James the KEVLAR (TM) would allow greater physical spacing, therefore
more transparent RF-wise, as KEVLAR is (relatively) transparent to EM/RF
energy.
Spiral wrapping the OEM (original equipment manufacturer) antenna seems
to offer the greatest range, and the least blanking at specific
attitudes relative to the the TX while airborne.
As far as form drag on the vertical whip you mention:
For the conditions you give and using standard viscosity and density of
the air, and say 35m/sec airspeed (please excuse my math):
u (velocity) =1.79 x 10^-5 N x s/m^2 (3.75 x 10^-7 lbf-sec/ft^2)
p (pressure) = 1.21 kg/m^3 (0.00234 slugs/ft^3)
Now the Reynolds number:
Re = VoDp/u
= 35ms x 0.0075m x 1.21 Kgm^3/(1.79 x 10^-5 N x s//m^2)
= 1.77 x 10^3
Total drag @ 305mm of length (let's say Cd is 0.20 for a cylinder):
Fd = Cd x Ap x pV2/2
= (0.2)(0.305m)(0.0075m)(1.21 kg/m^3)(35^2 m^2/s^2)/2
= 0.34 N
So, at ~126Km/hr (78mph) there is 34.7 gram-force (1.22 ounce-force) on
that vertical whip. As you can see, it is insignificant. The whip you
mention is of European size/length, significantly larger than what we
have here (I grabbed a Deans; it measures 1.48mm x 16.5mm - and another
made here in Canada; 0.81mm x 16.5mm).
Now if you lean any of them back to ~45 degrees (to the airflow), it is
now more of an airfoil, and form drag is significantly reduced (by
~x/.707). The idea of using vertical whips is to have the element 90
degrees to the GND plane. Technically range would be reduced by leaning
it, but would still be significantly better RF performance than an OEM
(wire lead) within a carbon boom...
regards
James Porter Jr wrote:
>
> > Given that all layups are hand-made the overlap, weave skewing, type and
> > density of CF and orientation makes even two fuses from the same
> > manufacturer significantly different EMI/RFI-wise...
>
> It appears that the carbon/Kevlar hybrid cloth is the real culprit, at least
> on 35 and 50 mHz. I've had pretty good luck with a few pure carbon booms
> and fuselages, but very poor range with the hybrid. Regardless, with either
> type of fabric it's best to get the antenna outside the fuselage as soon as
> possible. So far the best for me has been a spiral wrap of the antenna on
> small carbon booms and a straight line down the top of a carbon fuselage.
>
> I've not tried a vertical whip on any of my thermal duration or slope racers
> as I abhor the thought of adding that kind of drag, but I see many large,
> scale sailplanes in my area with them. Any comments/data on the drag of a
> .030" (0,75 mm) x 12" (305 mm) vertical wire whip?
>
> Jim Porter
> Neckargemund-Dilsberg
> Germany
>
> "The airplane stays up because it doesn't have the time to fall."
> Orville Wright
--
*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*
Simon Van Leeuwen, Calgary, Alberta
RADIUS SYSTEMS
Cogito-Ergo-Zoom
IAC25233*MAAC12835*IMAC1756*LSF5953*IMAA20209
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