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Drag From Cat Whisker VOR/LOC Antenna

flickroll

flickroll

Well Known Member
I have a Bob Archer antenna in my wingtip but am not satisfied with it's performance. The antenna is OK, however with HID lights in close proximity to the antenna the sensitivity of my SL30 Nav receiver is reduced significantly.

I am thinking about installing a RAMI AV-12L cat whisker antenna on the underside of my -8, close to the tailwheel, just aft of the F-810-B bulkhead. Does anyone have any data about the drag that will be induced by this antenna? I suspect it will be negligible, especially considering where I will mount it which will be in significant turbulence from the prop and the exhaust. But if anyone has before and after numbers with this antenna or the Comant equivalent I'd appreciate any insight.

Thanks
 
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Antenna drag

I have a Comant mounted on top of the vertical stabilizer, with the elements pointed forward (its an eye-poker if pointed backward). I take if off for racing, but can't measure any difference in speed. Antenna performance seems fine in this location.
 
Samo Samo

I have mine mounted with the elements pointed back (opposite of Alan) and I also remove the elements for racing most of the time but I have the same results as Alan - no measured difference. The beauty of mounting it on top of the VS is the center element is mostly contained within the vertical stabilizer cap.

Bob Axsom
 
flickroll said:
I have a Bob Archer antenna in my wingtip but am not satisfied with it's performance. The antenna is OK, however with HID lights in close proximity to the antenna the sensitivity of my SL30 Nav receiver is reduced significantly.

I am thinking about installing a RAMI AV-12L cat whisker antenna on the underside of my -8, close to the tailwheel, just aft of the F-810-B bulkhead. Does anyone have any data about the drag that will be induced by this antenna? I suspect it will be negligible, especially considering where I will mount it which will be in significant turbulence from the prop and the exhaust. But if anyone has before and after numbers with this antenna or the Comant equivalent I'd appreciate any insight.

Thanks
Click to expand...

Is it really that bad? Is it impacting your flying because the reduced performance? More of a pride issue?

Personally, I really only use the VOR reciever to fly the ILS. As long as I can get a good signal on the LOC/GS at the IAP, that is all I really need.

JMHO
 
I picked up a flier of all the RAMI antennas while at OSH and it says that the AV-12L has a drag force of 0.92lbs at 250mph.

Not sure how to convert that into speed lost, does anyone here know the formula?
 
Rodoc said:
Personally, I really only use the VOR reciever to fly the ILS. As long as I can get a good signal on the LOC/GS at the IAP, that is all I really need.
Click to expand...

Good point David. However, the Archer antenna is giving me some quirky indications on an ILS. If I can figure out how to stop that, then the Archer would be fine because any other navigating would be predicated on GPS.
 
Jim,

Go back and search from a few months ago - I had a long thread chasing down the exact same low performance. Long story short, I repackaged the wiring in my wingtip to keep it in line with Bob Archer's recomendations, and set up a dedictaed GS antenna to maximize signal strength to both GS and LOC receivers, and now I have pretty good performance! (I also don't think the cat whisker drag would be significant, I just wanted to avoid cuttign new holes and such....)

Paul
 
I always install the cat whisker antenna on the belly just forward of the tie down. When you need it, it needs to work. There are still some places in this country where you can only get to flight service via listening on the vor, including the Bahamas. I can get reception, depending on altitude, of 80-100 miles away. It's been nice to have it there when I need it. I seriously doubt one could tell a difference in speed with or without it.

Vic
 
vic syracuse said:
I always install the cat whisker antenna on the belly just forward of the tie down. When you need it, it needs to work. There are still some places in this country where you can only get to flight service via listening on the vor, including the Bahamas. I can get reception, depending on altitude, of 80-100 miles away. It's been nice to have it there when I need it. I seriously doubt one could tell a difference in speed with or without it.

Vic
Click to expand...

Wow!

