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 43 FLIGHT TESTING TECHNIQUES NON-MANEUVERING LONGITUDINAL static stability is a mouthful to say, and like many things with long titles, it just ain't that complicated. Still, "static long-stab" is an important stabil- ity characteristic, and it could be a bit insidious in how it affects your flying. Quick review: Static stability addresses whether your airplane initially tends to return toward its pre-disturbed condition. For example, when you move the pendulum of a grandfather clock away from its hanging position and release it, its first move is back toward its hanging position. This is positive static stability. Dynamic stability addresses whether your plane actually does return to its pre-displaced condition and how it does or does not get there. Release that clock pendulum, and it will swing back and forth several times, eventually coming to rest at its original position—positive dynamic stability. While static and dynamic stability are the two basic kinds of stability, there are additional classifications for the longitudinal (pitch) axis and the lateral-directional (roll- yaw) axes. This discussion is limited to non-maneuvering longitudinal static stability. Specifically, we're going to explore how an airplane reacts to changes in airspeed. Let's say you've trimmed the plane for 100 knots in straight and level flight. You'd expect to hold a little back- stick to fly 90 knots and a little forward-stick to fly 110 knots, if your airplane exhibits positive longitudinal static stability. Holding back-stick implies that if you let go, the plane would initially tend to accelerate back to its initial trimmed airspeed and vice versa for the forward-stick, faster airspeed case. If you have to push the stick to maintain 90 knots in an airplane trimmed for 100 knots, your airplane would be statically unstable at that flight condition. The implication here is that if you release your push, the plane's initial ten- dency would be to decelerate more, moving further away from its trimmed airspeed. Clearly that's an undesirable, nonintuitive situation. If you released the stick and the plane remained at 90 knots or 110 knots when it was initially trimmed for 100 knots, it would appear to exhibit neutral static stability, be- cause other factors—such as control system friction—might be overpowering your plane's static stability. Figure 1 shows a static stability plot for airplanes with positive, neutral, and negative static stability. WHY NOT RETRIM? Why would you fly off-trim? Well, you always fly off- trim, until you retrim. Whenever you change your flight condition, the airplane's trim requirement probably changes, and you retrim after establishing the new condition. For example, you approach the downwind leg of a land- ing pattern at 100 knots and slow to 80 knots on downwind. Power and airspeed changes often cause pitching moments, which you counter by trimming. When those moments are balanced, you don't have to hold forward or aft stick to remain at that airspeed. After you slow to 80 knots, you know you have to retrim because you're holding back-stick. This is probably the most important aspect of long-stab— the stick force cue that tells you the airplane is not at its trimmed airspeed. Pilots receive information through every sensory chan- nel. We process and react to these separate puzzle pieces without even thinking about them. Some cues to an in- Static Stability Intro Does your airplane return to its pre-disturbed condition? BY ED KOL ANO E A A E X P _ F e b 1 4 . i n d d 4 3 EAAEXP_Feb14.indd 43 2 / 3 / 1 4 3 : 2 0 P M 2/3/14 3:20 PM