Flight Controls


Turning the yoke moves cables, guided by a series of pulleys, that deflect one aileron up and the opposite down.

When you turn the yoke left or right, you activate a spocket connected by a chain and a cable to a complementary chain and sproket on the copilot side -- that's why the yokes turn together.

Pushing or pulling the yoke moves other cables that either lower or raise the elevator, causing the airplane to descend or climb.

How your airplane turns, climbs, and descends

By Marc E. Cook

We take it for granted that a twist on the airplane's control yoke or a nudge on its stick raises the nose or drops a wing tip. We take it for granted that a light push on the rudder pedals compensates for a crosswind or centers the ball in the turn coordinator. In much the same way we anticipate how simple it is to steer a car by the wheel or a bicycle by the handlebar, pilots seldom give a second thought to the airplane's flight-control system. And yet in the 100-plus years since the Wrights experimented with kites and gliders, basic aircraft flight-control systems have evolved tremendously and have been developed to be simple, lightweight, and-perhaps most important-utterly reliable.

The aim is simple: Enable the pilot, sitting comfortably in the cockpit with intuitive controls, to make the airplane go precisely where he or she desires. A tremendous amount of design work and testing goes into a control system, partly because it is a critical system-and therefore cannot be allowed to fail-but also because of the inherent contradictions in the mission. The control system must be operable by a pilot of average size yet must apply sufficient leverage to move the control surfaces to maximum deflection within a prescribed range of operating speeds. At the same time, the system must also provide intuitive feedback to the pilot and present control forces that are neither too light (causing overcontrolling of the airplane) nor too heavy (preventing the pilot from obtaining full movement of the control surfaces). Although certification standards prevent aircraft with extreme forces from getting into production, there's still a wide range in which the engineers are allowed to work, and as a result, some airplanes feel better than others.

Three axes, three sets of controls (mostly)

With a few exceptions, light aircraft flight control systems are very much alike, using hinged flaps along the trailing edge of each wing (you already know them as ailerons) to control the airplane around its roll axis. That is, the ailerons are responsible for one wing descending and one wing rising as you initiate a turn. To control the airplane in pitch, there is either a combination of a fixed horizontal stabilizer with a hinged, moveable elevator, or something known as a stabilator-basically a single-piece movable elevator whose leading edge drops as the trailing edge rises. And for yaw, the common form is a hinged rudder following obediently behind a fixed vertical stabilizer. (Incidentally, extensive aerodynamic studies and years of flight testing have shown the shape of the vertical stabilizer to be of comparatively minor importance to how the airplane flies, hence the longstanding tradition of making the vertical element sexy rather than purely functional.)

How Cessna does it

These controls need your input. Because Cessna's Skyhawk is arguably the Model T of contemporary light aircraft, and its longevity (and sheer sales volume) naturally causes every other aircraft designer to take notice of how Cessna did things, we'll use it as the central example.

The Skyhawk uses steel or stainless-steel cables almost exclusively to transmit movement at the yoke and rudder pedals to the flight controls. Cables are comparatively light and durable, made up of multiple lengths of wire wound into strands and then bundled together. For example, a common control cable is a 7319 flexible cable, made up of 19 wires in a strand, with six strands wound around a single central strand. The benefits of multiple strands and wires are strength and redundancy; for most applications, the system will tolerate multiple broken wires without the whole cable breaking. Moreover, cables can go just about anywhere in the airframe where there's a bit of free room between the cabin and the control surface.

Let's follow the 172's control path from the yoke through the airplane and see how the system works. Naturally, the yoke is free to move back and forth as well as turn left and right. To permit this, the end of the control column behind the panel contains a simple universal joint-a kind of double pivot that allows the column to rotate (for roll control) while also tilting slightly-attached to a large, stylized Y. One upper stem of the Y connects to the pilot's yoke while the other upper stem lies behind the copilot's yoke. The vertical element of this Y extends down to the cabin floor.

When you command the ailerons to move, you are actually rotating a small sprocket at the top of this Y, which is connected through a chain and a section of cable to a complementary chain and sprocket on the copilot side; this is why the yokes turn together. (It's important to understand that any looseness between the two yokes is because this interconnect cable is not tight enough, or the chains and sprockets are worn-not because of some control-system malady elsewhere.) The interconnected yokes are in turn hooked to the main control cables through special cable terminators and a series of pulleys-metal discs with grooved edges in which the cables ride. These pulleys rotate around a simple shaft bolted (in this case) to the control Y.