Putting the Cat Whisker Antenna on the belly is an idea worth stealing.

Would you have a photograph or two that you could post?

Thanks!
 
Bit of fun

I'm posting an old post. Basic formulas are not mine. I'm not engineer. So, take what follows as a joke...

RV9A with O-320
Antenna RAMI AV-529 (COMM, straight) rated lbs. 3,43 drag @250 mph (should recalculate for VOR antenna)

"How to calculate speed loss.

First of all, we have to determine drag @195 and @187 mph. (which is max and cruise 75% speeds for RV9A)

Drag formula: Drag (lbs.) = (V*V*area*0,5)/56.403
Antenna area (sq. inch.) 3,43
So, [(250*250)*X*0,5]/56.403 = 3,43
And: (3,43*56.403)/31.250 = 6,1907932
This is RAMI AV-529 antenna area (useful for obtaining new drag @ different speeds)

I) V max. (195 mph.) @ sea level

Drag @ 195 mph. [(195*195)*6,1907932*0,5]/56.403 = 2,0868

Power req. = Speed * drag
Speed (ft./sec.) [195*5280 (ft/mile)]/3600 (sec/hour) = 286 (ft./sec.)
Power req. = 286*2,0868 = 596,8248 (lbs.-ft./sec.)
1 HP = 550 lbs.-ft./sec.
Req. HP: 596,8248/550 = 1,085136 HP
Propeller efficiency 80%: 1,085136/0,8 = 1,35642 engine HP

If we need 160HP to maintain 195 mph.: 160-1,35642 = 158,64358 engine HP available
Speed loss formula: [(engine HP av./engine HP)^(1/3)]*speed (mph.)
[(158,64358/160)^(1/3)]*195
[(0,991522375)^(1/3)]*195
0,997166*195 = 194,4473
195-194,4473 = 0,5527 mph. loss

II) V cruise 75% (187 mph.) @ 8000 ft.

Drag @ 187 mph. [(187*187)*6,1907932*0,5]/56.403 = 1,9191
Density alt. correction(79%): 1,9191*0,79 = 1,516

Power req. 286*1,516 = 433,576 (lbs.-ft./sec.)
HP req.: 433,576/550 = 0,78832 HP
Propeller efficiency 80%: 0,78832/0,8 = 0,9854 engine HP

If we need 120HP to maintain 187 mph.: 120-0,9854 = 119,0146 engine HP av.
[(119,0146/120)^(1/3)]*187
[(0,99178833)^(1/3)]*187
0,997255*187 = 186,4867
187-186,4867 = 0,5132

So, if the above calculations are right, we can say that on the given airplane such antenna makes airplane loose, at worst, between 0,51 and 0,55 mph.
It has a maximum Vswr 1.8:1, which means a max. loss of 12% signal".

A VOR antenna is much thinner. It should produce low drag. Maybe its drag will be less than 0,5 mph.
 
Ironflight said:
Jim,

Go back and search from a few months ago - I had a long thread chasing down the exact same low performance. Long story short, I repackaged the wiring in my wingtip to keep it in line with Bob Archer's recomendations, and set up a dedictaed GS antenna to maximize signal strength to both GS and LOC receivers, and now I have pretty good performance! (I also don't think the cat whisker drag would be significant, I just wanted to avoid cuttign new holes and such....)

Paul
Click to expand...

Paul, as I recall you put HID lights in your tips, and on one of those tips you have the Bob Archer antenna. What did you do differently to increase performance? On my installation, all I have is LED based nav and LED based strobe wires (power for strobe, power for nav, common ground and sync) running out the diagonal member on the nav antenna before they turn forward to the nav/strobe lights. No other wires cross the antenna so I don't what I could do differently, and with the HID light turned on range is cut in half, at the least. I would be reluctant to shoot an approach until I get full performance from this antenna. Thanks
 
Originally Posted by vic syracuse
I always install the cat whisker antenna on the belly just forward of the TAIL tie down. When you need it, it needs to work. There are still some places in this country where you can only get to flight service via listening on the vor, including the Bahamas. I can get reception, depending on altitude, of 80-100 miles away. It's been nice to have it there when I need it. I seriously doubt one could tell a difference in speed with or without it.

