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  #1  
Old 02-12-2009, 08:01 PM
Bill Wightman's Avatar
Bill Wightman Bill Wightman is offline
 
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Location: OKC, OK
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Default FIX: Putting a hole in your control stick for wiring

Problem: How to run the wiring out of your control stick(s) when Vans says its bad to put a hole in the wall of the tube, especially near the pivot point.

Here's the warning from Vans:

Any hole anywhere in the control stick will reduce its strength. The nearer this hole is to the pivot point of the stick (where the bending forces are higher), the greater will be the severity of the strength loss. Van's recommends that builders do not drill holes anywhere in the control stick, particularly near the pivot point. Generally, the accepted routing for electrical signal wires is through the opening on the bottom of the control stick.

So now we have a problem that nearly 100% of RV builders must face: how to run the wires out the stick, and most of us don't want to run them out the bottom of the stick for reasons explained elsewhere. Also, there's no "opening" at the bottom of the RV8 stick mentioned in the warning from Vans.

This problem was also discussed at length in another thread when a builder asked how to route wires out of the control stick. Since nearly all of us have wiring in the control sticks these days, a solution is needed.

Solution: The solution is to use a reinforcement "washer" around the hole. The purpose of the washer is to reduce and disperse stresses around the hole in the stick.

This work-around assumes 3/8" diameter hole is necessary to route control stick wiring into the stick. The location analyzed for this work-around is 3/4" up from the longitudinal pivot axis of the stick (the pivot bolt). The hole may be placed on the front, back or side of the stick.

The "washer" used to distribute stress is 4130 steel, .050" thick, and has an outer diameter of 5/8". As mentioned, the inner hole is 3/8" diameter. The washer should be rolled into a radius so that it fits the stick tube's 1-inch outside diameter. To carry the stress loads, the washer should be welded around its outer and inner edges. Edit: Its not recommended to braze 4130 (see post below)

Exploded view of RV8 front stick tube with washer


The resulting tube with washer will have very close to the same bending strength as the tube with no hole in it. Smooth the inner edges of the hole after the washer is fastened. There may (probably are) better ways to do this, but this is one solution that will work.
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Last edited by Bill Wightman : 02-13-2009 at 12:02 AM. Reason: removed option to braze in place
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  #2  
Old 02-12-2009, 08:18 PM
cwreeves cwreeves is offline
 
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Location: Tucson, AZ
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Default Control Stick

Bill,
I'm not an engineer and I can't see how drilling a hole in the control stick will weaken it to the point of failure. After all, we're not using the stick as a jack handle or cheater bar on a pipe wrench. Your only putting enough force on the stick to deflect the control surfaces. I'm sure we're not applying many lbs of touque. I drilled a hole in my RV-7 stick and installed a very small rubber groumet I bought at ACE. Then I installed the wiring.
Charlie
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  #3  
Old 02-12-2009, 09:55 PM
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DanH DanH is offline
 
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Default

Quote:
I'm not an engineer and I can't see how drilling a hole in the control stick will weaken it to the point of failure.
Ahh, the power of "TLAR"
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  #4  
Old 02-12-2009, 10:25 PM
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db8 db8 is offline
 
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Default 4130

So where does one get a 4130 washer from? Make it? Local stores carry them?
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  #5  
Old 02-12-2009, 11:33 PM
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Greg Arehart Greg Arehart is offline
 
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Default

I'm not an engineer either, but I agree with Bill that a hole MAY create a dangerous situation. Yes, it is unlikely that anyone will put the requisite force on a stick to bend it during normal flight, given the hole size that Bill tested. Are all wiring holes the same size? I also note that the pressure tested was only 6" above the pivot - its easy to be higher than that on the stick and not really notice the pressure. Final bit of info: I just tested my 9 to see flyability with the trim tab full up and full down and it was shocking to me how much pressure was required on the stick to stay level. Again, not likely in normal flight but...

my two pennies worth.

Bill, I would be interested to see the model for a hole in the SIDE of the stick.

greg
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  #6  
Old 02-12-2009, 11:41 PM
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sprucemoose sprucemoose is offline
 
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Default

Quote:
Originally Posted by Bill Wightman View Post
I'd probably choose to braze it (any thoughts on this?)
Bill,

Very interesting, thanks for the work. I've got a small hole in my stick exactly like you describe. Since I'm down for annual, I might just pull the stick and do the washer mod like you describe.

