Their policy on permissible trim tab "slop" percentages is interesting and should be sobering to builders.
Maybe we can find some more specific info on a drilled hole near the edge of a curved form on a critical control surface component, that is absent of any protective redundancy....
SheepdogRD wrote:Please understand, Dave, I wasn't taking a shot at you with my comment; I was pointing out that the RES guidelines are for punching, not drilling. And I was reiterating advice that's been given to me by several aircraft builders whose approach I respect: "Check AC 43.13; it's the bible." This is my first aircraft project, and I learn a lot every time I open the "bible".Dave Krall CFII SEL SES wrote:Do you have the chapter and verse in AC 43.13 for trim tab bellcrank hole placement and drilling?
So I looked in the bible, and I find AC 43.13 is oriented to inspection and repair, not design. Initial design is up to the manufacturer. AC 43.13 says, in 4-60a(2), when repairing aircraft after an accident or in the course of a major overhaul, to "inspect all highly-stressed main fittings, as set forth in the manufacturer's instruction manual." So, even for repairs, it returns the repairman to manufacturer specs.
In fact, I was surprised to read last night that welding over a worn bolt hole isn't approved. In 4-60a(4) it says "Elongated or worn bolt holes in fittings, which were designed without bushings, are not to be reamed oversize. Replace such fittings, unless the method of repair is approved by the FAA. Do not fill holes with welding rod." Still, I don't think that comment was aimed at simple tab holes; instead, it's focused on "elongated or worn bolt holes in landing gear, stabilizer, interplane or cabane-strut ends".
After thinking about it, I suspect they're concerned about embrittlement, because brittle metal can crack and then fail suddenly, rather than showing wear, which is easier to spot in a preflight or condition inspection.
For something like the tab, where we need to redesign because the hole is simply too big and/or too far from the edge to fit the non-standard clevis, we become aircraft manufacturers. We have to do what we think will work.
Over the years, what works has been codified in engineering guidelines. For instance, boiler design guidelines kept getting tighter and more specific until boilers stopped blowing up, but there were a lot of boiler explosions until they figured out what worked.
While AC 43.13 may be the bible, it seems to be mute here, so it appears that I've referred you to a dead end. No chapter, no verse. But Steve said the magic words:Right now, if I can't find a clevis that works better, my plan is to trim the front of the tab a bit so it fits the RAC clevis, and see if I can find -- or machine -- a bushing so the RAC pin fits the hole. Ideally, the tab would be L-shaped, so the end could be rounded. We're the designers, now, so we can do what we think will work.stede52 wrote:I'm just telling you what will work for that thickness of steel, you can do whatever you wish.