Vic

Just clarifying that it on the aft belly, forward of the tail tie down.

Vic
 
flickroll said:
Paul, as I recall you put HID lights in your tips, and on one of those tips you have the Bob Archer antenna. What did you do differently to increase performance? On my installation, all I have is LED based nav and LED based strobe wires (power for strobe, power for nav, common ground and sync) running out the diagonal member on the nav antenna before they turn forward to the nav/strobe lights. No other wires cross the antenna so I don't what I could do differently, and with the HID light turned on range is cut in half, at the least. I would be reluctant to shoot an approach until I get full performance from this antenna. Thanks
Click to expand...

Jim,

I originally built my own Archer clones, and installed them fairly far aft on the tip ribs, with the idea to get them as far away from the wiring for the lights as I could. When I decided to track down my poor performance, I replaced them with REAL Archer's first, but didn't move them forward according to the instructions, so it didn't make sense to actually attach the wires to the diagonal member. What I did notice is that my wire service loop was sort of "randomly" draping over the antenna, so I attached them to the tip rib with Adel clamps until they were well forwardo of the antenna. This seemed to help performance. But the real key for me was the separate GS antenna (nothing more than a stripped piece of Coax running along my roll bar. I justify it by deciding that the signal aquired by the Archer antenna doesn't get split before going to the radio, and therefore, the radio gets more signal strength.

paul
 
GS/VOR splitter loss?

Ironflight said:
I justify it by deciding that the signal aquired by the Archer antenna doesn't get split before going to the radio, and therefore, the radio gets more signal strength.
Click to expand...

Couldn't hurt, but I doubt it is a big effect. The glidslope band is at three times the frequency of the VOR band (around 330 MHz vs. around 110 MHz), so there's no reason a well-designed GS/VOR splitter needs to 'steal' power from one to feed the other. (I'm assuming that at $140 a piece, something like the Comant CI-507 is well designed :). Anyway, it is specified at 0.5dB maximum insertion loss, and that is not much.)

--Paul
 
There is an RV-7A in the Classifieds section right now that has the VOR antenna mounted under the tail: http://www.vansairforce.com/community/showthread.php?t=46893

Looks like a pretty good place in the A-model. For a taildragger, I'd think you'd have to mount the antenna further forward, point the elements the other way, or otherwise do something to keep the antenna from getting beat up by grass and weeds... hmm...

mcb (wavering between an Archer in the wing and cat whiskers under the tail)
 
mburch said:
Looks like a pretty good place in the A-model. For a taildragger, I'd think you'd have to mount the antenna further forward, point the elements the other way, or otherwise do something to keep the antenna from getting beat up by grass and weeds... hmm...
Click to expand...

It actually hasn't been a problem on Louise's -6 Matt - been there (under the tail, pointed aft) probably since it was built, and only very occasionally has a small piece of grass on it.

Paul
 
mburch said:
There is an RV-7A in the Classifieds section right now that has the VOR antenna mounted under the tail: http://www.vansairforce.com/community/showthread.php?t=46893

Looks like a pretty good place in the A-model. For a taildragger, I'd think you'd have to mount the antenna further forward, point the elements the other way, or otherwise do something to keep the antenna from getting beat up by grass and weeds... hmm...

mcb (wavering between an Archer in the wing and cat whiskers under the tail)
Click to expand...


