<<The dimensional stability makes this worth the effort. You really don't want
compliant material sandwiched in a bolted joint. >>
<< The reason to not have fiberfrax under the engine mount has nothing to do with fire protection. Instead, it is to avoid any crushable material that is being compressed by the bolt.>> So what is the compressibility of a thin layer of fiberfax under several hundred pounds per square inch, negligible. It is actually the ¼ inch plywood firewall that is yielding. I agree that the bolts must be in tension to work properly. Builders should check bolt torque frequently until it stabilizes, and at each annual. Mine stabilized in the first few months. The firewall is a flexible membrane. The real strength is in the hard points behind the firewall. Crushing the plywood is not elegant or professional, but I am not convinced that it is dangerous as long as the builder maintains bolt tension. When I built my plane in 1990 there was very little experience with this design, so I decided to add some reinforcement for peace of mind. Plans call for hard points made of plywood gussets with many layers of fiberglass cut to a given template. The result is a very strong stiff bracket. Around the perimeter of that bracket is an abrupt transition in strength and stiffness forming a virtual dotted line labeled, “fatigue here / break here”. I increased the length of the gussets about 1 inch and increased the footprint of the first 2bid lay-up about 30%. Each subsequent lay-up was a bit smaller, resulting in a gradual transition in strength and stiffness to eliminate the dotted line. A few weeks later I met a fellow building a Glassair. Glassairs are noted for the strength of their massive one piece spar and slab sides. When I looked behind the firewall I burst into involuntary laughter, the hard points were substantially less substantial than the ones in the Lancair plans. It took some quick thinking diplomacy to explain my reaction to the builder, “I am amazed at how compact your engine mount hard points are, mine are much bulkier”, we are still friends. Now hundreds of planes have been built, presumably most of them to the plans, and they have accumulated hundreds to thousands of hours. How many
failures have there been on planes built to the manual. Why haven’t we been hearing and talking about these failures? Where is the Tech Bulletin on this problem? Are my brackets better than the ones in the plans, no. They took longer to install and added unnecessary ounces. A better design might be to keep the original footprint and taper the lay-ups to eliminate the dotted line and save weight. The most
complicated, labor intensive, heavy solution to a problem is not necessarily the best solution. One of the great things about homebuilding is that people can build the airplane they want. You can build a showplane, a basic transportation plane, or an engineering tour de force, whatever gets you off. Telling people they have to do it your way or they will be unsafe requires documentation in the form
of; A Incident /Accident reports or B An engineering analysis (not just an opinion) proving that the plans are unsafe.
BILL HANNAHAN
WFHANNAHAN@YAHOO.COM
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