You’re probably familiar with your airplane’s primary control surfaces, what they are, where they are and how they work. (If not, now’s a good time to research the topic.) According to the FAA, primary controls are those “required to control an aircraft safely during flight,” and are the rudder, ailerons and the elevator/stabilator of a conventional airplane. The pitch-control surfaces of a canard-configured airplane usually are considered primary controls, also.
Meanwhile, your airplane probably has some other control surfaces—known as secondary controls—which serve various functions. They often augment the primary controls but also can be used to change a wing’s shape, for example, which can improve low-speed performance. They also can be used in emergency situations. In some airplanes, moving what you think is a primary control actually changes a secondary control surface’s position, which then moves the primary surface. Let’s explore.
Perhaps the most familiar secondary control surface is the trim tab, two of which are pictured in the image of a Cessna 172 Skyhawk’s tail, below. One of these two trim tabs is fixed and only adjustable on the ground while the other one can be adjusted from the cockpit while airborne. The fixed trim tab is the triangular piece of sheet metal on the bottom trailing edge of the rudder, while the movable trim tab is the one in the foreground, which is attached to the elevator and connected to the trim wheel in the cockpit.
Both of these trim tabs are used in the traditional fashion—to help alleviate control pressures. They do so by helping deflect their associated primary control surface. The direction the trim tabs themselves deflect—and the desired primary surface’s reaction—decide whether they are servo trim tabs or anti-servo. The sidebar on the opposite page distinguishes between these two basic types of trim tabs.
Fixed tabs also are used in other locations on an airframe, notably the ailerons. In both cases, also, they are servo trim tabs. Ideally, you wouldn’t need to adjust them at all; someone else likely has found the fixed tab’s sweet spot for that particular primary surface and adjusting it may result in that surface being out of trim. An example, again referring to the image below, would be if the Skyhawk pictured flew hands-off with its inclinometer ball slightly out of center, indicating too much or too little rudder deflection.
To adjust a fixed trim tab, you need two hands: one to hold the primary surface being trimmed and the other to bend the tab itself. You then need to fly the airplane—in the configuration and at the speed for which you’re trimming—to determine of the adjustment is correct. It’s entirely possible you bent the tab in the wrong direction, or bent it too much, resulting in an even worse out-of-trim condition. If you find an airplane that won’t fly hands-off with a centered ball, it might be time to get a mechanic to look at it and confirm it’s rigged properly.
Meanwhile, the movable trim tab on the elevator is a common feature on the personal airplanes many of us fly, and we know how it works, at least from the cockpit. Variations on controlling pitch trim include the Mooney and its all-flying tail—the entire tail assembly pivots to provide pitch trim—plus canard-equipped airplanes and those with three lifting surfaces, like the Piaggio P180 Avanti.
Flaps and slats
Wing-mounted flaps and slats are another secondary control surface. The flaps are usually mounted to the wing’s trailing edge while slats are leading-edge devices. Leading-edge flaps also can be employed, but usually are reserved for heavier transport airplanes. Boeing’s 727 sports a good example of a leading edge flap. The sidebar on page 22 has more details on this and other leading-edge devices. Of course, they’re usually used only during takeoff and landing, or slow flight.
Wing flaps come in many shapes and sizes—or not at all on popular airplanes like Mr. Piper’s J-3 Cub. Common types include the Fowler flaps of a Cessna 172, slotted flaps of a Piper Cherokee and split flaps of the Cessna 310. Extension and retraction is performed in a variety of ways, also, with manual “Johnson Bar” systems, electric motors turning jackscrews or hydraulic cylinders doing the actual work.
No matter how they’re placed, designed or actuated, wing flaps exist to increase the airfoil’s camber, thereby boosting lift at low speeds. The trade-off—which we learned in Aerodynamics 101—is that with increased lift comes increased drag. The different designs increase both airfoil characteristics to different degrees. Generally, the more slots, the greater lift, and the more deflection, the greater drag.
In addition to their work enabling lower approach and landing speeds by creating greater lift and drag, flaps may also change the wing’s center of pressure.
Panels mounted atop a wing can be used for primary and secondary roll control, as well as to minimize lift and maximize weight on the wheels upon landing. In gliders, they deploy symmetrically and are used as speed brakes; retracting them in a glider almost could be thought of as adding power. Thanks to their complexity, however, they’re rather rare on personal airplanes, with the Mitsubishi MU-2 series being a notable exception.
When used as a primary control surface as on the MU-2, one benefit of a spoiler is the absence of the adverse yaw experienced with conventional ailerons. You may also see spoilers in use aboard jet transports, which may use them in conjunction with ailerons for roll control at low speeds, or as the primary roll control when mounted inboard and flown at high speeds.
