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Freeplay & Centering: Flight control characteristics that affect pilot workload
By Ed Kolano (originally published in EAA Sport Aviation, May 2000)
In April’s "Test Pilot" we introduced flying qualities and described the concepts of stability and control from two perspectives-in terms of design goals and how they affect you when you’re flying your airplane. After integrating these number-based airplane characteristics with pilot opinion, we called the result handling qualities, the focus of which is how your airplane feels to you when performing piloting tasks.
We also touched briefly on the pilot-airplane interface-the physical connection between you and your plane. This included the control stick/yoke, rudder pedals, switches, knobs, instruments, and cockpit environment. This month we’ll explore two aspects of the physical connection through the stick and pedals known as flight control freeplay and centering characteristics.
Touch the tips of your index finger and thumb to form an OK sign (turkey beak for you shadow puppeteers). Do the same with your other hand, but have your index finger and thumb meet within the circle of the first hand, forming a chainlike link. Now hold your arms in front of you so your forearms form a straight line parallel to your chest.
You’re now simulating a portion of the longitudinal flight control system, with your right elbow pointing toward the stick and your left elbow pointing toward the elevator. Simulate forward and aft stick displacement by moving your right arm to the right and left. Depending on the size of the circle your thumb and index finger create, your right arm moves an inch or so before the finger-link takes up the free space and pushes or pulls your left arm. This is control system freeplay.
In an airplane 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 can allow one component to move a short distance before it moves the one connected to it. The more linkage connections there are, the more opportunities there are for tiny gaps, and they combine.
The freeplay band is a range of positions within which you can move the cockpit controls without changing the airplane’s pitch, roll, or yaw. Freeplay is always described as a band because pilots usually don’t know where in the band the control system is. If the control is at the forward end of the freeplay band, moving the stick farther forward instantly deflects the elevator trailing-edge-down. To deflect the elevator trailing-edge-up, you have to move the stick all the way through the freeplay band before the elevator will deflect. If the control is somewhere in the middle of the band, you have to move it through the remaining portion of the band before the elevator will deflect.
Precisely flying an airplane with an appreciable freeplay band is difficult, and with a decrease in precision comes an increase in pilot workload. For example, in formation flight pilots constantly make small adjustments to stay in position. If the control system freeplay exists, pilots must continually experiment with stick position within the band to make the desired adjustments, a tiring increase in their workload.
Freeplay exists only near the control’s trimmed or hands-free position. Once you deflect the control surface, you’re pulling the stick in one direction and the air load on the deflected control surface is pulling in the opposite direction. In airplanes with an annoying freeplay band, some pilots intentionally fly slightly out of trim, which essentially removes the “slack” from the control system. If they trim a bit nose down, they must maintain a bit of back stick and can make small elevator deflections without having to traverse the freeplay band. The U.S. Navy’s Blue Angels used this trick when they flew the A-4 Skyhawk, and it’s hard to argue with their results.
Freeplay does not involve control forces. When you move the stick within the freeplay band, the stick force changes. As you displace the stick, the force increases until it reaches the breakout force. The breakout force (the minimum force needed to make the airplane respond) is reached at the end of the freeplay band, where the control surface deflects and the airplane responds.
Adding springs to your control system changes the force you feel, but springs won’t do anything about the tiny gaps in the linkage and the freeplay they create.
By the Numbers-Freeplay
- Trim your airplane for hands-free flight at the condition of interest (i.e., level cruise or full-flaps final approach speed).
- Watching your airplane’s nose relative to the horizon, slowly increase your pull force on the stick.
- When the nose begins to move upward, note the stick location. This position is the aft end of the freeplay band for the established flight condition.
- 4. As soon as the nose begins to move, relax your pull so you can maintain your test condition for the next test. If the plane has already begun to climb and decelerate, reestablish the test condition without adjusting the engine controls, configuration, or trim.
- Repeat the test, but this time push on the stick until the nose begins to drop. Note the stick location. This position is the forward end of the freeplay band. The freeplay band is the difference between the aft and forward ends.
- Repeat the test for left, then right stick, looking for the start of a roll rate.
- Repeat the test for left, then right pedal, looking for the start of a yaw rate.
- You can make these measurements by securing a tape measure to the instrument panel or cockpit sidewall and holding the free end next to the stick with your nonflying hand. This might be overkill for your purposes. Visually noting the results of your freeplay tests might supply all the information you need. You can also perform the tests without watching the stick. Can you detect the stick movement within the freeplay band by feel alone? If you detect this movement, especially when not specifically trying to, the freeplay band is probably hampering your flying precision. Even if you only notice the movement when specifically looking for it, the freeplay can still affect your flying.
The cure for excessive freeplay is to minimize all those linkage gaps. This doesn’t mean you should install over- or undersized components, but you can replace worn components with new ones and make sure control cables have the recommended tension. Don’t overtension the system; it reduces freeplay, but it adds control system friction, which introduces its own flight control and apparent stability complications.
When the airplane is trimmed, the stick/yoke and pedals are at specific locations. Absolute centering occurs when a displaced flight control returns to its exact predisplaced position. If the control returns to a position close to the exact position, it has positive centering.
