Structural limits
I think we've done weight and balance to death, and by the sound of it most people would rather diet regimes wasn't revisited either.
But, I thought I'd get the rulebook and slide-rule out and see what the implications are of an MW6-S(*) in an aerobatic aeroplane.
Just to recap, the classic Cessna / Piper trainers are stressed to +3.8g and 77kg (170lb) per seat. But, when you play with weights and safety factors, you find that the seat will give way at about 2.75g - which is well within normal g-limits, and also significantly slightly under the 3g value normally used as the hard landing case for undercarriage strength. So, there's a very good case for a 25 stone pilot not being permitted to fly in one of these for reasons of seat structure.
Now we talked about the Bulldog, which would have been certified to BCAR Section K, which was the UK light aircraft safety requirements up until the early 1990s. That (for aerobatic aeroplanes) requires a minimum of a +6g limit which means that the structure will actually be good for +9g (+50% is the usual safety factor for metal aircraft structures).
Section K also uses 170lb for seats normally, but 190lb (86kg) for seats designed to be used with a parachute (which I think we can take as a given for any aerobatic trainer).
So, assuming the usual safety factors, and no particular over-engineering, the Bulldog seat should fail at 86kg x 9g = 774kg downwards load.
Now 25 stone comes out at 159kg. Divide 159 into 774 and we get 4.85g, that's the loading at which the seat is likely to collapse, and certainly far more g than any heavy landing (or at least one in which the maingear is likely to have survived intact).
However, putting my aeronautical engineer's hat on again I'd still rather see than 1.5 safety factor being applied. So, dividing 4.85g we get 3.23g. That's more than most of us have any desire to pull if we aren't flying aerobatics, and beyond the likely design case for the undercarriage.
The American rules aren't significantly different and I think that you can assume that the same sums would apply to, say, a C150 aerobat (although the narrow cabin may present an entirely different challenge).
So, as long as our large pilot can fit in, and there's no problem concerning overall weight and balance (which is easily checked in 5 minutes) I think that structurally speaking, there shouldn't be any particular problem with a Bulldog - or for that matter any other aeroplane certified for aerobatics, albeit using it for non-aerobatic training.
Apologies if anybody things that this post probably belongs in tech log, but the thread did start here and hopefully this is of some use to somebody.
G
(*) MW6-S is a type of PFA homebuilt aeroplane, I leave you lot to find it's other name.