Each correction is welcome, look forward to more
I tried going a bit further with those last ones but found that the final stage (conversion back to 8-bit, I can't find any 'native' 8-bit sound editors) was adding in too much hiss so I have tried a different approach - creating a few experimental sounds from scratch - hiss still appears in the final stage but not so much as to be a problem.
The results - whilst being far from perfect and probably not to everyone's taste - are better than I thought they would be, and I have uploaded a few of them for anyone who want to see what they are like (zips tagged with '20120210' [Edit2: NB duff mp3 versions with silence-weirdness now fixed and re-uploaded). The filenames are a bit obscure and obviously have to be renamed to be used but I hope it is clear what they should be - they are all for '_int' sounds, just set the name for whatever you want to fly. NB rotor and slap must be matched, i.e. both should be e.g. 'trem24' as this is the beat rate.
FWIW Currently I am using:
rotor_brn_trem25g4.pcm as hokum_rotor_int.pcm
slap_wht_trem25cont90.pcm as hokum_slap_int.pcm
turbine_brn_trem212_bs.pcm as hokum_turbine_int.pcm
which seem almost good enough to me though I haven't gone through the whole range of mix-and-match yet.
The turbine ones could definitely be better, the slap is probably a little loud and there's an occasional small crackle but it seems better than the hiccup (I can only speak for a headphones-only setup) though this set is primarily experimental and therefore - in theory - easily fixable. That said, a more advanced sound system than mine could well reveal all sorts of hidden horrors!
On the plus side, I now no longer have to turn the volume down for just this one game. I did find that the voices (especially co-pilot) were now comparatively (too) loud, so I extracted them, put them through the mangler and poked them back into the sound.dat file, which seems miraculously unharmed. A bit of extra hiss as a by-product but it's all radio talk so I can probably forgive that...
Comments and/or suggestions gratefully received, especially if I'm now heading in entirely the wrong direction!