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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. |
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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4130
So where does one get a 4130 washer from? Make it? Local stores carry them?
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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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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 |
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. |
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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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 |
More than one way to skin a cat
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Another solution would be to run the wiring harness externally along the outside of the control stick. McMaster-Carr and MSC sell nice spring steel clips for this purpose. The clips are shaped like a C, with a bulge (for the harness) in the middle of the C. Two or three clips (or other securing methods) will keep the harness in place. Charlie Kuss |
Just to see
I tested a 3/4 x .035 section of moly I had handy, drilled 7/16 hole. Stuck it in a vice, Hole 1" high and pulled-- Did fail at the hole!( ripped) Just started to bend at 100 lbs on my crude pull scale and failed at about 120 lbs pulling 12" from the hole. 1" would do a little better.
It would be good to have all of Van's different sticks types for distructive testing. |
I'm guessing that Pat Tuckey is glad his stick didn't bend or break:
http://www.vansairforce.com/communit...hlight=trutrak I wonder if Pat has any holes drilled in his stick, and if so, how large are they and where are they located? |
Tested!
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I want to make sure VAF readers here don't take this result to be the gospel. The real stick tube assembly is loaded and supported differently (not like a tube in a vise) and is a 1" OD tube. This does show how relatively easily a steel tube with a hole in it can fail in bending. The most important thing Frank's test shows is the failure mode was what I feared would be the "worst case". The wall of the hole began to rip, which then lead to rapid buckling of the tube. |
Test, Test, Test!
I love the fact that someone did a test! Too often these days, we engineers tend to rely on analysis alone to "solve" problems. In the old days, before sophisticated analysis tools, guys just did testing! I was thinking that I might have an old RV-8 stick that I didn't use sitting in my box, and that I should rig up a test - then I came back to this thread and found Frank had beaten me to the idea. If I get the chance, I'll look to see if I still have a"test article" and see if I can rig one up.
Analysis confirmed by testing is golden - this is a GREAT effort all! Paul |
Dittos Paul...
Sure would be good to test the real article. |
After catching up on this thread, I took a look at a pic from when I installed my AP roll servo. This is the left stick in a bought-flying '98 -6. Please forgive the dust and my runny nut creep indicators...cleaned it up since snapping the pic! :eek:
The hole (which was there before and I re-used when installing a new stick grips) is drilled very close (in my estimation) to the pivot point, which is discussed in the warning referred to above, so this has my attention! Just has a snap bushing in it to protect the wires. ![]() Not sure if the design has changed or if side-by-sides have a different design, but it looks like the attachment for the cross tube, with its fore-aft weld and the triangular gusset would provide some strength to prevent bending and failure at the hole. Just wanted to ask the brain trust for an opinion on that, as I don't want to assume that's the case, and I certainly don't want to have the stick fail there in the event of a lock up! Thoughts? Look OK, or should I suck it up and weld the washer? Don't want to mess up the nearby welds either, though not sure that would be an issue. Another thought would be, if there is room, to weld a vertical gusset on the back of the stick (sorta like the ones that are on the rudder pedal vertical attach points). Sort of like this (bad drawing, but shows the concept...and this is not an engineering fix recommendation...not qualified to do that...but rather question of applicability to this situation): ![]() Just trying to make sure all is well, and thanks for the great info here! (Hey Dan, OT here, but note I followed your advice and inserted the long bolt to the servo rod with the bolt's shank in the control stick hole where the force is applied...learned a good one from ya there! :)) Cheers, Bob |
Bob, hard to tell from the photo, but it looks like the stick is double walled at the position you drilled.
In addition, the pivot point is pretty well reinforced by that gusset plate. Looks pretty strong compared to the modeling Bill did. How is the snow doing??? |
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Greatly simplifies installation as well. The grip, cable and connector can be wired externally then fitted to the stick later on. Thanks Charlie! |
Hole location?
This is a very interesting thread.
As a mechanical design engineer I use finite element analysis (FEA) as a guide. Testing is the ultimate decision factor for me. When pulling out of that steep dive just after you released that 500 lb bomb puts quite a load on the stick so we need to be careful :D I my opinion the aileron loading on the stick is much less, so why not put the hole on the side of the stick????? That is where I am putting mine. Your mileage may vary....... Paul |
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Snow is off and on. Cold though! Small window for flying yesterday! Woo hoo! Quote:
One thought though...what you said about the lower force on the ailerons (or side stick pressure used) makes sense...but if your roll servo or aileron locked up, you might have to use a pretty good amount of force to break it loose. Maybe that's an unlikely scenario, and is different from "putting your feet on the panel and pulling", but perhaps the reinforcement of the hole on the side couldn't hurt either...if it's ultimate safety we're after. Not challenging your assumptions...I'm not an M.E.! :) Quote:
Running a wire down the back or front of the stick wouldn't bother me if I was re-doing this, and the handlebar tape would make a nice cover too. FWIW to consider, my boots actually come up to the bottom of the stick grip anyway, so such an external install might just be covered up anyway. Just thoughts! Cheers, Bob |
You could easily put a hold in the stick and do it in such a way as to not reduce the strenght at all.
