Sunday, August 19, 2007

AV - Chugger's Progress - IV

A recent thread ('REAMING') on the Usenet Newsgroup devoted to homebuilt airplanes brought to light the fact that a basic tenet of building with wood was so misunderstood as to make the job of attaching fittings far more difficult and time consuming than it needs to be. The basic tenet is this: When a fastener penetrates a wooden member for the purpose of transferring the primary load, the hole for the fastener ALWAYS begins as a loose, over-size fit.

There's two reasons for this rule, the first being the fact wood isn't very strong, the second that 12% to 15% of any piece of wood consists of water.

For example, in the first case let's suppose you're attaching a wing-lift fitting to a spruce spar. Sitka Spruce is only good for a few hundred pounds in tension. If we were using quarter-inch bolts to secure the fitting we might very well drill a 3/4" hole instead of the expected 1/4". Why? Because that would allow us to install an aluminum or hardwood bushing into the spar, increasing the tensile load-limit per fastener by approximately 8x for hardwood and more than 20x if we use aluminum. The fit of the bushing in the spar doesn't have to be especially precise since it will be installed using a gap-filling epoxy such as JB Weld for the lo-buck homebuilder or 3M's 'Scotch Weld' structural adhesive for the Rich Folk. (The aluminum bushing would of course be reamed to matched the fastener.)

In the second case, since the rule is to never allow a fastener to come into direct contact with wood, we would start with a hole at least 1/64th over-size and butter a suitable sealant into the hole. Additional sealant is applied to the fastener which is then installed. If the fitting requires periodic dismantling (and all primary load carriers do, to facilitate inspection) then the bolt would be treated with wax or other release agent before being coated with sealant.

The sealant would typically be an epoxy or resorcinol glue -- something 100% waterproof (*). The engineering behind this procedure is based on the fact that all modern-day adhesives are stronger than wood.

Back in the Good Ol' Days, whenever that was, the typical sealant was varnish and frankly, it didn't do a very good job. A hole is mostly end-grain and to ensure it was adequately sealed you'd often have to flood the hole with varnish for an hour or more, allow it to cure then re-drill the hole. Not many bothered to do so. Instead, they'd hook a patch on a piece of safety wire, saturate the patch with varnish and pump it back and forth through the hole a few times. This was a virtual guarantee that the fastener would corrode down inside the hole and become almost impossible to remove.

Often times a cursory examination of an airframe or set of plans reveals fittings and fasteners that appear to violate sound engineering practice. In such cases it's always wise to take a closer look. A massive landing gear fitting that appears to use nothing more than a couple of AN3's in tension usually proves to bear the landing loads in compression, the AN3's serving only to hold the fitting in position and not subjected to any portion of the landing load. I mention this because these are areas where home-builders tend to improve on the design by replacing the perfectly adequate #10 fasteners with quarter-inch or even 5/16".

Finally, I've included a couple of url's that will be of benefit to anyone thinking of duplicating the 'Chugger.'

* - 'Waterproof' as defined by Forest Products Laboratory testing procedures.