Originally published February 1997
Russ Erb, our newsletter editor and a plans-based builder of a Bearhawk project, was our featured speaker. If you know Russ, you know he's got a few ideas about how to build airplanes. As a flight test engineer for the Air Force, he also has a lot more credibility that the average home builder who knows more than the designer, so we were fortunate to get a data dump on his effort.
The Bearhawk is an aluminum wing, tube-framed four place high wing with the look of a Cessna 170. Russ took on the wing ribs first and his presentation was based on the tools and techniques he's developed to do the part fabrication. He had the plans in hand last year while an instructor at the Air Force Academy, but didn't have the time or place to really get started. A lot of thinking, a not inconsiderable pent-up desire to get going, and Russ's knack for manufacturing engineering problem solving, and you get a pretty good home factory that could probably turn out a couple thousand identical articles after the first.
Russ gave us a pretty good run down on getting ready to build parts. Tips on how to buy materials, transfer designs to them, cut various thicknesses, and mark the cut shapes for later identification and set up were helpful. Did you know that UPS will ship material lengths up to eight feet? Did you know the common stock length is twelve feet? How about using a "removable" glue stick to fix a paper template made from a CAD printout? Why not use a pencil or a metal scribe to mark out a design? (Answer: the pencil will cause corrosion and the scribe creates a stress concentration that could lead to failure later.) This gives you an idea of the preparation Russ has done.
As part of the presentation, Russ actually made a wing rib. Using a master form block for consistent positioning and shape, he checked a roughed piece for its intended position in the wing structure and facing based on his marking scheme. The piece was mounted in the block and we saw how it would be shaped using a router with a laminate bit mounted. Russ noted he prefers a hardwood for the block because it's easier to cut to a true shape than softwood, and doesn't deform in ways that could cause the bit to travel improperly. Once the piece is shaped, the edges should be smoothed using a Scotch Brite pad to remove surface irregularity that could lead to cracks later.
The next step was to use a two piece shaping block to sandwich the formed part to permit flange fabrication. A flange gives the rib strength without added weight. Again the markings were checked and the part loaded in the proper orientation. Using a dead blow hammer, Russ gradually bent the material edges to form the flanges. The dead blow hammer imparts true force with minimum bounce, so it's the preferred tool. When removed, the part is sprung slightly away from the flange direction. This is a stress condition that can be relieved by using a fluting tool. Russ uses a Avery vise grip with the flute on the holding surface because of the good mechanical compression a vise grip gives. If a surface is over fluted, a corresponding seamer can lessen or remove a flute to bring the part into proper alignment.
As a last step, Russ fabricated lightening holes for weight reduction. A large hole can be cut using several tools, but a fly cutter will do the job best. However, caution is strongly recommended in using one. Never try to set up a fly cut with a hand drill; only use one with a properly adjusted drill press. As one guy quipped, it's also not a bad idea to turn on the drill press and run as well. Once the hole was cut and smoothed with a Scotch Brite wheel, Russ again flanged the edge of the hole. This time he used a rounded wood block cut out from the master shaping block. This was done by using bench vise as an arbor press to push the block into the hole.
Russ has quite a process worked out and he even impressed the composite
builders who, for the most part, swallowed hard and stayed to watch all
this metal bending heresy. (Not all mind you, but most.) He's got an interesting
project going and I suspect we'll get several good programs from him as
he progresses. (Gawsh, and I didn't even have to punch that up to make
it sound good -- ed)
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 11 August 1997