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Stick Grip Wire Routing

Veetail88

Well Known Member
I recieved my grips from Infinity the other day. (fighter pilot noises to ensue very soon!)

The multi wire cable they supply is pretty thick!

I'm wondering how others have exited this wire from the control stick?

Just drill a hole in the side of it that's large enough? That would be a pretty big hole! Will it affect the integrity of the stick? Wouldn't want it to break off in my hand!

It seems like the only option though. Probably need a rubber grommet in there too.

The back seat stick (RV-8) will have the exit hole above the split point for stick removal.

Just wondering what others have done.
 
Consolidate Grounds

There is a ground wire for each switch/button in that wire bundle. There is not a need in my opinion (and others that have been flying for years with this grip) to run a seperate ground for each. I consolidated all of my ground wires up inside the grip, this drastically reduces the diameter of the bundle. Yes you will have to strip off the pretty blue outer jacket in the process. I simply placed a long piece of heat shrink over the wires and wrapped them with spiral wrap where it exits the stick. the other benefit of this is that is makes the bundle more flexible for less resistance on the stick.

Just an option.
 
....The multi wire cable they supply is pretty thick! I'm wondering how others have exited this wire from the control stick?....Wouldn't want it to break off in my hand!
If it breaks off in my hand...I'll let you know.:)

2e2mc0y.jpg
 
Thanks

Thanks Guys!

Now that I see the installation, the hole dosen't look that big. I guess it'll be ok.
 
Infinity Wire Routing

I have installed four of these Infinity grips in RV-8s including the Doll finishing the last one this week.

I found the blue Infinity cable to be too large, and I feared the cable would put a load on the stick causing control input to a responsive RV. I decided that each switch did not need its own ground wire. I disassembled the grip and removed the outer blue cover and shielding from the cable. I cut the ground wires from each switch, and grouped them together in the open area in the middle of the grip. A single ground wire exits the grip to handle the ground function. The remaining wires from the original cable are now down to a manageable size, and can exit the stick through a small hole just above the pivot bolt. I then ran them along the top of the control quadrant through the center section to a small 12 contact strip connector mounted to the floor rib on the right side of the control quadrant just aft of the aileron push tubes.

The Doll has over 1050 hours and I have not experienced any problems with this set up. The Infinity grip provides the following functions in the Doll:

Trigger- Push to Talk
Left switch- Flaps
Coolie Hat Switch- Aileron and Elevator Trim
Right top button- Engine Starter (when enabled)
Middle left button- Auto pilot disconnect
Lower button- Boost pump
 
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I'm just about to install my Infinity grip and this is good info and I will probably follow Danny's advice.

I have another question regarding these grips. Do people tend to rotate the grip on the stick to match the angle of their arm toward the stick, or do you just install it facing forward? It seems to me if it is installed facing forward then you would have to bend your wrist a little for it to fit well, however if you rotate it to match your wrist angle, you might tend to apply a little pitch when rolling as it would be more natural to move 90 deg. to your wrist rather than 90 deg. to the plane.

I would be interested in peoples experieinces. Danny?

Paul
 
I rotated mine Paul - I did it very simply by sitting in the cockpit with the grip not bolt in place, and seeing where it comfortably fit my hand. I'm very glad I have it rotated, and would definitely do it again.

Paul
 
Thanks Paul. That was my inclination.

I rotated mine Paul - I did it very simply by sitting in the cockpit with the grip not bolt in place, and seeing where it comfortably fit my hand. I'm very glad I have it rotated, and would definitely do it again.

Paul
 
Van's says you might not want to do this, Rick.

Howdy Rick,

Got a note from the engineering side of Van's about your control stick hole and thought I'd pass it along.
b,
dr

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 anwhere 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.
 
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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 anwhere 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.
Very interesting. Normally, I consult with Van's when faced with puzzling technical issues such as this one. In this particular case, I drilled the hole in that location after consulting with the popular joystick grip manufacturer. When I noted that there is no way that blue cable could physically get past the pivot point, I naturally asked how other builders deal with the problem. Upon reflection, I must conclude it is always best to heed Van's advice, it is after all their design and I regret any confusion I may have caused. :eek:
 
Electronics to the rescue!

I also have the Infinity grips and found the fat blue cable problematic for several reasons already mentioned here. Mainly, that it requires a BIG hole to exit the stick which could significantly compromise strength, and that it is too bulky and inflexible to transition from the stick to a fixed part of the airframe without imparting some noticeable force on the stick in some positions (perhaps it wouldn't be noticeable in flight, I don't know, but definitely noticeable on the ground).

I ultimately decided to completely do away with the big blue cable. How? By putting active electronics right inside the stick tube to continuously scan all the switches and send down the data over RS-232 to my main board behind the instrument panel. So instead of the big blue cable, I'll have just a tiny 3-conductor shielded cable (power, ground, RS-232) exiting the stick and making their way to the forward cockpit. Much more manageable: very small hole in the stick (or just exit out the bottom), easy to make the transition to the airframe without noticeable force on the stick, and even easy to put a little connector right there to make the stick easily removable. I haven't weighed my setup or the blue cable yet, but I'll bet that my setup is also quite a bit lighter. That blue cable is a beast!

No pictures available yet. My circuit board that'll go inside the stick is actually in fab as we speak. But I'll post some pictures later after I've put it together.

P.S. I know this doesn't necessarily help those of us that aren't electronically inclined. Sorry. But for those who are, feel free to plagiarize the idea, AT YOUR OWN RISK of course.

P.P.S. I also rotated my stick grips to the natural position of my hand. Turned out to be about 22.5 degrees.
 
Stick Forces

I have no problem with the statement from Van's. My question is what are the maximum stick forces that could be a problem and how do they occur?

I was concerned about drilling in the stick so I used a piece of 3/4 tubing (co-pilot side), and drilled a hole the distance from the grip and put the other end in a vise. Pulled until my back started to hurt. No bending or deformation.

Am I missing something?

