra radiator was introduced on the Mk IX. Even then the issue wasn't totally cured. The problem was not so severe in the Hurricane because its radiator was directly in the prop wash. Have a read of this, this is taken from the chief pilot of the BBMF. They fly both Hurricanes and Spitfire as well as a Lancaster bomber.

http://www.key.aero/view_feature.asp...ection=airshow

Note the very large radiators directly under each ungine for the Mk II Wellington.
http://upload.wikimedia.org/wikipedi...ington_Mk2.jpg

Fighters, by definition cannot run for more than about 40 - 50 minutes at 12lb boost/3000rpm because of fuel limitations, and yet if a fighter could only be flown for a few moments at 12lb/3000rpm then there would be no need to point out the increased fuel consumption at 12lb/3000rpm (see point 4 in AP1590G above at 12:27PM)

Cooling on the ground and cooling in the air are two separate issues. This is a test of a Spit V at the normal and combat ratings, and the plane was flown repeatedly to its service ceiling at 16lb boost:
http://www.spitfireperformance.com/aa878.html
and the aircraft was specifically tested for cooling suitability at 16lb boost and was found acceptable for English summer conditions and this required considerably more cooling than at 12lb.

I don't have any "agenda" other than historical accuracy. Every Hurricane and Spitfire in RAFFC could use 12lb boost, and there was no 'magic' limit after which the engine caught fire or blew up or seized up. Using 12lb boost simply increased the wear and tear on the engines and probably kept the ground crews up late at night doing engine checks but this probably caused little concern to pilots whose lives and/or aircraft were saved or who ensured kills by "pulling the plug" and going all out. It is quite telling that the Merlin III could be modded to accept 16lb boost on the Sea Hurricane I with essentially identical engines and cooling as per BofB Hurricanes.

If COD is going to be an accurate simulation then it has to allow 12lb boost at the pilot's discretion, and then factor in some kind of admin penalty for using it without justification.

I am not saying the engines should suffer from major overheating issues if pushed for a few minutes but they should and did overheat if pushed to high for too long. The Spitfire Mk Vc had a larger radiator and had better cooling than a Mk I and Mk II, so while it is very relevant it is not a perfect example. I know these engines could take more punishment than just a few minutes at full power, but they weren't immune to breaking if pushed. Definately more punishement than a mere admin warning. I also know that cooling on the ground and cooling in flight behave differently, I am just pointing out the fact that CoD does not model overheating on the ground very well. I can sit indefinately with the radiator open on the ground at 1200 rpm, something that couldn't be done in real life in a Mk I Spitfire.

I know these planes ran at +12lb boost, I have no problem with that. My only issue is with the fact that you seem to be advocating they should run without problems at +12lbs boost until the fuel ran out. ALL of the scenarios you refer to would not require +12lbs boost for more than a few minutes. In reality it would be unlikey that someone would follow your full power dive for more than a few minutes. I know if I was in a Spitfire Mk Ia in 1940 I would not dive after a 109E in a full power dive for very long, especially considering that in a few short minutes I would most likely be flying over France at low altitude with an engine starting to run hot.

In the Mk Vc full power test you linked to the radiator temperature reached 124 C at 12.000ft after 3 minutes, OAT was +5 C. This is already 4 C higher than the recommended maximum, at 130 C the Merlin had to start venting to reduce coolant pressure (small hole on right side of engine cowling just behind the prop). If this test had been done at lower altitude in the hotter air then it would almost certainly have overheated without pilot intervention. It was easier to keep these planes cool at 12,000 or 24,000 ft than it was at SL.

See my response above. I do not want unrealistically hard overheating modelling for any of the aircraft. What I do want is the fact that sometimes if you push too hard for too long that there MAY be repurcussions that go beyond an admin reprimand. Apart form the ground cooling issues CoD seems to do quite well in this regard.

An aircraft might fail if run too long at 12lb/3000rpm, but then again it might not, and failure is not an automatic consequence, and in fact unless the engine has suffered battle damage it probably won't fail if run until fuel exhaustion, but of course running out of fuel is likely to ruin your day, in any event.

Regarding the Spit Vc test:


You will note that the test made allowances for increased temps during English and Tropical (IE Egypt) summers.

Dowding did take his pilots to task for probably abusing the use of 12lb boost:
http://www.spitfireperformance.com/dowding.pdf
but this was only one facet of abuse that could lead to engine failure, as Dowding's memo points out, but RAFFC had lots of spare aircraft and engines, and I doubt that pilots felt that they were using 12lb boost unnecessarily.

