Flexible Honing for Airplane Parts

Photo by Art Zemon

The Bede BD-4 is a light aircraft for general aviation that hobbyists with little or no metal fabrication experience can build at home. Manufactured by Bedecorp, this kit-built airplane measures over 21-ft. long, over 7-ft. high, and has a wingspan of nearly 26-ft. Powered by a single Lycoming O-320 engine, the BD-4C variant weighs over 1000 lbs. and takes approximately 700 hours to build.

Although the aluminum wings of the original BD-4 were easy to assemble, newer designs such as the BD-4B and BD-4C feature a more conventional metal wing with a tubular spar bonded to honeycomb ribs. In fixed-wing aircraft, the spar is the wing’s main structural member. Metal ribs attach to the spar and help carry flight loads, as well as the weight of the wings while the aircraft is on the ground.  

Cylinder Honing

As one hobbyist explains, the BD-4C’s tubular spar has a 6.5-in. inner diameter (ID) and runs the entire length of the wing. At a blog called Cheerful Curmudgeon, the author of an entry entitled Wings Fit then described his frustration with being unable to connect the spar in the wing to the spar in the fuselage. Each end was slightly oval instead of perfectly circular, so wing installation required “a lot muscle”. 

“Once on,” the amateur aircraft builder added, “they were thoroughly stuck”. More muscle and some vigorous “persuasion” helped with the wing’s removal, but disassembly was an exercise that the Bede enthusiast did not want to repeat. The solution, he was told, was to enlarge the ID of the spars in the wings so they would slide onto and off of the fuselage’s center section.   

“What do you use to hone the inside of a 6.5 inch aluminum tube?” the blogger then asked his readers. “A 6.5 inch Flex-Hone, of course,” he answered. The airplane assembler’s use of a flexible cylinder hone was somewhat unconventional, however, and demonstrates how hobbyists sometimes use Flex-Hone® tools from Brush Research Manufacturing (BRM) in unusual but effective ways. 

Flex-Hone® Selection and Use

The Flex-Hone® features abrasive globules attached to flexible nylon filaments. Available in a variety of diameters, abrasive types, and grit sizes, these flexible cylinder hones remove burrs and improve surface finish at the same time. With their soft cutting action and low-temperature, low-pressure abrading process, BRM ball hones are self-centering, self-aligning to the bore, and self-compensating for wear.

Typically, hobbyists such as the author of the Wings Fit blog entry chuck flexible honing tools into a handheld electric drill. The use of a good-quality lubricant is required. In the case of the Bede BD-4 build, the ball hone user followed these standard practices. He then used this BRM deburring tool for material removal and geometry correction – applications for which the Flex-Hone® is not designed.     

Ultimately, the aircraft builder was successful – even though he spent more time honing than BRM recommends for a standard finishing application. The user’s experience with a 180-grit Flex-Hone® and then a 120-grit tool is also instructive. As the Flex-Hone® Resource Guide explains, it’s better to start with a coarser grit tool, and then use a finer-grit one. Choosing the right abrasive type for the base material is also important.

Flying High in the Bede BD-4     

Eventually, the Bede BD-4 airplane builder accomplished his surface finishing mission. Once equipped with a new 120-grit Flex-Hone® tool, he performed “a couple more honing sessions” and discovered that “the wings finally slide on and off the center section of the spar very easily”. BRM salutes him for his dedication, and hopes he’s now flying high in his light aircraft. This hobbyist was looking for a versatile, portable surface finishing solution, and found what he was looking for in the Flex-Hone® tool.

Image Credit: Art Zemon