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

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! 😛😂😵

But I have flown with you, Cliff, and the drag from the antenna is only low when aligned with the airflow. Your yaw string looks like a windshield wiper! You don't go faster by using the rudder pedals like bicycle pedals.

Thanks. I needed a laugh.

Just mount the blade antenna by the front lug only, leave it loose a bit. Cut a slight smile slot for the coax, use a bit of grease, and,voila!
Now your blade style antenna can feather with the wind, netting you the least amount of drag possible.
;-)

On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote:

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

I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html
which may help sort things out for you, though these numbers are for 3D
shapes while what you really want is drag comparisons for 2D shapes with
the shape extending right across the wind tunnel or (better) with one end
attached to a side wall that's in the airstream.

Other, less well written web pages, suggest that a teardrop shape with a
blunt LE and max thickness at around 30% chord and slightly convex
surfaces behind max thickness that terminate at a sharp TE should have
about half the drag of a cylinder with the same frontal area.

As others have said, drag will rise sharply if the AOA of the streamlined
shape isn't zero and this will get worse as the max thickness moves back
toward 50% and/or the LE becomes sharper.

If you're really worried, find a University with an aerodynamics dept and
try to persuade a student to write a term paper based on wind tunnel
tests on a rod antenna and the best shaped blade antenna you can find.

Now, you can calculate your glider's drag at, say, best glide and max
cruise (easy to do from its flying weight and glide polar) and then
calculate how much each adding each antenna would affect the glide angle
at those speeds.

We'll all be interested to see just how much effect the antenna has.


--
Martin | martin at
Gregorie | gregorie dot org

Martin Gregorie wrote on 1/1/2020 1:56 PM:
On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote:

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

I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html
which may help sort things out for you, though these numbers are for 3D
shapes while what you really want is drag comparisons for 2D shapes with
the shape extending right across the wind tunnel or (better) with one end
attached to a side wall that's in the airstream.

Other, less well written web pages, suggest that a teardrop shape with a
blunt LE and max thickness at around 30% chord and slightly convex
surfaces behind max thickness that terminate at a sharp TE should have
about half the drag of a cylinder with the same frontal area.

As others have said, drag will rise sharply if the AOA of the streamlined
shape isn't zero and this will get worse as the max thickness moves back
toward 50% and/or the LE becomes sharper.

If you're really worried, find a University with an aerodynamics dept and
try to persuade a student to write a term paper based on wind tunnel
tests on a rod antenna and the best shaped blade antenna you can find.

Now, you can calculate your glider's drag at, say, best glide and max
cruise (easy to do from its flying weight and glide polar) and then
calculate how much each adding each antenna would affect the glide angle
at those speeds.

We'll all be interested to see just how much effect the antenna has.

A 50:1 900 lb glider at best L/D has 18 lb of drag; the rod antenna at 62 knots
has a drag of 0.41lb/16 = 0.025, or 0.14%. That's about 0.07 L/D loss.

The rod drag quadruples at 125 knots, but the drag of the glider would increase
substantially, also, so maybe the percentage doubles, which is still only 0.3% of
total drag.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1