Experimenter

October 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/194874

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your right-hand component of the link traverses the gap. Move your stick the other way. Again, your righthand fingers must move across the gap before you get any elevator deflection. Control system freeplay is that simple; it's how far you have to move the cockpit control before it moves the control surface. Freeplay is always described as a band because you usually don't know whether the control is at one end of the band, the other end, or somewhere in between until you move the control. Let's say the stick is at the forward end of the freeplay band. Any forward movement of the stick from here will deflect the elevator trailing edge down. To deflect the elevator trailing edge up, you would first have had to move the stick all the way through the freeplay band. If the stick is somewhere in the middle of the band, you'll have to move it through the remaining freeplay in either direction to change the elevator's deflection. An airplane with an appreciable freeplay band, as you can imagine (or maybe have already experienced), can be difficult to fly precisely. For example, formation flight requires the pilot to make continual, small corrections to remain in position. Doing this in a plane with too much freeplay can tire a pilot in short order. Freeplay causes increased pilot workload, and precision suffers. Most of us don't fly formation aerobatics, but we still want to fly as precisely as conditions require without resorting to flying off-trim. Not knowing how far to move the cockpit control to achieve the desired result can lead to frustration and precision difficulty. So, the next time you're having a hard time making small pitch, roll, or yaw adjustments, check for freeplay before blaming your own skills. To check for freeplay in the pitch control system, start with the airplane trimmed for hands-free flight at a steady airspeed. Then slowly ease the stick back until the plane's nose just begins to move upward. Note the stick position, then slowly move the stick forward until the plane's nose just begins to lower. Note that stick position. The difference between the two noted stick positions is the pitch control system freeplay band. It works the same way for roll and yaw. When you perform this test, keep in mind you're looking for the first small change in pitch attitude. Because you're moving the stick slowly, the airplane's pitch attitude change will likewise be small. You'll have to make sure you keep your head still. Any up or down movement of your head (your eyes, really) will change the perspective of the plane's nose against the horizon and appear to be that pitch change you're looking for. You'll also need calm air to spot the tiny change in pitch. Once the control is outside the freeplay band, the connection feels solid. You feel the force caused by the air load on the deflected control surface. As you relax the force you're applying to the cockpit control, that air load restores the control surface to its trimmed deflection. The chain link stays tight because the system is outside the freeplay band. Try it with your arm model. What we described so far is more correctly called position freeplay because we're talking about the range of stick positions that do not cause a change in pitch attitude. Notice control stick force doesn't enter the picture when talking about position freeplay. An airplane might have springs in its control system that require you to exert a force on the stick to move it. You'd still ignore the force and measure position freeplay the same way. Freeplay exists only near the control's trimmed position. Once you deflect the control surface, you're pulling (or pushing) in one direction, and the air load is pushing (or pulling) in the opposite direction. That's why it feels solid outside the band, and you can make precise, tiny adjustments to the cockpit control and consequently the control surface. Force freeplay is simply what's commonly referred to as the slop in the stick. It's that (hopefully) small band where it takes virtually no force to move the stick…a dead band, if you will. We'll talk more about control system forces in the coming months. Some pilots intentionally fly out of trim to take advantage of the fact that freeplay exists only when the control is near its trimmed position. They might use a little nose-down trim and maintain a compensating slight pull on the stick so they can make small elevator deflections without having to traverse the freeplay band. The Navy's Blue Angels used this trick when they flew the A-4 Skyhawk, and there's no arguing about their precision. Next time we'll tackle control system centering. Stay tuned. Ed Kolano, EAA 336809, is a former Marine who's been flying since 1975 and testing airplanes since 1985. He considers himself extremely fortunate to have performed flight tests in a variety of airplanes ranging from ultralights to 787s. EAA Experimenter 39

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