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Pushrods too short?

Paul Eastham

Well Known Member
Hey all,
I did some initial fitting of my pushrods, with controls and control surfaces in place, and came up a bit short (i.e. less than half of a bearing engaged) on two of the pushrods:
- right aileron (stick to bellcrank)
- elevators (stick to bellcrank)

I'm pretty sure my pushrods were constructed to the lengths specified on the plans. I'm curious if other people have encountered this? I didn't get a chance to debug this problem too much yet so I thought I'd check here first.

One thing I want to try: re-chcek the aileron with the elevators in neutral position. Another thread was talking about some small changes in aileron position when the elevators are fully deflected.

Thanks - Paul
 
RV7a was the same

Yup...If I were doing it again I would make the pushrods to the actual length required rather than what the plans show.

Mine needed to be 3/8th's" longer from memory.

I managed to adjust everything to make it so they would not afll off when screwed one way or the other...Just.

Frank
 
too short

Yeah I ordered new ones, be sure to install the angle/wood straight edges
on the alignment holes to line up the alierons, use good marks , tape
the alierons if your working alone, use the correct holes also, the 1st time
I was on the wrong holes.. :) make sure the belcrank are on the correct
sides,part number R on left and L on right I think I remember.. double check.. I made another alignment tool for the other wing like the one provided..
this locked the stick centered once the tubes were done and made it easier
for me to complete the rigging..
anyway mine were pretty close to half of the
threads but it was not enough for me to feel good about it.. also too long
will not allow them to unscrew all the way.. I don't know (not flying yet)
how much adjustments these will need in the long term but I don't think
a lot, if the alieron is not in trail I am sure the little tube can do about all
the ajustment you will need to correct any problem.. this error on my part
(cutting the tubes too soon) cost me about 150.00 for new tubes / cones
and rivets.. wished I left them uncut till the rigging and then welded them together..


Danny..
 
Oh, I thought of another possible source of error, does anyone else have this on their airplane?

It looks like the bottom of the passenger stick isn't square (tilted clockwise), when the stick itself is held in a neutral position:

20061220_IMG_4094.JPG


This seems to lead to needing additional length on that aileron pushrod.

Also...I was wondering if adding additional nuts on the opposing bearing would allow you to get by with less bearing engagement.
 
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My dwg 15 has a revision dated 10/9/2000 which adds 1/4" to the pushrod length. Does this fix the problem or is the revised length still too short? I already cut mine. :mad:
 
Paul,
If you have access to a lathe, you could make new pushrod ends slightly longer. Addition nuts will not solve the problem. You need at least 1/2 of the thread in the pushrod.
 
ROD END BEARINGS

couldnt you used slightly longer rod end bearings? yes i know they are 28$ dollars apiece :eek: but all you got to do is buy em. :) the ones on the plans are 14 or so why not use 16s? as far as 1/2 inch isnt it the full dia +1 thread
 
Mel said:
Paul,
If you have access to a lathe, you could make new pushrod ends slightly longer. Addition nuts will not solve the problem. You need at least 1/2 of the thread in the pushrod.

I have a small lathe and I can't count the number of items I have made for my 9A, both "optional" stuff and assembly aids. Thankfully, the pushrod ends aren't some of them, but Mel has a very cost-effective idea here. Buy a few bucks worth of 6061 or steel as needed and find someone with a lathe--it shouldn't cost over $20 and you could add whatever length (within reason) that you need. You probably only need to do one end of each tube. Sure beats shipping costs! Just my thoughts, YMMV.