Exactly, pushing too far for too long COULD lead to consequences a pilot would not like to face. That is what I want modelled, not some arbitrary limit of say 5 minutes = bang, conversely I do not want it modelled that I can run at +12lbs boost constantly without some potential consequneces. Your whole point seems to be that a Merlin could run at +12lbs boost until the fuel ran out. No pilot in real life WOULD EVER do such a thing from take-off until landing.

Read what I said again. My point was that at 12,000ft or 24,000ft it is easier to keep an engine cool than at SL. Had the test been done at SL the temperatures would have been much higher. Or to put it another way, flying full power +12lbs boost without taking circumstances and consequences into account is not possible. It is silly to assume there would be no potential issues running +12lbs boost until the fuel runs out.

OK, where did I say we shouldn't be allowed to use +12lbs boost? Also what use is a spare engine sitting in a workshop when you are 12,000ft over enemy territory? Like it or not the pilot notes have engine limitations for a reason, yes they may be conservative numbers but it doesn't mean they could be ignored.

These engines required carefull attention, not constant but they certainly weren't balls to the wall and forget about the consequences.

Give them back their Starwar cruise ship and let's end this discussion. They won't give up until they got back their 25lb spit. Years of stupid mods have to get a justification.;)

Hmmm, it comes to mind that the publication date of 20 March 1940 for A.P. 1590B/J.2-W, Merlin II and III – Use of + 12 lb./sq.in. Boost Pressure followed actual unit conversion to 100 octane/+12 in the field in some cases; see for example 602 Squadron Operations Record Book, 151 Squadron Operations Record Book, 111 Squadron Operations Record Book & 74 Squadron Operations Record Book - Fwiw.

Remember your linking to info on the Merlin II and III, the document Seadog is linking to is for a Merlin XX. The Merlin XX was not used by fighters during the Battle of Britain so its boost rating is irrelevant since we don't have any flyable plane in CoD with a Merlin XX.

Oh, I certainly got that, sorry if I didn't communicate the point effectively. The issue I was addressing was that the operational use of a modification may precede, or follow for that matter, the official Air Publication document announcing the clearance or mod. Since Merlin III's were modified to operate at +12 lbs at the unit level before the publication date of the mod, it follows that the Merlin XX may also have been operated with +12 lbs emergency at the squadron level prior to the 15 November 1940 publication date.

For example:
16 February 1940 Operational conversion to the modification
http://www.wwiiaircraftperformance.o...rb-16feb40.jpg
20 March 1940 Publication of the modification
http://www.spitfireperformance.com/ap1590b.jpg


I agree that the Merlin XX has little to do with anything with respect to Viper’s initial subject or the sim FMs. Try telling that to TomcatVip though ;) I’m less certain about your position on introduction into service of the Hurricane II with Merlin XX. See Seadog’s post #40: "The Hurricane/Merlin XX was introduced to combat in August 1940, but not given the designation Hurricane II until Sept 1940." McKinstry writes of the Hurricane II: "The first models emerged from the Langley works in September 1940, going into service initially with 111 'Treble One' Squadron and then replacing other Fighter Command units over the coming months."

I'm with ICDP on this one. Options should be there to accommodate everyone, that includes the players who want to fly a more "by the book" recreation of WWII air combat that follows real-life constraints a bit closer.

The issue is not so much if an engine could run flat out for a few hours, but the fact that there are no underlying features in the sim yet to model the chain of penalties and drawbacks that would occur in reality when doing so. We jump into a fresh airframe every time we spawn and unless there is some continuity between sorties, it's a bit too much of gaming the game for my taste to be able to easily exceed the limits.

On the other hand, if we get a complex dynamic campaign engine that can be used both offline and online, one that tracks the supply and strategic side of things then by all means i'm in.

I could be pushing the engines all i want, but if the lufties bomb the convoy carrying the spare parts to my base i would have to take off on the next sortie with an abused, inadequately serviced engine and that might give me trouble. That's an entirely different kind of gameplay though than the one which will occur if we opt for selective realism only where it suits us as players. Statistically it might even out (ie, every aircraft would get the same treatment), it just draws a sizable part of the tactical considerations and problems faced by a pilot out of the picture.

This is what bothered me most about IL2 as well. Our FMs might have been accurate to the Nth degree, but we were all constantly mucking around with amounts of horsepower that were in reality severely limited in their duration. And since so many parameters of any given FM, no matter how accurate, rest on the amount of available power, the end result is that the entire fight changes and non-realistic tactics are adopted, simply because we can.


