a.s.elliott(at)cox.net
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 Posted: Thu Jan 29, 2009 6:39 am Post subject: Aileron push rods - replies to questions |
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(long post again) Gang:
First, let me say that the amount of additional wear due to the pushrod change is *not* quantified. That's why the second alternative, regular inspection, exists. There are many possible scenarios.
[1] Bill may find that, after regular checking and lubricating every 25 hours for 100 or 200 hours, the wear is acceptable for making the bearing a "replace every 250 hours"-type part. At this point it would be reasonable to present the "improvement" to the community to let other people experiment if they wish. For many people, that would be 5 years of flying and would be a perfectly reasonable solution. If continued testing shows acceptable wear, perhaps the TBO can later be increased. We've all seen this happen.
[2] Bill may find, after the same checking, that there is no discernable wear at all. Then the experiment is a success! He continues checking and lubricating, but has little concern. At this point it would be reasonable to present the "improvement" to the community as a "tested and works" design change with the maintenance instructions included.
[3] Bill may find, after the 2nd or 3rd or n-th check, that the wear is already too large. Then the experiment is an educational failure. He replaces all the parts and puts back the cables, and advises the community that it didn't work. This doesn't mean it *couldn't* work, only that this particular implementation failed.
[4] Bill might choose, instead, after the failed check, to replace the bearings with a more appropriate style, make the necessary changes to rest of the parts, and start the testing process all over again...
A couple of more notes:
Other people making a similar modification might have different results. This could depend on your environment, your flying style and your fabricating/installing skills. It might be different if you have flex hinges or piano hinges. I would hope that all such results would be reported.
There is always a time & cost trade-off to be made between design attributes, up-front analysis and prototype testing. The key, IMHO, in a life-critical field like aircraft design, is to not be premature in publicizing and promoting ideas to others, not to jump to conclusions based on minimal testing, and to *always* be quick and honest about publicizing any problems.
Gig also asked if leaving in the balance cable would reduce concerns. I'm not sure what is gained then by doing the mod. You still have all the maintenance concerns of a cable and are really just treating the pushrod as a "stiffer" cable. Of course, you could then use a *much* lighter rod, but the problem with the ailerons has never been slop, it's always been feel, IMHO.
FWIW,
Andy Elliott, Mesa, AZ
N601GE,601XL/TD,Corvair
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