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



 
 
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  #1  
Old January 1st 20, 02:32 PM posted to rec.aviation.soaring
India November[_2_]
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed.
  #2  
Old January 1st 20, 08:36 PM posted to rec.aviation.soaring
MNLou
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

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
  #3  
Old January 1st 20, 09:56 PM posted to rec.aviation.soaring
Martin Gregorie[_6_]
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

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

 




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