I must corect my post below. I incorrectly grabbed .3 as the drag
coeff for a round shape from a book on design, then managed to
corroborate that with Hoerner's "Fluid Dynamic Drag," by missing the
fine print there re critical Reynolds # for round shapes. So .3 x .04
is .012 sq.ft. FPE (flat plate equivalent) looking like a slipped
decimal.
Correct drag coeff is 1.17 (worse than a flat plate), so 1.17 x .04
(the small Aeroflash beacon lens common to Cessnas) is .05 sq. ft.
FPE. Whether or not more than the drag of its tail, or a jelly donut
for that matter, is not important. We want to know effect on
velocity.
To compute FPE, we need know only HP, speed, and altitude. I put the
formulae into an Excel spreadsheet, and for a sample Europa in the
cruise, I get 2.5 sq. ft. FPE.
In comparison, a Mooney 201 is 2.8 sf; the record-holding Mike Arnold
AR-5 (200+ MPH on 65 HP is about 1.0 sf).
A .05 sf beacon on a 2.5 sf Europa sounds like 2%, but I got only
a bit less than 1 knot hit. This is because the relationship between
drag and velocity is not linear. Per my calcs (and a published text),
a Cessna 172 checks in at about 6.0 sf. Spreasheet predicts a 1/3
knot
penalty for its beacon.
Sooo, if I'm doin' all this right, it says appendages (and surface
imperfections, gaps, and lack of fairings on appendages) affect slick
airframes much more than dirty ones. But a beacon by itself won't
hurt much, and fixed gear can be an acceptable hit even unfaired,
though
in frontal area they are many, many beacons in size.
Regards,
Fred Fillinger, A063
> I think Barnaby slipped a decimal point there. I can't do the math
> on the Europa vertical fin/rudder, but I researched and found entire
> tails (all 3 surfaces) on light aircraft come in with a D/q of about
> .3 sq.ft. Cylinders are very draggy, but a beacon lens of say .04 sq.
> ft. frontal area requires a huge drag coefficient to exceed even .1 in
> drag area.
>
> > Fred Fillinger, A063
> > I remember Baraby Wainfain stating at a Oshkosh seminar that the
> > Cessna style fin top rotating beacon or strobe had more aerodynamic drag
> > than the entire fin and rudder....
> >
> > Tom Friedland A 079
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