Experimenter

April 2014

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 light aircraft.

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EAA Experimenter 15 I had a regular job at the time, plus kids in college. My wife is real supportive, which helps." It was important to Bruce to make the airplane as safe as possible and easy to fly. To that end, he incorporated a wing airfoil similar to a Clark Y, but he modified it so that the wings are tapered yet flat on the bottom for ease of building. Bruce lengthened the ribs in the middle portion of the wing, and the airfoil at the wing root has a lower camber and stalls before the outboard portion of the wing, which has more camber. "This airplane does not have a stall break," Bruce said. "You can pull it up at a 45-degree angle, and as it runs out of airspeed, you can see the vibration of the stall going out the length of the wing. But the airplane just noses over a little; with power off, it does a little bit of a falling leaf, but you can pick it up with aileron because the wing won't stall at the tip. The spars have been analyzed by a structural engi- neer, who stated that they were 'grossly overbuilt.' It's built really strong, but we haven't approved it for aerobatics; we'd do a lot more testing and structural analysis before that." The BK 1.3 has a sleek, flush-riveted fuselage of mono- coque construction with full-span flaperons, which simplify the building process (as opposed to having separate aile- rons and flaps). Bruce said, "The ailerons go up twice as far as they go down, so you don't have to be on the rudders as much. You still have full aileron function with the first notch of 10 degrees; I use that for takeoff. The full 20 degrees steepens the approach to landing and decreases the air- plane's tendency to float in ground effect." Instead of using a separate fiberglass or aluminum fuel tank, Bruce decided to create "an integrated tank." He said, "The nose is sealed with Pro-Seal tank sealant, so I'm rein- forcing the nose of the airplane even more, saving weight, and increasing the fuel capacity, while still providing gravity feed to the engine." N88BK's powerplant is the Great Plains' Flywheel Drive Air-Cooled VW conversion engine, with a flywheel-mounted propeller hub and an Edward Sterba 52-by-44 wood propel- ler. "This engine weighs about 158 pounds and is less expen- sive and lighter weight than the other type with the accesso- ry case," said Bruce. "Steve Bennett and I collaborated with Robert Hoover, and we worked with him until he passed away in 2012. So I had the two pre-eminent VW people working together on this installation. I'm really happy with it; it runs smoothly and has less vibration than the other VW engine, due to the way it's mounted. The mounts are under- neath the engine, in front and in back." Bruce fabricated the aluminum spring gear legs by first purchasing a 4-inch-wide-by-¾-inch-thick-by-4-foot-long aluminum bar. Then he used a Craftsman band saw with a fine-toothed plywood blade to make an approximately di- agonal cut (a pattern is available) that created two gear legs. He rounded the corners with a router and round-over bit, then bolted them into the fuselage's center section. The end result was a set of stout aluminum spring gear legs for about one-fourth the price of ready-made legs. He used 500-by-5 wheels for the main gear and a 400-by-4 for the nose wheel. The BK 1.3 can break ground in about 600 feet by ac- celerating at 40 mph indicated in ground effect, according to Bruce. Its stall speed is 48 mph (no flaps) and 44 mph (20 degrees of flaps), and 60 mph is the best climb speed, with 10 degrees of flap. The airplane climbs to altitude at 80 mph, and when set up for level flight at 3,200 rpm, it cruises happily along at 130 mph. Traffic pattern speed is around 80 mph, with 10 degrees of flaps added on base leg. Bruce said "Final approach is flown with full flaps at 50 mph over the fence. It rounds out and flares about 45 or 40 mph, and settles nicely onto the ground." Bruce distributed 80 sets of early builder plans, and of those, about a dozen have turned into active projects. "That's about right for a plans-built airplane; usually one in ten ever reach completion," he said. "I've gone and seen several BK 1.3 projects, and they're looking good. Some experienced builders say they see it as a 1,500- to 2,000-hour project, making all the parts from scratch. "I retired in November 2012, so I'm working full time with the BK Flier, which means that builders get phone and e-mail support. I want to help the builders get in the air because my real motive is to have other people be able to pull it of like I did. I don't think I'm all that great; I just went and did it. I joined EAA Chapter 35 in 1998, and Paul McReynolds was my technical adviser. I had a lot of support from my EAA chapter; I felt like I had joined the world of builders and fl iers." As far as completing the required FAA paperwork, Bruce recommended that builders visit the FAA website to view the FAA Advisory Circulars . "Download and read them," It wasn't just the f ying bug that had bitten Bruce, though; it was also the building bug. Photography by Sparky Barnes Sargent Interior view of the all-metal monocoque fuselage. E A A E X P _ A p r 1 4 . i n d d 1 5 EAAEXP_Apr14.indd 15 3 / 3 1 / 1 4 9 : 4 2 A M 3/31/14 9:42 AM

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