I think the only reasonable answer to a question like that is "It depends...".
Really this "hit compressibility" concept is horrid. Air is compressible all the time. If you go fast then compressibility becomes more important. But I'd generally be inclined to consider compressibility in calculations for M>0.2 if I wanted accurate answers. Really the Mach number at which you elect to consider compressibility is a bit like the temperature at which you decide to account for variation in the Cp of dry air - it's arbitrary, and depends upon the computational resources available and the effort you're prepared to put in.
I'd generally expect the tail to be lower aspect ratio than the wing. Therefore it can be thinner. I'd also expect it to be at lower absolute CL than the wing. So, if we assume the same amount of sweep for wing and tail, I'd expect the tail to remain subsonic to a higher freestream Mach number than the wing.
But the generalisations associated with the above are dramatic. Control deflection, variations in wing downwash angle and so on could easily make quite a big difference.
In the WWII context, with manual controls you're likely to find that the pilot runs out of bicep before shockwave development over the tail starts to bite.
Here's a quick list of the factors causing "Mach tuck":
- Migration of the wing's centre of pressure
- Changing wing downwash angle
- Rapidly increasing stick force for constant elevator deflection
- Reduced elevator effectiveness due to shock developement over the tail
You can see that 3 out of the 4 don't require shock development over the tail.
OTOH, the underlying problem is the tail; because that's where the control surfaces are, and the problem is a control problem.
So the brute-force approach towards the end of WWII and immediately thereafter was to hydraulically boost everything. Once you do that then the new problems are dealing with the failure modes and providing Q feel so that the pilot doesn't break the aeroplane.
Then you might get into trouble with lack of elevator effectiveness, because the elevator can't affect the pressure distribution upstream of any shock which may have formed over the tail. But really it's unreasonable to think of the flying tail as a "solution" to this problem, because going to a flying tail in 1940 wouldn't have solved fighter stability & control problems. Without irreversible screwjacks to drive the thing, you'd find that either it fluttered off or else the control forces were impossibly high.
Really the concept of the "flying tail" is a sort of marketing thing rather than reality. There's nothing magic about it. The simple reality is that if you're supersonic the you need the movable bit of your control surface to do all the work by itself, and you therefore have to size it appropriately. Note that delta winged fighters do just fine with elevons - no need for the surface to have its own private leading edge to work - it just needs to be big enough and to be driven by a sufficiently strong set of jacks.
In subsonic flight the elevator can be quite a lot smaller because it affects the lift curve slope of the whole surface it's attached to, which is great if you've only got a relatively limited actuating force available. Hence the rapid ubiquitous adoption of the elevator in subsonic aeroplanes quite early in the evolution of the aeroplane.
The other thing which people got used to is the idea that in general a nice set of aerodynamic controls will give you stick free stability which may well mask any nasty stick-fixed behaviour your design may exhibit. During WWII the added friction associated with pressurised cockpits started to show up some of the stick fixed problems that had been lurking under the rug for decades, and this caused people to start working on tweaking the control system itself via bob-weights etc to affect the subjective handling characteristics of the aeroplane, rather than just expecting the pilot to tolerate what he was given.
This gradually took us towards full FBW/manoeuvre demand systems, which allow you to design the apparent handling of aeroplanes to be almost independent of their aerodynamics. Which is great, though it does open all sorts of philosophical cans of worms since you now have to think about what the ideal set of control laws would be, rather than just asking the test pilot whether he can fly the thing or not...
The loss of stick free stability also rather reduced the importance of the fixed portion of the tail from a handling perspective, and since the elevator had to be big enough for the supersonic control task, it was almost always going to end up big enough for the subsonic case and therefore why not just bin the fixed tail?
Actually there are several reasons, especially for larger aeroplanes; essentially a fixed tail with an elevator is likely to be lighter, though you may have to also vary its incidence for trim, which takes some of the advantage away (though the trim change can be an order of magnitude slower than the control trim, since effectively the trim change has to damp the phugoid whilst the control change has to damp the short period oscillation, and therefore the trim actuators can be smaller).
But I digress; time to get a cup of tea & get back to my thesis...
05-22-2011, 03:03 PM
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