Did your FEM take into account the heat affected zone of the weld of the washer?

As for brazing 4130, this is a no-no. I know that bicycle frame builders do it, but this is not a bicycle we are talking about. Below is a quote from Richard Finch, welding guru and author of several welding textbooks:

"Always avoid brazing 4130 steel. The reason is because the chrome-moly steel has a definite grain structure that actually opens up at medium-red brazing temperatures. When brazing alloy is melted onto the steel surface, it flows easily into the many small cracks and crevices in the chrome-moly steel. Then, as the braze joint cools, the brass will not compress, and it causes major cracks to form in the 4130 steel. Often, a brazed 4130 steel part will crack completely before your eyes as it cools."

- Richard Finch, "How to Weld **** Near Anything" pgs. 115-116
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Last edited by sprucemoose : 02-12-2009 at 11:53 PM.
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  #7  
Old 02-13-2009, 12:01 AM
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Bill Wightman Bill Wightman is offline
 
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Default Don't braze

Jeff - WOW thanks for the pointer on NOT brazing 4130!!!! I had no idea. Guess that leaves welding as the best option. I'm editing that option out of my opening post. THANKS.

Charlie - Everybody here ultimately makes their own decision on how they put these planes together, within the scope and context of AC43.13. I'm only offering an engineered work-around to a problem many of us must solve somehow. This remedy isn't intended to be taken as marching orders for anybody. Keep in mind the warning was issued by Vans, not me.

*************************

Where to find the washer? You could use an AN960-616 aircraft grade washer. Its close enough in material properties to work for this purpose. Specs are .390" ID .625" OD and .063 thick in standard thickness. You could work the thickness down to .050 and put the roll bend on it easily. Aircraft Spruce has them.
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Last edited by Bill Wightman : 02-13-2009 at 12:20 AM. Reason: where to find...
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  #8  
Old 02-13-2009, 12:31 AM
chunt0 chunt0 is offline
 
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Default Welding Washers

While we're at this, don't try welding cad plated washers. I understand this would produce highly toxic fumes.
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  #9  
Old 02-13-2009, 09:23 AM
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DanH DanH is offline
 
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<<Yes, it is unlikely that anyone will put the requisite force on a stick to bend it during normal flight, given the hole size that Bill tested.>>

We heard that argument a few times in the other thread. However, Bill carefully explained all his assumptions and conditions. He expects the material at the edge of the 1/2" hole to reach yield with 120 lbs of force applied 6" above the hole. He applied a safety factor of 1.5, which lowers the value to 80 lbs. The safety factor is accepted practice, because all materials exhibit some scatter in testing for ultimate and yield strengths, and because field fabrication is seldom perfect....a tiny nick in the edge of the hole changes everything.

Mr. Grigson offered a link to FAA standard control system loads. The standard maximum applied load for an elevator is 167 lbs and the minimum is 100 lbs. Control systems are expected to be able to withstand these applied loads; they approximate what the bio-chemical meat servo can actually apply.

Here's a simple model:



40 lbs applied at 12" from the hole is the same as 80 lbs applied 6" from the hole as modeled by Bill. That's less than half the FAA design minimum. It is also just 10 lbs away from 5 lbs per G x 6 G's = 30 lbs.

Take out the safety factor and you're at 60 lbs, still less than half. Removing the safety factor approximates going to ultimate strength rather than yield, as noted by Alan's in-house engineering professor. The stick fails at an applied force less than accepted design standards. Given a control jam, anybody here can apply 60 lbs at the grip.

Given no fabrication compromises, never seeing 6 g's, and no control jams, yep, your stick probably won't fail. Everything will be fine as long as everything stays fine....but good engineers assume worst case. Van's says don't do it because they applied accepted design standards. Bill explains why and offers a very easy solution. I find the advice from both to be prudent. Our goal is an airplane better than standard.

Nice job Bill.
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  #10  
Old 02-13-2009, 09:46 AM
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mburch mburch is offline
 
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Default

Okay, so now I'm scared to drill a hole in my stick. But, I still need a way to get wires out. As somebody asked in the other thread, how is this done on Van's demonstrator airplanes?

mcb
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