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09-27-2013, 01:51 PM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Quote:
Originally Posted by DanH
All true. However, Mark is sure his bearings were not misaligned, and reports no binding or odd feel. Right now I'm inclined to discount misalignment as a cause.
Bill, perhaps the cycles are vibratory, not aerodynamic. Explanation as we go along.
But first, here's a sketch with what I think is the bending mode when cycled:
In this illustration the tip of the elevator has been forced upward in relation to the fixed stabilizer pivot. The resulting load is as shown. The lower rivet is in tension. The spar material below the rivet is unsupported.
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Dan I really like and appreciate your sketches. They're always very helpful for all of us to visualize otherwise complex concepts. In this particular case, though, I think the load path may be a little more complicated.
What is not shown in this sketch is what the actual load path "should" be. The jam nut should be transferring all of the rotational torque load into the spar itself, and then into the skin throug all the spar/skin rivets. In reality, some portion of load will always be picked up by the nutplate into the doubler and only then into the spar - really an indeterminate load path. The idea, I think, is for most of the load to be transferred directly to the spar by sandiching it between the nutplate/doubler on the aft side, and the jam nut on the forward side.
Someone earlier mentioned the idea of a star lockwasher. Another poster thought, in my opinion correctly, that a lockwasher directly on the spar would dig in and possibly exacerbate a cracking issue. I find that I agree with both of these posters. What strikes me as a rather good idea, though, is what Bob suggested: using a washer against the forward side spar, a lockwasher on top of that, and finally the jam nut. Other than adding a little weight and possibly decreasing the flutter frequency, a washer and lockwasher would spread the load over a slightly greater area into the spar.
Of course with literally thousands of aircraft flying per the original design, due diligence and inspections (definitely a torque stripe witness mark) are the easiest, cheapest, and likely best way to prevent problems with the control surfaces.
__________________
Kurt W.
RV9A
FLYING!!!
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09-27-2013, 01:58 PM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by Bob Kuykendall
...Actually, and I know I'm not alone in this, what I think would be best would be to ditch the threaded rod ends altogether and go to a machined part with a sizable footing attached to the spar at multiple points so that it does not rely so much on clamping forces. But that cuts close to the kind of idle speculation that we're trying to avoid...
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No, you are most certainly not alone. I'm toying with doing a machined ball bearing hinge fitting as a replacement for the rod ends. I am not a fan of the stock Vans setup, but I fully understand why it was done.
And in the end, while the stock setup is far from an example of engineering elegance, it is obviously "good enough" if assembled and maintained properly.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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09-27-2013, 02:37 PM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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Quote:
Originally Posted by DanH
All true. However, Mark is sure his bearings were not misaligned, and reports no binding or odd feel. Right now I'm inclined to discount misalignment as a cause.
Bill, perhaps the cycles are vibratory, not aerodynamic. Explanation as we go along.
But first, here's a sketch with what I think is the bending mode when cycled:
In this illustration the tip of the elevator has been forced upward in relation to the fixed stabilizer pivot. The resulting load is as shown. The lower rivet is in tension. The spar material below the rivet is unsupported.
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Dan, again, a picture is worth a thousand words, thanks! Since the stiffener is not cracked, your view seems the most likely loading case, and it also indicates that the stiffener is NOT more stiff than the spar with channeled and skins attached. If it was more stiff (channel shape) the one lonely rivet would not be carrying so much load. I am assuming it is to scale. I was loosely referring to a (yet unidentified) cyclic load overlaid on a static load, thus it failed only on one side where it is biased in favor of your sketch. (Imagine a sine wave where the "zero load" line is not centered between the peaks) It remains unknown why this bird has the failure when so many others don't and have more hours on them. Reference - 3800TT on N477RV flown by Mike Seager and many others, including 6's. This point, from a design perspective, would not indicate a fundamental design deficiency. But - it ain't over until it's over.
There has been a lot of discussion about the jamb nut, but it was tight and seems to have no relation to this failure mode.
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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09-27-2013, 03:57 PM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Quote:
Originally Posted by DanH
All true. However, Mark is sure his bearings were not misaligned, and reports no binding or odd feel. Right now I'm inclined to discount misalignment as a cause.
Bill, perhaps the cycles are vibratory, not aerodynamic. Explanation as we go along.
But first, here's a sketch with what I think is the bending mode when cycled:
In this illustration the tip of the elevator has been forced upward in relation to the fixed stabilizer pivot. The resulting load is as shown. The lower rivet is in tension. The spar material below the rivet is unsupported.
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Great picture Dan, but I would suggest another theory.
If you regard the hinge pint as fixed, and put a large over travel on the elevator so that the stop gets hit hard -- the elevator spar will become twisted since the stop is at the fuselage and the tip is unrestrained.
A twist of the spar would put the stress at the circle as drawn.
Perhaps these two cracks come from the control surface banging very hard to it's stops, and perhaps only a few times, rather than an ongoing event?
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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09-27-2013, 06:40 PM
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Join Date: Oct 2007
Location: Macon, GA
Posts: 499
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I can count on zero fingers the number of lock washers that I've seen on (structure on) big airplanes.
The reason is that they don't work, and can actually cause clamp load to decrease over time, by digging in. if you think about the mechanics of a lock washer, it's only different from a regular washer when the nut is REALLY loose already.
My first impression is that the rodends are fine. IF (big if) there is an issue, it's with the fact that the rodend is really just bearing on the doubler and the jamnut.
Lockheed/Boeing/Gulfstream would (if they were to use the rod end) machine the spar from bar and leave a thick spot where this joint is. They'd probably use a real nut on the back side, for much higher bearing area. I think on the RV, there might be reason to have a doubler on both sides of the spar, especially one that doubles the Web and caps.... IF there's a fundamental issue, and I'm not sure there is.
