Control

Control is the ability of a pilot to change the airplane's flight conditions. It is brought about by the use of devices that alter the lift force on the surface to which the device is attached.

Familiar controls include the elevator to provide longitudinal control (in pitch), the ailerons to provide lateral control (in roll), and the rudder to provide directional control (in yaw).

The pilot's link to the control surfaces is by use of the control stick and rudder pedals. From the pilot's point of view, if he pulls the control stick back, the elevator turns upward, giving a negative camber to the entire horizontal tail surface and producing a downward lift. This, in turn, produces a nose-up moment about the airplane's center of gravity and the airplane pitches upwards. A sideward motion of the control stick results in the movement of one aileron up and the other down. This reduces the camber of one wing while it increases the camber of the other wing. One wing then produces more lift than the other and a rolling moment results. This condition causes the airplane to roll about its longitudinal axis in the direction toward which the control stick was pushed. Pushing on the rudder pedals will deflect the rudder. If the pilot pushes the right pedal forward (the left pedal comes back), the rudder deflects to the right. This movement increases the vertical tail camber and a tail force to the left results. A moment arises that yaws the nose to the right and hence, the airplane turns right.

Control effectiveness is a measure of how well a control surface, e.g., the rudder, elevator, or aileron, does its job. In general, the larger the control surface is with respect to the entire surface to which it is fitted, the greater the control effectiveness. Also, high-aspect-ratio control surfaces possess greater control effectiveness than low-aspect-ratio surfaces. 

Whenever a pilot deflects a control surface into the airflow, a pressure distribution will be set up that tends to force the control surface back to its original position. The effort necessary to hold a particular control surface in the desired position may vary depending upon the control surface design. Not only must the pilot be able to deflect the surface at will, but the effort should also be small enough to ensure that the pilot does not tire. One way to reduce the necessary effort is through aerodynamic balance. With aerodynamic balance, the hinge of the control surface is set so that when its surface is deflected, the air that strikes the surface in front of the hinge creates a pressure distribution, hence a force, that helps turn the surface even more. This counteracts the force behind the control surface tending to reduce the control surface deflection. By careful design, the pilot-supplied effort is considerably reduced. However, care must be exercised so that the controls are not "too light" (too little effort needed to move them) lest the pilot unwittingly overcontrol the airplane to its destruction. The control systems of today's airplanes are power-operated and, whether aerodynamic balance is used or not, the pilot-felt control forces are small. In fact, artificial feel is incorporated into the controls so that the pilot has a sense of feel in the controls.

Another way to reduce the effort required by the pilot is through mass balance, which is used to prevent flutter of the surface that may occur due to accelerations of the airplane. It is a dynamic effect, and a control surface that moves about on its own may lead to dynamic instability of the airplane. The solution is to move the control surface center of gravity near or in front of the hinge line. This may be accomplished by adding lead in front of the hinge line or by using small mass balances.

Tabs are auxiliary control surfaces placed at the trailing edges of the primary control surfaces. Tabs serve two purposes: (1) to balance and (2) to trim. Balance tabs are set up to move opposite and proportional to the primary control surface movement. They are used to assist the pilot in moving the control surface and in reducing the amount of force that the pilot needs to apply to the stick. If the pilot wishes, for example, to move the elevator down, the balance tab will deflect upward as the elevator deflects downward and the pressure distribution set up will create a force, hence moment, to move the control surface down. Because they are placed at the trailing edge, balance tabs possess long moment arms and are very powerful in action.

Trim tabs are used to reduce the force the pilot applies to the stick to zero for particular chosen flight conditions. They are very important since they ensure that the pilot will not tire in holding steady flight. Trim tabs may be set when the airplane is on the ground or may be manually operated and set by the pilot.

Some control devices do not fall into the conventional categories outlined above. They are used in unusual flight circumstances or for added control advantages. These include spoilers, all-moving surfaces, reaction controls, and the butterfly tail.

Spoilers are used to reduce or "dump" the lift on a wing by altering the pressure distribution. They are useful on gliders to vary the lift-to-drag ratio for altitude control and on airliners on landing to reduce lift quickly to prevent the airplane from bouncing into the air. Spoilers are also useful in lateral (roll) control. At low speeds, ailerons are the primary lateral control devices. However, at high speeds, ailerons may cause bending moments on the wing that distort the wing structure. At transonic speeds, compressibility effects may limit their effectiveness. Spoilers may be used to avoid these disadvantages. By reducing the lift on one wing, the spoiler will cause a net rolling moment to roll the airplane about its longitudinal axis.

Conventional control surfaces are, as a group, considerably less effective at high speeds where compressibility effects are dominant. The all-moving control surface, found on high-speed aircraft, operates differently and is more effective. Whereas the conventional control surface changes lift by a change in camber, the all-moving control surface controls lift by changing the angle-of-attack. Examples are to be seen on the horizontal tail surfaces of the F-4 Phantom and the F-14A airplanes. By being able to change its angle of attack, the all-moving surfaces can remain out of a stalled condition. The all-moving horizontal tails may be moved independently as well to provide lateral control.

At low dynamic pressures, aerodynamic control surfaces become largely ineffective because only small forces and moments are present. Under these conditions, reaction control devices may be used. These are small rockets placed at the extremities of the aircraft to produce the required moments necessary to turn the airplane about each of its axes. At zero or low speeds, the Hawker Harrier vertical takeoff and landing (VTOL) airplane uses reaction rockets placed in the nose, wing tips, and tail.

The North American X-15 rocket plane used reaction controls for its high-altitude flights where the air density was so low as to render the aerodynamic control surfaces useless. In the same manner, the Space Shuttle uses reaction controls for the same reason to change its pitch, yaw, and roll attitudes.

The butterfly tail is an interesting variation of the conventional control system since it combines the functions of the vertical and horizontal tail. The advantages claimed are reduced weight and drag. However, there are increased problems in cross-coupling of the pitch, yaw, and roll motions and reduced directional dynamic stability. To pitch up or down, both control surfaces are moved up or down together. To yaw right or left the "ruddervators" as they are called are moved in opposite directions through equal deflections.

In summary, many factors influence the design of an airplane. The final design is at best a compromise to often-conflicting requirements. As one moves toward multimissioned airplanes, the compromises become more frequent.

—Adapted from Talay, Theodore A. Introduction to the Aerodynamics of Flight. SP-367, Scientific and Technical Information Office, National Aeronautics and Space Administration, Washington, D.C. 1975

For Further Reading:

Smith, Hubert. The Illustrated Guide to Aerodynamics, Blue Ridge Summit, Pa.: TAB Books, 1992.

Wegener, Peter P. What Makes Airplanes Fly? New York: Springer-Verlag, 1991.

“Ailerons.” http://www.grc.nasa.gov/WWW/K-12/airplane/alr.html

“Aircraft Rotations.” http://www.grc.nasa.gov/WWW/K-12/airplane/rotations.html

“Horizontal Stabilizer – Elevator.” http://www.grc.nasa.gov/WWW/K-12/airplane/elv.html

“Vertical Stabilizer – Rudder.” http://www.grc.nasa.gov/WWW/K-12/airplane/rud.html