BoB Spitfire 100 Octane fuel

[klem]

Originally Posted by winny

There is a slight problem with k9787 ( 1st production Spitfire ) as a reference for mk 1a's. It was much lighter than a mk 1 in BoB trim. All up weight of k9787 was only 5819 lb, mk 1 in BoB trim 6155 lb ( source Alfred Price Spitfire in combat). It didn't have the bullet proof windscreen, which cost 6 mph, the IFF aerials cost about 2 mph, no armour plate.. Etc.

Top speed according to price of a fully equipped mk 1 in the summer of 1940 was nearer 350 mph. It's a minefield!

Just chewing the cud now...

Yes and I actually made a mistake over the K9787 prop - it was a fixed pitch wooden prop, quite light. 5819lbs all up, Merlin II, +6.25lbs, 361mph at 18,000 (360.5 @20k).

K9793 with the MerlinII and the 2-pitch metal prop and weighing only 5935lbs put up 366mph at 20,000 feet.

N.3171 weighed 6050lbs during its tests (231lbs heavier than K9787), Merlin III (same power as II) same boost +6.25, heavier Rotol prop, blown canopy and bulletproof windscreen, 354mph at 20,000. I don't think Alfred Price would argue with that. The ~6mph loss was attributed to the bullet proof screen but I can't help feeling that weight had something to do with it and perhaps the canopy too. Just to keep the Spit/109E comparisons in order, here's that level speed chart again:-
http://www.spitfireperformance.com/spit1vrs109e.html

One final point on N.3171, the speed tests were made at 3,000 rpm but they also reported:
"The results show that the maximum level speed is reached with the airscrew controlling at 2800 engine r.p.m. On increasing the r.p.m. to 3000 the speed was reduced, on the average by 4 m.p.h." So perhaps we could add 4mph to that 354mph result.

And again this shows the danger of just reading off max speeds as gospel. All three aircraft had different weights, different props plus a couple of other bits on N.3171.