The roller bearing bracket idea has merit as well. This would let you make a bracket with say 4 3/16 hikes so that you can have 4 AN3'S metal lock nuts, and generate some nice clamp load over a nice large area.
Quote:
Originally Posted by krw5927
Dan I really like and appreciate your sketches. They're always very helpful for all of us to visualize otherwise complex concepts. In this particular case, though, I think the load path may be a little more complicated.
What is not shown in this sketch is what the actual load path "should" be. The jam nut should be transferring all of the rotational torque load into the spar itself, and then into the skin throug all the spar/skin rivets. In reality, some portion of load will always be picked up by the nutplate into the doubler and only then into the spar - really an indeterminate load path. The idea, I think, is for most of the load to be transferred directly to the spar by sandiching it between the nutplate/doubler on the aft side, and the jam nut on the forward side.
Someone earlier mentioned the idea of a star lockwasher. Another poster thought, in my opinion correctly, that a lockwasher directly on the spar would dig in and possibly exacerbate a cracking issue. I find that I agree with both of these posters. What strikes me as a rather good idea, though, is what Bob suggested: using a washer against the forward side spar, a lockwasher on top of that, and finally the jam nut. Other than adding a little weight and possibly decreasing the flutter frequency, a washer and lockwasher would spread the load over a slightly greater area into the spar.
Of course with literally thousands of aircraft flying per the original design, due diligence and inspections (definitely a torque stripe witness mark) are the easiest, cheapest, and likely best way to prevent problems with the control surfaces.
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__________________
Macon, GA (KMCN)
RV-7, Niner Fife Victor
Last edited by gtmule : 09-27-2013 at 06:49 PM.
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09-27-2013, 06:48 PM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Quote:
Originally Posted by gtmule
I can count on zero fingers the number of lock washers that I've seen on (structure on) big airplanes.
The reason is that they don't work, and can actually cause clamp load to decrease over time, by digging in. if you think about the mechanics of a lock washer, it's only different from a regular washer when the nut is REALLY loose already.
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Don't tell your Lycoming engine or any GA type-certified aircraft manufacturer that lockwashers don't work. They're all over.
I also work in the big aircraft world. You're right, few lockwashers. If our RVs all had two or more turbine engines I agree that we wouldn't be having a lockwasher conversation.
Another thing I see few of in big aircraft is AD material rivets. Only to attach nutplates. Certainly you wouldn't suggest we get rid of those too?
As for "really loose", we've heard multiple reports just within this thread that the jam nuts on several aircraft were just that: really loose. Would lockwashers hurt? I don't see how.
__________________
Kurt W.
RV9A
FLYING!!!
Last edited by krw5927 : 09-27-2013 at 06:54 PM.
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09-27-2013, 08:31 PM
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Join Date: Oct 2007
Location: Macon, GA
Posts: 499
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Quote:
Originally Posted by krw5927
Don't tell your Lycoming engine or any GA type-certified aircraft manufacturer that lockwashers don't work. They're all over.
I also work in the big aircraft world. You're right, few lockwashers. If our RVs all had two or more turbine engines I agree that we wouldn't be having a lockwasher conversation.
Another thing I see few of in big aircraft is AD material rivets. Only to attach nutplates. Certainly you wouldn't suggest we get rid of those too?
As for "really loose", we've heard multiple reports just within this thread that the jam nuts on several aircraft were just that: really loose. Would lockwashers hurt? I don't see how.
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Might not hurt, I just don't think they help. They do show up on engines, especially on accessories, less so on major parts. Might be the engineers designing for maintainers. Most Mx guys will properly torque rod bolts to spec, but not necessarily vacuum pump nuts.
As far as rivets, the lower wing skin is like 1/2" 7075-T6 at the centerline of a c-130, it takes a lot of 1/8" AD rivets to splice that. Taper-loks would be great on RVs, but you'd spend more on those $250 reamers than you'd spend on all your other tools 
__________________
Macon, GA (KMCN)
RV-7, Niner Fife Victor
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09-27-2013, 09:44 PM
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Join Date: Jul 2009
Location: Lee's Summit, MO
Posts: 747
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Elevator pushrod bolt
I don't currently have a dog in this fight as I'm a long way from flying, but.....
several here have reported loose nuts on the elevator pushrod bolt and mentioned adding the second bolt. While the second bolt may be a good idea, it doesn't address the loose Jesus nut. If the pushrod bolt comes out, your second bolt isn't going to help.
There have been discussions in the past regarding the nylon lock nuts. My testing indicates these nuts only provide 1 to 2 inch pounds of friction. Should there be any conversation about improved captivation of these nuts?
__________________
Robert Williams
Lee's Summit, MO
RV-8 - Empennage & Wings Done
Working on Fuse
O-360-A1A
1946 Cessna 120
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09-27-2013, 09:59 PM
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Join Date: Aug 2007
Location: Wichita Falls, TX
Posts: 2,182
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Quote:
Originally Posted by DaAV8R
Should there be any conversation about improved captivation of these nuts?
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What about a Nord-Lock washer between the nut and spar? Nord-Locks seems to be plenty good enough of a lockwasher for the prop bolts on ground adjustable props.
__________________
Neal Howard
Airplaneless once again...
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09-27-2013, 10:33 PM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Quote:
Originally Posted by Neal@F14
What about a Nord-Lock washer between the nut and spar? Nord-Locks seems to be plenty good enough of a lockwasher for the prop bolts on ground adjustable props.
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This is a "belts and braces" nut that is an elastic stop nut and is also a castle nut ---
http://www.aircraftspruce.com/catalo...es/ms17825.php
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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