February 2013

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.

Issue link: http://experimenter.epubxp.com/i/108002

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Page 12 of 44

performance, but numbers are just numbers, and it isn't until the airplane is in the ultimate wind tunnel—the sky—that a designer knows whether his calculations work or not. "I was really pleased with the way the airfoil worked on the Patrol," said Bob, "and the 6-inch-wider cabin and higher-mounted wing made the cockpit really comfortable. I'm 6 foot and guys much taller than me are comfortable in it. "The span went up a foot while the chord came down from 66 inches to 60 inches," Bob said. "At the same time, since we'd be feeding a smaller engine, fuel came down from 55 gallons to 30 gallons. That alone dropped a lot of weight, and the fuel lines didn't need to be as big, so they dropped from 3/8 inch to 5/16 inch. I know these sound like small changes, but they all add up quickly. Just narrowing the wing also had the benefit of reducing the fiberglass wingtip weight by ten percent because they are shorter. You can't find a piece of the "When I decided to do the LSA, everything about the Patrol looked as if it would work well as an LSA, if I could get it light enough. However, when your goal is to remove weight from what is already a fairly light airplane, you start looking for ounces, not pounds. The Patrol, with a 180-hp engine, big flaps, and constantspeed propeller, is right at 1,000 pounds, and I wanted to get it down to 750 pounds to make it work as an LSA and still have a significant useful load." Since the structure of any airplane is designed around various load parameters, such as speed and gross weight, Bob's first move was to reduce those general parameters and re-engineer new loads for everything in the airplane. "First, I reduced the gross weight to 1,320 pounds, as opposed to the 2,000 pounds of the Patrol," he said. "I also designed it to a VNE (do-not-exceed speed) of 145 mph IAS (indicated airspeed), as opposed to the 165 mph IAS on the Patrol. These two changes greatly reduced the load requirements of the structure so I could maintain the same strength margins but with lighter materials." Bob admits to being an analog guy in a digital world and thinks simple and round is the way to go in panel design.  Reducing the overall requirements of the structure set in motion a whole chain of changes Bob could make that would get the airplane weight down, and at the same time, result in better performance. He said, "The list of really small changes is long, but they got me down to the weight we needed with a reasonable margin. I took weight out of everything. The ailerons, for instance, are dynamically balanced, not statically balanced, so they have no heavy counterbalance weights. Also, there are no balance weights in the elevators. The wing skins came down from 0.020 to 0.016. The spar cap strips are smaller and the wing struts smaller." Because he was working with less power, Bob chose to increase the aspect ratio and lower the span loading of the wing by making it both longer and narrower. Photography by Jim Raeder It's not evident in this view, but the fight deck is a full 6 inches wider and quite a bit taller than a J-3.  EAA Experimenter 13

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