[Viper2000]

Originally Posted by Kurfurst

'Fighter' seems to make the sentence untrue, given that the supposed .89 Mach figure was measured on an unarmed photo recce aircraft (ie. aerodynamic windshield, no cannons stubs, no MG ports ruining the flow over the wing); besides the fact that the said report of the PR XI dive measured does not mention a thing about control behaviour..

I have a report of a proper Mark IX (ie. Fighter) dive trial, and it shows exactly the same symptoms of loosing control as any other fighter above 0.80 Mach. Add to that the instruments were also inaccurate at these speeds, and you have a myth liked by fans, but with very little root in reality.

I didn't claim that a Spitfire fighter was capable of Mach 0.89 in a controlled dive. You seem to have set the PR.XI Mach number up as a straw-man.

The Spitfire Pilots Notes put the dive limit at 450 mph IAS (after position error; so really it's more like CAS but without the modern compressibility correction) or what was effectively Mach 0.85, the limit being defined by a lookup table due to the absence of a Mach meter.

I would suggest that Eric Brown is probably the best reference for relative performance of fighter aeroplanes because he flew so many types.

It's fine to argue instrument error when you're talking about squadron pilots diving in the heat of battle and seeing fantastic numbers on their ASI. Indeed, I'm more than happy to offer up the alleged Mach 0.92 dive by a Griffon Spitfire in the vicinity of Hong Kong post war as likely erroneous.

However, RAE were a competent flight test organisation, and they were perfectly capable of correcting for compressibility. The same goes for NACA, though it is notable that the USAAF went to RAE for an assessment of the high speed handling characteristics of their fighter aeroplanes (See Wings on my Sleeve).

Compressibility correction for a pitot tube really isn't that hard, especially subsonic when you can just say that gamma = 1.4.

Therefore I have considerable confidence in the Spitfire PR.XI dive data showing Mach 0.89; if you look at Morgan & Shacklady you'll see that the aeroplane was rather impressively instrumented for these high speed dives. I also note that this tended to break engines due to overspeeding, resulting in several serious accidents, despite the fact that the propeller was modified to feather in an attempt to contain rpm.

So I wouldn't claim that a Spitfire fighter could be safely operated by a squadron pilot at such a high Mach number.

But I have no reason to believe that it was unsafe to operate the aeroplane within its published envelope (i.e. the lower of 450 mph after position error correction, or Mach 0.85), not least because pilots tend to get quite vocal if aeroplanes scare them within the published envelope, and I also have no reason to disbelieve the tactical Mach numbers quoted by Eric Brown in his various books.

IIRC there may be some dive test data from a Spitfire IX showing a tactical limit of about Mach 0.83 out there somewhere. This would be fairly reasonable.

AFAIK the tactical limit for the Mustang is about 0.80, Thunderbolt about 0.72, Bf-109 and Fw-190A 0.75, whilst the P-38 was only ok to about 0.68. However, my books are at home; the numbers can be cross-checked in Wings on my Sleeve, Wings of the Luftwaffe, and Wings of the Weird & Wonderful.