The aileron cables follow the control Y down to the floor and turn toward the tail over two more pulleys (each cable, left and right), back to a point even with the rearmost sill of the cabin doors. (In high-wing Cessnas, the structure around the door posts, in front of and behind the doors, carries not just the bulk of the structural loads of the airplane but houses several systems, including fuel lines, wiring to the wings-for lights, the flap motor, etc.-and the control cables). Then the cables are routed up the door sills to the ceiling, where they cross over-the cable running up the left door post crosses to head out to the right wing, and vice versa-and continue out to approximately the midpoint of the ailerons.

The cables terminate at something called a bellcrank-though it looks nothing like a bell-that transfers the motion of the cables to a small rod that pops out of the trailing edge of the wing and then attaches to the aileron itself. This is the small rod you gently twist during preflight to make sure the system is not bound. There is a third cable, strung between the ailerons, which ensures that there is sufficient tension in the system and that when one aileron is moving up the other absolutely, positively is moving down. (Incidentally, the geometry of the bellcrank in the wing is often designed to create differential control; that is, one aileron moves upward more than the opposing aileron moves downward. This differential action helps to counteract the aerodynamic effects of the aileron's being hinged at the top and to reduce adverse yaw.)

That's the roll system; let's look a bit closer at pitch. The bottom of the control Y pivots at the floor of the 172 and is attached there to a small metal tube (called a pushrod) that then connects to another bellcrank under the floor. This bellcrank accepts a pair of cables that make a 90-degree twist and travel into the tail cone toward the elevator. (This is one advantage of a cable system; it can be made to twist and turn at will.) The left and right elevator halves are bolted to a crossmember and then to another bellcrank that receives the aft end of the elevator cable. Pull back on the yoke, the control Y tilts back at the top and actually pulls the small linking pushrod forward, rotating the bellcrank and pulling on the up-elevator cable. Almost magically, the trailing edge of the elevator goes up, and the houses get smaller. (Ask your CFI for an explanation.) Similarly, the rudder system uses a pair of long cables that run back to the bottom of the rudder; the mechanism under your heels also ties to the nose gear. Finally, there's the elevator trim system, again operated by twin cables to an actuator in the tail that gradually draws the trim tab up or down. All told, there are six cables running down the tail cone of a 172.

How the other half lives

Cables are a fine, efficient way to move flight controls, but not every airplane uses them. The main reasons are that cables could fray and break, and they must be kept under proper tension for the system to work properly. Also, every time a given cable winds over a pulley, there's a bit of friction added to the system, which impairs feedback from the control surfaces. Improperly designed or maintained, a cable system can make the controls feel stiff and dull.

One alternative to cables is the so-called pushrod control system, which uses a series of metal tubes to carry all back-and-forth motion from the yoke to the control surface; they're much more solid than steel cables and anchored in such a way as to reduce lost movement. If you've ever had a chance to fly a Mooney and like the way it feels-a tight, direct response to your inputs-you can thank the pushrod control system. Unfortunately, the pushrod system demands careful alignment of all the bits and pieces, and becomes quite complicated in anything but a low-wing airplane. Finally, the geometry of this type of system inherently limits available leverage, so the careful dance between feel and control becomes an even more demanding ballet.

Built to be safe

Perhaps no scenario is more frightening than loss of basic airplane control, something all airframe engineers (and the FAA certification people) keep in mind. That's why most systems are either redundant by nature or extremely durable. For example, it's possible to control the airplane even if you lose rudder or aileron control. Because these are totally separate systems, the odds of losing both aileron and rudder control simultaneously are extremely low. Thankfully, it's fairly easy to keep the wings level with either the ailerons or rudder (see "Losing Control," June 2002 AOPA Flight Training).

What about pitch? For the most part the pitch system stands alone, but there are important safeguards built in. For example, if one pitch cable breaks, it's still possible to trim the airplane to maintain tension on the good system and complete the flight safely. Should both cables come adrift, it's still possible to fly the airplane with the trim system. Finally, should the pitch system jam, it's possible (although by no means easy) to fly using the trim system alone.

Do your part to minimize the risks with a thorough preflight inspection. Every control system should move freely but should not be loose. Each control should respond to the yoke smoothly, without jumps or tight spots. You should not hear cables slapping around or nasty crunching noises inside the wing or empennage as you move the surfaces from the outside. Verify that the controls are operating correctly by actually looking at them during the runup-so many pilots, both new and thoroughly experienced, literally just go through the motions. Now that you know a bit more about how the system works, don't be one of them: be curious and be smart.

Marc E. Cook, a pilot since 1987, has logged 3,000 hours flying a variety of light aircraft. A former senior editor for AOPA Pilot magazine, he now writes about aircraft, automobiles, and motorcycles. He is based in Long Beach, California.

Illustrations by David Diamond


Advertisement

Advertisement