I agree that is way too far aft for a tail dragger. I saw a few -8's at OSH that had cat whiskers on the bottom of the fuse just aft of bulkhead F-810-B which is close to directly below the leading edge of the HS. This location is far enough forward that grass, weeds, etc. won't be an issue, and the fuselage skin is thicker here than it is forward of the bulkhead. I talked to one of the -8 owners at Osh and he said that you can mount the antenna without climbing into the 'coffin' by reaching down through the large hole in the HS 'platform'. I'm gonna take a close look at it this weekend and if what he said is true I'm going to go with a cat whisker antenna. My local avionics shop said they recommend the Comant over the RAMI equivalent because the Comant has the balun built into the antenna base and the RAMI has the balun in the antenna cable. They said the built in balun would give better performance but I imagine either would work well. The RAMI is a little less expensive.
 
Ironflight said:
Jim,

I originally built my own Archer clones, and installed them fairly far aft on the tip ribs, with the idea to get them as far away from the wiring for the lights as I could. When I decided to track down my poor performance, I replaced them with REAL Archer's first, but didn't move them forward according to the instructions, so it didn't make sense to actually attach the wires to the diagonal member. What I did notice is that my wire service loop was sort of "randomly" draping over the antenna, so I attached them to the tip rib with Adel clamps until they were well forwardo of the antenna. This seemed to help performance. But the real key for me was the separate GS antenna (nothing more than a stripped piece of Coax running along my roll bar. I justify it by deciding that the signal aquired by the Archer antenna doesn't get split before going to the radio, and therefore, the radio gets more signal strength.

paul
Click to expand...


Thanks for the explanation Paul. I think at this point I'm going to try a cat whisker. I've tried lots of things with the Archer antenna and just can't get the performance to the level I want. The downside of the cat whisker is I am sure the installation in an already flying airplane is no picnic, plus the holes that get drilled in the skin which I don't like...
 
ccrawford said:
I picked up a flier of all the RAMI antennas while at OSH and it says that the AV-12L has a drag force of 0.92lbs at 250mph.

Not sure how to convert that into speed lost, does anyone here know the formula?
Click to expand...
Assuming that this RAMI drag claim is true:

Drag will vary as the square of the CAS (I'm ignoring the tiny difference between CAS and EAS), so let's calculate the drag at 213 mph CAS (claimed top speed of a 180 hp RV-8). Drag = 0.92 * (213/250)^2 = 0.67 lb.

Power required = drag times TAS = 0.67 lb * 213 miles/hr * 5280 ft/mile / 3600 s/hr = 209 ft-lb/s. 209 ft-lb/s / 550 hp/(ft-lb/s) = 0.38 thrust horsepower (thp) required to pull this antenna through the air at 213 mph (I'm assuming we are at sea level, standard day, so CAS = TAS).

Let's assume a 180 hp engine, and a prop with 85% efficiency. The engine has 180 * 0.85 = 153 thp available. If we add this new antenna, we'll use 0.31 of that thrust power, leaving 152.62 thp to overcome the rest of the drag. Speed varies as the cube root of the power available, so our new speed would be 213 * (152.62/153)^(1/3) = 212.82 mph. This RAMI antenna costs us 0.18 mph in top speed.

If we wanted the drag at a cruise condition, we would calculate the CAS for the cruise case, and calculate a drag value for that CAS. This will be less drag than the top speed at sea level case. Then we calculate power required using the cruise TAS, and thrust power available using the cruise power and an assumed prop efficiency. 85% prop efficiency is a good SWAG if we are close to the prop's design point. The loss in cruise speed should be slightly less than the loss in top speed.

Note: This calculation is for one specific antenna. Other antennae may have different drag values, so the results would be slightly different. Bottom line - one antenna probably doesn't make a measurable difference. But the effect of several antennae might add up to a difference that the racer guys would care about.
 
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Kevin, I have often wondered if one molded small flexable airfoil shaped covers for the rods on external antennas, would there be a noticable speed gain? Assuming the comm antennas have similar drag as in your example (may be a bad assumption), looks like it's not worth the trouble (less than one mph for all three antennas)...
 
