[Viper2000]

Originally Posted by Kurfurst

No straw man here, the PR.XI PR Mach number is just happens to be the most commonly referenced.



What is a 'tactical limit'?

I'm using the nomenclature adopted by Eric Brown, which is that the tactical Mach number is the maximum Mach number at which the aeroplane may be used tactically, whilst the Critical Mach number is the maximum Mach number at which the aeroplane may be controlled. I am personally of the view that tactically in this context probably means offensively, because you can obviously frustrate an enemy's gun solution after having departed from controlled flight...

Obviously, with manual controls, both of these Mach numbers depend upon pilot strength.

Limits in the Pilot's Notes are there to protect the airframe and engine from harm, so they will tend to correlate more closely with what Brown would call the critical Mach number than with what he would call the tactical Mach number.

N.B. - This nomenclature is inherently confusing the aerodynamicists, who would tend to think of the critical Mach number as the lowest freestream Mach number at which sonic flow is seen around whatever shape they're examining. Therefore, for example, you can't look up aerofoil critical Mach numbers in Abbot & Doenhoff and compare them with flight test reports.

Originally Posted by Kurfurst

Basically I think the Mach 0.85 dive limit is arbitrary, ad hoc, ex stomach etc. - a bold guesswork that was set well before they would test the actual capability, just like many of the era's limits, though a bit bolder..

I am inclined to disagree, because Henshaw et al routinely conducted dive tests to the limiting CAS as part of their production testing at Castle Bromwich. This actually involved diving to 470 mph IAS because the instrument error was assumed to be 20 mph IAS. So there was no great shortage of knowledge as to the behaviour of the aeroplane at the placarded limits of its envelope.

Originally Posted by Kurfurst

But, personally I believe the behaviour shown by fighter Mark IX BS 310 was certainly no greatly different - better or worse - than just about any WW2 fighter: controls functioned normally up to about .70 Mach, then all sorts of anomalies began to appear.. and at 0.815, there's already a longitudal pitching motion - and 0.85 is still rather far away..

By Mach 0.70 a P-38 would already have departed from controlled flight...

I'd say that it's probably reasonably safe to place the tactical Mach number of the Spitfire at approximately 0.80, perhaps higher for the Griffon Spitfire due to its longer nose and correspondingly higher fineness ratio.

Additionally, comparison with the P-51D is interesting:

http://www.wwiiaircraftperformance.o...-divetest.html

http://www.wwiiaircraftperformance.o...-27-feb-45.pdf

IMO the Spitfire comes out of this comparison looking pretty good.

Originally Posted by Kurfurst

Do you have the rest of this report? It's interesting stuff.

Note, however, that it's a somewhat different animal from the PR.XI tests. The PR.XI tests were aimed at getting the highest possible Mach number on the clock. They therefore involved dives from about 40000', the maximum Mach number being reached at about 29000' with the aeroplane unloaded; recoveries were pretty gentle at about 2 g.

Meanwhile, your report appears to be an attempt to investigate the effect of Mach number upon CLmax; it talks about 5 g recoveries, which is pretty brave out at the edge of the envelope; the Mustang dive tests emphasise the risk of structural failure unless extremely gentle recoveries are made from high Mach number dives.

It's hardly surprising that the maximum Mach number at which you can really throw the aeroplane around would be lower than that which can be reached if gentle flying. I'm actually quite surprised that the Spitfire would tolerate this sort of handling at such high Mach numbers; there's no mention of rivets popping, things bending, breaking or falling off etc. I can't think of any other WWII fighter that would repeatedly tolerate >5 g at M>0.80 without complaint.

So I suppose that a lot of this stuff is in the eye of the beholder, but if I had to spend my time at the top right hand corner of the envelope for real in a WWII fighter then I'd pick a Spitfire to do it in.

The caveats regarding date collection in the report are important; it's quite hard to work out exactly what the uncertainties are with this sort of test. They might easily be as high as +/- 0.03 M.

