Inaccurate performance data for BOB fighters in COD comparing to RL data

[Skoshi Tiger]

Considering those test certificate shown earlier (one issues back in '37) clearly stating that the test engines, even using 87 octane fuel was capable of over 10 1/2 lbs boost at 3000rpm and would give about 200 extra horsepower (at the expence of engine wear and possible failure at some point) at those settings, is there any wonder that in combat situations some of the pilots would try to use it?

Even going so far (before the official modification and the introduction of 100 octane fuel) as modifying their boost cutout controls with match sticks to obtain that extra performance.

I'm sure it was the same for pilots on both sides of the Channel.

[Seadog]

Originally Posted by Skoshi Tiger

Considering those test certificate shown earlier (one issues back in '37) clearly stating that the test engines, even using 87 octane fuel was capable of over 10 1/2 lbs boost at 3000rpm and would give about 200 extra horsepower (at the expence of engine wear and possible failure at some point) at those settings, is there any wonder that in combat situations some of the pilots would try to use it?

Even going so far (before the official modification and the introduction of 100 octane fuel) as modifying their boost cutout controls with match sticks to obtain that extra performance.

I'm sure it was the same for pilots on both sides of the Channel.

No, the engine couldn't run at 10.5lb boost with 87 octane fuel. The story about the pilot modding his engine with a match stick pertained to a Merlin III using 100 octane fuel and an unauthorised mod to obtain 16lb boost at low altitude Here's the some info on the use of 100 octane for development at RR:

RR was using 100 octane fuel for testing and development from 1937 onward:

Appendix IV

The Merlin and 100 Octane Fuel

Questions have been asked on the early use of 100 octane fuel and in particular on its influence during the Battle of Britain. Until 1937 the Merlin had been confined to 87 octane fuel to DTD230, because it was felt that in the event of war 100 octane. which was being developed by the Americans, might not be available to the British. This anxiety arose from the American Neutrality Act. which could prevent supplies being shipped to this country. Probably as the result of a paper by Rod Banks in January 1937, the Air Ministry agreed to proceed with engine development to take advantage of high octane fuel.

At that time the American 100 octane did not suit the Merlin because it lacked a good rich mixture response. Esso undertook the development of a suitable fuel, using 10% aromatics, and the driving force behind this was Dr Bill Sweeney whose fuel mix became known as Sweeney's Blend. Three months before the start of the war an Esso tanker Beaconhill delivered a full cargo of the special 100 octane fuel to Britain and by March 1940 the decision had been taken to switch Fighter Command to this type. Bomber Command changed over early in 1941.

The effect of 100 octane was to allow the Merlin to run at 12lb boost putting up the power of the Merlin III from just over 1000 hp to 1300 hp. However, this high power was obtained at between 8 000 and 9 000 ft and above this altitude, at a max combat power rpm of 3000, the boost and, therefore. power advantage was progressively declining. On 87 octane fuel and 6lb boost, using 3 000 rpm, the maximum power was 1030 hp at 16 000 ft. At this point on either fuel the engine was giving the same power, so above this height 100 octane fuel offered no advantage. The majority of the air fighting in the Battle of Britain was at 18 000 ft and above and the engine in most common use was the Merlin III. The gain in performance from 100 octane was entirely at lower altitudes. Before the end of the Battle Spitfire IIs with Merlin XIIs were in service, with the supercharger gear ratio increased from 8.58 to 9.09:1 giving a better full throttle height at 12lb boost and a small number of Hurricane IIs fitted with two-speed Merlin XXs. with ratios of 8.15 and 9.49:1 for MS and FS gear, these engines could take much greater advantage of 100 octane fuel and in the case of the Merlin XX were capable of maintaining 12lb boost to over 20 000 ft at 3 000 rpm, thanks to the new central entry supercharger.

This set the pattern and without 100 octane fuel the further power development of the Merlin would not have been possible. As an example the two stage blown Merlin 66 was capable of over 1600 hp at 16000 ft using 3000 rpm and 181b boost. The pioneering work of Esso to produce a suitable 100 octane fuel was the key to the high power Merlins in all spheres of operation and it was not until 1944 when 150 grade fuel became available that further advances in boost pressure to 25lb were made, allowing over 2 000 hp to be used in squadron service.

The opening paragraphs of this appendix are the result of information supplied to Michael Evans, Chairman of the Rolls-Royce Heritage Trust by Alexander Ogston, Historian of the Wings Club in New York, who has had a lifetime in fuel technology and a conversation which I had with Rod Banks shortly before his death.

The Merlin in Perspective, p87.

as has been pointed out the boost over-ride needed to be modded to allow for greater than 6.25lb boost, and this was only done when 100 octane fuel was used.

[41Sqn_Banks]

Originally Posted by Seadog

as has been pointed out the boost over-ride needed to be modded to allow for greater than 6.25lb boost, and this was only done when 100 octane fuel was used.

