Excellent observation, Graham, but I was sort of figuring on that,
maybe even correctly. ;-) If perfectly in trail, the drag of the aft
hinge is much less, and the lateral distance is mostly beyond where
"mutual interference" is a factor - yet another of Hoerner's graphs.
Offset it's still in the wake, so I called it .2" total, but either
way, even a 1.25" max thickness fairing requires a Cd .of .16-.20 to
break even. Below that is achievable with a proper streamline
section, not flat on the sides, and with correct trailing edge, not
open at the back. Further, in part the fairing is in turbulent
airflow, where length is the greatest, making wetted area more of a
factor. That's why they usually just butt tail sections against the
fuselage, without any fairing to reduce interference drag, since the
net effect can be a penalty back there.
Above could all be wrong, but I walk around a bizjet and see things
that appear to violate the rules, but look it up and find probable
rationale for making little difference. Or like why are the engines in
the back on bizjets, but under wing on most transports?
Regards,
Fred F.
> > The 1/8" thick flap hinges just will not have much adverse effect (drag
> > coeff. about 1.0). A fairing's drag is its frontal area, multiplied by its
> > drag coefficient, and too wide requires a good aerodynamic design just to
> > break even.
> > Fred F.
>
> However, there are two of them, one behind the other but not quite in line.
> I would guess that the interference effects may well add up to more than a
> straight addition.
>
> graham
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