Control freeplay usually comes from "slop" in the control system. Worn rod ends, loose cables, elongated bellcrank holes, and a variety of other loose connections allow one component to move a short distance without moving the other. Even brand new airplanes with "tight" control systems can have freeplay. The more linkage connections an airplane has, the more opportunities for tiny gaps, and they are cumulative.
We'd all like absolute centering and no freeplay, but we don't always get it. Fortunately, flying is a dynamic evolution. As pilots we do whatever it takes to achieve the result we want. Usually, we're not thinking about centering and freeplay bands. We nudge and bump and tweak the controls until the airplane is trimmed. And we do it again after every gust or turn or upset of any kind.
We work continuously toward the desired altitude, airspeed, bank angle, or whatever flight condition parameters apply. That's the challenge of flying. Understanding the mechanisms of centering and freeplay enables us to remove some of the guesswork, fly the airplane to the desired condition more expeditiously, and direct more of our attention to other piloting tasks.
To see control "slop" freeplay in action, touch the tips of your index finger and thumb to form an OK sign (turkey beak for you shadow puppeteers). Now do the same with your other hand but have your index finger and thumb meet within the circle of the first hand forming a chain-like link. Now hold your arms in front of you so your forearms form a straight line parallel to your chest. We're simulating a portion of the longitudinal flight control system with your right elbow pointing toward the yoke and your left elbow pointing toward the elevator. Move your right arm alternately to the right then left, simulating the forward and aft yoke displacement. Notice your right arm has to move an inch or two before it takes up the space in the finger link and pushes or pulls your left arm. This is control system freeplay.
Refining our freeplay definition, we can say freeplay is how far the cockpit control (right elbow) moves before it changes the control surface deflection (left elbow).
Freeplay is described as a "band" because you usually don't know where it is in the control system until you move the control. If the control is at the forward end of the freeplay band, any forward yoke displacement instantly deflects the elevator trailing-edge-down. To deflect the elevator trailing-edge up, you'll have to move the yoke all the way through the freeplay band before the elevator moves. If the control is somewhere in the middle of the band, you'll have to move it through the remaining portion of the band before the elevator will deflect. Any control motion within the freeplay band wouldn't change the control surface deflection.
You can imagine how difficult it would be to fly precisely with an appreciable freeplay band in your airplane's flight control system.
Formation flight, for example, requires the pilot to make constant small corrections to stay in position. The continuous yoke motion necessary to traverse the freeplay band can make this task quite tiring. In short, flight control system freeplay causes precision to suffer and pilot workload to increase.
With the control outside the freeplay band, the connection feels solid. The linkage connects the cockpit control (on which you're exerting force) to the deflected surface, which transmits the air loads acting on the surface. As you relax your input force, the air load is already pulling in that relaxed direction - toward the trimmed position - so the chain-like link remains tight. The system doesn't have to traverse any freeplay gap because the system remains in tension or compression the whole time it's outside the freeplay band.
Here's how it works in the airplane. From straight and level flight you pull the yoke back. The first little bit of yoke travel may do nothing because you're moving the control through a portion of the freeplay band. Then the elevator begins to deflect. As you continue to pull the yoke back, you have to use more force to balance the increasing air load. When you relax your pull, the elevator deflection begins to decrease immediately because the air load keeps the control system tight. When you pull the yoke past the end of the freeplay band, you don't have to cross it because you took up the slack when you first started pulling the yoke back.
Freeplay exists only in the vicinity of the control's trimmed position. Once you deflect the control surface, you're pulling one direction and the air load is "pulling" in the opposite direction. Try it with your OK-sign/arm flight-control model. Note the position of your left (elevator) elbow. When your right (yoke) arm displaces your left arm, your left arm should be pulling toward its original elbow location, simulating the air load on the elevator. Notice that your "control system" remains tight until your left elbow returns to its original, trimmed position. At the trimmed position, the system doesn't transmit any air load to the cockpit control. That's why we trim - to eliminate the cockpit control force necessary to maintain the control surface's deflection.
If an airplane has an annoying freeplay band, some pilots will fly it slightly out of trim - intentionally. This takes advantage of the fact that freeplay exists only when the control is near its trimmed position. They might trim a little nose-down and maintain a slight back force on the yoke so they can make a small elevator deflection without having to traverse the freeplay band. The Navy's Blue Angels used this trick when they flew the A-4 Skyhawk, and it's hard to argue with their results.
Most of us don't fly formation aerobatics, but we still want to fly as precisely as we can without resorting to flying off-trim. Not knowing how far we must move the cockpit control to achieve the desired effect is frustrating and makes flying harder than it needs to be. The next time you're having a hard time making small pitch, roll, or yaw adjustments, check for freeplay before you blame your piloting skill.
To check for freeplay while you're flying, start from a trimmed position and displace the control slowly in one direction until the airplane's nose just begins to move. Note the cockpit control's location. Then do the same thing in the other direction, and note the control location. The difference is the freeplay band.
What we've been talking about here, is control system position freeplay, or how far the pilot must move cockpit control before it changes the control surface's deflection and, consequently, causes a change in pitch, roll or yaw attitude. You may or may not feel control forces within the freeplay band.
Control force is a separate issue. To satisfy the technical purists, we need to mention another type of freeplay called "force freeplay." We'll cover force freeplay in an upcoming article about flight control forces.
