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!


I am not a tehnofreak, but I understand that Transponder
aerials have to be base loaded and require a metal
ground plane to work properly. Obviously not a problem
for a spam can or a metal Schweitzer glider, but something
else that has to fitted into the already crowded centre
section of a FRP sailplane.

The ground plane can be as small a 6" in diameter, and internal
antennas have been installed sucessfully in gliders with less room and
tighter access than the Std Cirrus. Drag is not a valid concern for
your situation, but it sounds like you can't/won't afford the
considerable cost.

--
Eric Greenwell - Washington State, USA

www.motorglider.org - Download 'A Guide to Self-launching Sailplane
Operation'