Drag of Transponder Antennae compared

[AS]

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_UKzqsCE&feature=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'

[India November]

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.

[AS]

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'

[andy l]

One or two people with aerodynamics knowledge have said before that a delta is one of the highest drag shapes there is, if the airflow is not perfectly in line, and thus they personally won't have those fence fairings at aileron ends or flap end by wing root. Some blade transponder aerials look quite like a delta shape

How does the size and drag of a rod type transponder aerial compare with the short near verticle part of a total energy tube, and how often have people worried about that?

Simpler than the 3d printing would be take a short piece of curved mylar of suitable width, wrap it around and tape the edges together with capping tape. It might need a brief wave of a heat gun to enhance the curvature at the front (which is how it's curved in the first place).. It might take 3 or 4 tries or might not work. Don't do this for the TE tube.

Is there a prize for the first person to glue a nice mini Kamm-tailed fairing on the back of the ball, which doesn't fall off in the next few months? Am I joking?

[Eric Greenwell]

Remove the TE portion of the probe, and use electronic TE for the vario. Put the transponder
antenna in the fin. I've done the first part; not going to do the second part until I get a new
glider :^)

But what to do with the tip wheels? Got to be worse than a blade antenna. Take one off and taxi
only on the left wing? And that scoop on the sliding canopy window - better glue that shut
before I even think about using it again! Yikes - the yaw string - better mount it waaay back
on the canopy, behind your head, so it doesn't trip the laminar flow too early (or mount it
inside, towards the front, where I can still see it).

$200,000 for a new glider, and the manufacturer doesn't do any of these things! They must think
we are rich and bereft of technical knowledge. Also, why isn't there MK VII Laminar Flow Yaw
String I can buy? Did bumper retire?


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

[Eric Greenwell]

Excellent video.

[Mark Mocho]

To really make a difference in parasitic drag, learn how to use your rudder pedals. One little boot that misaligns your yaw string is probably equivalent to two or three years worth of drag from your transponder antenna!

[Dan Marotta]

...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

[jfitch]

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.

[stephen...@sympatico.ca]

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=https%3A%2F%2Fwww.twistedmind.nl%2Fsample%2Fanten.htm&psig=AOvVaw2wwGZOQpinqISH7e0xc0nx&ust=1613061524918000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCNCp0dTg3-4CFQAAAAAdAAAAABAD

[Bob Hills]

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

[Kenn Sebesta]

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.

[andy l]

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.

[Eric Greenwell]

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]

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.

[stephen...@sympatico.ca]

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.

[Eric Greenwell]

stephen...@sympatico.ca wrote on 2/11/2021 6:49 AM:
> 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.
>

You'd avoid a long RF cable, but would have run 12VDC power and the communication cable to it,
plus provide the access to remove it for repairs. I think mounting the box behind the gear
well, and running 15' of coax to it is easier and work just as well. 15' is not far with good
quality coax.

[Hank Nixon]

These comments appear to be by people that have never had to work in the the rear of a fuselage or in the fin.
Mounting a thin antenna, without the little ball as far back as can be reached from the front area is a practical trade off. Slightly behind the gear and up the side a bit helps access and lets the gear doors protect the antenna. A simple wrapped fairing would be a clever small improvement. This far back the boundary layer is pretty thick so drag is likely minimal.
As far as drag reduction, sealing the canopy well would yield a far greater return on time invested.
BTDT
UH

[Eric Greenwell]

Hank Nixon wrote on 2/11/2021 10:52 AM:
> These comments appear to be by people that have never had to work in the the rear of a fuselage or in the fin.

My original comment was I would not personally mount the antenna in the fin, but I would select
that as an option when buying a new glider :^)

It was hard enough getting the coax from the panel to just behind the gear...

[Chip Bearden]

Back in the 70s, the original 201 Standard Libelles didn't have the little plastic half-embedded wingtip wheels you see on most of them. Woody Woodward (W1 for those with good memories) got the kit from Glasflugel to install them on his glider. Concerned about drag, lazy, or maybe just with an inquisitive mind, he installed a wheel on one wing only and flew around with it for a year. Unable to tell any difference, he stopped worrying about it and installed the second one. :)

Three years ago when we were refinishing my ASW 24 wings, UH built the molds and installed a similar set of wingtip wheels in my glider. I love them! Now I can move the glider around by myself on the ground, even (especially) on pavement. They're half embedded in the wingtips so the profile is actually slightly less than that of the factory's rubber wingtip skids.

Chip Bearden
JB

[Eric Greenwell]

I had one of the tip wheels fall off my ASH26E while pushing it out of the tie down (how did it
hold on through the previous day's taxi, takeoff, flight, landing, taxi?). Flew the next two
days with one wheel; awkward, but fortunately, the winds were light.

The other drag I accept gladly is the steerable tailwheel. Makes taxiing easy, also pushing
around by hand (forward only, pushing backward is a PITA) and steering with the rudder, but a
big bonus is the terrific control of the ground run when taking off or landing, especially in
cross winds or tailwinds. If I went back to towed gliders, I'd still want a steerable tail wheel.

[kinsell]

Refinishing my Ka6-CR wings a number of years ago, we made the
surprising discovery the leading edges had been made more pointy by the
addition of balsa wood strips. Somehow they had never gotten dented
over the years.

Glider was owned initially by Rudy Moser, and apparently he had the
habit of doing experiments like this on one wing and flying it for a
while. If it wanted to fly in circles of the correct direction, he'd
go ahead and do the other wing.