I'm not preaching to get everyone onto the full real boat, heck i fly with externals on when i'm offline because i like pausing, watching all the pretty effects and the aircraft and taking screenshots. There should always be options to "pull" this piece of software away from sim and more towards game, this is how the series sells and new "converts" join our ranks. I'm just saying that such implementation should be optional and part of the realism settings, not a default across the board change that applies to the highest difficulty settings.


Case in point, what ICDP mentioned about the overheat modeling. It's far too forgiving while idling on the ground, to the point that it's sometimes hard to properly warm up the engines and they choke when applying throttle smoothly, forcing the player to do it in steps of "throttle up and wait for the temp to catch up".

The most glaring example is the Blenheim which i've taken to studying with a bit of an obsession (i like radial engines :-P). The Mk.IV pilot's notes specifically state keeping the cowl flaps full open during start-up, warm-up and taxi because lack of airflow at near zero airspeed will make the engines overheat. It also advises closing the cowl flaps completely to improve acceleration during the take off run, because the airflow is sufficient to cool the engines once the aircraft is moving.

In the sim it's the exact opposite. To warm up i step on the brakes and gradually increase throttle with the cowl flaps closed and once i hit the magic 200 degrees of cylinder temps i apply take off power and fully open the cowl flaps.

Like i said before, if we get merlins running a constant +12 lbs it won't be long before we get DB601s running a constant 1.45 Ata as well and it will be IL2:1946 all over again: less need to plan ahead and adopt real life tactics, because my magic horsepower is always there for me.

I'm sorry lane, I got that part wrong, I stand corrected. The Merlin Mk XX was used introduced in August, first it was fitted to modified Mk Is made to Mk II standard at the factory, then on the Hurricane Mk II which was introduced to squadron service in September 1940. So it was indeed present in the BoB.

Regardless the salient point still stands in that CoD does not have a flyable aircraft with a Merlin XX. I think it is worth mentioning that the Merlin XII of the Spitfire Mk II introduced a 70/30% water/glycol coolant mix. This new mix improved cooling abilites and was safer than the 100% glycol mix of the earlier Merlin Variants as used in the Mk Ia. Having said that, it was not a magic mixture that = no more overheat.

I agree with Seadog that we need +12lbs boost for the RAF fighters, but I dont agree it could be used all day long without potential consequences. Incidentally the Spitfire Mk IIa is already running at +12lbs boost speeds.

All excellent points Blackdog, to have this as part of a Dynamic campaign would be great. Unfortunately I would not expect it as a feature in Cod at anytime.

This a so true, every time you took off in IL2 it was in a perfectly working factory fresh machine. In real life it was very common for one aircraft to have a significantly different speed than another so called identical one. I remember reading a passage from Jerry Crandall's book "The Focke-Wulf Fw 190 Dora. Vol 1" where two Dora pilots went to see what their new mounts could do. One pilot was the squadron commander and the other a flight leader. Both pilots flew in formation and opened their throttles to max at treetop height, the flight leader remembers hitting 607 kph IAS and he watched the other Dora flown by the squadron commander being left in his wake. After the flight the squadron commander approached him and asked if he could have his Dora, needless to say the guy refused. Luckily the squadron commander didn't pull rank :)

Sorry for going OT there.

Some thought about wear modelling, might as well continue the discussion in a seperate thread, this one is titled "Effect of boost control cutout prior to +12 psi boost modifications"
;)

http://forum.1cpublishing.eu/showthread.php?t=23618

According to Mason in The Hawker Hurricane, Hawker had received 14 Merlin XX engines by June 1940, and from Sept 4 to Sept 30 1940, 32 Hurricane IIs were delivered to operational squadrons. Birtles states that the first Hurricane Mk II was flown on June 11 1940 and the first production Mk II with a 12 gun armament flew in August 1940.

Good info, thanks for posting. I have a few Hurricane books and have read them a few times, it was very silly of me to forget when the Mk II was introduced.

This doesn't follow. The air temperature is lower, but so too is the air density.

The worst case for cooling is generally likely to be minimum speed at FTH.

Above FTH the power decreases in proportion to air density, but the problem is reduced because:

• FTH is likely to be <11 km and so the air temperature continues to decrease during the climb.
• minimum TAS will be higher, and mean TAS will also be higher because airframe drag is less.

BTW, the last time I tested the Spitfire II it was just wrong, and if anything was faster than it should be with a Merlin XII at +12, with the wrong FTH.

Speed is a pretty meaningless metric IMO. It's much better to plot a TAS vs altitude graph and then take the error to be the area of the gap between the lines of real test data and sim data. Of course, this should be an absolute figure, because +ve and -ve errors obviously don't cancel each other out.

