Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other other light aircraft.
Issue link: http://experimenter.epubxp.com/i/81495
Under the Cowl Materials are generally a black polymer that feels like traditional rubber. Other mounts are made of nitrile, silicone, and other materials. Each has its benefi ts and compromises, and each is understood by the applications engineers at the manufacturers' companies. Some are highly resistant to heat; some are better for their resistance to petrochemicals and ultraviolet (UV) rays; all require proper mount design, assembly, and maintenance. Some Pitfalls While most mounts will easily handle the generally smaller engines on our homebuilt aircraft without problems, there are several common mistakes that can shorten the lives of mounts precipitously. "Doughnuts" may also be used vertically in compression. T is example is on the Corvair-powered Panther prototype on display at EAA AirVenture Oshkosh 2012. There are two basic mounting confi gurations: tension and shear. Tension mounts ultimately rely on the strength of the threads of the fastener (whether the threads go into structure or are in a nut) that holds the assembly together. Shear mounts depend on the "sideways" strength of the bolt. Envision this: If the nut should fall off but the bolt (if it were to stay in place) would continue to do the job, that's a shear application. An example of an assembly held in place in tension would be a propeller mounting bolt; a wing strut usually mounts in shear. Traditional mounts of both types are common; they both work, when properly designed, built, and maintained. (The opposite is also true.) Conical mounts, with the narrow ends of the cones facing each other in a cast mount, differ from concentric mounts, which are cylindrical, having the same diameter through their length. Clearly, conical mounts lend themselves well to tension applications; cylindrical mounts work well in shear; a simplifi ed cylindrical mount can also work well in tension, as a "compression donut." Some fastener basics are ignored by those in a hurry or those who don't know better. The reason is immaterial when your engine falls off. The correct washers are essential, and don't leave them off because your bolt is too short. Retention, whether with safety wire, cotter pins, self-locking fasteners, a chemical locker, or a combination of these, is obviously critical; yet I often see people reuse self-locking nuts or even cotter pins. These things hold your engine on! Think big. Spend a dollar! Some Tips As a natural rubber mounting ages, its surface will become covered with a waxy fi lm. This protects against ozone, UV, and some contaminants. Leave it on! Old mounts may sag. You'll notice this when the spinner doesn't line up with the cowl the way it used to. Mounts also compress over time. When they do, the original spring rate changes, and when the spring rate changes, the performance of the mount changes. When the mount's performance changes, it's usually not for the better. When you're buying a "new" mount, check the date. For Lord mounts, it's molded into the mount, usually right near the part number, and it's usually diffi cult 30 NO. 1 / SEPTEMBER 2012