Bob Kelly
 
cytoxin said:
couldnt you used slightly longer rod end bearings? yes i know they are 28$ dollars apiece :eek: but all you got to do is buy em. :) the ones on the plans are 14 or so why not use 16s? as far as 1/2 inch isnt it the full dia +1 thread

A good idea, but after browsing the ACS, Aurora and Heim catalogs it seems that they don't make any longer versions of the sort specified. The specified 3414 and 3614 show up on the "special purpose" Heim page and only come in one length per set of other parameters (thickness and bore size)

I'm really enjoying all the unnecessary "critical dimensions" that start popping up at the end of the kit. No tolerance for variations at all, even though it would have been simple to build in a little room for slop...parts with razor-thin edge distance, pushrods that can have no less than half bearing penetration, but were designed for exactly half penetration rather than 3/4... Ok, rant done. :)
 
md36-16

this part number is the one i was speaking of the D in in the number means its drilled. (not for grease fitting) that would be the L one. i have two of these cause i was having rudder fitment issues and ended up not using them. they are identical to the vans ones.(and magnafluxed)

Paul Eastham said:
I'm really enjoying all the unnecessary "critical dimensions" that start popping up at the end of the kit. No tolerance for variations at all, even though it would have been simple to build in a little room for slop...parts with razor-thin edge distance, pushrods that can have no less than half bearing penetration, but were designed for exactly half penetration rather than 3/4... Ok, rant done. :)
ditto ditto ditto :rolleyes:


AURORA BEARINGS
MALE
GMM-M & GMB-M Series Male Rod Ends, General Aviation - Precision

Body - Carbon steel, protective coated for corrosion resistance.
Race - Carbon steel,protective coated for corrosion resistance.
Ball - Alloy steel, heat treated, hard chrome plated(AMS-QQ-C-320 Class 2).
Units are traceable and are magnetic particle inspected after assembly.
Grease Fitting available for all sizes. Units are supplied without grease fittings. When a grease fitting is required, specify by adding suffix "Z" or "F". Ex.: GMM-3MZ-470 = Zerk Type Fitting GMM-3MF-470 = Flush Type Fitting.
Load ratings apply only to rod ends without grease fittings. For load ratings with fittings, consult our engineering department.
For rod end bearings which are non-traceable or non-magnetic particle inspected, consult factory.
 
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Update

Update after staring at the problem some more:

The short aileron pushrod seems to be due to a bent stick weldment (see picture a few posts ago). If I set the bearings to the plans lengths, then attach to the copilot stick weldment, the bottom of the stick weldment becomes square to the center section, but the full length of the stick is off-square. Can anyone confirm that theirs is all linear rather than bent like mine?

I guess I'll be asking Van's for a new weldment if I've guessed the problem right.

As for the short elevator pushrod, I'm also wondering if the (center) weldment was built incorrectly. I have confirmed that the pushrod is the correct length, but with the bellcrank in neural position, one bearing is on the verge of falling out if I set the sticks to vertical (1/16" of the bearing engaged). The sticks are pretty much inline with the horn that attaches to the pushrod -- my theory is that that angle is incorrect.

Either that or my fuselage is an inch too long -- the first RV-9A-stretch edition! :) It would be a good theory too except that all the prepunched parts in the fuselage fit pretty well...

Thanks for the input so far--
Paul
 
hmmm

paul, i am having a hard time seing how there could be this much error. are all of the push rods set to the proper dimension? small ail tubes, brackets on correctly. my weldment looks much like yours and when i trial fit them they were all good. i feel something is wrong elsewhere but i could be wrong. i know youve probally done this but set the P/Rs (ALL)to the plan length then install if the stick is crooked then get another stick. good luck and merry christmas.
 
All bearings and pushrods are set to plans length. Originally I didn't think this was the problem either, but when there's a small error at the bottom of the stick, it gets amplified quite a bit due to the length of the stick.

