Experimenter

OCT 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.

Issue link: http://experimenter.epubxp.com/i/401344

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EAA Experimenter 45 Rigging information in the aircraft's FAA type certifi- cate was in degrees of movement up/down or left/right. It doesn't matter if you are building a homebuilt or putting together a disassembled certificated plane; you must get the flight controls rigged correctly. This means that the controls will displace to their maximum limits and do so with no interference from anything. At an aircraft factory, with multiple aircraft deliveries per day, they typically use devices called "throw boards" to check the deflection of the control surfaces. These throw boards are usually made of plywood or aluminum and are designed to fit on the primary flight surface contour and hang over the movable surface. They rapidly and easily show the deflection angles of the flight control surfaces. You could make throw boards for your project, but the time and effort usually are not worth it unless you are making multiple planes, such as on a production line. The alternative is to use a protractor and measure the actual angles achieved. Put the control surface in the neutral or zero degree position, and shim the protractor to also read zero and tape it in place. Then deflect the surface to its maximum limits and read the angles. Adjust as necessary. If you have some extra bucks in your budget, you can use a digital angle finder, such as Home Depot's Model No. 822, for about $20. You can simply tape it to the aileron or elevator, zero the reading, and then do the maximum deflections. A less expensive alternative is to buy a Johnson Pitch and Angle Locator, Model No. 50 (Home Depot, $8). (See photo.) Again, tape it on the horizontal flight control surface, shimmed to read zero. Do the deflections and take the readings. Adjust as necessary. If you have been following closely, you have no- ticed that I only covered the ailerons and elevators. A mechanical protractor will work for the rudder but not the digital unit or the Angle Locator. Probably the easiest way to rig the rudder is to calculate the inches of deflec- tion at a measurable point on the control surface for the equivalent degrees of deflection. You will need to go back to high school trigonometry. For instance, for a 25-degree deflection on a rudder that extends 12 inches, the math shows the sine of 25 degrees to be 0.422, so the 12-inch surface would deflect 0.422 x 12 = 5.06 or about 5-1/16 inches. Set a step ladder, chair, box, or similar item just behind the flight control surface at the 12- inch deflection point, set the surface to zero or neutral, mark the location on the ladder or whatever, and then deflect to the maximum and measure from the zero point to the edge of the deflected surface. Adjust as necessary. Again, this approach works best for the rudder, but it can be used on ailerons and el- evators just as well. Of course, remember that the ailerons work differentially. I did see a plane with both going up and down together! Smooth uninterrupted controls are an absolute must. Maximum deflections should be achieved, usually ±2 de- grees. Overdeflection can be just as bad as not enough. For instance, overdeflection on a rudder can lead to structural failure on many planes! After you get the controls properly rigged, check full deflection in the cockpit by "wiping out" the cockpit with the controls ensuring full deflections and no binding or "funny" noises. Do it in a quiet area and carefully listen. There should be no rubbing, scraping, or chattering noises. If so, investigate and correct. I have seen several RVs where one or the other of the control sticks hit something in the cockpit, such as seat cushions, instrument panels, and human body parts. In one particularly bad situation, the pilot's stick would jam under the throttle, if you put in full down elevator and right aileron! The wrecked Navion had many bent parts and pieces from at least two different airframes, so rigging it was a challenge. But we completed it with all surfaces perfectly in limits. When it flew, the pilot said it was rigged perfectly and flew hands off. The time and patience we spent in the rigging paid off with a happy owner. Hope this little discussion helps you with the building or repair and maintenance of your bird.

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