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W-716 Aileron Pushrod too short

grayforge

grayforge

Well Known Member
Arrgghhh... Test fit my W-716 stick to aileron bellcrank pushrods today and found them too short. :-( Ailerons in trail and bellcrank jig in-place. I did a test: turned the jam nuts all the way loose, then spun the pushrod in one direction to see if it would stop before... plop... the rod end fell out.

I found a number of folks with this issue on VAF. A common solution is to use rod end bearings with longer threads (MD3616M), but these look to have a thicker ball requiring adjustments to washers at the stick and spacers at the bellcrank. Has anyone found a rod end bearing that's identical except for the length of the threaded shaft?

I could also build another set of pushrods :-( or add an extra jam nut to each rod end bearing to use up available thread space for the pushrod to twist off.

Thanks!
Russ
 
Aileron Pushrods

Russ,

It's been a few years since I dealt with this same issue. Although I added a bit to the length (knowing that others came up short), they were marginal. I ordered the rod ends (likely the one you referenced) and that provided ample thread to satisfy my comfort level. These endpoints are way too important not to do whatever it takes (reordering the rods, if necessary) to feel comfortable that you have adequate thread in the rods.
Good luck

Jim Diehl #71976
7A @65 hrs TT
 
Thanks for the input Jim. Did you replace all 4 rod end bearings and were they, in fact, thicker, requiring alternate washers/spacers?

I've seen people replacing only one bearing on each pushrod, but I'd think it could still twist towards the longer rod end bearing, freeing the short one.

I'll be replacing all 4 rod end bearings. :)
 
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Any idea how this is happening? The drawings specify the length to cut the rods down to the 32nd of an inch!
 
smiller said:
Any idea how this is happening? The drawings specify the length to cut the rods down to the 32nd of an inch!
Click to expand...

Yep, it is another one of them well known long term issues that never results in a drawing update. The lucky ones are the ones that read about it on here and make em longer than the plans...

Another thing one can do to stop em from being able to screw themselves off is to add an additional jam nut on each end. (oops, Russ already said that)
 
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How much too short is the pushrod? I ask because I just installed the F-665 interconnecting pushrod between the sticks, and at the prescribed length between the bearing centers, the sticks lean toward each other by about an inch at the top. If my math is correct I would have to shorten that pushrod by about 7/32 total to make the sticks parallel. If the sticks are actually supposed to lean inboard, would that extra 7/64" (three full turns on both 1/4-28 rod ends) below the stick pivot be enough to keep the long pusrod from being able to unscrew itself?

Also, since I cut my pushrods per plan before I knew about this potential problem, is there a way to safety the bearings? Someting like double jam nuts, lock washer(s), or drilled nut and safety wire?
 
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That's a good point that some adjustment is available by lengthening F-665. You could also lengthen the bellcrank to aileron pushrods for a bit more reduction in W-716 length.

However, my thinking on the F-665 interconnecting pushrod is that you adjust its rod end bearings to make the sticks parallel. To me, it wouldn't make sense for them not to be.

Ditto with the bellcrank to aileron pushrods. These should be set to what the bellcrank jig specifies for proper play in each direction.

Just about to pull the trigger on longer rod end bearings.
 
grayforge said:
That's a good point that some adjustment is available by lengthening F-665. You could also lengthen the bellcrank to aileron pushrods for a bit more reduction in W-716 length.

However, my thinking on the F-665 interconnecting pushrod is that you adjust its rod end bearings to make the sticks parallel. To me, it wouldn't make sense for them not to be.

Ditto with the bellcrank to aileron pushrods. These should be set to what the bellcrank jig specifies for proper play in each direction.

Just about to pull the trigger on longer rod end bearings.
Click to expand...

I agree with you on not playing with the W-818 push rod length. That would affect the differential aileron travel, though I really don't have a feel for how much.

The long push rod connects to the stick about 3-1/2" below the pivot, so three turns on the F-665 rod end only moves the stick about 1.75 degrees. You can see it, but it's not overly noticeable. How much longer does the long push rod need to be?
 
I'm guessing my pushrods are about 1/4" too short. If I build new ones, I'll probably make them 3/8" longer. This will ensure very little of the threads show.

I recall constructing according to the drawings. If I pull them back out. I'll re-measure.
 
Well, I decided to build new pushrods, instead of using longer rod end bearings. It's the best option, I think, and what Ken at Vans recommended.

The bellcrank and stick were not designed for the larger rod end bearings. Ken warned against dinking around with the washers & spacer to make larger rod end bearings work.
 
I'm confused. Is this a known problem or a problem that is specific to this particular build? I'm right at the point of making these pushrods and normally I wouldn't deviate from the plans on something like this. Is Vans suggesting that we make them longer?
 
