Testing Of Wood For Strength... By Jerry Norton, EAA 32855 780 Marble Broomfield, Colo.
T)AST ARTICLES on non-certified wood may be ex-l tremely timely and important. As the cost of certified wood continues to rise, the savings that can be gained by the use of commercial-grade wood increase and cause more persons to consider the use of commercial-grade wood in spite of the risks involved. The only problem that really exists is: what is the actual strength of this piece of wood, certified or uncertified? We can inspect for grain structure, rot, compression failures, etc., but still haven't found out the strength of a particular piece of wood. ANC-18 uses an average value instead of a minimum value, and doesn't include some of the commercial varieties available today. A simple inexpensive method is to actually test the wood ourselves. The test equipment is quite simple and probably everyone already has the necessary equipment. A jig, built from scrap 2x4 lumber, and a pair of bathroom scales are all that is needed. The mathematics of derivation of the jig dimensions are given so that anyone can build a different size jig if
desired. Consider a beam:
Extreme fiber stress is given by: F = Me where c = h
~r
T
Combining all equations: F = 3WL 2bh2 To make the scale easy to read we will assume that when the scale reads 100 Ibs. we have 10,000 psi fiber
stress developed in the wood. The equation then becomes100 =
3L 2bh2 We will now let b = h (it is easier to saw a square piece of lumber, as we only have to set the saw once). We will also let L = 14 in. (I happened to pick 14 in. as I figured this out backwards first). The equation now becomes: h = 3 3 x 14 = 3 0.21 2 x 100 Solving for h we get: 0.5943 or 19/32 in.
W/2
W/2
The largest error comes in sawing the lumber. If it is made oversize we will get too high readings, and if we make it undersize the readings will be low or on the conservative side. The jig is made as follows:
2x4
L/2W The moment about the center is given by the following equation: M = WL
The moment of inertia of the beam is given by: I = bh3 12 With the following dimensions: PAGE 20
NOVEMBER 1969
TOP PIECE
Fly Baby Added To Museum Collection
SLIGHTLY ROUNDED TO
APPROXIMATE WOOD CURVATURE
ANVIL
To use, place the anvil on the scales, place the sam-
ple on the anvil, and press the top piece on top of the sample, and observe the reading on the scales until the piece breaks. The reading in pounds multiplied by 100 equals the modulus of rupture in psi. For example, quarter-sawn Douglas fir should test between 115 to 130 IDS., while flat-sawn Douglas fir will test between 105 to 125 ibs. For a very light person, the 14 in. length can be extended to 28 in. and multiply by 200 instead of 100. -..PRESS KITH /HMDS HERE
Since the Bowers "Fly Baby" was first prc^iiiuJ to the world of amateur-built aircraft, it caught on among builders for its unity of simplicity, low cost, safe characteristics and folding-wing features. Several dozen have already been completed. Thus it is right that one of these airplanes take its place in the EAA Air Education Museum to preserve its history for years to come. The "Fly Baby", designed by Peter M. Bowers of Seattle, Wash., won first place in the 1962 EAA Design Contest
for an "easy to build and fly" airplane. The "Fly Baby" pictured here, N-1340, saw its builder, Robert J. Junker of Northbrook, 111. pass away before the first flight could be made. It was the wish of his family that the airplane be preserved, and it was donated accordingly to the Museum. ®
How Crowded Is the Sky? By Edward D. Williams Aviation Reporter THE MILWAUKEE JOURNAL SAMPLE UNDER TEST
I have used this set-up to check out many different kinds of wood and they are in agreement with the table in ANC-18. It will be noticed that samples from the same board will give different readings but, unless there is a large discrepancy, the explanation is probable grain structure. I use this jig to check a few samples from every board I buy, and I have found several boards that broke at low values. Upon real careful examination, I was able to discern the fiber breakage talked about in the article on compression failures. Without the jig I wouldn't have caught
the bad wood and might have had a failure after the plane was built.
I would seriously suggest that anyone using commercial grades of wood build this tester and use it on every board. The loss of a small strip 19/32 in. wide is small
compared to the difference in the prices of certified and
non-certified wood. It could also mean the difference between an accident report and a safe airplane. ®
J
UST AS WATER comes to a point in a funnel, so does air traffic congregate at certain points — the airline airports. Because some of the points are more popular than others — such as the airports in Chicago, New York and Washington — a maddening air-congestion problem develops there. Unfortunately, this has led to the mistaken idea that the rest of the sky is black with airplanes, leading to the inaccurate phrase "the crowded sky" currently in vogue. The situation has been put into perspective by the general-aviation industry with this example: If all the 126,000 private planes in the country were put into the air at the same time and collected over Arizona, there
would be a full square mile of separation between each
aircraft and the skies over the other 49 states — more
than 96 percent of the land area — would be completely empty. The real problem, as any experienced airline traveler knows, is how to get the nation's 2,000 airliners in and out of the high-density airports which do not have adequate facilities. — Reprinted from "The Milwaukee Journal", July 28, 1969 SPORT AVIATION
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