Hi,

If I remember my fluid dynamics courses correctly, spheres and round rods
produce a relatively large amount of drag when compared with teardrop or
flared objects. The low cost "stub" antennas are popular transponder
antennas because of their low cost. However, it is my belief that the more
expensive "shark fin" or "blade" antennas will produce much less drag, even
though they are a bit larger. They also look much nicer on a glider. You
can see examples he
http://www.soarmn.com/cumulus/comant.htm

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

"Eric Greenwell" wrote in message
news:2gpLg.4072$%k5.472@trnddc08...
wrote:
wrote:
Derek Copeland wrote:
Whether I would be able to mount the aerial internally
or not would depend on the engineer certifying the
installation. I understand that the latest generation
sailplanes with carbon fibre fuselages will have to
have externally mounted aerials, probably top or bottom
so that they will transmit both up tp TCAS equipped
airliners and down to Air Traffic Control. Although
the aerials are quite short, they do produce a significant
amount of drag. Remember that a 500 kg glider with
a 50:1 glide angle will only have a drag of 10 kg at
best glide speed.
Here is what an aeronautical engineer wrote on our ASH 26 E newsgroup,
responding to the same concern of another owner:

"As a sanity check assume 1/8" by 2" wire (projected area .25 square
inch) with a drag coefficient of 1 (normally a round wire is less) then
the
drag is (.25/144)*1*60*60/295 = 0.02 lbs at 60 knots or 0.08 lbs at 120
kots. (At 60 knots the flat plate drag is about 12 lbs per square
foot).
Even if the antenna was twice as long or twice as thick we are still
looking
at around .04 pounds at 60 knots or 0.16 lbs at 120 knots."

That's very small compared to 10 kg, and it's at 120 knots!


But it is not the drag, but the drag relative to the lift.

NO Antenna
500 kg glider
10 kg drag
L/D = 50.0

Antenna
500.2 kg glider (add antenna weight, liberal guess)
assume shape of antenna is straght wire, no tip (as above)
assume drag is 0.16 lbs = 0.073 kg
assume no interferance drag
L/D = 500.2/10.073 = 49.66

A third of a point loss is significant for those who know how to use it
(I am told).
Fine tune these results with a real antenna and try again.

(Standard Cirrus
330 kg
L/D = 35
9.43 kg drag

L/D w/antenna:
330/(9.43+0.073)=34.73
Noticable if you have done all your gap work, sealing, airfoil tuning,
etc...?)

The .073kg is at 120 knots, so at 60 knots best L/D, it would be 0.01825
for an L/D of 34.927. At thermalling speeds, where induced drag dominates,
it's insignifcant (one extra bug may cause more loss!), but perhaps that's
a good enough calculation for the UK, where you probably aren't batting
along in a Std Cirrus fast enough to make profile drag the dominant
factor.

--
Note: email address new as of 9/4/2006
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org