A Simple Wood Testing Rig By Don Cookman any of us are looking for ways of saving materials M costs in our planes, and my own searching has led me into making a careful study of commercial lumber types as a possible substitute for aircraft specification material for certain parts. This business of using substitute materials, of course, has to be approached with caution and judgment. Most lumber yards carry high grade Douglas fir porch flooring, a strong, light, hard wood. It's a rather quesBEND STRESS

tionable substitute for spruce in wing spars as it is apt to contain hidden pitch pockets. However, when ripped down into pieces suitable for wing ribs, practically all such faults show up either from inspection or when bending to shape. Thus, I felt it was a possibility, for ribs and perhaps other parts. I made up a simple testing rig, which I used to see if the wood I bought locally might really measure up to published strength figures. Happily, I found that my stock checked out right on the nose as compared to data book figures. The accompanying sketch shows how I made my test set-up, and it is self explanatory. As an example of how the figuring is done, suppose we wish to test a piece of Douglas fir cut right out of a section of porch flooring. The sample is % in. thick by 1 in. wide by 5 ft. long, which allows for clamping length plus a 36 in. arm for testing: Y

1) Bending stress=M x—?—where M=weight x arm

I = b x - 12 a h -for a rectangle or 2 -for a square .5 Y Bending stress = M x = M x .062 - = M x 8.064 2) Transpose formula to solve for M: Bend stress 8000 (from chart) = 992 M= 8.064 8.064

Bend Stress #/sq. in. Sitka Spruce . . . . . . . . . . . . . . . .6,200 Douglas Fir . . . . . . . . . . . . . . . . .8,000 African Mahogany . . . . . . . . . . . 7 , 9 0 0

3) To determine W on a 36" arm: M 992 W = Arm ~ 36 - = 27.5 Ibs. Conclusion: To test a strip of Douglas fir of the dimensions given, suspend a weight of 27.5 Ibs. on the end of a 36 in. arm. If the wood is up to strength, it should not break.

TIPS One light airplane factory uses a trick you wouldn't believe if you had not seen it done. They strike longerons with a rubber mallet to bow them out about a quarter of an inch. When dope tightens the fabric, it pulls them in so they are straight rather than bowed in between cluster joints. On fuselages where the longerons are quite thin and long between joints, put fabric on with less than normal tension to prevent dope from pulling it too tight for the good of the longerons. If too much heat is applied to Ceconite it will shrink even more when doped and can even make the structure collapse. & '•'• * * * It is considered good aircraft practice to drill holes slightly undersize in vital fittings and then ream the holes to true and accurate final size. Due to shifting of the work, bending of the drill, variation in grind, increasing dullness with use, etc., twist drills cannot be relied on to make consistently accurate holes. M-M-A, Inc., Lancaster, Pa., makes tap guides sold under the trade name 3-I-Q which hold hand-turned taps at exact right angles to the work and insure true, uniform tapping. When

taps go in crooked, they bite a lot of metal from one side of the hole and too little from the other side, giving unreliable threads and causing tap breakage. # •'.• & -'.- -iDon't use a scriber to mark steel tubing for cutting; scratches that deep are sure to be starting places for cracks. Get a silver colored pencil of the type used to mark blueprints. It marks steel tubing well, even when oily or greasy, and can be seen even when the metal is heated for tack welding. •;••

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To make the cutting of steel tubing faster and neater try one of the chromeplated tubing cut-off wheels for table saws, available from Sears and power tool dealers. An ordinary plumber's tubing cutter works well, too. All steel tubing fits should have gaps not over 1/16 in. Slight looseness at the ends of tubes is used to allow for heat expansion and avoid a weld at one end pushing things out of alignment at the far end. But too-large gaps lead to excess use of rod for filling, with weak joints and danger of burning through tube walls. SPORT AVIATION

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