There is a popular idea that it is possible to say which aeroplane was faster, flew higher, was the most heavily-armed, and the internet is full of ‘Top Trump’ (remember those) arguments about comparative performance.
In reality, all new fighter designs went up against the edges of what was possible and the differences were so marginal as to make the answers always ‘it depends’ – which is never a popular answer but rooted in fact.
A recent discussion arising from conversations between WFP member Jim Munro (who is making a film about the Whirlwind) Dodge Bailey (Chief Pilot at the Shuttleworth Trust), John McClure, a veteran Whirlwind pilot who shot down a Ju-88 while with 137 Squadron, and myself has produced interesting results.
The acknowledged ‘fastest fighter’ in 1940, when the Whirlwind entered service, was the Spitfire Mk. I. A key difference between the Peregrine engines of the Whirlwind and the Spitfire’s Merlin was ‘Full Throttle height (FTH).
FTH is the height at which the throttle could be opened to its full extent – it had to be restricted lower down to avoid ‘detonation’. This was also the altitude of maximum performance – above FTH the reduction in air pressure into the ‘gate’ progressively reduced ‘boost’ – the additional ‘charge’ of air into the cylinders giving extra horsepower (the point of superchargers). That’s as technical as this gets.
The Peregrine had a lower FTH. Essentially, available horsepower peaked lower down. It wasn’t a failing, it was designed that way.
The Aircraft and Armourment Experimental Establishment (A&AEE), where performance figures were carefully established, pegged the Whirlwind and the Spitfire as pretty much equal at 354 m.p.h. at their respective FTH’s.
However, the nature of the curves meant that the Whirlwind was faster than a Spitfire when tested below its FTH, and slower above it.
The problem for the Whirlwind in competition with the Spitfire was ‘boost’. Improved fuels and wartime conditions meant that units were permitted to operate at higher supercharging rates (increased pressures) than those at which the aircraft had been initially tested by the A&AEE.
When tested, the Peregrine was rated at 6.75 lbs. permissible boost, with a FTH of 15,800ft. John McClure recently kindly volunteered the information that by 1941 the maximum permissible boost had been raised to 9 lbs.
Similarly the Spitfire I was tested at 6.25 lbs boost. At some point early in the Battle of Britain the maximum permissible had been raised to 12 lbs.
This graph shows the effect of all this – the black lines to the right are my additions to a 1940 graph of Spitfire performance from the A&AEE. The thick black lines are the test results for the Whirlwind prototype L6845 with 6.75 lbs , the thin black line is my extrapolation to 9 lbs.
The blue line is the ‘official’ initial Spitfire I performance, slower low down than the ‘official’ initial Whirlwind performance. However there are pencil lines put in at the time that include performance at plus 12 lbs, faster than the WW, even at plus 9..
Please ignore the watermark, this is Crown Copyright with my additions.
So, literally pound-for-pound, the Whirlwind was initially faster low down but due to the accelerated development and ‘proving’ of the Merlin into higher boost regimes it may never have had a chance to be so in service.