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FM/DM threads Everything about FM/DM in CoD

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Old 05-06-2012, 11:53 AM
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Osprey Osprey is offline
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Default Bug 174 on 12lbs boost. Review please.

I'm about to update bug #174 on the Merlin 100 octane issue. I have re-written the bug for Artist as requested and would like a review. Kurfurst and Crumpp will be ignored, I am interested in ratification from one or more from Glider/Banks/NZTyphoon/Robo/lane. If this is ok I'll post the update, if there is anything I can add such as a good graph or table or test please post. Thanks.

Presently if the Automatic Boost Control (ABC) is pulled on Spitfires and Hurricanes and full throttle applied there is no increase in boost above 6.25lbs even when the ABC is enabled. This results in reduced and inaccurate performance of RAF fighter aircraft.

During the BoB all frontline fighters had been modified to use 100 octane fuel in their Merlin II and III engines which allowed the engine to achieve 12lbs boost under strict limits. The Merlin XII engine fitted to the Spitfire II was designed to use this fuel from the factory. The approval and introduction of these changes gave the RAF fighters a performance boost in top speed, acceleration and rate of climb up to the Full Throttle Height (FTH) of about 18,000ft. At sea level a Spitfire was about 30mph faster when the ABC was enabled.

Merlin II and III (fitted to Spitfire Ia and Hurricane Ia)
The Merlin engines have a mechanical supercharger and can deliver up to 20lbs of boost at S.L. with the throttle valve fully open using either 87 or 100 octane fuel however this would cause serious engine damage by pre-detonation in the cylinders. Because of this the Merlin II and III have a boost controller fitted which limits the boost to only 6.25lbs. With modification to use 100 octane fuel the pilot could achieve 12lbs boost by pulling the Automatic Boost Control increasing the ‘safe’ power that the engine could produce. When the pilot applied the ABC on the Spitfire a thin wire was broken to indicate to the ground crew that ABC had been used and to make necessary checks. The Hurricane had a valve which was pulled (“pulling the tit”)

Merlin XII (fitted to Spitfire IIa)
The Merlin XII did not require modification in order to use 100 octane fuel. It had a slightly different throttle system and used a ‘gate’ on the throttle control. This allowed the pilot to achieve 12.5lbs boost on takeoff and is allowed up to 1000ft when he moved the throttle past the gate. Up to the gate 9lbs boost was achievable up to the FTH of 17000ft and later clearance was granted to use 12lbs boost (see pilot notes) There are lots of combat reports supporting this.

Here are the engine power ratings for given boost vs altitude

Spitfire Ia prop tests using 6.25lbs boost and 87 octane
It contains information on the clearance and usage of 100 octane fuel including this graph for the Spitfire Ia using 12lbs boost

Here is the performance test for the Hurricane Ia using 100 octane fuel and 12lbs boost

Here is the performance for the Spitfire IIa
Note that the boost for the tests is 8.8lbs, that is up to the ‘gate’ and this falls away from 17,500ft, the FTH.

Pilot notes for the Spitfire IIa indicate the limitations

This report is evidence for the approval of 12lbs/100 octane prior to the Battle of Britain start

This is supported by lots of evidence of its use in combat reports (supplied by others in this bug report)

These reports not only supply data on the speed per altitude but also rate of climb and acceleration, plus some information of spin characteristics, dive ability and rate of turn. This should all be used to improve the current flight models for RAF types.

Last edited by Osprey; 05-07-2012 at 03:47 PM.
Old 05-06-2012, 12:28 PM
Glider Glider is offline
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looks good to me
Old 05-06-2012, 01:25 PM
41Sqn_Banks 41Sqn_Banks is offline
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- IIRC ABC stands for "automatic boost control" and not the cut-out.

- Spitfire II had "gate control" for fixed take-off boost (it gave a fixed throttle valve setting) and "boost control cut-out" like the Spitfire I, see this thread: http://forum.1cpublishing.eu/showthread.php?t=31319
Old 05-06-2012, 01:28 PM
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fruitbat fruitbat is offline
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I think 1C will be as stubborn about this as they were about the FW190 bar in il2.
Old 05-06-2012, 02:49 PM
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Could you add any indications of the duration for which running at 12lbs / 12.5 lbs was allowed? They would have to model engine damage if the time was exceeded and is essential for any correct FM/DM
Old 05-06-2012, 03:48 PM
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The official limit was 5 mins but there was nothing to stop pilots going for a lot longer than that. Dowding issued a memo to all pilots that warned them of overusing the boost. In some of the combat reports you sometimes see words along the line of switched guns to fire, lowered the seat and pulled the plug.
Old 05-06-2012, 03:52 PM
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@Banks: Thanks.
@Stormcrow: Pilot notes specify the limits.
Old 05-06-2012, 04:56 PM
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Bonkin Bonkin is offline
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Wow. That's a wealth of information that you have accumulated there Osprey.

I have an original Air Publication 1565A "The Spitfire I Aeroplane Merlin II Engine" manual which I thought might be useful - but I think those references you have must cover it all. I went through it specifically looking for performance charts but unfortunately there are none. There are change bars around the fuel and pitch sections though... and it states:

Fuel ....... Specification D.T.D.230
Note:- 100 octane fuel may be used, if the engine has been suitably modified.

Under the Automatic boost control it does mention +12lb/sq.in at sea level but does not tie this to 100 octane fuel.
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Old 05-06-2012, 08:02 PM
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Just had a speed test in the Rotol Hurricane. Couldn't get more than 230mph ASi out of it, trimmed, level flight, at various RPm (best 2650). According to this:


I should get 261mph with the Rotol which is 290mph TAS.
Old 05-06-2012, 08:18 PM
Biggs Biggs is offline
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i have a feeling as well that we will not have correct speed performance for these engines for a while.

I applaud the efforts of you and others to try and keep this issue near the forefront... 1C needs to get it right.
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