Even so, the tilt of the stick in the neutral position is not that bad, maybe 5 degrees? You could fly with it, but the pasenger may wonder about your build quality :)
 
Paul-
It's hard to tell (from the pics) whether your weldment is a problem or not, but I will say from my research before I cut the pushrods (from many places all over the internet), the dimensions on the plans are significantly too short, especially for the stick-elev. bellcrank. (Perhaps I should say that the dimensions could be correct, but only if the stars are aligning that day for a given project. The margins for error are, at best, non-existent.) If I remember correctly I cut that one almost a quarter inch longer than the plans suggested (maybe 3/16") and my rod end bearings are just over half way engaged on the threads. I cut the bellcrank-elev. horn rod slightly longer (maybe 1/8") than plans as well and it worked out pretty good. If nothing else, calling Van's and complaining about your situation MIGHT lead them to explore whether or not to review/revise the drawings as necessary. 'Tis the season of miracles you know!
 
Maybe I'm thinking about this wrong but if it were really just the weldment, wouldn't that make one pushrod too long and one pushrod too short? Just an offset doesn't change the total length, it just changes the relative lengths.
 
jcoloccia said:
Maybe I'm thinking about this wrong but if it were really just the weldment, wouldn't that make one pushrod too long and one pushrod too short? Just an offset doesn't change the total length, it just changes the relative lengths.

This can be compensated for by the central pushrod which is also adjustable.

If I set all pushrods including the central pushrod to the specified lengths, both ailerons and bellcranks go to neutral, but that right stick is tilted.

If I want to get the right stick to neutral, I can elongate the right aileron pushrod and shorten the central pushrod. This is the case where I don't have enough bearing in the aileron pushrod, though.

I've sent off the question to Van's to see if that bent weldment is normal. I sent them these pictures...

All bearings in position per plans, bellcranks neutral:

img1.jpg


Sticks and bellcranks neutral, causing poor bearing position (note a lot of thread showing on the right):

img2.jpg
 
next time

well i tell you i wish i could have built this plane one time already.. i hear ya and i know that alot of things are an the ragged edge of minimums. i complain of this all the time. even though i feel mine are good to go what would a 1/4 inch more hurt other than weigh a gram or so. then i know if the jam nuts get loose it cant walk off.
ket us know the end fix please
 
Follow up ?

I've sent off the question to Van's to see if that bent weldment is normal.

Paul, I realise this was a while ago but what was the response from Van's ?

I am at the trial rigging stage and have started with the L wing......aileron to bellcrank pushrod length and pushrod bearings engaged just over half distance all OK BUT it appears bellcrank to P1 control stick lengths too short for sufficient bearing engagement - only approx 1/3 on each !

As was said elsewhere it appears pushrod lengths on plans are a starting point only.............luckily I haven't made my pushrods for the elevators yet but I will not be pleased if I need to order 2 new wing pushrods and bearings.
 
I did not really get a satisfactory reply. Ken Scott's first response was:

"""The stick base looks fine...just like our production drawings and
the one I have in my hand. The squashed bottom portion results in
a small angle between the base and the stick. We set the stick
portion vertical and take what we get on the base. In our
experience, the pushrod lengths shown work, and we almost never
hear anything different from the field."""

After I told them about how I couldn't get more than half engagement on all the pushrods, they said I needed to have more than half engagement, but didn't offer any more assistance. They didn't reply to subsequent requests for help.

In my case, I think three things finally got me "close enough":
1. Make sure sticks are in neutral pitch position when making these measurements. Elevator position does influence aileron position. Watch the ailerons when you move the elevator, on a rigged RV, if you don't believe me.
2. Tiny adjustments all the way out to each aileron, to take up the slack, so to speak. It took a lot of fiddling but finally I was able to get to a happy place, and the aileron bellcranks were still at neutral, at least within the precision afforded by the alignment tool.
3. If you are really close to halfway-engaged on any pushrod, you can consider putting an extra jamnut on both bearings so that the thing cannot rotate loose. But you want to have something very close to half engagement for strength reasons, too. Van's later told me that this was an ok practice.
 
Thanks for responding Paul,

It seems that this is a situation that has cropped up more than a couple of times.......I personally feel that for the pushrod lengths the plans should have a caveat to warn about slight variations for builders instead of giving an absolute value.
 
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