Drag From Cat Whisker VOR/LOC Antenna

flickroll · #1 08-16-2009, 05:51 PM

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

Alan Carroll · #2 08-16-2009, 05:59 PM

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.

Bob Axsom · #3 08-16-2009, 06:24 PM

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

Rodoc · #4 08-16-2009, 06:44 PM

Quote:

Originally Posted by flickroll

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

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

ccrawford · #5 08-16-2009, 07:17 PM

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?

flickroll · #6 08-16-2009, 08:29 PM

Quote:

Originally Posted by Rodoc

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.

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.

Ironflight · #7 08-17-2009, 06:15 AM

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

vic syracuse · #8 08-17-2009, 06:18 AM

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

Crossbow · #9 08-17-2009, 06:26 AM

Quote:

Originally Posted by vic syracuse

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

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!

Camillo · #10 08-17-2009, 07:46 AM

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.