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a.s.elliott(at)cox.net
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 Posted: Wed Jan 28, 2009 8:23 am Post subject: Aileron push rods - an engineering comment |
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Folks:
Let me preface this comment with an anti-disclaimer: I *am* an aerospace engineer and I *do* do analysis for a living.
OK, I applaud Bill Phillips for his innovative idea and professional execution. This is the core value of experimental aviation. But I want to raise a design issue I think is very important: When you change part of a system, you have to consider whether you are going against the designer's intent in other related parts.
The existing aileron control system is based on having tension loads in all the cables. This pulls the aluminum aileron bellcranks firmly against the steel hinge pins, creating a nice bearing surface that is easily and properly lubricated with a drop of oil every 50 or 100 hours.
Further, the balanced tension loads plus the cable weight mean that the lower "thrust" bearing surface between the bellcrank bearing blocks and the aluminum bracket is evenly and very lightly loaded. Once again, with regular simple lubrication, this should last a long time.
And with both bearing surfaces, light wear will not significantly effect the operation
On the other hand, the pushrod is only attached to one side of the bellcrank. This is not good for the bellcrank bearing, which is now both *not* loaded radially *and* has to support a steady bending moment (torque) about the span-wise axis. Both *NOT* conducive to long life and not fixed by lubrication. Furthermore, the lower aluminum-aluminum thrust bearing surface is also now loaded unevenly and will wear steadily.
Even more, every bit of wear in this system makes the situation progressively worse. This is not good design practice.
Of course, lots of aircraft use pushrods instead of cables, BUT they also have bearings designed for those kinds of loads. So my "best" recommendation would be to replace the simple bellcrank bearings with more appropriate ones designed for this kind of loading, if you do the pushrod modification.
My "alternate" recommendation would be to carefully inspect the bellcrank bearings on a regular basis and as soon as you notice any significant vertical free play in the pushrod end, replace the bellcrank, brackets and bearing *using the more appropriate bearing*. I would guess that every 25 hours would be a good interval and any more than +/- 1mm vertical play would be enough to consider the maintenance action.
FWIW,
Andy Elliott, Mesa, AZ
N601GE,601XL/TD,Corvair
[quote][b]
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Gig Giacona
Joined: 10 Jan 2006
Posts: 1416
Location: El Dorado Arkansas USA
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| Posted: Wed Jan 28, 2009 8:48 am Post subject: Re: Aileron push rods - an engineering comment |
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Excellent post Andy. Would the issues you raise be alleviated or reduced if you left or installed the balance cable as originally designed?
In other words just replace the rear cable with push rods and not the front.
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_________________
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR |
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japhillipsga(at)aol.com
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| Posted: Wed Jan 28, 2009 9:01 am Post subject: Aileron push rods - an engineering comment |
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Andy, thanks for the info. Sure wish you had spoke up a couple months ago when I broached this subject several times on the list. I did have several experienced APs examine the bellcranks as the bearing surfaces were a concern to me, but they assured me the amount on face material on th XL was far greater then the face of several certified aircraft they presently owned which reassured me greatly. Sure wish you had been there as an aero space engineer doing analysis. Say, how's that Corvair car engine doing for you? Crankshaft holding up okay? Best regards, Bill
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rsteele(at)rjsit.com
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| Posted: Wed Jan 28, 2009 2:38 pm Post subject: Aileron push rods - an engineering comment |
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Great comment. I was also thinking that the rivets that attach the bell
crank supports to the rib could also be put into tension. This could not
happen with a balanced load using cables. Is this a potential problem
also?
Ron
Engineering school was long ago and far away...
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