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Old 01-31-2019, 09:07 PM
burrm burrm is offline
Join Date: Oct 2018
Location: Maryland
Posts: 1
Default Vertical Stabilizer Wiring Conduit

Hello all,

This is my first post here, although I have been lurking for some time. I have been a pilot for many years but I am a complete "NOOB" when it comes to home building. Nonetheless, I am embarking on an RV-10 build, the first step of which I am sure you all know is the vertical stabilizer.

Looking around the web, I have seen a few folks who have modified their vertical stabilizer to include one or more wiring conduits by drilling one or more 3/4” holes into the web of the VS-1006 top rib and adding a doubler in order to facilitate the penetration of the wiring conduit(s).

I like the idea of having the flexibility for wiring extra lights, cameras, antennae or whatever in the future, and especially like the flexibility that homebuilding affords us to make such modifications as desired.

That all being said, and given my total lack of experience with such matters, I have not been able to find any engineering references regarding the implications of a modification such as this. For example, how is the appropriate size and thickness of the doubler determined, what are the risks or implications of going too small/big or too thick/thin? Are there implications (other than aesthetics) of placing the doubler either on top or on the bottom side of the rib? How many rivets should be used to attach the doubler? How is this determined?

I’ve consulted AC 43.13, as well as FAA-H-8003-31A but didn’t find any specific reference that was relevant, although admittedly given my lack of experience I might not recognize it even if it is there. The closest I could find was toward the end of chapter 4 in the latter book starting on page 4-105 “Rib and Web Repair”, where it talks about patches being the same gauge as the original material or one gauge higher, two rows of rivets properly spaced, etc.. However, this is talking about repairs to damaged ribs as opposed to intentional modifications with doublers.

Perhaps I’ve watched too many episodes of “Air Disasters” on TV, but before making a seemingly significant modification like this, I would like to make sure that I have fully done my due diligence and fully understand all of the implications, have convinced myself that I will be doing it “properly” and with proper references noted in the builder’s log etc.. I guess given my lack of experience I'm not comfortable jumping into it merely based on reading a few internet posts, any more that I would start sawing holes through load-bearing walls of my house without understanding the implications and/or consulting with a professional.

I know the obvious answer is "if you're not comfortable and don't know what you're doing, don't do it and just stick to the plans". I get that, but in addition to making decisions regarding this one specific potential modification, part of my larger goal here is also to educate myself. I feel like if I am going to be a successful homebuilder, I should know how to research such items, what data to consider in order to make informed decisions and choices, etc.

Thanks in advance for any wisdom and/or opinions anyone cares to offer.

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Old 01-31-2019, 09:26 PM
charrois charrois is offline
Join Date: Jan 2019
Location: Legal, AB
Posts: 8

Hi Mike. Good question, that I don't necessarily have the answers for, but can talk about the approach I used.

I'm not sure of documentation for the proper size, thickness, and rivet spacing of doublers. In the past, when I added doublers not according to plans, I just followed the rough dimensions and spacings of other doublers that were in the plans. But a person does have to be careful to not overdo it either - a doubler will naturally increase the stiffness and strength of a part. Increasing the strength should have no ill effects, but if the increased stiffness is too abrupt of a transition from the part on its own and the part vibrates a lot, that could lead to fatigue at the transition point.

As a "rule of thumb", in the "custom" doublers I've made, I've made the doubler out of material the same thickness as the original material. But I know this doesn't help with specific documentation like AC43-13. I also would be interested if someone finds a proper engineering reference for it.

I installed a conduit in the VS as you described and it turned out to be invaluable - I have 3 devices up on the top of the VS that I had wiring runs to.
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Old 01-31-2019, 09:44 PM
rocketman1988 rocketman1988 is online now
Join Date: Jun 2010
Location: Sunman, IN
Posts: 1,355
Default AC43-13

Rivet spacing, offsets, and patterns can be found in AC43-13...
Aerospace Engineer '88

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Old 02-01-2019, 05:36 AM
jliltd jliltd is online now
Join Date: Sep 2013
Location: Rancho San Lorenzo
Posts: 414

As charrois pointed out, in the vernacular "doubler" = doubling the the total thickness. Therefore the doubler is the same thickness and material as the part it is reinforcing. In a different example, if the reinforcement patch was double the thickness of the original part it would be a "tripler" since the end result would be a total stack up of 3 times the original thickness. And so forth. Again we are talking rules of thumb.

One of the reasons for sticking to the original thickness (doubling) is to provide adequate reinforcement without over-doing it. So a "tripler", "quadrupler" or other thicker reinforcement might be stronger in and of itself but this increase in thickness can cause a loading spike at the edge of the reinforcing patch resulting in localized stress concentrations and sometimes uneven fastener loading (like the proverbial highway construction crew, not every rivet pitches in to help). A generalization for sure as there are many more factors about the geometry and loading that can affect the design. Thankfully these aren't usually a problem with an aircraft the scale of an RV-10, especially at a tip rib which isn't as loaded as root rib structure. AC 43.13 helps with generalized accepted procedures for light aircraft.

The fact you are even asking about this bodes well for a good build. Good job.
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Old 02-01-2019, 06:26 AM
MIKE JG MIKE JG is offline
Join Date: Sep 2016
Location: NE Ohio
Posts: 75

Good discussion on this thanks. I've never thought about the problems that having a "doubler" that was too thick might cause.

Learning, learning, learning, always learning!
-Mike G
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Old 02-01-2019, 09:09 AM
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Mike S Mike S is offline
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 14,530
Default Welcome to VAF

Originally Posted by burrm View Post
Hello all,

This is my first post here, although I have been lurking for some time.

Mike, welcome aboard the good ship VAF
Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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Old 02-01-2019, 10:34 AM
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ppilotmike ppilotmike is offline
Join Date: Jan 2010
Location: Denver, CO
Posts: 1,904
Default Welcome!

Congratulations on your decision to build an RV! Another Data Point. The 10's vertical stabilizer's ribs have lightening holes in them, all except for the top-most one. You can pass your conduit through them without having to drill any holes in the structure (this is best). You will have to drill a hole in the top rib (cap), so keep that hole and its conduit as small as possible (like 3/8" or 1/2"). If you do this and keep that hole well away from the edges of the cap, I would think you'd be okay without a doubler. You might need a doubler if you're going to mount an antenna or something up there. A lot of 10 builders have used this area to mount their "cat whiskers" NAV antenna up there. Personally, I did not change the vertical stabilizer structure at all from the plans (no conduit, no rudder trim servos, no nothing), but there are certainly a lot of people who have. One thought about cameras: One benefit to going with an exterior mount, like the ones sold by some 3rd party vendors, is that 1) they've likely done some engineering to design the mount, 2) they've got some history with people flying with the mount, and 3) you can change out the camera as technology improves and the cameras get inevitably smaller and lighter. If you hard mount something, you're stuck with the mount, the location, and often-times the camera model. MY $0.02. Happy building, and welcome to the madness.
Mike Rettig
EAA Chapter 301
VAF Dues Current: Paid for 2019 on 12/03/18
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