Ken
 
Ouch!!!

Wow! I guess I just learned a lesson.

Seeing Rick's installation, it looked to me that the hole in the stick wasn't all that significant, so I charged ahead and did the same.

Problem is, as I'm not experienced with this type of engineering, I consider my own opinion on this stuff worthless. I knew enough to question drilling such a hole, but quickly accepted another's solution.

While I think the question of "how much stick force" is actually required coupled with a simple test of sticking the thing in a vice and seeing just how much stress it will take seems valid, one word from an engineer at Van's sends my judgement and decision into a tail spin!

I hope to see more logical ideas about this, but for now, I'm thinking I'll be ordering a new front stick and starting over.

I drilled the back one too. At first thought I think, if that one breaks off, the front seater (me) will simply continue to fly the airplane. But what if it just bends and binds on the back of the front seat, not allowing any down elevator? Will this line of paranoid thought keep me from moving forward on my project, or might it save my life?

Unfortunately, I want to get my airplane built so I can fly it rather than just enjoying the everyday pleasure of building it.

I hope I remember this hasty decision during the countless additionaly important decisions I'll be making along the way.

Rick, in the end, your installation may be fine. I'm not criticizing you or your work, rather I'm just questioning my own decision making haste. Gotta cool my jets! ;-)

 
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I also have the Infinity grips and found the fat blue cable problematic for several reasons already mentioned here. Mainly, that it requires a BIG hole to exit the stick which could significantly compromise strength, and that it is too bulky and inflexible to transition from the stick to a fixed part of the airframe without imparting some noticeable force on the stick in some positions (perhaps it wouldn't be noticeable in flight, I don't know, but definitely noticeable on the ground).

I ultimately decided to completely do away with the big blue cable. How? By putting active electronics right inside the stick tube to continuously scan all the switches and send down the data over RS-232 to my main board behind the instrument panel. So instead of the big blue cable, I'll have just a tiny 3-conductor shielded cable (power, ground, RS-232) exiting the stick and making their way to the forward cockpit. Much more manageable: very small hole in the stick (or just exit out the bottom), easy to make the transition to the airframe without noticeable force on the stick, and even easy to put a little connector right there to make the stick easily removable. I haven't weighed my setup or the blue cable yet, but I'll bet that my setup is also quite a bit lighter. That blue cable is a beast!

No pictures available yet. My circuit board that'll go inside the stick is actually in fab as we speak. But I'll post some pictures later after I've put it together.

P.S. I know this doesn't necessarily help those of us that aren't electronically inclined. Sorry. But for those who are, feel free to plagiarize the idea, AT YOUR OWN RISK of course.

P.P.S. I also rotated my stick grips to the natural position of my hand. Turned out to be about 22.5 degrees.

Hi Roee.

All of my circuits in my HRII will be controlled by relays with coils switched to ground. This was done so that I could use serial controllers to manage them.

Phase 1 is discrete wiring (22 gauge wires). Later, I will swap in the serial controllers and reallocate some of the wires to serial links and power for the controllers.

My objective is to develop a redundant control architecture with error checking and automatic cut-over. Might be overkill for a simple aircraft but then it allows the next stage of integration (triple super secret).

Vern
 
Hi Roee.

All of my circuits in my HRII will be controlled by relays with coils switched to ground. This was done so that I could use serial controllers to manage them.

Phase 1 is discrete wiring (22 gauge wires). Later, I will swap in the serial controllers and reallocate some of the wires to serial links and power for the controllers.

My objective is to develop a redundant control architecture with error checking and automatic cut-over. Might be overkill for a simple aircraft but then it allows the next stage of integration (triple super secret).

Vern

Hi Vern,

Cool, sounds like we have some similar ideas. On my 7A, the accessory circuits will also be controlled electronically, although with solid state devices rather than relays, on a main board of my own design. I'm not planning to go quite as far as you, it sounds like, on fault tolerance. Only where it's safety critical in my aircraft -- my architecture won't be scalable to the space shuttle :). My objectives are more oriented toward intelligent functional integration of airframe functions and avionics, which is no big secret.

I'm also taking a phased approach in development, although a bit different than yours. I'll be wiring up my plane in its final configuration from the get go, and the wiring harnesses will be pretty clean and simple, being nothing more than "I/O". Ideally, my wiring should never need to change. The functional magic will all happen on the main board, and that's the piece that will evolve in phases.

Anyway, I don't want to take this thread on too much of a tangent. If you want to share notes on this, shoot me an email: roee at kalinskyconsulting dot com.

Cheers,
-Roee
 
Whoa, not so fast!

Wow! I guess I just learned a lesson.

Seeing Rick's installation, it looked to me that the hole in the stick wasn't all that significant, so I charged ahead and did the same.

Problem is, as I'm not experienced with this type of engineering, I consider my own opinion on this stuff worthless. I knew enough to question drilling such a hole, but quickly accepted another's solution.

While I think the question of "how much stick force" is actually required coupled with a simple test of sticking the thing in a vice and seeing just how much stress it will take seems valid, one word from an engineer at Van's sends my judgement and decision into a tail spin!

I hope to see more logical ideas about this, but for now, I'm thinking I'll be ordering a new front stick and starting over.

Whoa, not so fast guys.

If Vans engineering staff is like everybody else, they are very busy, and don't have time to analyze every potential modification that every builder might make to their kits. Oftentimes, engineering departments will simply issue a CYA statement that they don't recommend changes rather than actually consider the physics involved. This is fine, it covers their butts, and limits their liability. We can all understand that...

However, this has nothing whatsoever to do with whether your stick retains sufficient strength if you drill a hole in it. I'm not saying that builders should modify their control systems willy-nilly, without considering the implications because they are significant. But on the flipside, don't assume that you aren't capable of thinking this problem through, just because you aren't an engineer, and don't just take somebody's sweeping generalization that your plane will fail if you don't build it exactly per plans!