Take the hole you have drilled in the 4130 control stick, take another piece of 4130 steel tubing the same diameter as the stick and fishmouth cut the small piece of tube. Use AC43-13 1B to see how it is done and for pictures. Now weld the piece over the hole that you have drilled to where the smaller piece is 90 degrees to the control stick with the fishmouth cut fitting over the hole you have cut in the control stick. Now file and sand down the 4130 tube piece to create a 1/8 lip or the desired height you want of the lip. This is somewhat along the lines of the washer idea except its the FAA way. And we all know that the FAA way is the ONLY way.. :D This is the same type of welding tech used to build airplanes so I think it would be ok. Just my two cents. If you really want to know if it will hold up, I'm sure someone would test it and see. |
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Pat |
I ran my wires down the tube and out the bottom (carefully), so no hole at all. I only have a PTT pair and obviously a larger set of wires might not do so well. Anyone see a potential issue with just having the wires out the bottom?
greg |
Another concept to strengthen tube
Hi all,
Another concept to strengthen tube. Buy some larger diameter 4130 tube from spruce, cut a 2" long piece, slide it down your control stick and drill a hole where you want the wires (telescoping tube). For example, if I wanted to strengthen my 1" OD RV-8 front stick around the hole I drilled, I could but some of the following: 1 1/8 OD .058 Wall Part No: 03-07100 $3.70/ft It has a 1.009 inch inside diameter. Slide a 2" piece down your tube, drill the hole and call it a day. You could weld it on, bond it on, or even have a couple of screws. Take a closer look at the RV-8 rear stick and you'll see what I mean. Cheers Robert Grigson |
Since the -8 and side-by-side models have very different sticks, I'd be interested in seeing what the FEA looks like on the side-by-side sticks with the offset pivot points. Seems like the analysis may be applicable to the centerline models with the pivot through the stick.
Jim |
Hole location - worst vs. best case
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I'd be interested to see similar analyses with a hole a few inches higher and on the side. |
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I went out the bottom
I have the thick wire bundle for the infinity grip and I went out the bottom. It does rub on the belly skin but there is not much pressure on the wire, the bundle has a thick outer sheath and if I see wear I will use spiral wrap to reinforce it,
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redundancy
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In case of joystick failure. I'll just leave the wireless passenger stick, firmly bolted in................no matter how fat the passenger is! :D L.Adamson --- RV6A |
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But mine's not wireless...that's pretty hi tech...what'd ya use, 803.b or .g? :D Cheers, Bob |
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L.Adamson --- RV6A |
below pivot
Why not put the wire exit hole BELOW the pivot point? This way there are NO elevator loads, only aileron loads which are, I would think, considerably less.
Bevan |
Alternate hole locations
A few of you have asked about putting the hole in a better place in the stick. The analysis I did had the hole in just about the worst possible location, and it was 1/2" diameter to boot.
I have a couple comments about alternate locations: First, consider putting the hole in the side of the stick rather than the front or back. This is simply because the roll forces on the stick are less than the pitch forces. I'd guess off-hand that the roll forces are maybe 2/3 or even 1/2 the pitch forces. That would result in a corresponding decrease in stress around the hole. Second, placing the hole further UP the stick is better than closer to the pivot. The controlling distance is what's measured from the hole to the application of load, ie the stick grip. Locating the hole higher up on the stick does give us some other problems to deal with and those have already been discussed. Finally, there's no substitute for an actual test of this part. The FEM analysis is pretty and it has merit for showing how stress is distributed. BUT its hard sometimes to draw the correct conclustions from this type of analysis. I'll try and get a few tests in for y'all. |
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Do you have a part number or a better idea of what these are called? I'm interested in something like this, but I've searched all over McMaster and MSC's websites and I can't find them. Obviously I'm not using the right search terms. Thanks, PJ Seipel RV-10 #40032 |
loads below pivot point
Hi all,
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Still, drilling holes in this area of the stick scares me a lot, I'll have a close look at mine (side by side) when I come to that task. Kind regards, Mario |
try fig 4-38, p 4-172
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I think I have a better idea now after re-reading the post. What you are talking about is making a T-joint right over the hole in the stick (similar to putting holes within a cluster joint for line oil to penetrate) then cutting off the vertical leg of the T after welding, so that only 1/8 inch or so remains. This seems like a good alternative to welding on a washer. My next question is- does the vertical have to be the same diameter of the stick? If the hole is 1/4, why not use 5/8 or so tubing for this? |
Mario,
On my stick (RV7A), the lower portion of the stick (below the pivot) is aileron loads only. Bevan |
Infinity Grip Wire Routing - Control Stick Hole Doubler
I added a hole to my control sticks for the Infinity Grip under the pivot and on the side. Under the pivot for reduced stresses on elevator loads. On the side to reduce moment caused by lighter aileron loads. TIG welded doubler made from trimmed portion of stick.
https://drive.google.com/file/d/1v79...ew?usp=sharing https://drive.google.com/file/d/1TVZ...ew?usp=sharing Installed cable at hole interface with silicon tape wrapped and cable tied to stick to reduce any movement at cable exit through hole. Cable routes to bottom with a loop on inner skin, wrapped in snake skin to prevent wear or chafing. https://drive.google.com/file/d/13zQ...ew?usp=sharing https://drive.google.com/file/d/1Gph...ew?usp=sharing |
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