In the end, this thread wasn't intended to be an argument about +12 boost.

It was intended to point out that given that we have 87 octane boost indications, we should have 87 octane boost cutout behaviour, which was very different from 100 octane boost cutout behaviour. IMO it's worth simulating this simply because it's interesting.

Obviously we should have 100 octane fuel and +12 boost, but that's for another thread (indeed it's pretty sad that such a thread should be needed at all, but obviously it is).

*edited to remove engine wear stuff to Kurfürst's thread*

Great idea!

Historically, if you were in a Merlin III powered fighter and you needed to use 12lb boost, you did so, and you damned the consequences because staying alive and/or destroying the enemy was more important than explaining away a broken engine. 12lb boost/3000rpm will not cause overheating in level flight. It might cause overheating in prolonged max angle of attack climbs as per Dowding's memo of Aug 1, 1940 but other combat manoeuvres were probably the prime culprit in causing increased engine wear leading to bearing failure, also as per Dowding's memo.

I must admit to being kind of lost on what your stance/opinion is. Now you are saying you want to be able to fly at +12 lbs boost becuase there was a war to win and that was more important than a broken engine. So you accept that flying around in combat at +12 lbs boost could wreck your engine, but that the pilots didn't care?

I am curious on what do you base the above, especially as the engine's coolant system was designed for some 30% lower engine outputs..

Because +12 in level flight = fast, and the cooling system was designed for the max climb case. Flying twice as fast doubles the mass flow rate through the radiator, and thus the heat rejection capability at constant radiator matrix temperature.

If you've got enough radiator for the climb case, you've almost always got too much for high speed level flight - hence the need for variable geometry.

Pity it doesn't work as such in CoD... I mean temp goes up like mad in a moment if you set the radiator flaps too narrow, almost regardless of aircraft speed. :/

The list of bugs/questionable model behaviours is quite a long one... As has been mentioned earlier, at the moment the cooling behaviour is also far too forgiving on the ground; you can idle a Spitfire for ages without boiling the coolant...

Dowding's memo reports a number of combat situations that led to increased engine wear and thus to bearing failure and basically these centre on high G/low oil pressure manoeuvres and inverted flight, both of which led to oil starvation due to low oil pressure and obviously have nothing to do with 12lb/3000rpm flight. Prolonged steep climbs at high boost/rpm was a potential problem area (but why would Dowding mention this if pilots weren't doing it historically?). I previously posted a test report showing that 8.5 hours (in repeated 5min intervals) at 12lb boost/3000rpm did not cause increased engine wear:
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
Basically, it is a question of modelling the cooling and lubrication capability of the aircraft and in straight and level flight 12lb/3000rpm will not cause problems but pilots need to keep a watch on their oil pressure, temp and engine coolant gauges during prolonged steep climbs, and their engine RPM during dives.

So if I am chasing/being chased by a 109 and I'm in level flight and I pull the boost override, I have a very high probability of being able to run at 12lb/3000rpm until I run out of fuel. If I make repeated steep climbs and let my temps and pressures stay in the red, then my probability of engine failure increases. It is a question of accurate flight modelling.

Then it seems I owe you an apology, I have assumed all along that you meant +12 lbs boost was always available without consequence in all flight regimes, combat or otherwise.

Indeed it is far too forgiving on the ground. I just sat on the ground in a Spitfire Mk I idling at around 1200 RPM. If I left the radiator closed the temperature would slowly rise too 120 C, if I opened the radiator it would drop to 80 within seconds. It seems that the radiators have the exact same level of effectiveness regardless of airspeed or airflow.

I should have been clearer from the start in stating that straight and level and/or turning flight was unlikely to be problematic at 12lb/3000rpm because the cooling and lubrication system could cope with it (gauges stay within normal parameters). I suspect that there might be differences between the various aircraft in steep climbs and 12lb/3000rpm in terms of cooling capability but accurate flight modelling will provide danger warnings to the pilot as his gauges go into the red.

The Merlin was cleared for +12lb on Take off and that could be used for "short duration" in an emergency. It was so limited, that it was not cleared for even ONE full minute much less FIVE.

It clearly states that the engine is highly overloaded when using +12lb boost. Furthermore, using it for short duration outside of take off, immediately deadlines the engine until it is inspected by a mechanic and cleared for re-entry into service.

How do those very clear instructions get translated into "could use +12lbs continuously"?