Your logic of determining the worst case stick forces, applying a healthy safety factor to that, and testing the stick to that value and determining that no damage results is perfectly logical and acceptable. I would do that before replacing your sticks or going to extremes building or buying electronics to send all of those signals via RS-232. YOU are the builder of YOUR aircraft and YOU have the ultimate authority and responsibility to build an airworthy aircraft. Just because you aren't capable of performing a stress analysis of your stick doesn't matter. Ten analyses are worth one test, and you can easily test the strength of your stick.

Vans aircraft are not required to be built or certified to FAR Part 23, but this is a good guide to help you think this through. Here are a couple of hints:

FAR PART 23?AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES Subpart C?Structure. Control Surface and System Loads. ? 23.395 Control system loads. (a) Each flight control system and its supporting structure must be designed for loads corresponding to at least 125 percent of the computed hinge moments of the movable control surface in the conditions prescribed in ??23.391 through 23.459. In addition, the following apply:

(1) The system limit loads need not exceed the higher of the loads that can be produced by the pilot and automatic devices operating the controls. However, autopilot forces need not be added to pilot forces. The system must be designed for the maximum effort of the pilot or autopilot, whichever is higher. In addition, if the pilot and the autopilot act in opposition, the part of the system between them may be designed for the maximum effort of the one that imposes the lesser load. Pilot forces used for design need not exceed the maximum forces prescribed in ?23.397(b).

(2) The design must, in any case, provide a rugged system for service use, considering jamming, ground gusts, taxiing downwind, control inertia, and friction. Compliance with this subparagraph may be shown by designing for loads resulting from application of the minimum forces prescribed in ?23.397(b).

(b) A 125 percent factor on computed hinge moments must be used to design elevator, aileron, and rudder systems. However, a factor as low as 1.0 may be used if hinge moments are based on accurate flight test data, the exact reduction depending upon the accuracy and reliability of the data.

(c) Pilot forces used for design are assumed to act at the appropriate control grips or pads as they would in flight, and to react at the attachments of the control system to the control surface horns.

Noah Forden, BSME
Mechanical / Aeronautical Engineer
US Navy
 
Stick

I too have put a hole in the sticks like Rick, and I know lots who have done it (mainly from this sight). I have never heard of a stick breaking there in any RV. Steel (especially 4130) is very strong stuff, and I just can't image the pressure is so great it will crack or break there, even over time. It's not like the sticks are continually getting slammed from stop to stop every time we fly. I am no expert though. I personally have no intentions of getting new sticks and have no reservations. Just a personal thought. Dave
 
Your logic of determining the worst case stick forces, applying a healthy safety factor to that, and testing the stick to that value and determining that no damage results is perfectly logical and acceptable. I would do that before replacing your sticks or going to extremes building or buying electronics to send all of those signals via RS-232. YOU are the builder of YOUR aircraft and YOU have the ultimate authority and responsibility to build an airworthy aircraft. Just because you aren't capable of performing a stress analysis of your stick doesn't matter. Ten analyses are worth one test, and you can easily test the strength of your stick.

Noah,

Thanks for your good insights. But just for the record, I'll restate that I went the RS-232 route to solve several stick-to-airframe transition issues as I described previously, not just the hole diameter.

Among those issues though, I did certainly have reservations about drilling a hole that is over half the diameter of the stick tube. That would certainly weaken the tube by a significant amount. How much? I don't know. Too much for this application? Maybe, maybe not. It is, of course, an answerable question. I could have gone the analysis route (yes, I'm also an engineer). I could have gone the test route, as you suggest. I chose a third route, which side-steps this problem altogether. I found what I believe to be an elegant solution that takes care of several issues, including but not limited to the hole diameter in question. I don't think of it at all as going to extremes. But if you don't like my solution, I certainly welcome you to take a different approach on your airplane.

P.S. I agree completely about Van's blanket statement likely originating from legal rather than engineering. As you said, we are legally free to take or leave Van's advice as we see fit. When deviating from the stock design though, a wise builder should exercise good judgment, which generally will require a combination of analysis, testing, and common sense.

P.P.S. With respect to maximum stick forces, consider that although normal stick forces are pretty light, a pilot could conceivably exert hundreds of pounds of force on the end of the stick when then the control surface is against the stop. That's something a prudent design may want to account for.

Roee Kalinsky, BSEE
Electrical / Systems Engineer
Independent consultant

Best regards,
 
Roee,

Didn't intend to ruffle your feathers. :D You're clearly very comfortable with the RS-232 approach, and have thought it through. I commend you, and wish I had your skills in digital design to do something like that. :)

My response was intended to address the original poster and his concerns about having ruined his control stick. My comment regarding "going to extremes" simply points to the fact that probably only 1 or 2% of people using infinity stick grips solve this cable bundle problem via multiplexing the switch signals, and that may not be the simplest solution for the average builder.

Good point about the max control system loads being the max that a pilot can exert, I believe that this is what paragraph 23.395 (1) is saying: " The system must be designed for the maximum effort of the pilot or autopilot, whichever is higher."
 
Roee,

Didn't intend to ruffle your feathers. :D

Noah,

No worries, no feathers ruffled. Hopefully likewise on your side :). I think we're in agreement that there may be many possible good solutions to a problem. And yes, we all naturally tend to exploit the particular skills that we possess. Nothing wrong with that. On the contrary, I think that's often what makes the diversity of this group so valuable as a problem solving tool!

Cheers,
-Roee
 
I too have put a hole in the sticks like Rick, and I know lots who have done it (mainly from this sight). I have never heard of a stick breaking there in any RV. Steel (especially 4130) is very strong stuff, and I just can't image the pressure is so great it will crack or break there, even over time. It's not like the sticks are continually getting slammed from stop to stop every time we fly. I am no expert though. I personally have no intentions of getting new sticks and have no reservations. Just a personal thought. Dave
Dave,

We know the Infinity grip is installed in a vast array of experimental aircraft designs, not just RV's. As we limit the discussion to the sheer number of Infinity grips sold and installed in the RV design, I submit (and I think you may agree) it is perfectly reasonable to suppose that many, many RV builders have drilled a hole in the side of the stick just as we have. The original poster simply asked what other builders have done.

