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

[Viper2000]

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:



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:



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.

[whoarmongar]

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

[Viper2000]

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.

[41Sqn_Banks]

AP 1590B Merlin II and III Aero-Engines

In its unmodified condition, the boost control cut-out valve permitted the boost pressure to rise very considerably. Mod. Merlin 154, however, limits the pressure to +12 lb. per sq. in. for use under emergency condition (see para. 359A et seq.).

[*Buzzsaw*]

Originally Posted by Viper2000

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.

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

[SYN_Flashman]

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?

[utu]

Originally Posted by *Buzzsaw*

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

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

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.

[Viper2000]

Originally Posted by *Buzzsaw*

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

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.

[=XIII=Wedge]

Originally Posted by Viper2000

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.

[Viper2000]

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.