Get to know this useful flight control
Airplanes have two sets of flight controls. Of course they do--left- and right-side yokes (or sticks), and rudder pedals. Well, yes, that's true of most airplanes, but what I have in mind is primary and secondary flight controls. As in elevator, ailerons, and rudder, the basic--primary--flight control surfaces that make the airplane pitch, roll, and yaw. Then there are the flaps and trim tabs, the special-purpose secondary flight controls. On heavier, high-performance aircraft secondary controls may also include wing leading-edge slats, spoilers, speed brakes, and other specialty devices. Secondary flight controls allow us to fine-tune aircraft attitude and configuration for more precise control. The secondary flight control we most often use--and sometimes abuse--is trim.
Ever heard an instructor declare, "Trim's your friend"? It's an apt description. A friend is someone you are in contact with frequently, someone who is helpful and can be relied upon to ease whatever burdens may be weighing you down. So it is with aircraft trim. We call on trim a lot when we fly because the results are helpful and reliable. In other words, trim is our friend.
The basic role of the aircraft trim system is to fine-tune the position of the primary flight control surfaces to zero out pitch, roll, and yaw moments. When the aircraft is trimmed it is in equilibrium; it does not want to rotate around its center of gravity. Trim eases the pilot's burden of having to apply constant pressure on the yoke or rudder pedals to maintain a desired attitude. It's like power steering--it makes driving the airplane a lot easier.
The trim control we are most familiar with, and most often use, is pitch trim. A relatively small movable tab on the elevator is adjusted from the cockpit to "hold" the elevator in the desired position for whatever we want the airplane to do--climb, fly level, or descend.
A few airplanes, notably Pipers and the Cessna Cardinal, employ a stabilator in place of the elevator. A stabilator is an all-moving horizontal stabilizer that functions as both elevator and stabilizer. As with an elevator, a stabilator has a movable pitch trim tab on the trailing edge, but there is a significant difference in a stabilator trim tab versus an elevator trim tab. Because the stabilator pivots laterally around its aerodynamic center, it gives the pilot a lot more control authority compared with a conventional elevator. That's the good news. The bad news is that all that power would make it easy to overcontrol in pitch because control forces do not change even as the pilot pulls or pushes harder on the yoke.
To prevent that overcontrolling, the pitch trim tab on a stabilator moves in the same direction as the stabilator. When you pull back on the yoke, the leading edge of the stabilator moves down and the trailing edge moves up, forcing the tail of the airplane down and the nose up. At the same time, the trailing edge of the stabilator trim tab moves up even higher than the trailing edge of the stabilator. This puts aerodynamic pressure on the trim tab, which tends to force the trailing edge of the stabilator down. The effect is to increase the effort required by the pilot to pull back on the yoke. Thanks to the movement of the trim tab, the increasingly heavier control forces serve as a natural impediment to overcontrolling in pitch.
Because it works against the stabilator, this type of trim tab is called an anti-servo tab. On a conventional elevator, the trim tab moves in the opposite direction from the elevator to assist its movement. This is known as a servo tab.
Whether elevator or stabilator, pitch trim is adjusted from the cockpit either mechanically by turning a wheel or crank, or electrically using a switch, or both. Few light aircraft have aileron (roll) trim or rudder (yaw) trim that can be adjusted in the cockpit. Instead, a small metal tab extending from the trailing edge of one aileron and another on the rudder must be adjusted--bent slightly in one direction or the other--on the ground to correct for an in-flight out-of-trim condition. This requires trial and error to get the adjustment correct--make an adjustment, fly the airplane to see if the adjustment is correct, and if not keep repeating the process until it is correct.
Proper use of pitch trim is simple enough. Before starting the take-off roll, make sure pitch trim is in the Takeoff position range. The exact position differs depending on the airplane's center of gravity and configuration (no flaps or partial flaps) on takeoff. What's the perfect takeoff trim setting? You'll know it when you feel it--the airplane rotates with ease and gently pitches up into an initial climb attitude.
Once the airplane flies out of ground effect, the pitch trim may need to be adjusted slightly to account for the different lift characteristics. Fine-tuning the pitch trim during climb makes for a smooth, constant-rate, minimal-effort ascent.
When leveling off in cruise, position the yoke so that the aircraft stops climbing. Sounds easy enough, but as the airplane accelerates you'll find it takes increasingly more forward pressure on the yoke to maintain level flight. That's the signal to adjust pitch trim. Roll in just enough so that the pressure subsides. The trim setting doesn't have to be perfect, because you'll soon be adjusting it again. In fact, as long as the airplane continues to accelerate, control forces will change. Only when the airplane has settled on a steady indicated airspeed can you make the final pitch trim adjustment.
How do you know the trim is set properly? Again, you'll know it when you feel it--no more holding pressure on the yoke to avoid climbing. In smooth air you should be able to take your hands off the yoke and marvel at how stable the airplane flies--no climbing or descending, no slight banking left or right, no heading creep.
You may have to make occasional small adjustments in trim to account for a slight shift in center of gravity as fuel is consumed, or fluctuations in airspeed caused by turbulence, but these will be minor.
When it comes time to descend, simply reduce power. With no change in trim the airplane wants to maintain the same airspeed, and the only way it can do that with reduced power is to descend. Some experimentation will reveal the correct power setting to achieve the desired rate of descent.
How is trim misused? By mistaking it for a primary control surface rather than a fine-tune adjustment of the proper primary control surface. The most common trim infraction is using pitch trim instead of the elevator to maintain level flight. Using pitch trim in place of the elevator means you'll always be chasing the correct trim setting, especially if airspeed is changing. Instead of it being a true friend, trim becomes an addiction that consumes all of your attention and concentration. Rather than you controlling the airplane using the primary flight controls, trim begins to control you because you are always adjusting, constantly searching for the right setting.
Get to know trim better. Treat it well and you'll be rewarded with a helpful, reliable new friend.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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