I think falling flat on the belly, is possible.
Ofcourse, just exactly what one has to do, to get in such a state, I have no idea(well, I do have an idea, but not straightened out yet).
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So I think there are two questions we're asking here:
1) Just how much does the benevolent characteristics of the P-38, help in resisting fatal spins?
2) Is the kind of stall we see in AH, possible in real life? Or is it due to limitations of computer generated FM?
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Actually, when I was testing the flat spin out myself, I kept having trouble because no matter what I did, it won't fall into that certain spin(I know what it is you talk of). In most cases, even in flat spins, the nose naturally came under the horizon, and a slight push on the stick, rudder work, and perhaps engine control, was enough to recover very quickly.
I don't remember exactly how, but in certain cases the flat spin would start out with the nose above horizon, and then, in that state, even when the nose came down it wouldn't recover - this one's almost impossible to get out without alt.
It could be because the FM has troubles with two-engined planes, and if it is indeed that.. maybe it'll be fixed soon(or perhaps, in the worst case scenario, it may be something that cannot be fixed at all...!).
But whatever its cause, I definately don't think its a common thing to happen.