Drag of Transponder Antennae compared

Just fly faster. On cylinders and spheres, the drag drops to about 1/3 above the critical Re. Around 220 knots should do it for the transponder rod.

On Wednesday, February 10, 2021 at 7:44:51 AM UTC-8, Dan Marotta wrote:
...And you'd tell the drag difference how? Would that be 10 oz vs 1 oz?
What's the overall drag of the glider at cruise? BTW, my Stemme has a
rod antenna for the transponder and a blade for the Flarm B.

Dan
5J
On 2/9/21 9:51 PM, AS wrote:
On Tuesday, February 9, 2021 at 9:14:09 PM UTC-5, India November wrote:
On Monday, February 8, 2021 at 11:06:31 PM UTC-5, AS wrote:
Interesting home-brewed investigation into the drag between the fin-type and rod-type transponder antenna by one of our friends in The Netherlands.
Seems like the fin type antenna - unless mounted exactly on the centerline of the glider, where the flow is expected to be parallel - may produce a lot of turbulence and drag.
https://www.youtube.com/watch?v=8ii_...ature=emb_logo
Take a look at this type of antennae installed under the belly of power planes. In most cases, you will find one side (typically the right side due to the prop-wash)) caked with soot and grime while the other side is relatively clean. Would be interesting to quantify the drag this causes and the extra fuel burn over the lifetime of the plane. I bet most power-pilots don't even know/think about it.

Uli
'AS'
Yes, theoretically the streamlined aerofoil section has a lower drag coefficient than a cylinder oriented at right angles to the airflow. However, the rod antenna has a smaller frontal area, and also if the aerofoil is misaligned with the local airflow it will disturb the flow and cause drag.

Ok - here is a follow-up question/challenge: The aerodynamic resistance of a cylinder vs. a tear-drop shape is about 10:1. How about a 3D-printed airfoil shape like a simple symmetric NACA airfoil made in two pieces, which snaps over the pole antenna? It could be retained/secured by the ball on the end but be free to swivel thus self-align with the airflow.
Gentlemen - Warm up your printers ... ;-)

Uli
'AS'

Just keep it out of the airflow ... here’s a mocked up solution with dual diversity antennae ...

https://www.google.ca/url?sa=i&url=h...AAAAAdAAAAABAD

Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna in that? I know it would mean an electrical connection from tip to fuselage but that is not insurmountable.

Just a thought for someone to expound upon.

Bob 7U

On Wednesday, February 10, 2021 at 5:47:39 PM UTC-5, Bob Hills wrote:
Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna in that? I know it would mean an electrical connection from tip to fuselage but that is not insurmountable.

Just a thought for someone to expound upon.

Bob 7U
I still say on many performance threads on RAS...the nut behind the stick is worth more than the money spent on the ship....unless normally at the top of the sheet....

On Wednesday, February 10, 2021 at 5:47:39 PM UTC-5, Bob Hills wrote:
Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna in that? I know it would mean an electrical connection from tip to fuselage but that is not insurmountable.

Just a thought for someone to expound upon.

Bob 7U

I think you could do that, but there are a couple challenges

1. RF cable is heavy and significantly affects the signal. On sailboats, it's suspected that a stern-rail mounted VHF radio antenna actually performs better than a masthead mounted antenna. So sending it 5-10m to the wingtip instead of keeping it close to the TX unit could noticeably decrease range.
2. Even when in perfect shape, RF connectors are a big source of energy loss, which further reduces range.
3. The connector would have to be properly connected/disconnected every time the plane is pulled out of its trailer. A powerful RF transmitter which is disconnected from an antenna can actually destroy itself, so the consequences of a forgotten connection are not necessarily limited to only being invisible that day.

On Thursday, 11 February 2021 at 05:10:10 UTC, Kenn Sebesta wrote:
On Wednesday, February 10, 2021 at 5:47:39 PM UTC-5, Bob Hills wrote:
Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna in that? I know it would mean an electrical connection from tip to fuselage but that is not insurmountable.

Just a thought for someone to expound upon.

