Hi Graham,
Another hard question, huh? I don't know the formula but there must be one. One
bit of related information, the sail on a nuclear sub is similar. They found
that with no fillet a vortex was formed each side which when it passed through
the prop caused noise, very characteristic noise. The fix, I'm not quite sure
but I think was some sort of vane to unwind the vortices.
No question though fillets help. I once asked Burt (messiah) Rutan if my Long
would go better with root fillets. Nah, he said, you're just increasing wetted
area. Just shows you even God isn't always right, although he's never wrong, I
hasten to add.
The best compromise must be a function of the pressure distribution round the
fuselage compared to that round the wing. a root fillet will give a gradual
pressure change plus a reduction in wetted area. OTH it could be that local
velocity (speed*direction) is the crucial factor. Now that I've found out that
it's too complicated for me to understand, I should say that we chose a 2"
radius fillet extended to the trailing edge of the flap. We haven't checked
sideslips yet but present indications are that we haven't spoilt the aircraft's
excellent handling. Pete Clarke's subjective impression so far has been that
this one feels nice. It seems to have less airframe noise too, at low speed it's
virtually silent. We haven't got the outrigger cuffs on yet,either.
Some time, when the dust settles I'll do some oil streak visualisations of the
flow round the trailing edge of the wing and maybe try and optimise the wing
fuselage transition. Might add 5 knots to the cruise.
BTW if you think 5 kts isn't worth the effort read Kent Paser's book Speed with
Economy; he improved his top speed by 64 mph (Mustang II) from 179mph without
changing the engine in any way.
None of his mods added more than a few mph.
Graham
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