Sunday, August 10, 2008

Chugger's Spar

Chugger's spars are built-up C-sections. The shear web is 1/8” aviation grade birch plywood with the face-grain oriented vertically. The par caps or booms are made of hemlock or Douglas Fir. The drawing shows the profile of the spars. Please note that the 144” dimension shown in the drawing is not correct. I know some of you have been following Chugger's progress and that my recent medical problems have left you in the lurch. This posting should provide enough information for you to fabricate your shear webs and spar caps, which must be scarfed.

Scarfing is a standard woodworking procedure used in all wooden aircraft. Solid members, such as the spar caps are scarfed at about 15:1 whereas plywood uses 12:1. You'll note that this is a much flatter angle than is used by boat builders. Mark Langford's web site offers what has to be the best explanation of scarfing you'll find on the internet. Please go there... and see what he has to say before continuing here.

It will take two 48" x 48" sheets of plywood to provide the material needed for the two front and two rear spars. We must also provide for aileron spars, gussets and various doublers, meaning we'll be using a lot more 1/8" plywood. But these two sheets are all we'll need for the spars.

The drawing of the spar (below) is not complete. I've posted it to give you some idea of our goal. I am still working on the best method of transferring the load into the lift-strut and wing root fittings. I would like to use the bay adjacent to the wing root for the fuel tank but this too is still under development.

Your basic guide to aviation woodworking is AC-43.13, the manual showing acceptable methods for the repair of aircraft structures. You can buy a printed copy or download the manual from the FAA's library (see

Wood and fabric is covered by the first three chapters.

The Department of Agriculture, which 'owns' the U.S.Forest Service, got out of the aviation wood inspection business in the 1950's when someone noticed that, since airplanes were now built of metal rather than wood, at our present rate of usage the government had about a three hundred year supply of aviation-certified wood on hand, stacked in warehouses all over the country.

Nowadays, if you buy 'aviation-certified' wood, what you're getting is a promise from the seller that the wood appears to meet those government specs from days of yore. (And when it doesn't? Well... tough darts, Charlie. Maybe they replace it. Or maybe not.)

Another interesting slice of reality for the newbies is to read AC-43.13 (or any of several other references) and see that Sitka Spruce has no magical aeronautical properties. Indeed, there are several commonly available woods that are superior. The reason for the Spruce Myth is buried within the historical context of aviation... and of sailing ships. At the turn of the century and for forty years thereafter, masts, booms and spars were a common item at any lumberyard, not only near sea ports but at any city having water-borne transportation. When those pioneers of aviation needed wood they simply visited the nearest lumber yard. If it didn't happen to have something suitable in stock it was never more than a few days away, thanks to Railway Express.

You can find all of the wood you need to build an airplane inside the wood at your nearest lumber yard or box store. To get at it you will have to re-saw the wood that is there but this isn't as great a disadvantage as it might appear. Using Chugger as an example, the most critically needed pieces are the four main spar caps, about fourteen feet long, three-quarters of an inch thick by an inch and an eighth in height. While the three-quarter inch dimension remains fixed, all other pieces used in the airplane are shorter or shallower than the spar caps. If our Donor Timber was a two by ten joist, for example, ripping it into 3/4 x 1-1/2 laths gives us ten chances to find the perfect stick. And if we don't find it, we can cut out any imperfection and splice around it.