Bob 7U
I think you could do that, but there are a couple challenges

1. RF cable is heavy and significantly affects the signal. On sailboats, it's suspected that a stern-rail mounted VHF radio antenna actually performs better than a masthead mounted antenna. So sending it 5-10m to the wingtip instead of keeping it close to the TX unit could noticeably decrease range.
2. Even when in perfect shape, RF connectors are a big source of energy loss, which further reduces range.
3. The connector would have to be properly connected/disconnected every time the plane is pulled out of its trailer. A powerful RF transmitter which is disconnected from an antenna can actually destroy itself, so the consequences of a forgotten connection are not necessarily limited to only being invisible that day.

Cable losses per metre are greater with higher frequency. This is why systems such as satellite TV and even some terrestrial TV installations convert to a lower frequency before the downlead.

Transponders are working at UHF, and hence a short antenna cable distance is desirable, and the unit may be remote mounted in the centre section rather than in or behind the instrument panel.

Winglets are probably made of carbon fibre, so the antenna would be screened and not work inside.

I'll consider putting the transponder antenna inside a glass fuselage, and wonder how far from spaceframe and control rods/cables is desirable or feasible. Might need some contortionism to fit it. Might end up outside anyway.

Kenn Sebesta wrote on 2/10/2021 9:10 PM:
On Wednesday, February 10, 2021 at 5:47:39 PM UTC-5, Bob Hills wrote:
Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna in that? I know it would mean an electrical connection from tip to fuselage but that is not insurmountable.

Just a thought for someone to expound upon.

Bob 7U

I think you could do that, but there are a couple challenges

1. RF cable is heavy and significantly affects the signal. On sailboats, it's suspected that a stern-rail mounted VHF radio antenna actually performs better than a masthead mounted antenna. So sending it 5-10m to the wingtip instead of keeping it close to the TX unit could noticeably decrease range.
2. Even when in perfect shape, RF connectors are a big source of energy loss, which further reduces range.
3. The connector would have to be properly connected/disconnected every time the plane is pulled out of its trailer. A powerful RF transmitter which is disconnected from an antenna can actually destroy itself, so the consequences of a forgotten connection are not necessarily limited to only being invisible that day.

A better location would be in the fin, and I think some manufacturers offer that location. The
cable would shorter than going to a winglet (about 22' vs 30'); it could be even shorter (15')
if the transponder box (eg, a Trig unit) was mounted behind the gear instead in the instrument
panel. The fin could not be made of carbon, of course.

--
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

Eric Greenwell wrote on 2/11/2021 5:19 AM:
Kenn Sebesta wrote on 2/10/2021 9:10 PM:
On Wednesday, February 10, 2021 at 5:47:39 PM UTC-5, Bob Hills wrote:
Thinking totally outside the box here and open to ridicule :).....

Blade antennas are similar in shape and size to a winglet. Why not incorporate the antenna
in that? I know it would mean an electrical connection from tip to fuselage but that is not
insurmountable.

Just a thought for someone to expound upon.

Bob 7U

I think you could do that, but there are a couple challenges

1. RF cable is heavy and significantly affects the signal. On sailboats, it's suspected that
a stern-rail mounted VHF radio antenna actually performs better than a masthead mounted
antenna. So sending it 5-10m to the wingtip instead of keeping it close to the TX unit could
noticeably decrease range.
2. Even when in perfect shape, RF connectors are a big source of energy loss, which further
reduces range.
3. The connector would have to be properly connected/disconnected every time the plane is
pulled out of its trailer. A powerful RF transmitter which is disconnected from an antenna
can actually destroy itself, so the consequences of a forgotten connection are not
necessarily limited to only being invisible that day.

A better location would be in the fin, and I think some manufacturers offer that location. The
cable would shorter than going to a winglet (about 22' vs 30'); it could be even shorter (15')
if the transponder box (eg, a Trig unit) was mounted behind the gear instead in the instrument
panel. The fin could not be made of carbon, of course.

Note: my length values are for a 15M glider. The winglet location would be even worse for 18M
and larger gliders.

--
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

Still thinking inside the box. There is no reason why the TX unit needs to be in the cockpit. It could easily be mounted in the fin with a remote head in the cockpit.