Antenna inside of fuel tank?

Is it possible to put an Archer antenna inside a tip tank, glssed over to isolate it from the fuel, or would the reception significantly be reduced?
 
deene said:
Kevin, I have often wondered if one molded small flexable airfoil shaped covers for the rods on external antennas, would there be a noticable speed gain? Assuming the comm antennas have similar drag as in your example (may be a bad assumption), looks like it's not worth the trouble (less than one mph for all three antennas)...
Click to expand...
Sure, an airfoil shape has much less drag than a cylindrical shape. But, as you noted, if RAMI's drag claims are valid, the speed increment from adding an airfoil shaped cover would be minimal.

Also keep in mind that adding any cover will increase the frontal area, as the maximal thickness of the airfoil will have to be a bit bigger dimension than the diameter of the antenna. This extra frontal area will give a bit more drag than we would get if we could somehow make a cover that had the airfoil's max thickness = antenna diameter.
 
You guys are making me question my penciled-in choice of an Archer antenna in the wing, as the more traditional VOR antenna under the tail sure seems to have a lot of advantages from an RF perspective. Regarding streamlining, I'd be more inclined to make a little fiberglass fairing to cover the "hockey puck" than to worry about the antenna elements... I know it's hanging out in airflow that's already disturbed, but that thing sure does look blocky!

mcb
 
mburch said:
... I'd be more inclined to make a little fiberglass fairing to cover the "hockey puck" than to worry about the antenna elements...
Click to expand...

Hi Matt

I've been thinking about the doing something similar if needed. The RAMI AV-12L has a rounded edge so I don't think it would be worth the trouble for that antenna. I have ordered a Comant CI 158-C which will come today. I don't know what it looks like, however if it is not radiused like the RAMI I'll probably make a fairing for it.
 
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I love my VOR antenna

I have a love hate relationship with Bob Archer Antennas. I HATE my com antenna (on com2). I LOVE the VOR antenna. It works well, no issues at all, and I have an HID landing light in the same tip. I have zero problems shooting ILS approaches, VOR approaches, etc. I can't imagine why you would want to put an antenna into the wind if you didn't have to.

The problem with the COM antenna is that com radio waves are vertically polarized, and there is not enough vertical distance in a wing tip to get the right antenna height. So, I guess for a while at least, I will live with poor reception on COM 2.
 
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Update

Installed a Comant CI 157P antenna on the bottom of my -8's fuselage, just aft of the F-810-B bulkhead. Reinforced the mount area with a doubler on the inside of the fuselage skin. This installation was a PAIN as I had to crawl way in the back of the -8's fuselage, which seems like a coffin :(, and it was hot (95 deg) and humid. Installed a 1/2" piece of plastic tubing with adel clamps on the horizontal fuselage stringers as a conduit for the antenna wire. Pulled the wire and made the connections. I still need to clean up aluminum chips from drilling and reinstall the elevator control rod, but at least the antenna installation is functional.

The performance on the ground seems to be better than with the Archer wing tip antenna. To be fair, the Archer antenna performs 'OK' as long as no lights are running. But when I turn on HID lights, the range of my SL30 Nav receiver is reduced dramatically. With the Comant antenna, I am able to pick up a VOR station about 15 miles away and on the other side of a 1000' 'mountain' range. The Archer could do this too. However, when the HID lights were turned on, with the Archer antenna, I would lose the VOR station. With the Comant, with HID lights, strobes, and nav lights on, I still receive the station. In other words, the Comant does not seem to be subject to the RF noise that the Archer antenna was.

I'll report back after I fly, but so far so good :D
 
We have a Comant CI-215 on our RV-6, mounted just aft of the F-610 bulkhead. It works very well, and grass doesn't bother it:
whiskers.jpg

If you need to deal with more substantial shrubbery, the whiskers on the CI-215 are threaded and easily removable. It does cost a lot more than the CI-157 though.

