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

[Crumpp]

Can I ask again for a link to the notes you are looking at

I don't have a link Glider. These are not posted on some website but part of my collection. I scan the pages and post them.

The notes you quote do not seem to tie up to what is know about the updated version of the Spitfire in use during the BOB.

Exactly. Quite a large "Fly in the Ointment" for silly claims like:

before the BoB started, ~30 squadrons of Spitfires and Hurricanes are known to have been converted to 12 lb boost. These squadrons would be those that would most likely come in contact with the Luftwaffe.

100 octane was the predominant fuel used by Fighter Command.

And depending on the dates you pick for the battle to be over:

by the end of the BoB, Fighter Command had converted to 12lb boost.

You wonder why I question what you claim "is known." Facts are it is not known.

[Crumpp]

there was never a shortage of 100 octane fuel.

What?? There was a shortage of 100/130 grade on several occasions during the war.

There are several reports on the aviation gasoline situation available at both Maxwell AFB and Dayton OH.

[Osprey]

Redbeard Rum.

[JG5_Thijs]

Hello all,

With great interested I’ve been following the discussion regarding the use of 100 octane fuel by the RAF in the Battle of Britain. I decided to look up some scientific articles, but could only find the following:

Gavin Bailey, ‘Narrow margin of criticality: The question of the supply of 100-octane fuel in the Battle of Britain’ English Historical Review volume 123 number 501 (200 p 394-411. (This article was quoted earlier by 28_Condor on page 98 of this thread, he, however, did not quote the article fully since there are some interesting points that Bailey brings up regarding the impact of 100 fuel use.)

There are some interesting things in this article regarding the use of 100 octane fuel and the performance of the Spitfire Mk I and II. A short summary. First a quick summary of the availability of 100 octane fuel, then the operational usefulness of 100 octane fuel.

Bailey on the availability of 100 octane fuel

Bailey states that at the time the war broke out there was 153,000 tons of 100 octane fuel in stock, compared to 323, 000 tons of other aviation fuels. In February 1940 the stock of 100 octane fuel had risen to 220,000 tons. In May 1940 fighter units began converting to 100 octane fuel and there was plenty of 100 octane fuel available for the duration of the Battle of Britain.(406)

Note by me about 100 octane being used in the game in this respect: This quote above, and the other information provided by other people on this forum, makes me conclude that use of 100 octane fuel was widespread during the BoB. It would therefore be fine to program RAF planes with 100 octane, or give the option to mission builders to choose between 87 and 100 octane fuel.
This, however, is not say anything on the great improvement of 100 octane fuel gives over 87 octane fuel as claimed by many authors and people on this forum.

Bailey on what other authors write about the use of 100 octane fuel:

He says that other authors argue that the use of 100 octane fuel from America was one of the critical advantages for British fighters during the battle, he does not agree with this vision.(394-395) Bailey argues that it tends to be forgotten that the widespread use of 100 octane fuel is in the same time period as the introduction of the constant-speed, variable pitch propeller which offers a much larger performance increase than the new fuel alone. (395)

To demonstrate this Bailey uses two tables:

The following table shows a test of a Spitfire Mk Ia and Spitfire Mk II (399)


Spitfire MK I test with a fixed propeller.


Table 1: There is only a marginal improvement in the rate of climb and maximum speed comparing both planes in the first table. There is however, a dramatic increase between a Spitfire with a fixed propeller and the newer variable pitch one. See table 2 (401)

Bailey concludes that the main advantage of 100 octane fuel was at lower altitudes, but was marginal at best at higher altitudes.(401) His table demonstrate that there is actually a drop in top speed at higher altitudes.

Bailey on the boost of the Merlin engine

The author gives the following information about the boost increase that was achieved by 100 octane fuel: Normal limitation on the supercharger compression of a Rolls-Royce Merlin III with 87 octane fuel was +6.25 inch above atmospheric pressure. The introduction of 100 octane fuel increased this to +12 for short periods, not exceeding 5 minutes.(39


Take off to 1,000 ft — 3,000 rpm at +7 psi/+12.5 psi;
Maximum climb (1-hr. limit) — 2,850 rpm at +7 psi/+9 psi;
Combat (5 min. maximum) — 3,000 rpm at +7 psi/+12 psi.
(This chart is about Spitfire MK II with 100 octane boost which Baily took from the following source: Air Ministry, Air Publication 1565B, Pilots Notes, Spitfire IIA and IIB Aeroplanes, Merlin XII Engine (anonymous Air Ministry publication, London, 1940, amended 1942).)

Conclusion by Bailey:

He concludes that the dramatic performance increase because of 100 octane is overrated and that other, earlier, authors wrongly claim that there is. These other authors forgot that the variable pitch prop was the real source of the dramatic performance increase of RAF planes which they contribute solely to 100 octane fuel.

