Monday, September 3, 2007

AV - Chugger's Rib

Labor Day, 2007.

Weather has been hot. A few miles north of us a mountain lion was seen drinking from a swimming pool and the coyotes are staying close to the few creeks that still have water. To hot to work in the shop, even with both fans going. Even the breezeway is pretty warm.

Too hot for welding and too sweaty for working on the fittings, I turned my attention to Chugger's wing. I have tentatively settled on the 4415 airfoil and needed some ribs for testing. I converted the 4415 coordinates into a rib drawing of the required chord, laid a few lines across it for alignment and printed it out. You'll find it in the Wing folder in the Chuggers Group, along with a pattern for the nose rib.

I didn't have a suitable piece of 3/4" plywood for the rib jig but did have some particle board that was wide enough. I don't like to use particle board for jigs because it's nothing more than thick paper and warps like a bitch but I glued a couple of stringers across the bottom and after the glue had cured, soaked it good with dilute varnish. That was a couple of days ago.

The several sheets that made up the pattern were trimmed along one edge using a straight-edge and razor. The jig board was given a coat of un-thinned varnish and each page of the pattern was painted with varnish on its back-side. The pages were then stuck to the varnished jig-board and aligned. Bubbles were chased to the edge of the sheet with my thumb and the whole thing was left to dry. But if you've never used this method, don't. It happens to be a quick & dirty method but varnish isn't a very good adhesive when applied to typing paper. It will hold the paper in position long enough to install the bits & pieces that will hold the rib's sticks in place. In doing so it will also fasten the pattern to the jig board. The whole thing will then get a coat of Deft Satin Finish Wax, which I understand is no longer available in the USA due to the tree-huggers. (The advantage of a wax finish is that nothing sticks to it.)

At this stage the thing looks like hell but it should work okay and only took a few minutes, if you don't count the clean-up :-)

I'm going to try using an Ison-type wing with wooden drag/anti-drag struts instead of wires or rods. The red hatch-mark is where the diagonal struts will pass through the rib. I'm also going to try building the ailerons in situ following the lead of Leonard Mulholland. I've not yet decided how to do the leading edge. I'd like to use 1/16" (1.5mm) birch ply but it is fairly expensive. Unfortunately the less expensive foam & fiberglas alternative, of which I've already built several samples, is about 3X heavier than the plywood.

I'm still tinkering with the leading edge but it looks as if I'm going to have to bite the bullet and run up to Corona (ie, Aircraft Spruce) for a couple of sheets of 1/16" ply.

After posting an article about building stick ribs in which I used 1/8" doorskin gussets attached with 1/4" aircraft nails and Weldwood 'Plastic Resin' glue I got several messages from people who found it impossible to use such small nails, having found they couldn't hold them with their fingers. The secret is to not hold them at all but to use the magnetic end of your tack hammer to pick them up and drive them into place. Unfortunately, that takes a bit of practice and since most of you are first-time builders I'll try using staples and/or pneumatically-driven 23 ga. wire brads.

The wing span will be a tad more than 28 feet, dictated by the available work-space (ie, about 15'). Chord is 56" so the wing's area will be approximately 125 square feet for a gross weight of 850 lbs, giving a 1-g loading of about 7 lbs per square foot. At 3.3-g that's about 22 lbs. With a rib spacing of 12" that's about 100 lbs per rib. Given the lift distribution of the NACA 4415 at its maximum angle of attack that means the portion of the rib between the spars will see about 80 lbs, the trailing edge will see almost no load at all and the remainder will be concentrated near the leading edge. One reason for cobbling-up a rib jig at this stage is that I want make and then break a few ribs to ensure they'll be strong enough.

According to the classic design formulas as published by Raoul J. Hoffmann (and others) in the 1930's, the aileron should be about 40% of the semi-span in length and 20% of the chord in width. As with the leading edge structure, this is another area I'm still tinkering with. If everything works out I'll post the required patterns in the Chuggers file archive.


PS -- Be sure to read Chugger's Progress - III posted on 4 July 2007. This post (ie, Chugger's Rib) produced a couple of comments that made it pretty clear their authors were not aware of what has gone before.