On 6/5/2012 9:20 PM, Steve Leonard wrote:
On Jun 5, 9:47 pm, Eric wrote:
It depends very much on the antenna; e.g., the dirty looking transponder
3" stubby rod with a ball on top has insignificant drag on an 18 meter
glider. Probably for a 15 meter glider, too, but I don't recall the
numbers. Blade style transponder antennas are even sleeker.

Com antennas are much larger than the 900-1090 MHz antennas we are
talking about, and can have significant drag, but it's a mistake to rule
out external antennas for the higher frequencies.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Guess it sort of depends on what you consider to be "insignificant",
Eric. 15 or 18 meter ship, dry, at best L/D only has maybe 20 lbs
TOTAL drag. 1 lbs may not seem like much, but it is 5%. In contest
terms, 50 points. Per day. If there was a really good, really
accurate TE system without using a probe (Schuemann B Box comes to
mind), the really hot pilots would all think just a bit more about
finding a way to put one of those in their plane and do away with the
now seemingly very draggy TE probe. The vertical portion of it is
about the same length as the transponder antenna, but bigger in
diameter.

It may seem like fly specs in the pepper, but over time, it adds up.
So, why do we have TE probes? Because they work, and nobody else has
come up with something that works as well. At least, not as far as I
know.

I recall it being ounces, not pounds, but I can't find my notes. Here's
a number from Rami Antennas for their transponder antenna:

"The AV-22 is a rod style transponder antenna utilizing its BNC
connector for mounting to the airframe. The antenna is designed to
operate at speeds up to 350 mph and altitudes up to 50,000 feet. It has
a drag force of 0.41 lbs @ 250 mph."

At 125 mph (108 knots), the drag would be 0.1 lbs; at best l/d of 54
knots, it would 0.025 lbs, or 0.125% of your 20 lb drag, rather than 5%.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)