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Transponder Blade Antenna Orientation

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AndyWW

Well Known Member
I'm about to drill my belly skin for the transponder antenna. If it matters it is a Delta-Pop blade, just aft of the baggage wall on the right side. I started trying to draw a line that was exactly in the fore-aft direction to minimize drag, but then I started to wonder if I need to account for prop-wash? For a clockwise-turning propeller that would suggest pointing the blade to the right. Has anyone else wondered about this? I did a bit of googling but haven't managed to come up with anything yet.

Given the lengths that some of us go to for drag-minimisation (fuel vent fairings etc) it seems like this could actually be worth considering? At least on a Saturday night after a couple of good reds...!
 
There was a lot of work put into minimal-drag orientation of comm blade antennas on V-tail Bonanzas. The guy who developed the blades tufted the fuselage sides and shot video of the tufts in flight. It turned out the tufts flowed parallel to the bottom of the fuselage, not straight fore and aft, which was a surprise.

So, if you really want to know, the thing to do is stick a GoPro on someone's belly and tuft it. It sounds like a fun project I'd be willing to volunteer my -7 for, but it appears you're pretty far away.
 
Tufting & videoing as mentioned would be a good approach. Another method would be to have others take pictures of any oil/exhaust streaking on their bellies before they clean it off.

My current 6A has a high time engine in it & the oil/exhaust pattern suggests the prop wash deflects the air stream off to the right (passenger) side slightly. My guess about a degree or two right.
 
The advertised drag for the Rami blade transponder antenna is 0.09 pounds at 250 mph. That implies that the drag at 200 mph is 0.06 pounds. 1 ounce.
 
BobTurner said:
I have wondered the same question about my nose gear leg fairings.
Click to expand...

On one trip to OSH, I was following a DA-40. Totally bored, I dipped down lower & noticed his nose gear fairing was deflected sideways. A call to him determined he had no yaw trim issues, felt normal.
The wheel pant rear edge was deflected at least 20 degrees off to the right to the center line!
 
More off topic...

Ralph Inkster said:
On one trip to OSH, I was following a DA-40. Totally bored, I dipped down lower & noticed his nose gear fairing was deflected sideways. A call to him determined he had no yaw trim issues, felt normal.
The wheel pant rear edge was deflected at least 20 degrees off to the right to the center line!
Click to expand...

Common issue on the DA40. I've owned two and they both did this. Look for pictures of flying DA40's and you will probably see the same thing.
 
Mikeyb said:
The advertised drag for the Rami blade transponder antenna is 0.09 pounds at 250 mph. That implies that the drag at 200 mph is 0.06 pounds. 1 ounce.
Click to expand...

However that is probably a theoretical calculation which assumes the airflow is parallel to the blade. Imagine it was instead perpendicular. It wouldn't be unreasonable to assume a number 100 times bigger which is then very significant. It's likely that if the flow is at angle theta to the blade then the drag is something like the (MaxDrag - MinDrag) * sin(theta) + MinDrag. Suffice it to say I don't think this is a life or death question but it could be significant if the angle was significant.

Ralph Inkster said:
Tufting & videoing as mentioned would be a good approach. Another method would be to have others take pictures of any oil/exhaust streaking on their bellies before they clean it off.

My current 6A has a high time engine in it & the oil/exhaust pattern suggests the prop wash deflects the air stream off to the right (passenger) side slightly. My guess about a degree or two right.
Click to expand...

I think this is probably the only way to really answer. If Ralph's guess is right then, and it seems reasonable to me, then it probably isn't all that significant for the transponder antenna. But it could be a lot more significant for gear leg fairings. Perhaps the most important conclusion is that it is actually quite important to get anything that hangs out into the airflow as parallel as possible to that airflow, especially if it is big (eg wheel spats).
 
A data point - the holes for the transponder antenna under the right side of the baggage compartment in an RV-14 are aligned fore and aft.
 
Alternative...

Fair warning -- this is experimental.

If you want to gain a microknot or so of airspeed, fashion a monopole and mount it in a wing tip or other RF transparent cover (Rudder bottom? Vertical Stab Tip? Front of Wheel Pant?)

The antenna in the picture, a 1/4L @ 1090MHz, the center radiator is 1.9 Inches, 4 ground plane elements are 2.12" at an angle of 45° (135°), assumes a Vf of 70%C (RG-400).

Or you can mount the existing antenna (RAMI, DP, COMANT) on a 4-5" diameter metal plate and mount that out in a wing tip.

Do pay attention to length of feedline -- RG-400 has ~1.8dB/10ft of loss at 1GHz.

Measure the Impedance and Vswr before firing up the transponder, Make sure it's ~50ohms and < 2:1 Vswr.

See attached:
 

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swirl angle

Ralph Inkster said:
Tufting & videoing as mentioned would be a good approach. Another method would be to have others take pictures of any oil/exhaust streaking on their bellies before they clean it off.

My current 6A has a high time engine in it & the oil/exhaust pattern suggests the prop wash deflects the air stream off to the right (passenger) side slightly. My guess about a degree or two right.
Click to expand...

I've got some ballpark numbers for swirl angle for a 67 inch prop driven by 110hp and a true airspeed of 150kt @ 10k from a recent simulation:

The swirl at the prop disc goes from about 3 degrees at 40% of prop radius to 1 degree. So, I guess, maybe less then 2 degrees on the belly going from right to left toward the tail.

The extra section drag of misalignments of +/-3 degrees will be near zero, but the induced drag will be a square law function. Could be computed.

Also, the flow direction on the belly is influenced by the wing lift and curls outward from the center. this would be added to the swirl angle, apposing on the right, adding on the left. No guess what that local flow is at cruise. Time for "dirty" oil testing.

When it comes time to install one of these blades on my relic, I'll just point it to the right about 1 degree and call it done - possible milli-knots gain and no extra work.

Ron
 
Cumulo said:
I'll just point it to the right about 1 degree and call it done - possible milli-knots gain and no extra work.
Click to expand...

This is what I have done. My thanks for all the thoughtful replies.

The discussion did also make me wonder how much drag our comm antennas cause and whether it could be reduced by adding some aerofoils to them (which would have to be carefully oriented)? Has anyone done this or seen any research?
 
Antisplat makes a little plastic fairing for this purpose.

The most scientific research I've seen done is a data point that said "looks cool"...but that was me
 
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