Given the routine installation of electrically functioning stick grips in general and the Infinity grip's widespread popularity with RV'ers in particular, I am puzzled that as far as I can tell Van has never added a cautionary note in the builders manual to discourage builders against adding a wiring exit to the stick. I am guessing that my detailed in-your-face installation photos moved Van's to comment. My admittedly untrained gut feeling is it would take an enormous and unlikely loading event to even begin to distort that 4130 steel stick and I too have no intention of replacing the stick. It is conceivable that potential crack propagation migrating from around a less than properly prepared hole can present other longer term problems. It follows that proper deburring technique would be all the more prudent. Still, these are merely the untrained opinions of a non-engineer and regardless of what I think, Van's has now relayed to us through VAF a cautionary if informal warning meant to discourage the practice. It is nice to know the good folks at Van's quietly monitor this forum and am sure they are occasionally amused (and in this case apparently horrified) by some of the things they read here! Because Van's has now issued a stated position regarding this matter, I cannot in good conscious encourage, recommend or suggest other RV builders drill a wiring exit hole of any size into the side of the stick. Hmm...is this YMMV or CYA?
 
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Well put.

Well put Rick. 4130 is supposedly (from a friend who most likely knows) 7% stronger than regular steel. My sticks are made of regular steel, though the wall thickness is thicker than the 4130. I did this because of some wierd bends I did with some larger seat cushions and stick clearances. I made larger seat cushions (maily front) because I have long legs and want the support.

Anyway, I had these bent and welded at a gocart shop and chatted with the guy about the hole size. Now he is just your average Joe, but he had no reservations at all. He sees a lot of wrecks and knows how hard these steel tubes get hit, and he didn't blink an eye. I am stating this for my own justification -- not anybody elses. One has to make their own decision and do their own homework. But, I have absolutely no reservations as I have mentioned before. Like you said Rick....there have been countless times this has been done on RVs and others and history speaks for itsself -- no sticks breaking off, or at least none I have heard about. Dave
 
Infinity Grip

My Infinity Grip just arrived so this thread made for good reading.

Van's advice "Generally, the accepted routing for electrical signal wires is through the opening on the bottom of the control stick" won't work in my case since Van's didn't provide a hole on the bottom of my control stick...

Looks like I'll be drilling a hole somewhere-I'll consolidate my ground wires as suggested by Danny King and make it as small as possible.

Mike
 
Stick stress levels - FEM modeled

Here's some detailed information for those of you who drilled the hole in the stick or are thinking about it. I just did a structural stress analysis of the tube, both with and without hole. This is a FEM (finite element model) analysis of the stick tube, and I believe the results are very accurate. The model does look bent in the views below because I used a magnification factor of about 1 million for displaying displacements in the model.

The front stick tube is 1.00" OD, .035" wall AISI 4130 steel tube. This material has a yield strength of 90000 psi which is typical for aircraft steel (its 4130 normalized at 1600F). I also modeled a .100" weld radius around the pivot tube, since this will affect distributed stresses in the area we're interested in.

Edit: The load used here is pulling AFT on the stick, and hole shown is on the FRONT of the stick.

Model 1: No hole
Without the hole, this analysis indicates the tube will yield with an applied load of 360 pounds, with that load placed exactly 6" above the pivot point. At that point, the stick will begin to bend, and tube shaped cross sections will typically fail due to buckling in the wall after a predictable amount of bending.

Here's a shot of the FEM model. Note the max stress occurs just above the pivot point, which also is where most of us would want to drill a hole.
RV8_stick_stress.jpg


Model 2: With hole
Placing a .500" hole with its center .750" up from the axis of the pivot bolt, I re-ran the model. Now the max stress levels go up around the hole edge, by a factor of about 3. With this, we get yield in the wall at a load of 120 pounds.

Here's a shot of the stress model:
RV8_stick_stress_holed.jpg


Conclusion: If you want to apply the same factor of safety to the stick tube as the rest of the plane then reduce the allowable loads by a factor of 1.5. This means the following design values would be our limits:

Stick no hole: 360 / 1.5 = 240 lbs
Stick w/ hole: 120 / 1.5 = 80 lbs

You may draw your own conclusions from here. I would advise against the hole. If you have a hole and must keep it, a washer of about .050" thick welded around the hole would help to distribute stress.

Note: There is probably a very sensitive relationship between the hole size and placement, and the max stress in the tube wall. If any of you have concerns about YOUR airplane, email me and I'll take a look at your specific situation.
 
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Wow

Bill,

It's awsome that you ran that analysis. The strength of the broad experience of the folks on this site is incredible and invaluable! Thank you!

In your data, you placed the load 6" from the pivot hole correct? It seems to me that as the load we would place on the stick in the real world is more like 13" or 14". That added lever would greatly reduce the failure load no?:(
 
Holy cow!

Thanks Bill! That analysis, valid or not, put the fear of God in me. I think I'll leave my sticks in their virgin state. Glad I saw this before I augered a big ole' hole in one of both of them!

I'm sure it's fine for most pilots, but I don't exactly have a velvet hand so I might need that extra strength in the stick! ;).

PS. Talked to Rick Blaes, he's ornery as every!
 
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In your data, you placed the load 6" from the pivot hole correct? It seems to me that as the load we would place on the stick in the real world is more like 13" or 14". That added lever would greatly reduce the failure load no?

Jesse, You are correct. If the load is placed farther from the pivot point, then the stresses would reduce correspondingly, for the same desired force applied to the elevator control surface.

I ran the analysis for a 6" distance from the load (center of the stick grip) to the pivot point. If the distance were increased to 12" (double the distance I used) then the stress would be reduced also by 1/2. (note this rough approximation assumes the "hole" is co-located with the pivot axis, which its not. But good for a rough estimate)

Also, I modeled a 1/2" hole which is quite large. A smaller hole, OR a hole that's on the side of the stick rather than dead-on the front would have a lot less influence on max stress.
 