When not restricted to ground use or used for roll control, spoilers come in handy if you need to get down quickly. By disrupting lift and adding to drag, spoilers allow airplanes to descend rapidly without building up speed. This capability is useful on faster airplanes, like jet transports and military aircraft, but many personal airplanes can be modified with speed brakes, which extend upward from the top of the wing, like a spoiler. Precise Flight offers a wide range of speed brakes for common airplanes, including experimental models. In addition to their use on jet transports, the speed brake-style spoilers are especially popular among operators of turbocharged piston airplanes, because deploying speed brakes can increase the rate of descent without the need to reduce power.
Failures of secondary control systems are not nearly as problematic as might be failure of a primary control system. Some failure modes can be inconveniences, while others may require a hefty input to a primary control to overcome.
Perhaps the worst-case scenario for a secondary control problem is pitch trim failure, especially one in which the system is powered to one extreme of its allowable travel. If it happens quickly, surprising the crew, maintaining control could be problematic. That’s one reason autopilots and electric pitch trim systems aboard personal airplanes have a quick-disconnect button or switch within easy reach of the pilot. Additionally, related pitch trim systems may be powered down individually or disabled by a circuit breaker. A more common pitch trim problem is failure of the linkage to the tab. This will leave the trim tab in its last position, perhaps resulting in heavy controls at low speeds and higher-than-normal landing speeds.
Flap system failures are a bit simpler, meanwhile: They’ll either fail to go down or fail to come up. The most challenging flap-system failure mode would be a split-flap condition in which one flap is fully or partially extended while the other is fully or partially retracted. Called a “split-flap” condition, it can exert a rolling moment on the airframe the ailerons may be hard-pressed to counter. For this reason, many consider it ill-advised to deploy flaps while banked, like turning in a traffic pattern.
The obvious solution in a split-flap situation is to attempt deploying/retracting the “good” flap to the same position as the one with the failed mechanism. If you find yourself with flaps stuck in a full or partially extended position, remain within airspeed limitations and land without any further adjustment to the flap position. If one is extended while the other is fully retracted, and that’s the way it is, you’ll likely need some extra airspeed to maintain control during the landing to follow. Consider a longer runway, where a faster approach likely won’t make much difference.
As we’ve seen, secondary control surfaces generally are designed to make flying the airplane at different speeds a more pleasant experience. They help us minimize control forces in all flight regimes while allowing us to fly more slowly during takeoff and landing than would be possible without them. And in a pinch, they can save the day.
Consider an airplane with a jammed primary pitch control, perhaps the result of water in the system freezing at altitude. Judicious use of pitch trim as described in the sidebar on page 21 will give you back some control of the airplane’s willingness to climb or descend. In such a situation and until you get the hang of how much trim input does what, we’d suggest staying at altitude and getting some practice before trying to land.
Meanwhile, and especially if the primary pitch control is somehow disabled, be careful with things like full-power go-arounds, and extending landing gear or flaps. The pitch trim system only exerts so much authority, and it does it slowly. If your airplane’s pitch attitude changes with gear extension, you may want to perform that task at altitude, giving you plenty of time and space to use the trim system and set the desired attitude. Same for flap deployment, which is guaranteed to change the wing’s center of pressure location, perhaps exacerbating the situation.
For all that, secondary control systems have a major role to play in making our airplanes easier to fly and more capable. In a pinch, they can be used as primary controls, but they also have failure modes of their own.
The range of leading edge devices is as varied as those for the trailing edge. Perhaps the simplest is a fixed-cuff or drooping leading edge, shown at bottom right. This design builds into the leading edge a slight downward curve, which helps keep the airflow attached over the wing’s upper surface at higher angles of attack. You’ll easily find a cuffed leading edge on Cessna 172 Skyhawks built in or after 1973, or on many airplanes modified with a Robertson STOL kit.
Another leading-edge device popular on personal airplanes is the fixed slot. You’ll find them on airplane wings as varied as the Stinson 108-3’s partial-span design and even the Zenith STOL CH 701. Like the leading-edge cuff, they work by delaying the onset of boundary layer separation above the wing’s upper surface, which really is a fancy way of saying the wing can be pitched to a higher angle of attack before stalling.
The principal variation on the fixed slot is the movable slot, more commonly known as the slat. Thanks to the extra drag produced by a fixed slot, extension and retraction mechanisms were developed to reduce the slot’s drag at high speeds by eliminating it. This basic concept is employed on many jet transports these days but really hasn’t trickled down to personal airplanes, thanks in part to their complexity. A spring-loaded design, which retracts as airspeed and dynamic pressure build, is used on the Helio Courier.