Radio Technique
George M. Myers
More about the safety question
A while back I commented on the way our models are getting bigger and faster, without visible evidence of any stress analysis having been done or pre-flight static strength testing. I asked whether you, my readers, agreed that Model Aviation (MA) should present articles on those subjects. So far I have received four well-written letters emphatically in favor of the idea, and none opposed.
Now, let me tell you a true story. Some time ago I undertook to build a sailplane as a review for a magazine (not MA). The wing had plug-in tip panels, each supported by a 1/4‑in.-dia. dowel at the dihedral joint. I looked at the plan and said, “Too weak.” On the assumption that no competent designer would put money into creating a kit leaving such an obvious weakness, I decided to perform some elementary stress analysis.
The sailplane was designed for use with a high-start launcher. Experience (and a fish scale) told me to expect a 12‑pound pull on the towline at launch (with the rubber at maximum stretch). With a good wind and careful control, it is possible to get the sailplane almost vertically over the high-start stake, keeping the rubber fully extended. Therefore it is easy to see that the wing must be able to support a 12‑pound pull, plus the weight of the rubber, towline, and parachute hanging from the tow hook. I figure about 20 pounds; you might want to add another 20% to account for wind gusts.
If the wing supported the weight in the usual way, the 20 pounds would be distributed over the projected span “elliptically”—that is, there would be zero load at the tip, increasing rapidly to an almost uniform loading as your point of examination moves toward the center of the wing.
As a first approximation, assume the wing is uniformly loaded and calculate the weight supported by the removable tip as: (projected area of one tip) / (projected area of whole wing) × 20 pounds. Allowing for the elliptical taper of the air load, assume that the whole tip force is centered one-third of the way from the dihedral break to the tip.
You can either analyze the dowel using data from an engineering handbook (which I did), or you can break some dowels (which I also did). I clamped a brass tube over the dowel, slipped another brass tube over the projecting part of the dowel, and hung a weight of the proper size at the proper distance (one-third of the projected span from the dihedral break to the tip). The analysis said the dowel would break — it broke.
I completed the plane, took it out for its first test flights, and flew it with brass tubes substituting for the dowels. It flew. Then I removed the brass tubes and inserted the dowels, to give the design an honest test. One tip tore off immediately after the launch, as expected. So I wrote letters to both the kit manufacturer and the magazine, giving both the engineering analysis and the test results. (I am a professional aerospace vehicle test engineer, and have been for 35 years.)
I gave the manufacturer a chance to explain where my review was in error, or to make a production change, or to publish a change notice before the review was passed for publication. All writers do that in order to rectify a situation gone bad. The kit manufacturer told the magazine that he was a “troublemaker.” The magazine declined to publish the review.
Commercial magazines depend on advertising to pay the bills, so you rarely see them publishing reviews critical of anything advertised therein—or likely to be. This leaves you, as reader and potential consumer, in a peculiar situation. If you want information on a product and you can't find anything published, must you assume the item is defective just to be on the safe side? (Maybe no one has gotten around to testing or reviewing it.)
Reviewers often get criticized for always saying things are great—when experience later proves that they aren't. Now I ask: who pays for work done on reviews that don't get published? If you (the reviewer) want to get paid, you must either sell it to a publisher or publish it yourself. Knowing that, you (the reader) now know what you should “read between the lines.” Sometimes the reviewer is trying to tell you, in a nice way, “Don't waste your money!”
The point of all this exchange is that you must protect yourself. When an obvious danger of a ruinous lawsuit exists—as it does for manufacturers of fuels and paints containing toxic or inflammable components, or manufacturers of high‑energy rotating objects like engines, propellers, ducted fans, helicopters, and the like—then you can expect that some testing has taken place.
When you buy one of these products, the object will most likely be accompanied with warning literature, and the manufacturer will be receptive to constructive criticism. When the object being sold lacks such an obvious liability/litigation potential, the manufacturer's interest usually concentrates on price and profit.
Static structural test
I performed a static structural load test of that airplane's tow hook / fuselage / wing combination. Jugs of water provided the test load (one pint of water weighs one pound). Note that it was done before the wing was covered—the wing should not depend on the covering for strength. (Besides, if something cracks, you want to see it as well as hear it.) The wing spar was strong enough. Only the dowel needed to be changed.
You can perform the same kinds of tests. If enough interest exists, I will spend a few columns on simple test techniques. If you want this, send a note to either me or to MA's publisher.
George M. Myers 70 Froehlich Farm Rd. Hicksville, NY 11801
Transcribed from original scans by AI. Minor OCR errors may remain.