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Actually Bill,

The big blue wire bundle from Infinity is 3/8". You need a 1/2" hole to get the cable through and to fit a grommet in.

Additionally, you show the hole oriented and located exactly as I drilled my stick! My hole is in the back though. (no comment invited here!) I suspect if the stress were in a pushing direction the result wouldn't be much better.

I'll be ordering a new one or having a welded outlet fitting added to disperse the stress more evenly through the structure.

While I really can't imagine slamming the elevator against the stop with that kind of force, this is one of those things that would just bug me if I didn't do something about it.
 
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Cool!

Hey Bill, that is really nice work! I'm curious how the limit load varies with hole size - is it easy to run a 0.25" and 0.375" ? I'm just wondering if the is a linear realtionship.

Paul
 
Paul - I ran the analysis for 3/8" and 1/4" holes.

Diameter .500"
Max Force 120 lb

Diameter .375"
Max Force 150 lb

Diameter .250
Max Force 180 lb

These forces are predictions for YIELD in the material due to stress. This model was not analyzed for buckling failure.

JHines - the RV8 has light stick forces, around 3-5 pounds per g (just a guess) and they vary quite a bit with center of gravity. Of course, forward CG's produce higher An/g values.
 
Very nice.

My first thought when I read this was if you slept at a Holiday Inn Express last night? That was very interesting, and you have me thinking now (thank you very much for doing that by the way). My hole is 1/2 inch, and I think I will find out the required hole size (for the Infinity) with one common ground as Danny did. Then I will weld a small tube that angles down & out the existing hole. That should tie it all in and firm up that area, do you think? Danny, do you know the size hole one needs to fit the smaller bundle with one common ground for the Infinity? Or, I can open it up and figure it out, though I'm not ready for it yet. Thanks. Dave
 
Cool! Gimme more ;-)

Bill,

That's cool! I wish I had access to finite element analysis tools!

I have a few questions/comments/clarifications regarding your analysis. Bare in mind I'm an EE, not an ME, so forgive me if I may be a bit rusty on my Newtonian mechanics :). I may very well be off base here, so please correct me if that's the case.

1. I don't think the normal pivoting action of the stick plays any role here, unless you're really trying to model the actual structural characteristics of the stick weldment in the area of the pivot. And even then, that should only become significant to this analysis if the exit hole is very close to that pivot structure.

2. If that pivot structure is not otherwise significant, then what we really care about is distance between the exit hole and the top of the stick (i.e. the grip, where force is applied), not distance between the exit hole and the pivot at the base of the stick. I think that a more representative model, which could even totally exclude the pivot structure, would be the following: A stick tube of arbitrarily long length, held rigid in space at its base, and with force applied at its top. The parameters of interest then are 1) still, obviously, the diameter of the hole, and 2) the distance of the hole from the top of the stick, where the force is applied.

3. Some previously discussed comments about distance between the exit hole and the pivot need clarification. Again, I think what really matters is distance form the exit hole to the stick grip. And closer, i.e. exit hole higher up the stick, is better. That's because the portion of the stick tube between the grip and the exit hole essentially acts as a lever arm, with the hole location as the pivot point. So for a given force applied at the grip, the further down the exit hole is located, the greater the torque that will be experienced at the location of the exit hole, and therefore the greater the stress in the portion of the tube surrounding the exit hole.

4. As you explained, your model examined yield failure, not buckling failure. I lack the background to back this up with theory, but my intuition says that with a large diameter hole relative to the tube diameter, the force required to cause a buckling failure may be lower than the force required to cause a yield failure (and of course the force would be applied in the opposite direction for the buckle scenario). That's my gut feel, but on the theory, I'm a bit out of my depth on this one. Could you (Bill, and the other ME's here) share any insights on this?

Thanks!
-Roee
 
FEM revisited

Conclusion: If you want to apply the same factor of safety to the stick tube as the rest of the plane then reduce the allowable loads by a factor of 1.5. This means the following design values would be our limits:

Stick no hole: 360 / 1.5 = 240 lbs
Stick w/ hole: 120 / 1.5 = 80 lbs

You may draw your own conclusions from here. I would advise against the hole. If you have a hole and must keep it, a washer of about .050" thick welded around the hole would help to distribute stress.

This is a really interesting analysis, so much so that I showed it to my wife. She is an engineering professor who teaches these topics for a living. She pointed out a few things that bear on this discussion:

First, the calculation for yield strength means this is the force that will permanently bend the stick, not break it. You could conceivably bend the stick and still return for a successful landing. The ultimate (breaking) strength of 4130 steel is about 50% higher.

Second, the application of a 50% safety factor is not really justified. Using yield strength rater than ultimate strength automatically gives you a 50% safety factor. For the RV wing between limit load (6G for the aerobatic models) and ultimate load (9G).

Third, the hole placement shown is a worst-case scenario because its placed close to the pivot. Moving it higher up on the stick increases the safety factor (this is what I did on her advice).

From my own (non-engineering) perspective, I can also see some practical problems with this analysis. First, on the RV-8 there is a seat cushion at 6" above the stick pivot. I couldn't apply a load to that part of the stick if I wanted to. Furthermore, 120 (or even 80) pounds is a LOT of force. Think about trying to pick up three 5 gallon cans of gas at the same time with one arm. I might be able to apply this much pressure with both arms, if I put my feet on the panel. The elevator would have to be on the stop to do this, which means the stick would be all the way back. I don't see how you could ever generate this much aerodynamic loading (without breaking the elevator first).

I'm curious about Van's advice not to drill a hole, whereas the factory RV-8s do have buttons wired on the top of the stick. Those wires have to come out somewhere, and the RV-8 stick as designed doesn't have any openings on the bottom. My own stick has about a 3/8" hole several inches above the pivot. Its still perfectly straight after about 500 hours of flight.
 
