SPIT MK I/II and over boost

[Kurfürst]

Originally Posted by CaptainDoggles

I'll agree that the Spitfire Mk.1 is underperforming, but not because of "87 octane".

Frankly I am not sure about them. Do they (Mark Is, they seem to be modelled with 87 octane boost levels) not meet specs for a 87 octane one? Climb too slow? Or..?

[VO101_Tom]

Originally Posted by klem

Possibly. I haven't looked into it but perhaps the 'undermodelling' of the Spitfires is because they are modelled with 87 Octane?

Originally Posted by Kurfürst

Frankly I am not sure about them. Do they (Mark Is, they seem to be modelled with 87 octane boost levels) not meet specs for a 87 octane one? Climb too slow? Or..?

No.
Couple of months ago i compared the www.spitperformance.org graphs with the game speeds (here). The graphs show the 6lbs boost (with 87 oct. fuel) speed of Spitfire. The 109 would be faster, but not that much – if we look at the proportions of course, because now the 109 is slower than it should).

[klem]

Originally Posted by VO101_Tom

No.
Couple of months ago i compared the www.spitperformance.org graphs with the game speeds (here). The graphs show the 6lbs boost (with 87 oct. fuel) speed of Spitfire. The 109 would be faster, but not that much – if we look at the proportions of course, because now the 109 is slower than it should).

Thanks tom, it was just a thought.
(I think this is the link you meant http://www.spitfireperformance.com/spitfire-II.html )

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@Kurfurst. Sorry if I was not clear. I was talking about the Spitfires that are universally claimed to be "underperforming", these are the Spitfire I and Ia. I did not mean the Spitfire IIs.

I did some quick checks on the Mk II a few months ago against the MkII report linked above. I couldn't get it to fly as fast as the report stated but it was very close, just a couple of % under. Mind you I was pretty crude about it, as it meant thrashing the engine at the maximum boost I could make at that altitude (the report suggests +9lbs boost was used) and eventually it broke. So not to be taken too seriously and probably also impatience on my part. Results:-

Alt Boost RPM ASI mph ASI in report
6500 +5.3 2990 280 between 306 @ 5,000' and 326 @ 10,000
10000 +5.3 2990 280 326
15000 +6.2 2990 270 345
(on another run at 10,000' I seem to have written down +5.3 and 290 mph {?!} )

All below the reported data but of course I could not get +9lbs boost.

You may find this site interesting. I can't vouch for its accuracy butit looks pretty good to me.....
http://www.spitfireperformance.com/
http://www.spitfireperformance.com/spit1vrs109e.html
You can see from the level speed chart why we want the 100 octane - and I can understand why you would not be happy about that. By the way I have read several bio accounts of how, in the BoB, the 109 would escape in a dive but on occasions it was possible for the Spitfire to catch them in long chase.

Incidentally on use of 100 octane fuel, I know we seem to have agreed to disagree but that last link contains the following ...

"As of 31 March 1940 220,000 tons of 100 octane fuel was held in stock. The Co-ordination of Oil Policy Committee noted in the conclusions of their 18 May 1940 meeting with regard to the "Supply of 100 Octane fuel to Blenheim and Fighter Squadrons" that Spitfire and Hurricane units "had now been stocked with the necessary 100 octane fuel". The Committee recorded that actual consumption of 100 octane for the 2nd Quarter 1940 was 18,100 tons.

Jeffrey Quill recalled:

It was only shortly before the Battle of Britain that we changed over to 100 octane. It had the effect of increasing the combat rating of the Merlin from 3000 rpm at 6 1/2 lb boost (Merlin III) or 9 lb boost (Merlin XII) to 3,000 rpm at 12 lb boost. This, of course, had a significant effect upon the rate of climb, particularly as the constant speed propellers (also introduced just before the battle) ensured that 3,000 rpm was obtainable from the ground upwards whereas previously it was restricted by the two-pitch propellers. It also had an effect upon the maximum speed but this was not so significant as the effect upon rate of climb."

What is interesting apart from the date by which FC was converted is that quarterly use was 18,100 tons against a stockpile of 220,000 tons. There was no shortage.