This is not correct. In the unmodified condition there was no restriction of the maximum boost when the boost control cut-out was used, thus the boost could be increased to about +17 lbs at full throttle at sea level.

See:
http://forum.1cpublishing.eu/attachm...1&d=1337242614
http://www.spitfireperformance.com/spit1-12lbs.jpg

[NZtyphoon]

Having found a 1942 (?) copy of R A Beaumont Aeronautical Engineering: A Practical Guide for Everyone Connected with the Aero Industry (modern reprint http://www.pitstop.net.au/view/aviat...uery/plu/23199) it's interesting to read what he says about the "four degrees (types) of boost" used by British aero engines:

(1) Take-off Boost - ...an additional amount of induction-pipe pressure....accompanied by an extra richening of the mixture which prevents an unstable combustion termed detonation...(p. 105)

(2) The Rated Boost - often known as maximum climbing boost....boost pressure is reduced to a value which permits the engine to run continuously at that particular power output...(p. 105-106)

(3)Override Boost - For the greatest possible power output for take-off or emergency, an increase in pressure above the normal take-off boost is permitted on some engines. This condition is used in conjunction with a special fuel. (p. 106)

(4) Maximum Cruising Boost....

Thus it is specifically stated that override or emergency boost was used only in conjunction with a fuel that permitted the higher boost rating.

[Ernst]

Acctualy the SIM should be: SUPERMARINE SPITFIRE: CLIFFS OF DOVER. I almost sure the right the devs must go to BoM. At least no spitfires there. Most of data here come only from two sources: spitfireperformance.com or from Kurfurst site. I am almost sure that the DEVS had other data, maybe from URSS evaluations of the SPITS and 109s that not exactly match the data presented here.

[Crumpp]

No, the engine couldn't run at 10.5lb boost with 87 octane fuel. The story about the pilot modding his engine with a match stick pertained to a Merlin III using 100 octane fuel and an unauthorised mod to obtain 16lb boost at low altitude Here's the some info on the use of 100 octane for development at RR:

No Seadog, it was possible as we can see from the Operating Notes. I am sure that engine was trashed after overboosting to +16lbs but it was definately possible on 87 Octane fuel.

It definately was not good but it was possible. Pulling the tit and overboosting the engine is not proof of the use of 100 Octane fuel.

[Seadog]

Originally Posted by Crumpp

No Seadog, it was possible as we can see from the Operating Notes. I am sure that engine was trashed after overboosting to +16lbs but it was definately possible on 87 Octane fuel.

It definately was not good but it was possible. Pulling the tit and overboosting the engine is not proof of the use of 100 Octane fuel.

read:


The above story clearly pertains to an engine that was already modded for 12lb boost (and thus using 100 octane) and the match stick mod permit boost up to 18lb, and it has already been established that Hurricanes in France were using 100 octane fuel by May 1940.

Maximum boost with 87 octane was 7lb:

http://www.spitfireperformance.com/spit2pnfs3.jpg

[NZtyphoon]

Originally Posted by Crumpp

Pulling the tit and overboosting the engine is not proof of the use of 100 Octane fuel.

Wrong - there are two engine tests, one on 87 Octane and one on 100 which both ran up to 10.5 lbs boost - there was no boost override used which, as Beaumont observed, required special fuel - ie: 100 Octane - to achieve. Later Merlin engines, such as the 45 series, were capable of boost override of +18 lbs but needed, and were designed to use, 100 Octane to do so. Later still +25 lbs boost could only be achieved using 150 Octane.

[Seadog]

Originally Posted by Crumpp

No Seadog, it was possible as we can see from the Operating Notes. I am sure that engine was trashed after overboosting to +16lbs but it was definately possible on 87 Octane fuel.

It definately was not good but it was possible. Pulling the tit and overboosting the engine is not proof of the use of 100 Octane fuel.

Pulling the boost override with 87 octane would be equivalent to committing suicide; even if the engine didn't fail immediately, it would lose power due to premature detonation. Why in God's name would any sane pilot "pull the plug" if it resulted in loss of performance?

Yet, in all the combat reports of the pilots pulling the plug, the aircraft responded with increased performance; there are no reports where the engine began to suffer detonation and/or failed...

[klem]

Originally Posted by Ernst

Acctualy the SIM should be: SUPERMARINE SPITFIRE: CLIFFS OF DOVER. I almost sure the right the devs must go to BoM. At least no spitfires there. Most of data here come only from two sources: spitfireperformance.com or from Kurfurst site. I am almost sure that the DEVS had other data, maybe from URSS evaluations of the SPITS and 109s that not exactly match the data presented here.

Well a key point about the spitfireperformance website is it gives us data collected at the time in reports written at the time. It is unlikely that Russian test organisations would have got any nearer to the true performance figures than the companies building the aircraft and engines and their prime user the RAF and its test organisations.