ELT Antenna Doubler?

[jlfernan]

This is not a debate as to the best location for an antenna, I'm using the KISS method and plan to put the ELT antenna on top of the aft fuselage. It's the Ameri-King ELT. Do I need a double under the skin and if I do, do I need to rivet it in place or just leave it like a big washer? Just wondering.

[pierre smith]

Hi Jorge,
A small piece of rectangular aluminum, 2" or so is plenty. I'd probably use .062 and rivet the four corners after the piece is shaped to the rear fuselage contour.

Regards,

[terrykohler]

Jorge:
Don't know if you got my direct reply (and pictures) to your question. If you mount directly behind the baggage bulkhead, there's already a "doubler" in place. Just make certain if you use this, you also drill the antenna hole so there is sufficient clearance on the back side for the lock washer and nut. This location will allow you to use the vertical brace as a mounting surface for your coax.

If you don't use this location, I agree that you absolutely need a doubler under the rear skin. The loading on an external antenna coupled with the thin gage of the aft skin can lead to significant flexing. Plan to rivet the doubler in place. Make sure all corners have a very large radius.
Terry

[gmcjetpilot]

I agree with above, you need a doubler. Structurally if you don't rivet at all, it's just a huge washer and not very effective, so there is no need to make it big. If you go that way I agree 3" inches round or square is fine.

If you want something "structural" try go some thing bigger, at least longer. Try to get it near a bulk head. The doubler should have free flanges bend down into the long side edges and one flange at the bulkhead end, and if at all possible, attached that flange to the bulkhead. The other end should have a few rivets in it. Ideally a "True doubler" would go from bulkhead to bulkead but you can compromise for a small elt antenna. Also to make it a true doubler you would put many more rivets in, but I am making a compromise between a "washer" doubler (really no doubler) and a structural doubler. Just a couple of rivets at the end the doubler thru the skin will be fine if you attach the other end to a bulkhead.

You should look up the AC for standard a/c practice.

AC Number: AC 43.13-2A Date: 01/01/1977

Subject: Acceptable Methods, Techniques, and Practices - Aircraft Alterations (chap 3, pg 15-16)

http://www.airweb.faa.gov/Regulatory...2?OpenDocument

Chap 1-8 accepted practices and alterations (page 14 & 15 on adobe reader)

[Bob Hoffman]

Terry
Could you post the reply (and Pictures) here on the Forum so we all can learn ? I'm in the same boat as to what is the correct method.
Thanks
Bob

[terrykohler]

Bob:
Haven't been able to post pictures as I don't have them on a URL. In any event, the pictures I was trying showed the ELT antenna bowed in flight as well a a ground picture from the side of the AC. While you should be able to mount anywhere along the fuselage with proper support, I chose to go thru the F688 gusset on the right of center. While this gives a ready backing for the antenna, remember to drill near center to allow washer and nut clearance for the aft skin rivet shop heads. This antenna location also made it easy to route the cable down the F728A Bellcrank Channel. I located the ELT on an L-Bracket that I fabricated and riveted to the F729B on the top end and the floor pan on the bottom. I used round head rivets for the latter location with the shop heads on the inside of the fuselage. The bracket assembly is located aft of the elevator bellcrank. This ELT/antenna location allows very little fore/aft cable run and hopefully will minimize potential of disconnect in "the event of an off airport landing". Interestingly, the ELT bracket prevents oil canning of the aft bottom skin on the right side. I'm considering adding a similar L-Bracket/support to the right side to prevent the somewhat minor sound annoyance than seems to occur in ground effect, just before touchdown.
If you wish to see pictures of my AC in flight or on ground (interior/exterior) please send your email address to:
terrykohler@comcast.net

[terrykohler]

Bob:
Sorry if there's confusion. ELT bracket is on left (pilot's side) of the F729B.

[jlfernan]

I would love to use that spot but I'm going with a slider and it can be tough to open with an antenna in the way. Thanks anyway for your help.