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/254584
EAA Experimenter 45 plane has 6 pounds of friction in its longitudinal (pitch) control system, you might be able to release the stick at 80 knots and the airplane will stay at 80 knots. In this case, the friction masks the airplane's positive stability and makes it appear to be neutrally stable. If you slow to 70 knots, where it takes 8 pounds of back-stick, you'd have to keep some pull force on the stick because the required stick force is greater than the friction that is helping to hold the stick back. This airplane appears to be neutrally stable within a band of airspeeds around the trimmed airspeed and positively stable outside that trim speed band. Establishing an exact speed within that trim speed band can be dif cult and frustrating. It turns into a game of trial and error, nudging the stick, waiting to see what airspeed eventually settles out, and then nudging again. Frustrating. You can fly a statically unstable airplane just as you can balance a broomstick vertically on your up-turned palm. How long you can do it is another story. Balancing the broomstick takes total visual concentration and con- stant hand-eye coordination. Fortunately, statically unstable airplanes are usually only mildly unstable and usually only in a particular configuration. Still, the pilot must dedicate more time to airspeed control in these airplanes, and that means less time is available for other necessary piloting tasks. And yes, a few popular kit planes have mildly unstable characteristics during certain flight phases. The farther aft an airplane's center of gravity (CG), the less stable it is. This is why there are limits to the aft end of the allowable CG range. Figure 2 shows the effect of CG on a static stability plot. The forward CG limit is usually determined by how much nose-up authority the elevator has. The worst case for the forward CG limit is typically the landing flare in ground effect with the gear down and flaps down, all of which create nose-down pitching moments that the elevator (also less effective in ground effect) must overcome during the flare. HOW MUCH IS ENOUGH How much stability is good is a good question. The airplane should be stable enough to provide reasonable stick force cues to an off-trim condition. It should be sta- ble enough to remain at its trimmed airspeed rather than wander off. But it shouldn't be so stable that it takes two hands on the stick to fly a temporary off-speed condition. The Federal Aviation Regulations require general aviation airplanes to have positive static stability under most flight conditions: "The airplane must be longitudi- nally, directionally, and laterally stable…In addition, the airplane must show suitable stability and control 'feel' (static stability) in any condition normally encoun- tered in service, if flight tests show it is necessary for safe operation." How much "control feel" is suitable? According to the regs, "The stick force must vary with speed so that any substantial speed change results in a stick force clearly perceptible to the pilot." The stick force versus airspeed gradient is not a true measure of an airplane's static stability, but it is an opera- tional cue pilots use. You can increase the stick force by adding springs to your longitudinal control system; this doesn't make the airplane more stable, but it can provide a more obvious force cue to the pilot. Adding springs is not a simple solution because they will affect every control input you make; and the results may be good for static stability feel and bad for dynamic stability, flutter, and other airplane behaviors. The regulations also address the trim speed band. General aviation airplanes must have a trim speed band no larger than 10 percent of the trimmed airspeed. In oth- er words, when the airplane is trimmed for its 70-knot fi- nal approach speed, it can have a 7-knot trim-speed band, and when it's trimmed for its 180-knot cruise speed, it can have an 18-knot trim-speed band. Neutral and nega- tive static stability are not allowed in certificated general aviation airplanes. What's right for your airplane depends on how you fly your airplane. Neutral static stability might be good for an aerobatic airplane that undergoes large airspeed changes quickly when maneuvering. IFR pilots could benefit from an airplane with off- airspeed stick force cues should they inadvertently ne- glect the airspeed indicator in favor of the ILS needles during an instrument approach. Day VFR fliers can get by with a mildly unstable airplane, but they should be aware of that instability and maintain a rigorous airspeed indicator scan, particularly near the ground or other airplanes. Okay, that's the primer for longitudinal static stability. Next time we'll talk about flight testing techniques, start- ing with the trim-speed band determination. 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. Release that clock pendulum, and it will swing back and forth several times, eventually coming to rest at its original position— positive dynamic stability. 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