Centering
"Control centering" is another control displacement issue that can affect flying precision and pilot frustration. It concerns the return of the flight control to its original location after the pilot displaces it.
"Absolute centering" occurs when a displaced flight control returns to its exact pre-displaced position. If it moves toward its original position but doesn't quite get there, it has "positive centering." If the control continues to move away from its original location after the pilot releases it, it has "negative centering," which is obviously undesirable. It's extremely unlikely that you'll ever fly an airplane with negative centering.
A flight control system with positive centering has a centering band or range of positions that a control can occupy without you keeping your hand (or foot) on it. Say you're trimmed for hands-free steady flight. With positive centering characteristics, if you pull the yoke back slightly and gently release it, the yoke will move forward toward its trim position. If it does not return to its exact position, the location where it stops is the aft end of its centering band. Now push the yoke slightly and gently release it, and it will return to the forward end of its centering band.
Rather than "forward" or "aft," it's called a "band" because the yoke can occupy any position within it. When you first trim the airplane, you don't know where the yoke is in the centering band. It could be at the forward or aft end or anywhere between.
Let's say the yoke is at the forward end of the band when you trimmed for steady, wings-level flight. Then you make a heading change using a reasonable bank angle and no power change. During the turn you'll probably have to pull the yoke back a little to maintain altitude. As you roll out and release this yoke pull, the yoke moves forward a bit.
Now the yoke is probably at the aft end of the centering band. If the longitudinal flight control linkage has no freeplay, the fact that the yoke is now in a different position means the elevator deflection is also different, and the airplane climbs. You didn't change the power or trim during your gentle turn, but the airplane is now out of trim because the yoke did not center absolutely.
To return to your original level flight condition, you'll have to experiment by relocating the yoke within the centering band. This can be annoying and sometimes time-consuming because it's a trial-and-error process. You place the yoke somewhere within the centering band and note whether this position results in level flight at the original airspeed. If not, you try another yoke position. We're still assuming no freeplay exists.
Non-absolute centering can be caused by an improperly balanced control surface or control linkage looseness, but usually it's caused by friction. Friction prevents the displaced control from returning completely to its pre-displaced position, and it's up to the pilot to finish the job. To help overcome any friction and return the cockpit control to its original position some airplanes have springs built into the flight control system. Other airplanes rely on control surface air loads to return the controls to their trimmed position.
Whether control linkage looseness affects the cockpit control centering depends on where the looseness is and the location of other linkage components. An example is an airplane with centering springs attached to its control yoke. Centering springs are designed to do just that - center the yoke. Because the pilot must overcome the spring force to displace the yoke, they also provide a stiffer feel. If the linkage is loose and there's friction in the elevator hinge, the elevator may not center absolutely following a deflection. The yoke may center because that's what the springs are for. The pilot will still have to experiment by nudging the elevator to find its original trim position. With each nudge, the elevator deflection changes slightly, but the springs return the yoke to the same position.
If the springs are installed near the elevator and linkage looseness is near the yoke, the elevator may have absolute centering but the yoke might not. In this case the yoke's centering band will be a range of yoke positions within which elevator deflection does not change.
If we repeat our heading change, this airplane would remain trimmed for level flight following the turn because the elevator returned absolutely to its level flight trim condition when we released the back yoke. Depending on where the yoke was within its centering band before the turn, it may be farther aft than it was before the turn, and we probably wouldn't even notice the small change in its position.
Centering issues are just as valid for the ailerons and rudder. Positive, but not absolute, lateral yoke or aileron centering can cause residual roll rates, which you can overcome by using the same trial-and-error elimination technique. In roll, however, you'll see the result of a yoke nudge immediately.
A poorly centering rudder will probably cause a tiny yaw rate leading to a small sideslip. Sideslip is the relative wind coming from the left or right of the airplane's nose. If the airplane has a strong dihedral effect, even a small sideslip can couple into a roll away from the sideslip. If the sideslip angle is small enough, the pilot might think aileron centering caused the roll and try to correct it with lateral yoke. You can see how this one might take a bit longer to sort out.
Freeplay and Centering
Freeplay and centering can be confusing when you first explore it, but the situation isn't an unresolved chicken-and-egg argument. Let's say the freeplay band is within the centering band. After the pilot displaces it, the cockpit control will return to the edge of the centering band, but this location will be outside the freeplay band. That means the control surface will remain at a deflection different from its trimmed position - and the airplane will be out of trim. Depending on the control involved, the result will be a hands- and feet-free pitch or roll-yaw rate. The pilot must experiment with cockpit control locations until he (or she) finds a position within the freeplay band.
Things aren't quite as bad if the centering band lies within the freeplay band. In this case the cockpit control will return to a location within the freeplay band, so residual pitch, roll, and yaw rates shouldn't occur. In both cases we're assuming that the control surface returns to its pre-deflected position when the cockpit control is within the freeplay band. Again, this depends on the location of the flight control friction, looseness, springs, etc.
Yes, this can be confusing. The bottom line is how these issues affect you in the precise execution of a flying task. You've probably flown airplanes with varying degrees of control system freeplay and centering characteristics. If you rent, don't you have a particular airplane among the FBO's Cessna 172s you'd rather fly? Is it really the avionics or interior that leads you to this preference? Fly them all again and pay attention to their freeplay and centering bands to see if these culprits might be swaying your preference as well. Let us know what you discover.
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