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

[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.