A flight control system with positive centering has a centering band or range of positions the control can occupy hands- or feet-free. When you initially trim the airplane, the stick could be anywhere in the centering band.
Let’s say the stick is at the forward end of the band when you’re trimmed for level flight. Using a reasonable bank angle, you change heading (without changing the power setting). During the turn it’s likely you’ll have to pull the stick back a little to maintain altitude. As you roll out and release this stick pull, the stick moves forward a bit. Now the stick is at the aft end of the centering band.
If the longitudinal flight control (elevator) linkage has no freeplay and the stick is now in a different position, it 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 stick did not center absolutely.
To return to your original level-flight condition, you’ll have to experiment by relocating the stick within the centering band. This can be annoying and sometimes time-consuming because it’s a trial-and-error process. You place the stick somewhere within the centering band and note whether or not this position results in level flight at the original airspeed. If not, you try another stick position.
An improperly balanced control surface can cause nonabsolute centering in this case, but friction is the usual culprit. Too much friction prevents the displaced control from returning exactly to its predisplaced position, and it’s up to the pilot to finish the job.
Freeplay in the flight control linkage can also affect centering characteristics, depending on where in the control linkage the slop is and the location of other linkage components. For example, take an airplane with centering springs attached to its control stick, looseness in its elevator linkage, and a sticky elevator hinge. After deflecting the elevator, the stick returns to its centered position because the centering springs are doing their job. The elevator is not absolutely centered because of the hinge’s friction and the linkage’s looseness. To regain the originally trimmed position you’ll have to experiment by nudging the stick. With each nudge, the elevator deflection changes slightly, but the stick returns to the same position, thanks to the springs.
If the springs are installed near the elevator and the linkage looseness is near the stick, the elevator may have absolute centering but the stick may not. In this case the stick centering band will be a range of positions within which there is no change in elevator deflection. In this airplane, when you release the back stick after making the same small heading change, the airplane would remain trimmed for level flight because the elevator returned absolutely to its level-flight trim condition. The stick’s position depends on where it was in the centering band before the turn. It may be further aft than it was before the maneuver, and you probably wouldn’t even notice the small change in stick position.
Centering issues are just as valid for the ailerons and rudder. Positive, but not absolute, lateral stick/yoke or aileron centering can result in residual roll rates. Use the same trial-and- error technique to eliminate them. In the roll, however, you can see the result of a stick nudge immediately.
A poorly centering rudder will probably cause a tiny yaw rate leading to a small sideslip. Sideslip is when the relative wind comes 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 interpret the roll as an aileron centering issue and try to correct it with lateral stick or yoke. You can see how this one might take a bit longer to sort out.
By the Numbers-Centering
- Trim your airplane for hands-free flight at the condition of interest (i.e., level cruise or full-flaps final approach speed).
- Slowly pull the stick aft a little-an inch should be plenty-and slowly relax your pull. Note the stick position. This is the aft end of the centering band.
- Slowly push the stick forward a little, and slowly relax your push. Note the stick position. This is the forward end of the centering band.
- The centering band is the difference between the aft and forward ends.
- Repeat the test for left, then right stick.
- Repeat the test for left, then right pedal.
You can use the tape measure technique described for freeplay measurements, but again, this might not be necessary. For reasonably small centering bands, the important thing is the centering band size relative to the freeplay band.
Freeplay & Centering
Let’s say the freeplay band is within the centering band. After you displace a cockpit control it returns to the edge of the centering band, but this position is outside the freeplay band. That means the control surface is not at its originally trimmed deflection-and the airplane is out of trim. Depending on the control involved, the result will be a hands- or feet-free pitch, roll, or yaw rate. At this point you’ll have to experiment with cockpit control positions until you find a position within the freeplay band.
Things aren’t quite as bad if the centering band is inside the freeplay band. In this case the cockpit control returns to a position within the freeplay band, so residual pitch, roll, or yaw rates should not occur. In both cases we’re assuming the control surface returns exactly to its predeflected position when the cockpit control is within the freeplay band. Again, this depends on the location of flight control friction, looseness, springs, etc.
While we’d all like absolute centering and no freeplay, we don’t always get it, even when airplane assembly carefully follows the manufacturer’s instructions. Fortunately, flying is a dynamic evolution. As pilots we do whatever it takes to achieve the result we want. We usually don’t think about centering and freeplay bands, we just nudge and bump and tweak 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 can help us troubleshoot an apparent trim problem or relieve some frustration from less-than-desired flying precision. Once we know the source of the problem, it’s a lot easier to fix it or compensate for it. Either way, your flying becomes more enjoyable.
Next month we’ll take a break from performance and flying qualities flight testing techniques and talk about something equally significant-cockpit evaluations. We spend 100 percent of our flying time in this little room, so its fit and function have a dramatic influence on everything from comfort to how well we fly our airplanes. Whether you’re shopping for your homebuilt project, already building one, or flying your finished project, a thorough cockpit evaluation can reveal some important issues you may not have considered.