Correction: -14A was too
long so switched with -13A
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Unexpected RV-7A elevator differential movement
- Thread starter aerocroft
- Start date
long so switched with -13A
Maybe - Maybe not; the correct length in practice will be determined by the amount of grip showing beyond the hole. As this structure is loaded in shear, you cannot have ANY threads "inside" the loaded area (i.e. inside the hole). The excess grip & thread should be taken up with washers, so you can tighten the nut down, achieve the required torque and still be turning the nut on threads and not bottomed out on the bolt grip.
Got it. Thanks
It's possible that this is the only anomaly on this plane, and that it was found before it created a larger problem. Let's hope that this is the case, and I think we are collectively glad that you brought this to light. There have been many good questions and good discussion points. But I would strongly suggest that the OP find an RV-7 builder or an EAA Technical Counselor near you that's willing to give everything a good, thorough looking over. A condition inspection is a great time to do this and look at all the pushrods, connections, and other fasteners to ensure they're correct (even if this means going behind an A&P). Having the right type of hardware in the right spot is critical to long life (both yours and the plane). While he or she is walking you through their inspection, it's a great opportunity for you to ask questions and learn.
We were all new to aircraft building and maintenance at one time, so this isn't intended as a condemnation of your technical knowledge. But there's only so much the collective knowledge base on the forum can do with a handful of pics and videos. For the price of a hamburger and a little fuel to get an experienced person to your home field, I'll bet you can accelerate your learning curve and give you the confidence and knowledge you need to ensure your plane is being maintained correctly.
We were all new to aircraft building and maintenance at one time, so this isn't intended as a condemnation of your technical knowledge. But there's only so much the collective knowledge base on the forum can do with a handful of pics and videos. For the price of a hamburger and a little fuel to get an experienced person to your home field, I'll bet you can accelerate your learning curve and give you the confidence and knowledge you need to ensure your plane is being maintained correctly.
and, as @Ironflight and others have suggested, this is a good preflight activity, along with checking the throws, "short shake" the stick and see if it shakes back..
I did a pre-buy on an RV-6 that had this exact same issue. It was evident from this issue and a dozen or so others, that the builder/pilot didn't care enough to follow the plans, or maintain the aircraft correctly.
I appreciate the suggestion very much and I’ll follow through for my ACI in March
Can you explain this “short shake” in more detail?
Grab the stick and give it a short duration, high amplitude shake (1/4 - 1/2 the total available "throw") -- if everything is tight, you shouldn't feel or hear anything. I will bet you a beer that if you do this with your airplane in pitch, you will hear/feel a thunk as the heavier elevator (typically the left one) stops moving after the other one.
Yep. A fabulous example of why torque seal application by the builder is often a lie. IF used at all, it should be reserved for a qualified inspector.
5/16" diameter bushings are called out in the plans, however dumb it may be.
Note the 14A callout. Before anyone changes to a -13, re-read Mr. Decker 's post above. The correct length wouldn't have any threads in the joint, but rather would be long enough to move the root thread outside the horn, with a washer stack as needed to ensure the nut does not bottom.
Now, at great risk of being pilloried, I will admit to having a very close personal relationship with the wife of a builder who took a look at that particular joint many moons ago, said "No way", and installed an MD4616M, i.e. with an AN4-14A and -4 washers.
If we were to be really sinful, the right way to do this kind of joint can be seen in an example from a custom Pitts. The horn loads involved in tying the elevators together are separated from the pushrod shear.
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...and the travel stop isn't a bulkhead
All factory Pitts from the S1S to the S2B have a !/2" thick aluminum block between the horns and two additional AN3 bolts thru the spacers and horns. The Pitts rode end bearing is not a tight fit between the horns. That photo is very different than Pitts plans for S1S.
The VERY good news is that you found this out during inspection, The other good news is that it's easily remedied. In my -7A build I used the feeler gauges as Mike W. did and then cut stainless spacers on a lathe to fit the gaps - which were NOT symmetrical at all. This is due to assembly tolerance stack-up and it is common during the building process to substitute bolt lengths in order to get the grip length to be where you want it. The plan bolt lengths are based on nominal dimension tolerances and sometimes a substation is warranted. In the case of this assembly, it needs to have the thru bolt tight and the play eliminated- but you know that. I would advocate not using aluminum tube stock as the spacers.... My opinion.
bt3vex
Active Member
Meet Jesus Bolts to regularly check more frequently in my opinion:
Note: Some of these I do every flight some a few times a year
1. Elevator Horn (Remove inspection panel for access) (Few Times)
2. Flap attach bolts (The ones that go into the actual flaps) - (Lower your flaps and take a look, easy to see) (Every flight)
3. Aileron Bolts (Check the locking nut that goes into the push rod) (Every flight)
4. Aileron Bell crank (Open middle inspection panel on each wing). (Few Times)
4. Rudder Control Horn Attach Point (There are two, one for each cable) (Every Flight)
5. Throttle attach cable (Only when cowl is off)
Just my 2 cents,
Note: Some of these I do every flight some a few times a year
1. Elevator Horn (Remove inspection panel for access) (Few Times)
2. Flap attach bolts (The ones that go into the actual flaps) - (Lower your flaps and take a look, easy to see) (Every flight)
3. Aileron Bolts (Check the locking nut that goes into the push rod) (Every flight)
4. Aileron Bell crank (Open middle inspection panel on each wing). (Few Times)
4. Rudder Control Horn Attach Point (There are two, one for each cable) (Every Flight)
5. Throttle attach cable (Only when cowl is off)
Just my 2 cents,
While loss of any of the above is serious, I'm not sure they rise to the level of a "Jesus bolt", and, off hand, I'm not sure my RV has one. Compared to, for example, my former airplane, a 182, where there are 4 bolts (top, bottom, left and right sides) that attach the wing struts to the wing and the fuselage. Failure of one of these bolts is guaranteed to result in the loss of a wing. Fortunately, they are also one of the most over-engineered things I've seen on a GA aircraft.
Sam Buchanan
been here awhile
As a belt-and-suspenders approach when rigging the elevators on my RV-6 I added a second bolt similar to the example posted above tying the elevator horns together for the very reason of not having to depend on the pushrod bolt keeping the elevators in trail. I also have a plexi window in the access plate under the horiz stab so the bolts can be seen during preflight.
...yet...
… and it was discoverable in a basic preflight. Thousands flying and none have ever fallen out, Yet….. as bjdecker pointed out.
Good catch regardless, but this didn’t get to that point in one flight.
Tankerpilot75
Well Known Member
“Exciting” yes - but unfortunately also short!
Hopefully not already mentioned. I only did a quick scan of the thread.
The bolt length is only part of the problem, at this point. The holes in the horn proper are no longer acceptable. Proper spacers and torque would help but ultimately the installation shouldn’t be relying on friction to maintain position.
The horn material is too thin to bush properly. Hate to say it but the horns should be replaced or possibly weld filled (or weld doublers added) and re-drilled/reamed.
Good luck.
The bolt length is only part of the problem, at this point. The holes in the horn proper are no longer acceptable. Proper spacers and torque would help but ultimately the installation shouldn’t be relying on friction to maintain position.
The horn material is too thin to bush properly. Hate to say it but the horns should be replaced or possibly weld filled (or weld doublers added) and re-drilled/reamed.
Good luck.
You tighten those bolts to the values in chapter 5. They clamp the inner race of the bearings and allow the outer race to be free.
Thanks, wanted to make sure the thoughts were correct.
Yep. If you freeze frame the video you will see at the end that the bolt is indeed too long and only has one washer.
So it should have been blindingly obvious to anyone qualified that it was questionable not only at initial install but any subsequent “inspection”
Caveat emptor indeed
Or drill out one size larger provided a new bearing can be found to fit the larger bolt.
DanH provided a part number in post #59
that is wear that caused the hole to grow in size. You can see the black everywhere that is a byproduct of that wear. Tough to tell from pics if it is too excessive. First blush it will probably be ok, but really need to get a caliper in there and see what the hole diameter is.
I don’t think i will ever understand how folks will take on building a plane without understanding how threaded fasteners work. In prebuys, I routinely see bearing bolts loose, allowing rotation on the bolt itself.
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Actually, this hit me last night when I should have been sleeping.
Shoulder washers. It's doubtful you can find some that would work as supplied; but, with a little modification (most likely shortening the stem) they probably could fit just fine. Buy them from an aviation supply house vs. other. Material will determine if this is a potential short-time, long-time or permanent solution Ream the hole to a close tolerance fit; zero or slight interference would be even better.
Offered as a possible suggestion. Fire away. Comments would be appreciated. PULL!
Edit = If they can't be found, it would probably be easier to have some machined versus a weld fill. @PhatRV 's suggestion would be easiest if such could be found. In all cases, ensure proper ED is maintained.
Edit 2 = Sorry for the spit-balling but this is more entertaining than work. This would depend on the final dimensions of the holes in the horns after they've been reamed back into circles. A close tolerance bolt or even an oversized shank bolt (call General Aircraft Hardware) could be an elegant solution. The hole in the bearing could be reamed to the proper size and washers replaced accordingly. Before anyone panics over the bearing reaming suggestion, I can all but guarantee its margin would still exceed that of the connecting aluminum caps, long column control rods, etc. Once again, fire away.
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Interesting idea, and what it indicates is that there are probably a lot of solutions, most of which have been seen by the factory. That's why we pay them -
- to be a central source of best practices for unusual situations like this.If properly designed and torqued, there is no reason that some slop between the hole and shank creates a problem holding the joint in its place. Take your typical front lower control arm on a car. The hole is at least 2X bigeer than the bolt to allow for alignment adjustment. Everything holds its place via tension from the bolt. That takes the vehicle weight and a multiplier of 2 or 3 times that for impact loads, like pot holes and is under constant movement. If the designer was relying on something other than tension to hold position, they would specify reaming and call out tolerances for bolt size and hole size, as we see in the wing spar bolts. Also see this in the nose gear leg attach, where tension doesn't hold the joint and the proper fitting is doing all the work. Not an ME and don't know the principals, but see this in practice a lot. Have no idea how this joint was designed, so can't say what is required. Only saying you can't assume close fitting is required.
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Usually I agree with your wisdom, Sir. Not this time. While I obviously don't have the design details of either, on the surface you're comparing an OEM's specifically engineered joint to what is an most probably just an application. If assumptions were made for the latter, every effort should be made to return the boundary conditions to those where the engineer applied such. Don't get hung up on the "reaming" wording. My point was to get the fastener/hole tolerances back to original and from the video, I'm guessing that's not possible. As for joint application in shear, the load and the area in bearing are the only parameters. Friction adds margin. It is not (typically) considered except for specifically engineered solutions.
For background, look at the results from either the aforementioned formula, the application of 43.13, and the values in Mil-HDBK-5. They all give different values for related applications.
An oversimplification = The first is based on rudimentary math, the second on mathematical "equivalents", the third on empirical testing/validation. The last revealing any margin gained (or sometimes lost) from surface friction, dimpling, imperfections, etc.
My comments were meant to be a fun exercise to hopefully help the OP find a reasonably economic and safe solution. I doubt you would you accept such apparent situational slop in your aircraft.
Maybe we can entice the esteemed @David Paule to reply
So what are the tolerances? Your average bolt is a good .003" smaller than nominal and the average joe can easilly go .010 - .015 over through sloppy drilling. It would seem to me that .015 clearance with constant motion back and forth would wear the hole much larger over time, what prevents that? Perfect example here, where the hole grew substasntially by banging back and forth 1000's of times, due to no tension on bolt. Not really arguing beyond my training here, but I see all sorts of highly loaded structures where there is a good amount of slop between hole and bolt. Clearly there is more to this than just really close fitting fasteners. If that were the case, the designers would have to provide those tolerances.
To be clear, I am not suggesting that what the OP ius dealing with is acceptable; Only that I am not necessarilly assuming it is bad. Better understanding of joint design is necessary to determine that.
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In my glider club, the older gliders elevator control arms are made from aluminum and over decades of operation, the bolt holes are enlarged by the forces of vibrations from landing on rough surfaces. For older RV, I can imagine aircraft vibrations will put these holes out of tolerances as well, despite our best efforts to keep them tight, even with steel. I will make a point to preflight my RV knowing this can happen to my RV.
Since I built my RV, it's not difficult to replace or repair these control arms.
Since I built my RV, it's not difficult to replace or repair these control arms.
Outside of the force being applied, what makes this any different than the multitude of other push rods and bearings installed in exactly the same way? I think there are a dozen or so associated with the ailerons and elevators.
Seriously?
Your original anecdote of a specifically engineered joint that was intended to allow for adjustment is questionable at best.
No one here has suggested that joint preload isn't important.
At what point does the slop from poor workmanship, wear, abuse, etc. become a concern? I'll question again, do you accept this on your aircraft or other machinery?
What is wrong with returning the joint tolerances to drawing/design intent if other margins aren't reduced?
Sorry, not buying your arguments/rationalizations. Others can do as they wish. This is my last reply here.
We have rod ends installed in both single shear and double shear. Single shear installations are for low stress only, due to an obvious bending load on the bolt. A typical double shear installation eliminates that bolt bending. The forward end of the big elevator pushrod is a fine example. Note the two halves of the bellcrank are riveted together. The two arms, with the rod end sandwiched between them, cannot move in relation to each other.
The connection to the elevator horns looks like a double shear, but it's only true as long as the load is equal at each horn. Unequal horn load, i.e. unequal loads on the two elevators, results in a bending load on the cross bolt. If the bolt is tight and there are large diameter spacers on each side of the rod end ball, some of the bending is relieved. If the bolt is loose and/or the spacers are relatively small diameter, then bending is more severe or the bolt rocks in the holes in the horns.
The elevator loads are not always the same. Propwash beats them unequally. There is vibration, the wet dog shake at startup. And don't forget the trim tab, a force applied at one elevator only. These things make some of us uncomfortable, perhaps without reason. So, we upsize the rod end to -4 bore, or tie the horns together with another fastener to form a true double shear, much like the mid fuselage bellcrank.
The design as delivered is statistically adequate, so let's not be too critical. Just be sure to keep the statistics in your favor. Space with steel washers, not aluminum tube. Use a bolt which moves the root thread out of the joint, which may or may not be the length callout from the plans. Make sure it is tightened properly, then keyed.
How can you tell that's not grease from years of lubricating the rod-end bearing?.
And yes -- it appears that some builders don't understand that the ball part of the rod-end needs to be tight against the washers and move along with the horns, bolt and washers -- the only thing that should move is the ball against the outer body of the rod-end. They also need to be lubricated at each condition inspection or sooner if they appear to have dried out. The goal is for the ball to rotate smoothly in all directions (three degrees of freedom) relative to the bearing body, not for the bearing body to rotate on the bolt itself.
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Thanks to everyone for the input. Based on all of this and some insight help from mechanical engineer wife (NASA) I went ahead and added a single AN960-10 washer, a new AN365-1032 nut and torqued it to 24in-lbs and the elevators are solidly connected—no differential movement. I torque-striped the nut and will pay close attention between now and my upcoming ACI, where I’ll have a subject matter expert review the elevator and all other control systems etc. Again I appreciate all of the help. 
It's good that it "appears" to be all secure after your modification and re-torquing. However, if you zoom-into your own photo (below), you can clearly see that the bolt is not fitting tightly against the elevator horn on this side (i.e. horn hole appears oversized). I would still consider trying a close-tolerance bolt and having your wife take another look at this zoomed-in photo. This situation could actually make the condition worse by inducing a roll-moment following a strong pull on the stick.
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Just an FYI -- the corresponding close-tolerance bolt designation for an AN3-14A is AN173-14A, assuming the RV-7A uses the AN3-14A. Cost is $0.86 per bolt at Aircraft Spruce and Specially. If it were me and my airplane, I would fix this before flying again -- from one engineer to another. 
Slightly off-topic but while we're talking bolts, a question I've had since starting my project is why don't we put washers under the head of a bolt? When i was restoring my jeep I was taught to put a washer under the head and nut. The plans don't show washers so I haven't been putting any on. Or am I wrong?
Will a close tolerance bolt fit the rod end bearing?? I believe the proper repair for this is to have step bushings made with flanges, at least 1/2" diameter x .080 wide for the flanges and 1/4 od x 3/16 id x long for the portion that will fit inside the horn. x being the thickness of the horn. Weld the large flange to the outside of the horns with the small diameter inside an enlarged hole in the horns. This will maintain the proper relationship for the original rod end bearing and bolt The original holes should be line drilled and reamed to 1/4".Just an FYI -- the corresponding close-tolerance bolt designation for an AN3-14A is AN173-14A, assuming the RV-7A uses the AN3-14A. Cost is $0.86 per bolt at Aircraft Spruce and Specially. If it were me and my airplane, I would fix this before flying again -- from one engineer to another.
A less desirable alternative is to have the holes welded closed, redrill and ream the holes to a proper fit on AN3 bolts and file the welds flush with the horns.
Either of these methods will require removeable of the elevators but will not require replacement of the horns.
The reason for a washer under the head of a bolt is to prevent point loads from the sharp corners of the bolt head causing stress risers. AN hardware have a raised shoulder milled into the underside of the head to keep the points off of the base material. Sort of a built in washer.
Thanks!
Well, standard AN (Army-Navy) aircraft bolts feature a built-in "washer-like" surface on the underside of the bolt head. By using additional unnecessary washers on the head-end of AN bolts, you're just adding more weight to the aircraft.
Yes -- an AN173 close tolerance bolt will fit through a standard 3/16-inch (#10) rod-end bearing hole, typically with a zero-clearance or light interference fit. And, for the cost of only $0.86, why not give it a try. In addition, you don't want the bolt to rotate inside the rod-end bearing anyway -- right.
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Sometimes we do. There is no prohibition.
Generally speaking, if you are going to turn the BOLT instead of the NUT, you put a washer under the head of the bolt to prevent galling (even with the machined-in “turning surface”). Not all bolts on an aircraft are AN or Mil Spec - Lycoming uses other types of hardware for instance.
Agreed 100% !! (And Vans does use AN bolts for this application, (AN3-12A I think)) It's just my OCD I like the way the bolt head looks with a dedicated washer underneath but probably not needed ! (Tell my OCD this but sometimes not enough thread showing and then I need to live with no washer)
700 hrs. and still looks like new. (Pic on right)
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