Paul - I ran the analysis for 3/8" and 1/4" holes.

Diameter .500"
Max Force 120 lb

Diameter .375"
Max Force 150 lb

Diameter .250
Max Force 180 lb

These forces are predictions for YIELD in the material due to stress. This model was not analyzed for buckling failure.

JHines - the RV8 has light stick forces, around 3-5 pounds per g (just a guess) and they vary quite a bit with center of gravity. Of course, forward CG's produce higher An/g values.

So at 5 lbs. per g, even with a 1/2" diameter hole, you would break something else on the plane before you broke the stick, yes? (buckling considerations aside).
 
This statement confuses me:

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 anwhere 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.

Since they tell you to do this:

9u2l5h.jpg


Since this area is double wall maybe that makes it OK?
 
.....The model does look bent in the views below because I used a magnification factor of about 1 million for displaying displacements in the model.........The load used here is pulling AFT on the stick, and hole shown is on the FRONT of the stick.......
Bill,

The dramatic modeling you shared with us appears to be a truly fascinating and useful tool to say the least. I have little doubt the math calculated is the product of solid science and I am not ashamed to say that many of the conclusions offered in yours and subsequent posts shot right over my head. I'm just a high school graduate so please bear with me.

#1. If, for visual effect the model appears bent because of the magnification factor of 1 million is used in the virtual world simulation, does this mean in the real world the observer would not percieve a bending event at the forces modeled?

#2. What does YIELD really mean?

#3. Since it is unlikely a builder would drill a wiring exit hole on the forward face of the stick, wouldn't it be more appropriate to run modeling with the hole facing aft or nearly so? Would such placement even make any difference?

#4. If we accept the loading forces on the RV-8 are relatively light at approximately 3-5 lbs per G, at +6 and -3 g's, what maximum forces can be reasonably anticipated to be loaded onto the joystick? Forgive me if my assumptions are totally incorrect, but if I multiply 5(lbs.) X 6(g's)=30.The number doesn't even represent half of: Stick w/ hole: 120 / 1.5 = 80lbs. What am I missing?

#5. As a practical matter, as a pilot would I not be more concerned about the amount of force actually required to physically generate permanent deformation or worse...catastrophic failure of the joystick?
Van's recommends that builders do not drill holes anwhere 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.

#6. From what I understand as implicit in Van's statement, I conclude that as far as the RV-8 joystick design is concerned no additional hole added to the stick is approved so it must follow that the use of any joystick grip requiring wiring of any sort is not acceptable unless the builder routes the wire(s) to exit the bottom of the joystick.

#7. Short of drilling a wiring exit hole somewhere on the RV-8 pilot joystick, our choices are truly limited. Even if physically possible, would you really want the wires to exit at the bottom of the joystick? Doing so, the wire bundle would have to be extra long to account for maximum stick travel in all directions AND without becoming an unwieldy obstruction, do it while firmly secured to avoid potential conflict with other parts of the assembly. That would be a very neat trick if you can pull it off. For those who have done just that, photos please!

9bc1ec.jpg
 
Without reaching for the books, I'm guessing that a half inch AN fastener has a shear strength well in excess of 5,000 pounds. Just trying to keep the stick from sliding out of the socket. Do we really need a sledgehammer?
Terry, CFI
RV9A N323TP
 
control stick grip wire hole keep it simple

This whole discussion just puts a big grin on my face. In my brewery operating days, whether we were involved in a rigging operation (moving or loading a large and heavy fermenter) or discussing the proper method to place a repair tig weld on a stainless tank, there was always the problem of overthinking the issue. If my engineer friends would stop by and my crew and I were involved in the above mentioned activities, their suggestions were usually ignored in the final decision. Over thinking, over detailing, over complicating the task was their usual style. Don?t get me wrong, I like these guys, but we could not get them out of the shop fast enough so we could get on with the projects. (Sorry to all you engineers!) The issue of the hole cut in the control stick is also one that can be over thunk. From a rag and tube biplane fuselage builder, here is my 2 cents. Hey, pick the spot where you want drill your hole, grab a piece of 063 or 049 chrome moly plate, cut out a shape that when heated you can wrap around the hole area lengthwise(not the whole circumference of the stick, maybe half.) The piece my be 3? by 1? in maybe a diamond shape. Adapt it well with heat , and weld it to the stick. Clean up and paint. DONE! If you can?t tig weld, find someone that can.

Mark Wyss
RV4 fuselage
 
Reply to Roee

Roee, good insight from you. To the point:

1. I don't think the normal pivoting action of the stick plays any role here, unless you're really trying to model the actual structural characteristics of the stick weldment in the area of the pivot. And even then, that should only become significant to this analysis if the exit hole is very close to that pivot structure.

I agree the pivoting action of the stick doesn't play much of a role here. In the model I did constrain the pivot tube in all translational and rotational motions except for rotation about its axis. If not, the tube would be able to contribute stiffness to the structure if deflection is present.

2. If that pivot structure is not otherwise significant, then what we really care about is distance between the exit hole and the top of the stick (i.e. the grip, where force is applied), not distance between the exit hole and the pivot at the base of the stick. I think that a more representative model, which could even totally exclude the pivot structure, would be the following: A stick tube of arbitrarily long length, held rigid in space at its base, and with force applied at its top. The parameters of interest then are 1) still, obviously, the diameter of the hole, and 2) the distance of the hole from the top of the stick, where the force is applied.

Once again, we're in agreement for the most part. The pivot tube does play a role in the stiffness of the control stick tube when we're very close to the junction of the two due to the weld bead you see there. That bead adds some significant thickness to the area and since we're talking about .035 wall tube I decided to include it.

