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Elevator Pushrod is Rubbing

BSwayze

Well Known Member
I installed the elevator pushrods in my RV-7A fuselage and was very disappointed to hear something rubbing when I tested the forward-aft movement. I checked everything and I can't find anything wrong with my installation. I located where the rubbing problem is. It's the smaller forward pushtube, rubbing on the bottom of the hole through the F-705 bulkhead.

Here's a picture of the tube passing through the F-705. The tube moves up and down as it goes through it's forward-aft motion. In this shot, it's in the all-the-way-up position (stick full forward or full back). There's approximately 3/8" clearance remaining over the top of the tube.

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This picture is blurry (sorry) but here the tube is all the way down, roughly when the stick is in the neutral position. The tube rubs on the bottom of the hole.

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I put an inspection mirror under the tube in the bottom of the channel and took this picture. You can see how the rubbing of the tube is scraping the primer off the tube.

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Has anyone else had this problem?

My first thought was to file away the bottom of the hole a little bit and enlarge the hole, but this is the rear spar stiffener for the wings and I'm afraid of weakening it.

I'm also wondering if my control column weldment, WD-610-PC, was made wrong. If the short descending arm that holds the pushrod is too long, it explains everything. I measured mine, and it's 3-5/8" from the bottom surface of the weldment to the middle of the bolt hole:

100_7500%20(Small).JPG


Is this the proper length? I can't find any dimensions for any of Vans weldments. How do some of yours compare?

Any comments or help greatly appreciated.
 
Is your empennage on and the travel stops in place? I had rubbing where you did too, but found that when the tail was one and set to limits that there was no rubbing any longer. Hope this helps.
 
Very Common

I had exactly the same problem with mine and I've read of several others. Just grind down (and smooth) the F-705 enough to clear the pushrod. It doesn't bind by much and shouldn't take much grinding to clear. If I recall correctly, I had to grind ever so slightly into the rear spar stiffener, but not enough to weaken it.
 
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YEP!

Just file down the bulkhead a small amount, deburr, prime and build on!
 
I had the same problem with the push rod rubbing and resolved it by duct tapping a rasp on the bottom surface of the push rod, applying a little downward pressure on the rod while moving the control stick fore and aft. I didn't have to remove a lot of material to solve the problem and was glad to get the job done without having to remove the push rod to get access to the bulkhead.
 
Gentlemen,

Thank you so much for all your feedback. Steve, that's very interesting that yours changed when you mounted the empennage. In my case it's the forward smaller pushrod that's rubbing, and it won't change position by mounting the empennage or setting control stop limits.

Anyway, here's an update. I got 'er fixed. I had also written to Vans. I got a nice reply from Ken Scott. He said to go ahead as many of you have said, as long as it's no more than 1/8" of material to give the necessary clearance, making sure to radius and smooth the work.

Here's a shot of my work. It took less than 1/16" to give me all the clearance I need. I used my dremmel and a sanding drum, and my die grinder with a fine scotchbrite wheel to polish it up.

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I soaked a Q-tip in primer, making it real easy to apply a new coat of primer.

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I wrote back to Ken and took some time to give him some builder feedback. I explained all the reports I had received from these forums, and I urged them to shorten the arm on the control stick weldment by about 3/16", as this would solve the problem. It won't help any of us, but it would help future builders. If more of us do this, it will only help make things even better. My suggestion: be polite and respectful and stick to the facts. Ken said they would look into it.

Thanks again, guys, for all your help!
 
Bruce, it looks like if someone wanted to they could take that part to a local weld shop (either well trusted or properly certified for the job) and have the part shortened just a bit, right?
 
Probably not a good idea...

Bruce, it looks like if someone wanted to they could take that part to a local weld shop (either well trusted or properly certified for the job) and have the part shortened just a bit, right?

That would shorten the arm on the stick by almost 5%.

That would change stick forces and travel by a similar figure, in this case making the stick less sensitive. Certainly changing Vans original design intent.
 
