MY INTRODUCTION TO A HOMEBUILT
8-FOOT AIRCRAFT SHEET BRAKE
By Larry C. McFarland
Upon renewing my pilot license after a 25-year pause in flying, my best friend, Ed Hale and I flew to Zenith Aircraft Company, in Mexico, MO in a Cessna 150. Zenair gave me a flight- evaluation ride in a really neat all- aluminum Zodiac 601. I was really impressed with the beautiful view thru that large bubble canopy and the way it flew. I went home with a set of plans, a rivet gun and a metric tape. Within 2 months I was making wooden forms and cutting aluminum. Iím a scratch builder, like many of you, because I just like making parts. In the process, I soon cut a rudder spar that needed two long bends. The Zodiac requires good 1/8" bends in the most of its construction.
I took it to a heating contractor-sheet metal shop having an 8í bend brake. The owner wasnít experienced with bend radii and our first efforts had no bend radius. The spar was ruined. I made another blank, but the bend would just have to wait till I could simply acquire my own brake.
Curiously, no 8-foot brakes were available anywhere at reasonable prices. I soon found out that the commercial 8-foot manually operated sheet brake is a heavy piece, usually 1500 lbs to a ton at a cost of over $3,500.00, new. One could presumably buy one and sell it when done, but the used bend brake doesnít offer a reasonable percentage of resale value. Buying one also presents the problem of positioning and use. Seldom do we have the space or the kind of floors that tolerate moving 1500+ lbs. around a shop when only occasionally we must bend metal.
I checked the Internet and found nothing less than a century old for $3000 or one in Louisiana for $800 plus freight weighing 2500 lbs. No plans were available. Stretching a 48" model isnít good. An Internet-friend in Ontario building a 601 had a stretched brake, but he complained of it having too much "give". He was getting twisted and bowed parts for the inability to grip the sheet along its entire length. I was confronted with a "bend brake design necessity".
With frustration, from November to mid-December, 1999, I researched the numbers and designed a minimum-size, all steel, "compact" 8 ft. sheet brake that would bend .040 aluminum 6061-T6 at full-width. I found supporting the beams inboard of the ends reduced span deflection. Stresses get well beyond the short 4-foot and 6-foot types in a hurry. Limiting stresses to what must be bent reduced the problem more. Heavier materials to .060, of considerably shorter length (12") are easily bent but nothing heavier than that would be wise here.
In choosing standard-section steels that are personally weight-limited to 85 lbs each, one decides quickly what is available and reasonable. There are no special expensive materials, cold rolled steel, super-alloys nor special hardware requirements beyond the use of grade 5 bolts at main connections. In fact, the hot rolled steel was all obtained from a resellerís junkyard and sorted for straightness and minimal rust. I must caution and encourage any builder of this size brake to use all safe measures in lifting and handling during the build process. Common sense goes a long way. I loaded my trailer with half by six inch plate and two by six, ten-pound channels and some light four by four steel tubing. Construction from mid-December to January seemed a long way down a crooked road to build an airplane.
I enjoy the flavor of building more than design as I design equipment for a living. But at age 60, this would become a test of fitness as well as enabling my aircraft construction. This brake, like any other project, is just making one piece at a time and I knew that if I got bogged down, it would only be for muscle. I did require my wifeís assistance once while lifting the 165-lb. clamp weldment in place. Everything else remained at or near the original 85 lbs per part.
This is not a project that can be done without some machinery, owned, borrowed or rented. I have the minimum items, a metal cut-off band saw, drill press, oxy-acetylene torch, arc-welder and two 8" c-clamps. One should have some access to any reasonably sized horizontal or vertical milling machine and a small lathe of 9"swing. There are ways to get past the need for a lathe and even the mill, but 8-ft. requires too much handwork to try to avoid them. Someone in an EAA Chapter always has these.
Processes seem to get awkward when trying to weld and machine elements that will just stay straight. I spent an inordinate amount of time recalculating joint construction that reduced welding distortion to zero and I made a complete record of what worked best.
This sheet brake weighs in at less than 600 lbs on casters and is compact enough to be rolled to a wall when not in use. I was surprised that Iíd only spent 6 weeks, evenings and weekends in the build process. The brake turned out to be an investment of less than $600 materials. The brake has since provided me accurate bends with a consistently good 1/8" radius.
Since then, Iíve finished my rudder, stabilizer, elevator and wing spars. Iím now completing the skeletal construct of the center section of the Zodiac 601. Itís been just one year since the open house at Zenith 1999.
That said, Iím very pleased with the convenience of bending the long and short parts easily when I need them. The "pain of heavy work" is gone and its cost has proved to be more than worth the effort.
Iíve assembled the information into a 3-part, 49-page manual. It consists of drawings, photos, bill-of-material, process-methods and sequence that simply describes the project from start to finish. This package is offered to EAA Aircraft Builders as of this issue.