Comment by me regarding the information given above: It seems that an increase from 87 to 100 octane fuel (but with a variable pitch for both) only leads to a marginal improvement. Whether the planes in this game are modelled correctly is not within the scope of this argument.

Regards,

Thijs

[Seadog]

Originally Posted by JG5_Thijs

The following table shows a test of a Spitfire Mk Ia and Spitfire Mk II (399)

Table 1: There is only a marginal improvement in the rate of climb and maximum speed comparing both planes in the first table...

Bailey concludes that the main advantage of 100 octane fuel was at lower altitudes, but was marginal at best at higher altitudes.(401) His table demonstrate that there is actually a drop in top speed at higher altitudes.

Bailey on the boost of the Merlin engine

The author gives the following information about the boost increase that was achieved by 100 octane fuel: Normal limitation on the supercharger compression of a Rolls-Royce Merlin III with 87 octane fuel was +6.25 inch above atmospheric pressure. The introduction of 100 octane fuel increased this to +12 for short periods, not exceeding 5 minutes.(39


Take off to 1,000 ft — 3,000 rpm at +7 psi/+12.5 psi;
Maximum climb (1-hr. limit) — 2,850 rpm at +7 psi/+9 psi;
Combat (5 min. maximum) — 3,000 rpm at +7 psi/+12 psi.
(This chart is about Spitfire MK II with 100 octane boost which Baily took from the following source: Air Ministry, Air Publication 1565B, Pilots Notes, Spitfire IIA and IIB Aeroplanes, Merlin XII Engine (anonymous Air Ministry publication, London, 1940, amended 1942).)

Conclusion by Bailey:

He concludes that the dramatic performance increase because of 100 octane is overrated and that other, earlier, authors wrongly claim that there is. These other authors forgot that the variable pitch prop was the real source of the dramatic performance increase of RAF planes which they contribute solely to 100 octane fuel.

Comment by me regarding the information given above: It seems that an increase from 87 to 100 octane fuel (but with a variable pitch for both) only leads to a marginal improvement. Whether the planes in this game are modelled correctly is not within the scope of this argument.

Regards,

Thijs

The performance figures given in your post are for the MkI/IIaircraft using 6.25/8.8lb boost and the actual performance test results verifying this can be seen here:

http://www.spitfireperformance.com/spitfire-II.html

but the data you quote also states the maximum boost as 6.25lb/9lb respectively, which explains the small margin of improvement of the MkII over the MkI, especially as the MkII is somewhat heavier as well.

Thus neither aircraft was using the 5min/12lb boost combat rating of the engine which was only possible when using 100 octane fuel. By way of comparison a Hurricane I could achieve ~323mph at 10,000ft by using 100 octane fuel/12lb boost:



and here's the RAE data for a Spitfire I with various boost levels:


A RAF memo from 1939 confirms all the above:

It will be noted from the service reports that an approximate increase in speed due to the use of emergency 12lb boost of 28/34mph is obtained depending upon the altitude flown up to 10,000ft.
http://www.spitfireperformance.com/spit1-12lbs.jpg.

[Kurfürst]

Originally Posted by Seadog

and here's the RAE data for a Spitfire I with various boost levels:


A RAF memo from 1939 confirms all the above:

I've always wondered who had really drawn that graph.

[Seadog]

Originally Posted by Kurfürst

I've always wondered who had really drawn that graph.

Do you wonder who wrote the memo stating a 28/38 mph increase in speed under 10,000ft?

It will be noted from the service reports that an approximate increase in speed due to the use of emergency 12lb boost of 28/34mph is obtained depending upon the altitude flown up to 10,000ft.
http://www.spitfireperformance.com/spit1-12lbs.jpg.

Do you wonder who wrote this graph:




and do you wonder why these numbers are all so consistent? Are you trying to imply that 12lb boost will not result in an increase in speed over 6.25lb boost?

It also seems that you are trying to imply that the RAE Spitfire I data for 12 and 16 lbs boost was falsified? I really hope that this is not the case.

[28_Condor]

Originally Posted by JG5_Thijs

Gavin Bailey, ‘Narrow margin of criticality: The question of the supply of 100-octane fuel in the Battle of Britain’ English Historical Review volume 123 number 501 (200 p 394-411. (This article was quoted earlier by 28_Condor on page 98 of this thread, he, however, did not quote the article fully since there are some interesting points that Bailey brings up regarding the impact of 100 fuel use.)

Thijs

The download link didnt work

You can make the file available here? Thank you!

[Al Schlageter]

Originally Posted by Crumpp

What?? There was a shortage of 100/130 grade on several occasions during the war.

What the hell does 100/130 fuel, which wasn't even around during BoB, have to do with 100 octane fuel?

We still haven't seen from you the identity of the 16 squadrons that you say were the only squadrons converted to 12 lb boost.

[kendo65]

Great post JG5_Thijs. Thanks.