Av-17 antenna - do I need a hard point?

[Steve Sampson]

I am planning to put an AV-17 bent whip VFR antenna on the underside of the -4 I am building.

Question: Do I need to put a doubler to handle the vibration, or is the floor quite strong enough? I would be interested to know what is 'normal' and any problems people have run into.

Thanks,

[kentb]

But I put a doubler on my -9. All the sample that I was looking at had doubler so I made one also. Mine is about 1 inch larger on all sides of the antenna base. I dimpled and riveted to the bottom skin.

Kent

[gmcjetpilot]

Originally Posted by Steve Sampson

I am planning to put an AV-17 bent whip VFR antenna on the underside of the -4 I am building.

Question: Do I need to put a doubler to handle the vibration, or is the floor quite strong enough? I would be interested to know what is 'normal' and any problems people have run into.

Thanks,

If you go the FAA's AC for "Standard Structural Practices" it would be overkill and add more weight. We are talking about a little COM whip and not a WWII ADF LOOP antenna or flying wing antenna.

The floor under the cockpit has pretty thick skin and fwd/aft ribs or "intercostals" are close spaced, so you could get away without any doubler but wound not recommend it.

At min do what Kent said, doubler with same gauge or one gage up, 1 inch wider around antennas footprint. A few 3/32th or 1/8th inch rivets to tie doubler to skin with proper edge margin. Some omit the rivets, but suggest at least four or six rivets to "tack" the doubler down. A real doubler would be larger and have two rows of rivets around the antenna, but again its overkill and we are talking about a little comm whip antenna (on thick base skin and close spaced ribs). Also consider making the doubler rectangular (easier to cutout) and bending a flange into the stiffener sides. (see thumb nail below)



Just for discussion, the NEXT level up in doubler, say if the antenna was on thin skin, like aft fuselage, a larger doubler (with flanges) that ties into ribs/stringer, may be better, like picture below from FAA A/C Best practices. (see thumb nail below)

[Steve Sampson]

George / Kent thanks for your replies. What you both suggested was just about what I intend. I just want to distribute the floor load a little more widely in case it resonates (mechanically).

Your point about weight George is close to my heart.

PS George - I have just learned a new word. Brilliant! Doesnt often happen these days. "Intercostals"!

[chris mitchell]

Steve,

Technically the "intercostals" would be the spaces between the ribs! So your antenna (dare I say aerial on this web site?) would be sited in an intercostal space.

Chris