:confused:

The endurance testing quoted in this thread is extremely limited for an endurance test. Most engines are ran continuously for far longer time periods at the emergency conditions during endurance trails with resulting tolerance wear. The Merlin was run for only 5 minutes at a time with a 20 minute rest period between. In that context, the Merlin endurance trials at +12lbs were not successful and the results are far from the "idea" that the Merlin was cleared to run +12lb continuously.

Dowding states in his memo of Aug 01 1940:

http://www.wwiiaircraftperformance.o...ding-12lbs.jpg

so it was certainly cleared for 5 minutes. However, I previously provided a quote:

additionally
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
states:
The engine is not deadlined after using 12lb/3000rpm but such use must be noted in the log book and use beyond 5mins may prompt mechanics to check the engine's oil filters for signs of premature wear and/or bearing failure.

The above memo states that the endurance trials were successful and prompted the recommendation for the use of 12lb boost ASAP.

Well there you go.

I am sure that did not magically happen either. I believe one of the other snippets talks about measures to prevent the cylinder head cracks that appeared during the test in the form of replacing existing parts with new ones that were not prone to cracking.

That probably paved the way for an increase to 5 minutes.

Either way, the notion of +12lbs being using continuously is pure fantasy.

Who cares....

I am sure a few pilots on all sides rolled the dice and took their chances by violating limits placed on their engines.

We hear about the ones who gambled and won. Those who did not win are not around to tell their side of the story.

Facts are, according to the FAA's statistics in aircraft accidents, not adhering to published limits is a factor in ~85% of the engine failures in aviation accidents.

Believing that pilots routinely violate published limits is gamer thinking and not grounded in reality.

Ahh, no the trials were not successful. They pointed the way to modifications which allowed +12 to be used for 5 minutes just like the previous Emergency rating of +9lbs. In comparison most endurance trial engines have tolerance issues or things like worn supercharger blades. They don't have cracked cylinder heads like in the case of the Merlin trials.

Yes...Normal being the life of the engine in comparison to using +9lbs of boost for 5 minutes. It is not the gamer notion that 12lbs could be used continuously. It is just as the FC directs, when used, it must be dead-lined and cleared by a mechanic for return to service.

First off, I don't think it necessary to have the same discussion on two threads.

I pointed out to you that the gycol gasket leak reported at 49.5 hours of the 50 hr test was unrelated to 12lb/3000rpm operation as the memo states.


Again this does not state a requirement for automatic inspection or grounding of aircraft until the engine is inspected.

Dowding's memo states otherwise, and he states "...that some pilots were pulling the plug with little excuse on every occasion..."
http://www.spitfireperformance.com/dowding.pdf

An engine was flight tested for 49.5 hours of which 8.5 hours were at 12lb boost/3000rpm and the engine life was found to be little effected. I think that you might be having trouble translating some of the documents,
http://www.wwiiaircraftperformance.o...bs-14nov39.jpg
as there is nothing in the above to indicate excessive wear resulting from 12lb/3000rpm operation. The mods required for 12lb operation:
http://www.wwiiaircraftperformance.org/ap1590b.jpg
does mention the need for modified cylinder top joints, but then by definition these changes were all made prior to having the boost cutout modded for 12lb operation and the 14 nov 39 memo states that cylinder head problems were unrelated to 12lb operation, but I guess it was felt that more reliable cylinder head gaskets were a desirable feature in combat aircraft and the Merlin in Perspective discusses the problem with coolant leakage in early Merlins and the design changes implemented to correct it. Rapidly changing cylinder head temps were likely to cause leakage problems but by definition prolonged running at 12lb will result in a stable cylinder head temp.

And what did the Pope eat that day ?

Sure, eventually it will get to the ambient temperature....

;)

The title of this thread is "Effect of boost control cutout prior to +12 psi boost modifications".

Arguments about engine behaviour after +12 boost modifications belong elsewhere.

What ? No new doc released on that subject in Wwiiarcraft or spitperf dot com ?

:rolleyes:

*Buzzsaw* linked several links in 109 e3b against spitfire II topic, i noticed one thing:

http://www.ww2aircraft.net/forum/avi...2-a-20108.html

"The discussion was led by 'Glider' Gavin Bailey, who is a well respected historical journal whose material is subject to critical scrutiny by the best of English historians."

He wrote this:
1st August 1940 Memo from Downing re the Handling of the Merlin Engine
This note is advising the pilots that there is an increase in engine failures in the overuse of the emergency 12lb boost.
The interesting thing is that this memo was sent to ALL fighter groups. Had we been talking about the 16 squadrons or less this would not have been the case. It would have been sent to the squadrons involved.

This important information -i think - belongs to this topic. :cool: