RV-7(A) Flap Rod End Bearing

[macrafic]

I recently watched Vic Syracuse's presentation on buying an RV (and what to look for). I made a mental note to check my rod end bearings to validate that I installed large washers to prevent the bearing from coming completely detached. My question is about the rod end bearings on the flaps.

They are already pre-assembled (press fit) and there is no way to get a preventative washer on them. I would imagine it would be quite an exciting adventure to be flying along and have one flap bearing detach (asymmetrical flaps!!!!). Has anybody heard of that particular bearing coming apart? Anybody do anything at all to add to its security? Is this me overthinking/over concerned?

 

[Desert Rat]

I don't believe I've ever heard of one coming apart, but you're correct that it's at least conceivable.

I'm of the opinion that most of the time when you deviate from a proven design with 12,000 copies flying and no real history of that perceived problem being a real problem, you're probably doing more to make yourself feel better than anything else. I also suspect that in this case, if that thing came apart there would be enough aileron control to keep it level while you were getting sorted out or Vans wouldn't have built it that way, but I don't have any specific data to back that up.

All that being said, to alleviate that concern some folks have replaced that rod end with one you can get a washer on. The details escape me but there are previous threads on it...

 

[andoman]

I recently watched Vic Syracuse's presentation on buying an RV (and what to look for). I made a mental note to check my rod end bearings to validate that I installed large washers to prevent the bearing from coming completely detached. My question is about the rod end bearings on the flaps.

They are already pre-assembled (press fit) and there is no way to get a preventative washer on them. I would imagine it would be quite an exciting adventure to be flying along and have one flap bearing detach (asymmetrical flaps!!!!). Has anybody heard of that particular bearing coming apart? Anybody do anything at all to add to its security? Is this me overthinking/over concerned?

Yep, Some builders have replaced said bearing with a conventional bearing secured to the flap with a bolt.
I, too, am interested to know if there has been a documented failure of the stock bearing. In the pattern with flaps extended, I believe recovery would be impossible should one flap retract to trail position. I believe flap rod failures have resulted in at least one RV loss.

 

[JeremyL]

To parallel this convo, why does the rv 9 have the hex rod and the 7 just the tube? Seems the hex would be stronger.

 

[andoman]

To parallel this convo, why does the rv 9 have the hex rod and the 7 just the tube? Seems the hex would be stronger.

-9 has a Fowler flaps that are larger. The added forces require a stronger actuator rod. The other models’ tubes are adequate but leave little margin if weakened by chafing, bending, or poor tapping during fabrication.

 

[Rick_A]

To parallel this convo, why does the rv 9 have the hex rod and the 7 just the tube? Seems the hex would be stronger.

I used the hex tube in my 7A. I made the one’s specified in the plans and didn think they looked very robust. I saw the hex rods in the Van’s store one day and bought a set. I did this before I ever flew my plane.

I don’t why they are not standard. Maybe it was/is a way to save a few bucks on the price of the kit.

 

[JeremyL]

Good to know! Thanks guys, any idea where I could get a similar hex rod to use rather than vans store since they are probably 4 times the price they should be.

 

[andoman]

Good to know! Thanks guys, any idea where I could get a similar hex rod to use rather than vans store since they are probably 4 times the price they should be.

I have found Van’s to be reasonably priced. I wouldn’t trust the alloy or temper of most of what I could find online…

 

[JeremyL]

I have found Van’s to be reasonably priced. I wouldn’t trust the alloy or temper of most of what I could find online…

I found some on McMaster Carr, I would just need to know the wall thickness and such….. it’s aviation grade alloy and it’s 33 bucks a foot.

 

[vic syracuse]

First, thanks for watching the webinar! Second, the Vans-supplied flap bearing is a different type of bearing than I showed in the presentation. You are correct in that it is impossible to add a washer to it. I am not aware of any failures, nor have I seen any "imminent" failures on any of the RVs I have inspected. Yes, I know some have changed them, but personally I am not a fan of changing anything structurally in the aircraft, especially in the flight control system, without some advice from the kit manufacturer.

Vic

 

[rvbuilder2002]

I don’t why they are not standard.

Because the standard aluminum tube has been statically load tested in compression (the way it is most highly loaded in service on the airplane) and it was found to have a safety margin far and above what would normally be required.

The small # that I am aware of having ever failed were installed incorrectly, such that a bend had been put in them to keep them from rubbing on the edge of the opening made in the fuselage, rather than just adjusting the size / position of the opening.
[BTW, a failure of a kit builders bent tube is what precipitated doing the testing. The test results showed that if built per plans, the assembled push / pull tube was stronger than expected]

For anyone not aware, putting a bend in a column type structural member that is loaded in compression is a very bad thing to do.....

 

[bruceh]

I did the modification to use the rod end bearings, AN bolt and large fender washer to hold the flap actuation rod.

 

[Draker]

I did the same as Bruce, but I couldn't find a medium sized fender washer so I cut a slot in the flap skin for it to fit through. Kind of wish I took more time and identified a better washer but no big deal, I'm the only one who sees it.

 

[Desert Rat]

You guys are of course free to do whatever you want, but now if that heim joint starts to get stiff your failure mode is the bolt unscrewing a little bit. I know it's going into a nutplate, but it doesn't seem like that's a great idea to use a nutplate as the only safety on a bolt with potential rotational loads. I can't recall ever seeing something like that on a certified airplane, but of course I may be wrong. It wouldn't take much loosening before the aluminum tube spacer I see in Bruce's picture starts to wear, then you're side loading the bolt, then eventually maybe sheer the nutplate rivets, then we're off to the races.

I realize that's a lot of maybe's and what-if's. I think it's an extremely unlikely scenario and bet that you guys will fly for decades like that with no problems. I'd hope you would catch the wearing parts on a preflight, but then again I'd also hope you'd catch a loose ball and socket heim joint when wiggling the flaps on the preflight (we all still do that right?).

My point is I think the failure of the bolt installation isn't really any more out of the question than the idea of the kit supplied ball and socket heim joint falling apart. Pick your poison I guess. As for me I'm willing to bet that the original design was pretty well thought out.

While there have been some failures that started when builders "improved" the system by bending the pushrods, can anybody point me toward one where the root cause was the built-per-plan ball and socket heim joint that came apart? If not, I really think this is a solution looking for a problem.

 

[pauldan181]

I switched to the hex style rods a few years ago at annual, they didn't seem to be very expensive at the time.
I had access to a calibrated load frame and tested my old rods to failure just to see what they would take.
At 680 lb. force they started to bend and then MUCH less force to continue bending as was mentioned earlier.

 

[Taltruda]

I found some on McMaster Carr, I would just need to know the wall thickness and such….. it’s aviation grade alloy and it’s 33 bucks a foot.

I think it’s cheaper to just buy the Hex ones from Vans.. at least it WAS cheaper before the price hikes. The Vans parts are also pre tapped. On my -4, I used then full length, but on other models, you may have to trim 1/4 inch from each side.

 

[JonJay]

I built per plans. On the 6, that is a builder fabricated tapped tube. As mentioned by Scott, and tested by Vans and others, it is more than adequate, way more.
I get changing to the hex rod. It’s cheap and easy, is more robust. However, it has been proven unnecessary, and this continues to come up as a topic again, and again…… and probably will continue.
Something about that little tube……

 

[rvbuilder2002]

I get changing to the hex rod. It’s cheap and easy, is more robust.

And requires an even bigger opening in the fuselage, which is hard to seal so it makes the cabin heat performance even worse than what a lot of people already complain about

 

[scsmith]

I replaced the non-captive rod end fitting with a standard rod end bearing, AN970 washer, a spacer collar, and an AN bolt. Contrary to a previous post, it does not thread into a plate nut. At least not on my QB flaps. I had to make a special wrench to hold the nut. That would have been the case regardless of the bearing used. To those cautioning against deviating from the kit-maker's design, I would caution against deviating from AC 43.13. Yes it seems incredibly rare that a rod bearing fails. But they do once in a while. When my DAR saw that I had done that, he was very impressed and said it was one of the things he routinely flagged on RV inspections.

I used a 4130 steel tube for the pushrod rather than the specified aluminum. Scott is correct that if the tube is straight, and you do not exceed maximum flap extension speed, there is LOTS of margin with the aluminum tubes. BUT.... I calculated the load if you forget to retract your flaps and compared it to the calculated Euler column buckling load, and at V_a (maneuver speed) with full flaps, there was alarmingly little margin with the aluminum tube. I've never forgotten to retract my flaps on the RV, but I have on other airplanes. Have you? The 4130 steel tube, or the aluminum hex, are cheap insurance.

 

[Desert Rat]

"....Contrary to a previous post, it does not thread into a plate nut. At least not on my QB flaps...."

I don't know what to tell you Steve but mine are built per RV7 print 14A and they most definitely go into a nutplate. Here's a screen grab of that print to illustrate.

 

[rvbuilder2002]

I don't know what to tell you Steve but mine are built per RV7 print 14A and they most definitely go into a nutplate. Here's a screen grab of that print to illustrate.

Different models designed at different points in time...... they are not all the same.

 

[RV7A Flyer]

When my DAR saw that I had done that, he was very impressed and said it was one of the things he routinely flagged on RV inspections.

What do you mean he "flagged" it? I assume you mean he "flagged" the stock flap pushrod, but what does that mean?

 

[Malndi]

Since we’re discussing rods as well as rod ends…being in a hurry one day I found a way to string enough errors together to break the rod. As I was about to step up onto the wing I stumbled and bumped the flap - which of course was down. Didn’t appreciate that it was all that much of a knock and didn’t notice that the leading edge of the flap had popped out from under the wing skin. Strapping in I pressed the button and pushed down on the flap lever in the normal way then bang!


We made a replacement as per plans but felt a bit uneasy about the margin for error when drilling and tapping the rod. So I changed to hex, which comes already threaded.

 

[scsmith]

What do you mean he "flagged" it? I assume you mean he "flagged" the stock flap pushrod, but what does that mean?

By "flagged" I mean that the DAR said that when he saw the standard non-captive bearings installed, he pointed to them as an area of concern that he did not consider safe, with the strong suggestion that it be done in accordance with AC43.13. I don't know that he ever denied an airworthiness cert based on that, but if it was one factor among several, he may have. That DAR was Dave Morss.

 

[RV7A Flyer]

By "flagged" I mean that the DAR said that when he saw the standard non-captive bearings installed, he pointed to them as an area of concern that he did not consider safe, with the strong suggestion that it be done in accordance with AC43.13. I don't know that he ever denied an airworthiness cert based on that, but if it was one factor among several, he may have. That DAR was Dave Morss.

IOW, practicing TLAR "engineering". And perhaps another example of a DAR making up new rules on their own.

 

[Stevea]

Since we’re discussing rods as well as rod ends…being in a hurry one day I found a way to string enough errors together to break the rod. As I was about to step up onto the wing I stumbled and bumped the flap - which of course was down. Didn’t appreciate that it was all that much of a knock and didn’t notice that the leading edge of the flap had popped out from under the wing skin. Strapping in I pressed the button and pushed down on the flap lever in the normal way then bang!


We made a replacement as per plans but felt a bit uneasy about the margin for error when drilling and tapping the rod. So I changed to hex, which comes already threaded.
View attachment 54980

I wasn't crazy about the remaining wall thickness in the threaded area after tapping the originally spec'd AL tubing. Ended up using thicker walled AL tubing which worked well. Yes, the ID had to be drilled to size, prior to tapping....but otherwise no big deal. This was of course in the Jurassic -6/6A era, before the hex stock version had been "invented' by the mother-ship. Also, the AL tubing part of the flap links on the 6/6A are somewhere around 5 inches long, give or take.....column buckling strength is not an issue, even on the originally spec'd material. Maybe the other air-frames have longer links, don't know.