I did mention the grip-to-hole dimension in a kind of oblique way in an earlier reply to Jesse:

I ran the analysis for a 6" distance from the load (center of the stick grip) to the pivot point. If the distance were increased to 12" (double the distance I used) then the stress would be reduced also by 1/2. (note this rough approximation assumes the "hole" is co-located with the pivot axis, which its not. But good for a rough estimate)

What I was referring to with the "rough estimate" comment is exactly what you're talking about. I do agree its the distance from the point of force application to the hole that's controlling. But I also try to keep answers as simple and direct as possible so as to not loose track of the main issue. In this case, the hole is pretty close to the pivot axis, and this is where most if not nearly all builders will put it. Good job on your part though!!

4. As you explained, your model examined yield failure, not buckling failure. I lack the background to back this up with theory, but my intuition says that with a large diameter hole relative to the tube diameter, the force required to cause a buckling failure may be lower than the force required to cause a yield failure (and of course the force would be applied in the opposite direction for the buckle scenario). That's my gut feel, but on the theory, I'm a bit out of my depth on this one. Could you (Bill, and the other ME's here) share any insights on this?

You may be correct about buckling happening before yield. That's why I slid the CYA comments into my posts about this. I think a test should be run to really know since buckling is so hard to model.

And honestly, I'm not really at my best when doing structural problems. I specialize in aerodynamic work. But many of my clients and jobs have required some mechanical analysis so I got the FEM package for that. The ME's here will be able to apply a more competent outlook on this particular job IMHO.
 
Bill,

<SNIP>

#2. What does YIELD really mean?

<SNIP>

Take a look at this link:

http://www.auf.asn.au/const_images/stress_strain4.jpg

It is a generalized stress-strain curve for a metal. Stress (unit load, e.g. PSI) is on the vertical scale and strain (unit deflection, e.g. % or inch per inch) is on the horizontal. This is the kind of curve you would get if you put a test sample in a tensile testing machine and pulled it until it broke while measuring the stress and strain.

Notice the red dashed line? That represents the linear slope of the curve (black line) and describes the material's stiffness (the steeper the line the stiffer it is). For applied loads to the left of the blue dot "Sy", the material will "spring back" when the load is removed. This is called "elastic" deformation. For applied loads to the right of that dot, plastic deformation or "yielding" takes place and the material will not return to its original length after you remove the load.

Further to the right, you see the other blue dot "Su" or ultimate tensile strength. The material will not actually fail (snap or part) until you reach that load, which is much higher than the yield strength.
 
Last edited:
Reply to Alan

Hey Alan, The data I used came straight from the Aircraft Spruce catalog (!) I cross-checked the 90ksi number with my own material data and found it was in the ballpark.

As for the yield to ultimate margin, I'm really unclear about that. I'm finding huge differences in these values depending on whether I'm looking at 4130 cold rolled sheet, annealed, normalized, or tempered alloys. I think the tube we use is normalized... I know its not tempered. Cold rolled products are likely to be sheet or solid wire. Once again, this really isn't my best area but I'd agree that in the normalized alloy we have about a 50% margin.

For those who don't know how these terms are defined, here's what they mean:
TENSILE STRENGTH: This is the "strength" of the material in its elastic region. Meaning, that when stressed (stretched) the material will go back to its original dimensions when let go.
YIELD: means the material has stretched far enough that when let go it won't spring back to its original position. We call this "plastic" deformation. Its where the stress-strain curve begins to break over (like the stall point of a wing).
ULTIMATE: The highest point on the stress-strain curve.

I agree that from a pure material properties perspective, ultimate is higher than yield. I also agree that the stick *might* bend before breaking or buckling, allowing the pilot to land before it bends too far.

BUT - in a crippling failure mode like buckling, the structure can easily hit the yield point and then suddenly buckle and fail. I have no good way of analyzing this, unfortunately.

Second, the application of a 50% safety factor is not really justified. Using yield strength rater than ultimate strength automatically gives you a 50% safety factor.

This is true if there's a 50% margin in the material or somehow by design in the structure. This may very well be the case here, good point. But its clear the FAR's require a 50% margin above the published structural limits.

Also, in aircraft design, its common for structural margins of 2 or even 3 to be used in areas like landing gear, and engine mounts. Control sticks and systems? I don't know.

From my own (non-engineering) perspective, I can also see some practical problems with this analysis. First, on the RV-8 there is a seat cushion at 6" above the stick pivot. I couldn't apply a load to that part of the stick if I wanted to. Furthermore, 120 (or even 80) pounds is a LOT of force. Think about trying to pick up three 5 gallon cans of gas at the same time with one arm. I might be able to apply this much pressure with both arms, if I put my feet on the panel. The elevator would have to be on the stop to do this, which means the stick would be all the way back. I don't see how you could ever generate this much aerodynamic loading (without breaking the elevator first).

Alan, I know what you're saying. My purpose was to highlight the potential for a problem (as Van did) and let each builder decide. It might be possible to see stick forces in this range if an RV got pointed straight down, waaay over redline and the pilot panicked in the pullout. Maybe...
 
Reply to Rick

Hi Rick, I'll try to be as clear and concise as I can.

#1. If, for visual effect the model appears bent because of the magnification factor of 1 million is used in the virtual world simulation, does this mean in the real world the observer would not percieve a bending event at the forces modeled?

Correct, unless the stick began to buckle then you probably would see and feel that happen.

<< As an aside, those ME's who look at my FEM screen shots might note that the max deflection was only 8.75E-7 and the stress levels only reached a magnitude of about 200 psi in the critical area. This is because I used a unit load (one pound applied load) and then scaled the results by hand. Its easier than running the FEM over and over. >>

#3. Since it is unlikely a builder would drill a wiring exit hole on the forward face of the stick, wouldn't it be more appropriate to run modeling with the hole facing aft or nearly so? Would such placement even make any difference?

Yeah, I thought of that. But in the control system, the only difference between forward or aft hole placement would be if we were pulling or pushing on the stick.

#4. If we accept the loading forces on the RV-8 are relatively light at approximately 3-5 lbs per G, at +6 and -3 g's, what maximum forces can be reasonably anticipated to be loaded onto the joystick? Forgive me if my assumptions are totally incorrect, but if I multiply 5(lbs.) X 6(g's)=30.The number doesn't even represent half of: Stick w/ hole: 120 / 1.5 = 80lbs. What am I missing?