Bruce, it looks like if someone wanted to they could take that part to a local weld shop (either well trusted or properly certified for the job) and have the part shortened just a bit, right?

Danny,
I thought of that, actually. But as I evaluated the amount of work involved, it would have been a lot more work to remove the control stick weldments to get that part out of there and take it to a shop, not to mention the expense and ruining the powdercoating. Nevertheless, I would have done it in a minute if Vans had frowned on this solution. I still don't know what the proper length of the arm is supposed to be; Van's doesn't have any measurements on their drawings, understandably.

Which brings me to your comment, Gil. Given the length of the lever that the control stick is, it's hard to imagine that shortening the lower arm by a scant 3/16" would affect the stick forces by much, don't you think? My hunch is that mine is a bit longer than Vans designed it to be anyway. I think whoever is making these weldments for Vans probably has a jig that's just a tiny bit off. Why else would this be such a common problem? Again, it's just a hunch, but it would explain everything.
 
It's a 5%...

.....
Which brings me to your comment, Gil. Given the length of the lever that the control stick is, it's hard to imagine that shortening the lower arm by a scant 3/16" would affect the stick forces by much, don't you think? My hunch is that mine is a bit longer than Vans designed it to be anyway. I think whoever is making these weldments for Vans probably has a jig that's just a tiny bit off. Why else would this be such a common problem? Again, it's just a hunch, but it would explain everything.

...change in stick force and stick throw. If you did shorten it, you should also shorten the lower arm of the mid-fus. bellcrank to keep the throws the same, but then you would increase the loads on the tube. But do you really want to "diddle" with the control system design?

It is probably not really a welding jig problem, it seems to be a very minor design/measurement/tolerance problem. A lot of the -6 models had rubbing at the same bulkhead, but at the top of the hole. My measurements came from -6 drawings that give the full dimensions of the weldments. I have presumed that the -7 numbers are very similar. The lower fus. shape did alter between the two designs though. A quick check with a ruler should give you the percentage change.

Since nothing was pre-punched in the -6, the fix was easy...:)
 
Bruce,

if it was rubbing before the fix and you only removed 1/16 inch, doesn't that mean the clearance is something less than 1/16 now? That's probably fine, but it occurs to me that under increased G that thing will flex a little. Also during the occasional hard landing.
 
Interesting - Just at this part now on my RV6 project... and mine is rubbing on the top part ...

Jan
 
Gentlemen,

Thank you so much for all your feedback. Steve, that's very interesting that yours changed when you mounted the empennage. In my case it's the forward smaller pushrod that's rubbing, and it won't change position by mounting the empennage or setting control stop limits.

You can affect this clearance by mounting the elevators and setting the stop limits. It's hard to explain but it has to do with the position of the elevator bellcrank. Think about a little bit and you'll know what I mean. If the bell crank is more vertical you will have more clearance.

I did have a rub at first but once I got the elevators mounted, the control stops installed and the push rod lengths adjusted the rub went away. It doesn't work that way for everybody but you should probably save the dremmel tool until after you get all that done. It doesn't take much clearance and I doubt that the wing loading would affect it it much, because that push rod is quite stiff for it's weight.
 
Mine rubbed just like this even when all was installed.

I filed it a little and primmed just like the OP.

Problem solved....
 
Looking at it the other way, it is only an "issue" if you are concerned about the rear spar strength... else just file that down.

The RV-3 - and the 3-9 are essentially the same concept - requires a good % of the rear spar filed away here - see Paul & Louise Dye's website Link - Whilst rear spar strength is clearly important at the wing roots, the basic design does not appear to require that strength carried all the way through the fuselage?

Seems like leave adjusting the controls / stops etc. for more important issues ;)
 
Phil had the

same experience as I did. With the stick in the neutral position, adjust the fwd tube length to place the bellcrank at right angles to the fwd tube. This will maximise the clearence at the bulkhead.
 
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