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

[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