Locating Transponder Antenna on top of the fuselage instead of under it.

O
And since you asked about different style 1/4 wave dipoles. The rods, blades etc. should all behave the same from a RF viewpoint. They are effectively the same internally. Rods/single attach point style antennas may be easier to attach/mount on curved surfaces, but have higher drag than a blade if that small drag worries you. I've also seen A&Ps recommend the rod style so that if you do manage to crunch it will bend and hopefully do less damage to the fuselage/finish.

Transponder rod antennas spec drag at .41 lbs at 250 MPH and blade antennas at .09 lbs at 250 MPH. That seems to be a big difference but anyone have a comment to what this measures in the gliding world?

On Monday, December 30, 2019 at 2:52:49 PM UTC-8, wrote:
O
And since you asked about different style 1/4 wave dipoles. The rods, blades etc. should all behave the same from a RF viewpoint. They are effectively the same internally. Rods/single attach point style antennas may be easier to attach/mount on curved surfaces, but have higher drag than a blade if that small drag worries you. I've also seen A&Ps recommend the rod style so that if you do manage to crunch it will bend and hopefully do less damage to the fuselage/finish.

Transponder rod antennas spec drag at .41 lbs at 250 MPH and blade antennas at .09 lbs at 250 MPH. That seems to be a big difference but anyone have a comment to what this measures in the gliding world?

The simplest answer: parasitic drag ~ V^2. So at (100mph/250mph)^2 * 0.4 = ~0.07 lb. = ~1 oz.

I suspect this has been flogged to death on r.a.s. before.

"Honey I need a new glider with a tail mounted transponder antenna because it's lower drag. And I'll win more contests. Honey?" :-)

Although the faired blade antenna has a theoretical drage coefficient less than a rod, in practice you have to align the blade with the local flow in the area of the attachment point. If the blade is not parallel with the streamlines, I suspect it potentially could add more drag than a plain ole' rod.

I will let you know what I found and what decided.
Dan

I believe drag is related to the square of the speed.Â* How often do you
fly your glider at 250 kts?

On 12/30/2019 3:52 PM, wrote:
O
And since you asked about different style 1/4 wave dipoles. The rods, blades etc. should all behave the same from a RF viewpoint. They are effectively the same internally. Rods/single attach point style antennas may be easier to attach/mount on curved surfaces, but have higher drag than a blade if that small drag worries you. I've also seen A&Ps recommend the rod style so that if you do manage to crunch it will bend and hopefully do less damage to the fuselage/finish.
Transponder rod antennas spec drag at .41 lbs at 250 MPH and blade antennas at .09 lbs at 250 MPH. That seems to be a big difference but anyone have a comment to what this measures in the gliding world?

--
Dan, 5J

I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed.

On Wednesday, January 1, 2020 at 8:32:48 AM UTC-6, India November wrote:
I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed.

Assuming, as previously stated, that the blade antenna is aligned with the airflow around the fuselage at that point.

I would think that the drag of a misaligned blade antenna could be significantly higher than the perfect alignment drag.

For that reason, I'm personally going with a rod and ball antenna. The manufacturer's recommendation for mounting a transponder antenna is in an area where I have no idea how the air really flows.

Does anyone know if the difference between a perfectly aligned blade antenna and a rod and ball antenna is at all significant at, say, 100 kts airspeed?

Lou

Just anecdotal evidence here, but my Stemme came with a rod and ball
transponder antenna under the nose bowl.Â* I mounted a blade type antenna
about 6 inches aft of that for the Flarm B.Â* I haven't noticed any
difference in flying qualities, drag, etc.Â* Maybe it's measurable but
who cares?Â* I'm not in contention for a world record or title.

On 1/1/2020 1:36 PM, MNLou wrote:
On Wednesday, January 1, 2020 at 8:32:48 AM UTC-6, India November wrote:
I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed.
Assuming, as previously stated, that the blade antenna is aligned with the airflow around the fuselage at that point.

I would think that the drag of a misaligned blade antenna could be significantly higher than the perfect alignment drag.

For that reason, I'm personally going with a rod and ball antenna. The manufacturer's recommendation for mounting a transponder antenna is in an area where I have no idea how the air really flows.

Does anyone know if the difference between a perfectly aligned blade antenna and a rod and ball antenna is at all significant at, say, 100 kts airspeed?

Lou

--
Dan, 5J

From the Aircraft Spruce website, the drag spec on a rod and ball transponder antenna is listed as 0.41 lbs. at 250 kts. The blade type lists as 0.09 lbs. at 250 knots. Drag increases by the square of the speed, so at half the speed (125 knots), the drag is around 1/4 the numbers listed. If you are concerned about the .0225 lbs. in extra drag vs. the .01025 lbs., buy the blade type and keep the yaw string straight. You will probably increase your final glide numbers by at least a couple of yards.

Mark Mocho wrote:

"If you are concerned about the .0225 lbs. in extra drag vs. the .01025 lbs.., buy the blade type and keep the yaw string straight. You will probably increase your final glide numbers by at least a couple of yards."

But Mark, the blade type just looks cool! Especially when mounted on top! 😛😂😵