Rick, you're not missing anything. Like Alan, you're pointing out real world loads that should be well within our allowables. Its your decision to make; I do agree with your #5 comment in principle, but it should be the engineer who's concerned with design safety margins. We as builders should not have to apply our own amateur solutions to try and design fixes to traps set by the kit supplier.

#7. Short of drilling a wiring exit hole somewhere on the RV-8 pilot joystick, our choices are truly limited. Even if physically possible, would you really want the wires to exit at the bottom of the joystick? Doing so, the wire bundle would have to be extra long to account for maximum stick travel in all directions AND without becoming an unwieldy obstruction, do it while firmly secured to avoid potential conflict with other parts of the assembly. That would be a very neat trick if you can pull it off. For those who have done just that, photos please!

The stick as designed gives us problems that we now have to work around. Vans could alleviate this whole problem by supplying us with an engineered solution, like a 1/2" hole in the stick with a welded on washer as a gusset. (but will they?)

For me, a reinforced hole where ever you want it would be the answer. In fact, rather than highlight the problem over and over, I'm gonna take the time and engineer the fix right here and now. I'll post it as a new thread in the general discussion area...
 
Wireless?

Has anyone ever done a wireless stick grip control, perhaps the same technology used on car keys?

Just a thought.

Miles
 
Finite Elephant Modelling

Hi Bill,

I think its great that you bring attention to this detail. I too have seen large holes drilled near the pivot on the aft side of the stick, and like you would also recommend that builders either drill on the side of the stick, or better yet, reinforce the stick locally to restore the original strength (reinforcement is best).

Saying this, I dont think FEM is the best tool to use for this analysis. A hand calculation would be better suited. Your FEM is trying to predict the peak stress at the edge of a hole which does not predict a failure. This is because some local yielding may take place, but the net section remains stable and is still able to absorb / withstand further load. If you want to continue with the FEM, then consider breaking a piece of 4130 under test conditions to "calibrate" your FEM. FEM's sure look good, but they can also be mis-leading.

Jesse Bentley made a good point above also, could you modify your approach ao as to refer to the control force at the top of the control stick? (15 inch maximum according to Vans RV-8 drawings)

Our RV's are experimental, and we do not have to 'certify' our structures. However its useful sometimes to see what the certified guys are doing:
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=14:1.0.1.3.10.3.71.27&idno=14

One thing to note is that the maximum limit control force on ailerons are less than half that of the elevator. Therefore if you really have to put a hole in your stick, putting it on the side of the stick is a better than the fwd or aft. Reinforcement is better yet.

Note that the cases listed in the FAR's are not necessarily related to flight control surface loads - they also cover control jammed conditions whereby something prevents your elevator/aileron moving and the pilot gives it a real 'reef'.

I drilled a 0.25" hole on the side above the pivot also. I really should look at that also, but there are so many things to do....

Thanks for the nice FEM pictures.

Cheers

Robert Grigson
RV-8 Firewall Fwd
 
Just found the thread on the front page. I have not been reading / following it. My RV-6 has been flying 11.5 years and 2,198 hours. When I built it, I ran the wires for the PTT right out the bottom of the stick past the rod end bearings. I used wire from DMM props for the PPT wires coming down the stick as I wanted something VERY flexible. I purchased BULK wire for this so as not having to cut the probes of my DMM. Have not had any problems with the wires or the installation in the past 11.5 years and 2,198 hobbs hours other than replace the CHEAP Radio Shack switch on the pilot stick twice.

In other words, a complicated PPT installation is not necessary. IF you are using one of the fancy grips with all kinds of buttons, you will need more conductors but should still be able to route the wires straight down to a connector then out to the panel or what ever accessory that your stick gip button operates.
 
I wonder if Pat Tuckey had holes in his stick (control, that is)?

To Pat Tuckey:
RE: The issue of accomdating the "cable" the comes with the infinity stick grips.

Pat,
I would be interested to know if your stick was drilled for the Infinity or any type wiring. You obvioulsy put a very agressive force in gaining/maintiainig control of you aircraft when the auto pile servo screw went awol on you. Will you enlighten us?
thanks,
Don
 
Thank you

Robert, Thank you very much for your thoughtful and well written reply.

I have much to learn about using FEM as an analysis tool, being an aero guy for most of my professional years. Your commentary sheds insight into how best to use FEM -vs- hand calcs for this type of problem. I was tempted to break out my Roarks handbook for doing just that but the hole in the darn stick wall threw a wrench into that idea.

The stresses do build on the inside wall of the tube... and that's where I'd assume we'd get the first sign of plastic deformation. After that, the possibility of heading straight downhill into a buckling situation gets a little scary. Maybe I should look more at the general stresses in the area surrounding the hole and not get focused on the edge problems in the wall.

As for Jesse's comments, yes I could have modeled it with a longer handle on the stick. But it really doesn't matter when you consider we're reacting loads from the control surfaces. Those loads drive the stress levels in the tube no matter how long the handle may be.

Part 23, uggh. I know we need to design the aircraft so as to accomodate both "pilots" fighting against each other but jammed controls are only addressed in Pt 25 if I remember. At any rate, our control forces are really low. Much of the RV's basic structure wouldn't meet Part 23 anyway. I agree a hole on the side of the stick would be the best location for the least impact on the system. My RV8 has a hole (3/8") on the front-right quadrant of the stick. Very little to no impact on front/aft loads at that placement.

... and like you would also recommend that builders either drill on the side of the stick, or better yet, reinforce the stick locally to restore the original strength (reinforcement is best).

I absolutely 100% agree the best way to handle this is with a local reinforcement of the hole area, probably with a washer type of bead around the hole. I'm posting that fix tonight... good to see us agree on that.

Best wishes on your building and/or flying. Its great to have you here on VAF!
 
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