Vans isn't suggesting everyone make them longer, but enough people have this problem that a number of folks feel the spec should be 1/4" to 1/2" longer.
 
on my seven there is two dimentions given one for the tube and one for the total length if you measure the ends and the bearing in the end and add it to the tube length you see how far out the bearing threads need to be to get the total length I would say that too much threads are sticking out to get the total lenght the drawing calls for and that is the problem, I desided to make my tubes longer but now i suspect not long enough
 
Bill,

I remember the two dimensions. I'll see if I can do a test fit and check the lengths. I'd hate to cut off the tubing now to the drawing length, throw away the excess and then need to build a longer tube later when I try to fit the wings.
 
rapid_ascent said:
Bill,

I remember the two dimensions. I'll see if I can do a test fit and check the lengths. I'd hate to cut off the tubing now to the drawing length, throw away the excess and then need to build a longer tube later when I try to fit the wings.
Click to expand...

I cut mine over 2 months ago, not realizing the potential problem. What's done is done. I'm going to wait until I fit the wings and install the pushrods to sweat whether or not to build new ones. If I hadn't already made them I'd wait and try to figure some method of measuring what length is actually needed.

FWIW, there are at least two revisions to the length of these tubes on my drawing. Since my kit is relatively new, I've got my fingers crossed that Van's has already taken care of this.
 
To those who had problems with the pushrods being too short: which rev. of DWG 15A do you have, and to what lengths did you cut your tubes? I have R4 of DWG 15A, dated 10/16/03 (nearly 10 years ago!) and it shows the following lengths for RV-7 aileron pushrods:

W-716: 65-25/32" (tube itself); 69-9/32" (between rod-end bearing centers) [Delta = 3-1/2"]
W-818: 23-13/16" (tube itself); 27-5/8" (between rod-end bearing centers) [Delta = 3-13/16"]

I also just built mine about two months ago. Aarrgg!
 
Looks like we're in the same boat.:eek: You have the same Rev drawing 15A I got in my kit received new in April of 2012. I hadn't remembered that those revs were that old. The rev notes only reference lengths for the RV-8.

I hope I remember this thread when it come time to fit the wings...
 
Earlier this evening, I measured my pushrods. Sure enough, the tube portion is right on 65 25/32". The amount of thread showing is a hair over 1/2" total (adding both ends).

I received my replacement tubes and rod ends today and cut the tubes to 66 1/8". Theoretically, the extra 3/8" will mean that the threads showing with the new pushrods will be 1/8" total.
 
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I got a PM from a curious builder wondering if the new pushrods ended up the right length... sure enough, after test fitting on the wings, they were perfect. Just 4 or so threads showing on each end. A twist test to see if twisting the rods all the way onto one rod end bearing and back onto the other without one end dropping out passed with flying colors. :)

DSC03058.JPG


DSC03059.JPG
 
Grayforge
Your picture shows the bolts going in from the bottom and the nut on top if I am seeing it correctly. If the nut comes off the bolt falls out. For added safety I read somewhere ?? where it is better to always let gravity work for you and always put the bolt in so if the nut comes off the bolt will not just fall out. You may jiggle it lose and it still comes out with agressive movement but you have a better chance that you will feel it or find it before it comes out.
Just thought I would comment here nothing critial but could be a safety issue.
 
Actually, you're looking at my assembly from below. My bottom skin is not yet riveted on, while the top is. Good thought though. :)
 
Shoot, I am on here almost every day, I can't believe I missed this topic! Any engineers on here, some where I remember that a threaded fastener needed to engage 1.5 the bolt diameter for the length of depth into a hole? And on a nut, .5 Dia of bolt extending past nut?
 
Thread Engagement

Bret,

Normally 1.5 diameters is a good number to use for thread engagement. However, an aerospace industry excepted equation for calculating "thread pullout" is as follows:

P_pullout = 0.4*Fsu*Pi*D*L

where:
Fsu = Ultimate Shear Strength of the material (pounds per square inch)
D = Diameter -- in this case the outside diameter of the thread since the aluminum is much weaker than the steel rod end (inches)
L -- Length of engagement (inches)

The "weak link" for the W-716 assembly is tube buckling. The assembly should buckle at about 392 lbf. So, with only 0.250" of engagement (6 threads), the thread pullout is about 5X stronger than this. However, I wouldn't go much below that amount of engagement since the validity of the equations starts falling apart as you get down to only a few threads of engagement.

I hope this helps.
 
Hey! This is my first, welcome aboard the good ship:D. Thank you, that is some nice info, over my head but I like that kind of tech stuff. Are building yet?
 
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