--Paul
 
Update 2

Was finally able to fly today. Been installing a new set of wing tips, and also worked on a new empennage fairing, so it had been a while (3 weeks) since the -8 had been in the air.

The Comant antenna works well. I don't think it has quite the range of the Archer (which was excellent when no lights were on), but I am unable to do a side by side comparison so can't tell for sure. However two things make it a winner in my book:

1. Most importantly, the Comant gives rock solid performance while on an ILS, which is the primary purpose for my SL30 (vs SL40) in the first place. The Archer would occasionally cause my SL30 to 'swing' (never full scale, but occasional L/R U/D transients) both LOC and GS while on the ILS. I was not about to fly IFR with that performance. With the Comant both the LOC and GS are very stable. I may now have a new IFR traveling machine :D

2. Any combination of nav/strobe/landing/taxi lights made no difference with the performance. With the Archer, HID lights mounted in close proximity to the antenna dramatically reduced the range of the SL30. So much so that turning off lights would be a checklist item when shooting an instrument approach.

So....the effort was worth it. I don't like the look of the antenna, but the performance improvement makes it a win. And really, mounted where it is, you have to look at it to notice it. YMMV
 
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Thanks Jim for the update, and Paul for the photo. Based on this, I think I will change my plans from the Archer to the Comant for sure. Probably I'll buy the CI-158, which is like Jim's -157 but with detachable elements (should come in a smaller box too!).

cheers,
mcb
 
mburch said:
Thanks Jim for the update, and Paul for the photo. Based on this, I think I will change my plans from the Archer to the Comant for sure. Probably I'll buy the CI-158, which is like Jim's -157 but with detachable elements (should come in a smaller box too!).

cheers,
mcb
Click to expand...

The removable elements of the 158 is definitely a plus. I decided to go with the 157 however because the antenna elements are swept more. So the measurement from tip to tip of the whiskers of the 157 is shorter than for the 158. The drag coefficient is therefore probably a little less for the 157. Not that it makes that much of a difference, but it also gives a slightly sleeker look.
 
Comant Towel Bars....

I have these in the wingtips of my 6. I really haven't tried to Nav with them yet with my SL30. Has anyone else used this setup? If so what were the results?
 
Well, I did it...

20091101_vor_antenna1.jpg
20091101_vor_antenna3.jpg


Not flying yet, but seems like it ought to work pretty well down there!

mcb
 
If you airfoil the hockey puck part of the antenna it will be faster. Whether its measureable or not is a different matter.
 
I would be surprised if it's not measurable

That is a pretty big cylinder to attach to the high pressure flat bottom side of the fuselage which is being dragged through the air at some positive angle of attack even in level cruise flight. The faster you try to fly the more drag it will produce. A lot of people mount them there though so it must work fine and if you are not racing you will never know the difference.

Bob Axsom
 
Bob Axsom said:
That is a pretty big cylinder to attach to the high pressure flat bottom side of the fuselage which is being dragged through the air at some positive angle of attack even in level cruise flight. The faster you try to fly the more drag it will produce. A lot of people mount them there though so it must work fine and if you are not racing you will never know the difference.

Bob Axsom
Click to expand...

I have one mounted to my -8. I have not noticed any difference in speed (and my -8 is a fast one....). If there is a loss of speed you'd need instruments more accurate than what are in the airplane to measure it. What I have noticed is excellent nav performance. I tried and tried to get the Archer wing tip antenna to give good performance but finally gave up. My installation has HID lighting in close proximity to the wing tips....the HID's would significantly reduce performance of the Archer. They make no difference with the cat whisker antenna. YMMV
 
I have the whiskers antenna at the same place as paul under the tail on my RV-7. Anybody have trouble with localiser on procedure turn outbound. In this situation, i lost the localiser for about 15 second when i turn outbound to inbound in a hold or on a procedure turn on a localiser. I have a dynon and a sl-30.
 
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