Nuts & Bolts
Craft & Technique WHEN THE LARGER AIRCRAFT factories design an engine mount for a new engine installation, they have the benefit of structural engineers to design the mount, stress engineers to analyze the structure, and various methods of testing the prototype before it’s ever used in the completed aircraft.
A One-Off Engine Mount A method for doing it right—the first
Here, 1-inch PVC pipe is used to mock up a design for an engine mount to accept a 350-hp Chevrolet V-8 engine in a Callair ag plane to be used for glider towing. Some manufacturers will also call in specialists to perform vibration testing of the new mount design. It’s also common to test the mount under stress until it deforms or even breaks. However, we garage experimenters don’t have the luxury of such extensive testing. We must do the best we know how—and then hope for no failures. That means we 116
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RICHARD FINCH, SAE, AWS (SOCIETY OF AUTOMOTIVE ENGINEERS, AMERICAN WELDING SOCIETY)
might lose some sleep worrying about whether or not our new engine mount will support an The Chevrolet V-8 engine is hung from a crane to fit the plastic tubing engine mount design. The mock-up radiator is fitted just above and behind the V-8 engine.
engine without breaking, bending, or cracking. Fortunately, there’s a way to take a lot of the fear out of designing a new experimental engine mount. And time the American Welding Society Engineering Department has long recommended this method as a quick and reliable way to analyze the stress and design of welded structures, including engine mounts. In simple terms, this method involves first getting a design on paper and then building a scale model out of wood sticks attached with glue. Then, some basic semi-scientific loading of the structure will give you, the designer and builder, a real-time picture of where to add bracing and whether load deflections are within reason. That’s the same method that two of us used to design a new engine mount to connect a Chevrolet V-8 engine to the fuselage of an old crop duster that will eventually be used to tow gliders.
Once the engine mount design is completed, the PVC water pipe mock-up is mounted on a wooden frame for load testing. The mount design looks well braced.
Pipe Dream L.H. Donica owns the airframe—a Callair ag plane—and lives in Marana, Arizona. I live in New Mexico, so there was a logistics
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You wouldn’t believe it if you didn’t see it, but here are two 60-pound marine batteries sitting on the plastic pipe mock-up mount with hardly any deflection or bowing of the plastic pipe. Two-part hardware store epoxy was used to hold the mount together for testing. Sport Aviation
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Craft & Technique problem in designing the engine mount, but we used a system that worked well for us. I stopped by the El Tiro Gliderport in Marana and took measurements of the airframe that had once been powered by a 260-hp Lycoming IO-540 engine. Then I returned home to New Mexico, and with a three-
view 1/5-scale drawing of a Chevrolet V-8 engine to work around, I drew a 1/5-scale diagram of the new engine mount. Once the drawing was complete, I mailed it to L.H. in Arizona, and he started building the test mount out of plastic tubing. As could be expected, the engine
The American Welding Society Engineering Department has long recommended this method as a quick and reliable way to analyze the stress and design of welded structures, including engine mounts.
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had bulges and bumps that did not show on the scale drawing, and these protrusions had to be built around. Therefore, the actual engine was first hung in place in front of the firewall so that the test mount could be built accurately. We used 1-inch diameter PVC water tubing because it’s inexpensive—about $1.50 for a 20-foot stick—and because it works and fits a lot like the 4130 steel tubing that will eventually be used in building the actual engine mount. L.H. used
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the two-part epoxy that comes packaged in a double, hypodermictype container with push rods that mix the epoxy as it’s extruded from the tubing. The strength of the epoxy and the PVC tubing, when properly constructed, is amazing, as can be seen in the accompanying photos. The good thing about using PVC tubing to mock up the mount is that it works so easily. The tubing can be trimmed with a pocket knife or a linoleum knife, but it can also be sawed with a circular saw and fitted with a joint-jigger-type hole saw, just as 4130 steel tubing is fitted and cut. 118
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The tack welded 4130 steel mount is placed back on the Callair fuselage firewall as a final fit check before TIG welding is done at another welding shop in Tucson.
Stress Test After the plastic tubing test mount was fitted and glued together, the epoxy was allowed to harden overnight, and then the static test was performed. L.H. did not test the mount to destruction, because he had neither enough weight nor the safety procedures to determine what the actual failure mode weight would be. So he tested it by placing two 60-pound marine batteries on the bed of the mount. That totaled 120 pounds plus the wood platform for the batteries. The deflection of the plastic mount was nil. The fact that the mount supported the load of 120 pounds was sufficient proof that the mount design would support a V-8 engine, reduction drive, and propeller that would weigh only about five times more than the lead-acid batteries. The mount was somewhat over designed, but certainly safe. If the water pipe plastic mount had failed under a smaller load, such as after applying the first 60pound battery, L.H. could have analyzed the point of failure and strengthened the structure accordingly. The American Welding Society advocates this real-life design analysis method as well as computer-aided failure mode analysis and stress analysis design because it can be accomplished so easily and with very little expense. Another side benefit of building the test mount so quickly and inexpensively is that it was now possible to design a radiator for the watercooled V-8 engine so that the packaging of the new engine installation Sport Aviation
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Craft & Technique
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would be relatively simple and easily accomplished. Once the plastic pipe prototype engine mount design was finalized, the 4130 steel mount was fitted and tack welded with an oxyacetylene gas torch. The tack welded steel mount was fitted to the firewall of the Callair crop duster fuselage, and when everything fit just right, the
mount was taken to a local welding shop for heli-arc (TIG) welding. By first building a plastic pipe prototype test engine mount, a lot of the worry of whether it will be strong enough is taken away. This view of the tackwelded mount is from the bottom looking up and shows a strong crossbraced structure. All this design work saved many hours of cutting and fitting steel and also saved a lot of scrap tubing by doing it right the first time.
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The front view of the tack welded 4130 steel mount shows a tube or two that is slightly different from the plastic mock-up mount for easier building of the actual mount.
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