In this respect the PR.XI data is better because an auto observer was used AFAIK, and of course the aeroplane was quite heavily instrumented and modified in other respects as well. So I'm quite confident that their quoted Mach 0.89 is +- a rather small error (though I don't make any particular claim as to the applicability of this figure to an operational aeroplane on a squadron, beyond the qualitative implication that the basic airframe was selected for dive tests by the high speed flight because it had the best high Mach number characteristics available off the shelf).

However, it's important to make the distinction between the uncertainty due to observational difficulties, those due to lack of instrumentation, and those due to instrument errors.

For example, if the ambient conditions (especially static temperature) aren't recorded, there might be a considerable error in Mach number due to differences between the test day and standard atmosphere conditions. Not much can be done about this, because even a couple of hours later the weather can and probably will have changed.

OTOH, it's much easier to go back a few weeks later and correct for instrument error by careful calibration of the instruments used during the test, and measurement uncertainly can be reduced by repeating the tests and applying statistics.

I point this out because there is a tendency for certain sections of the community to suggest that just because some reported dive is considered bogus (usually a combat report for their favourite aeroplane citing 600 mph IAS or something) that all dive testing is bogus. The reality was of course that it was merely difficult to get good data in the 1940s; serious flight test organisations could do it, whereas the average fighter pilot could not, because nobody had seen fit to give him the necessary tools to do so.

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Tomcat et al, conventional elevators work by modifying the camber of the tail section.

Moving the relatively small control surface therefore affects the CL of the entire tail.

The force required to do this is set by the hinge moment.

At high freestream Mach number you get sonic flow over the tail. The control surface cannot affect the pressure distribution over the tail surface upstream of the sonic line. So the control effectiveness suddenly dramatically declines.

Since the control deflection was limited by pilot strength, the effect that the pilot perceives is a nose down pitching moment, because he's pulling as hard as he can, and the stick stays in the same place. But what's really happened is that the elevator effectiveness has declined, which is equivalent to reducing the absolute camber of the tail.

This means that high Mach number departure was often a 2 stage phenomenon. First the aeroplane starts wanting to pitch down due to shock formation on the wing changing the downwash angle over the tail. Then at some higher Mach number the effectiveness of the elevator fades away and the nose down pitching tendency gets worse. A lot of aeroplanes wouldn't really get into this second regime because they'd either break or start slowing down and getting into warmer air first.

Obviously, an all moving tail doesn't have this elevator problem because it just changes alpha rather than translating its lift curve slope with camber changes.

It has other problems due to the force required to actuate it, possible overbalance etc. But you can fix most of them by just throwing massive irreversible screwjacks at it, although it is advisable to combine this with Q feel so that the pilot doesn't inadvertently break the aeroplane when flying fast...

Yeager suggests that keeping the flying tail secret allowed the F-86 to have a technological lead over British and Russian aeroplanes of the period, but really this is an oversimplification because the idea of the all moving tail was not new. The rather more mundane reality is that getting the enabling technologies in the rest of the control system to work wasn't a trivial problem in the 1940s, and in Britain almost all of the funding for such work evaporated in 1945, whereas in America it kept on flowing. Meanwhile the Russians had slightly different priorities, but it's probably fair to say that the kill ratio achieved by the UN in the Korean war was probably more a function of pilot skill than aircraft performance differences (though the F-86 was superior at high Mach number, it was certainly far from perfect).

BTW, the first generation X-1 wasn't as clever as the M.52.

AFAIK the M.52 had no elevators and just moved its entire horizontal tail for pitch control "out of the box".

The X-1 had elevators for pitch and an all moving tail for trim (rather like a Bf-109, except that the X-1 moved the tail electrically, and the control was a coolie hat on top of the stick).

The elevators became ineffective somewhere in the transonic regime (0.9ish) and the workaround was to hold the stick still and control pitch with the trimmer.

I assume that the 2nd generation aeroplanes ditched the elevator...

Of course, despite its aerodynamic sophistication, it's not entirely certain that the M.52 would actually have been supersonic in level flight as drawn because this would have been asking an awful lot of its engine.

But now we're waaaaay OT.