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/307497
EAA Experimenter 37 6. Repeat Steps 2 through 5, targeting a test airspeed a few knots slower and faster respectively than your previous test airspeed until you've mapped the entire airspeed range of interest. Note: This alternating slow/fast method helps to keep your airplane at approximately the same altitude for all test points. If you are only interested in how the stick force increases as you fl y further and further from the trimmed airspeed, it would be more illustrative to fl y progressively slower or faster test airspeeds in order—for example, 110, 105, 100, 95, 90, 130, 135, 140, 145. Whether you opt to measure the control system friction for every test airspeed is up to you, but here's a guideline. If the nose moves with just the slightest stick relaxation and with the slightest increase in stick force, the dif erence between these two forces is small, and the friction is probably insig- nifi cant. In this case, you might opt to limit your testing to the basic approach described last month. If you fi nd you have to relax and increase your stick force substantially, there's prob- ably a signifi cant friction band. At this point, you could mea- sure the friction, but this qualitative assessment is probably all you need to answer your airspeed control dif culty question. We've overlaid the basic approach curve from Figure 1 on Fig- ure 2 to show that this is only one of many possible curves you might have come up with had you not checked for friction. Another note: Although we showed the blue and green curves as straight lines to better illustrate the friction band, this is not usually the case. Center of gravity (CG) can drastically af ect the shape of your airplane's static stability curve. The farther aft the CG, the fl atter the static stability curve, meaning it takes less stick force to fl y "of -trim" airspeeds. As the CG moves progressively aft, eventually it will reach a location called the neutral point. When the CG is located at the neu- tral point, the airplane will maintain any airspeed hands-free, that is, neutral static stability. With enough friction in an air- plane's longitudinal control system, the plane can appear to have neutral static stability even with the CG well forward of the neutral point. Because you're looking for tiny pitch attitude changes during this test, calm air is essential. If it's bumpy, save the test for another day. Early morning is usually the best time to fi nd calm air. Those same tiny pitch changes can only be accurately detected with an outside reference. The artifi cial horizon, altimeter, and vertical speed indicator are too coarse. The real horizon is the best reference. You can try using a distant cloud; but clouds move, and the closer you are to your external reference, the greater the likelihood of a parallax error. When mapping the airspeed range for this test, it is not necessary to stabilize on an exact target airspeed. Your goal is to feel how the stick force increases as you fl y further and further away from your trimmed airspeed, so it's the stick- force trend that's important. For our example, we targeted 110 knots for our fi rst data point. If the pilot stabilized at 112 knots or 108 knots when aiming for 110 knots, it would have been okay. If we were being rigorous about this test, we'd be fairing a curve through all the data points, anyway. When collecting data to create a curve, high-quality test data with a consistent airspeed spread between test points is more important than testing at the exact target airspeed. The same logic applies to our test. We're essentially constructing that curve in our heads as we feel how the stick-force requirement changes at the dif erent airspeeds. This wraps up our four-part series on longitudinal static sta- bility. Next time we'll take a break from the technical stuf and start exploring the fussier world of pilot decision-making. Ed Kolano, EAA 336809, is a former Marine who's been fl ying since 1975 and testing airplanes since 1985. He considers himself extreme- ly fortunate to have performed fl ight tests in a variety of airplanes ranging from ultralights to 787s. Performing the basic assessment will be tougher if there's a lot of friction in your longitudinal control system. E A A E X P _ M a y 1 4 . i n d d 3 7 EAAEXP_May14.indd 37 5 / 5 / 1 4 3 : 2 0 P M 5/5/14 3:20 PM