Stability Stability and Controllability
Besides being supported in flight by lift and propelled through the air by thrust, an airplane must be stable and controllable since it is free to revolve or move around three axes. These axes may be thought of as axles around which the airplane revolves, much like a wheel does. Each axis is perpendicular to the other two and all three intersect at the airplane's center of gravity (CG). The point around which the airplane's weight is evenly distributed or balanced is considered the CG of the airplane.

The axis which extends lengthwise through the fuselage from the nose to the tail is the longitudinal axis. The axis extending through the fuselage from wingtip to wingtip is the lateral axis. The axis which passes vertically through the fuselage at the center of gravity is the vertical axis (Fig. 3-8).

Rotation about the airplane's longitudinal axis is roll, rotation about its lateral axis is pitch, and rotation about its vertical axis is yaw.

   Because of their ability to revolve about these axes, all airplanes must possess stability in varying degrees for safety and ease of operation. An unstable airplane would require that the pilot continually vary pressures on the flight controls and consequently would be difficult to control. The term "stability" means the ability of the airplane to return of its own accord to its original condition of flight, or the normal flight attitude, after it has been disturbed by some outside force. A ball in a round bowl is considered stable because after being pushed to one side it will roll back and forth until it finally comes to rest at the center of the bowl.

   If an arrow having no feathered tail is shot from a bow, it usually will wobble or fall end over end as it travels, since there is no force produced to bring it back to its original point first travel. The arrow is made stable, however, by adding pieces of feather near the rear of its shaft. Then, when the arrow is shot and begins to wobble, turn, or yaw, the air strikes the tail feathers at an angle and deflects the feather end of the shaft to turn the arrow back to a straight path. This corrective action continues as long as the arrow has sufficient forward motion.

   An airplane wing by itself is also unstable. It would flip over and continue to flip end over end as it flutters to the ground. Like the unstable arrow, the unstable wing needs some kind of "tail feathers" to balance it and keep it on a straight course. Like the stable arrow, airplanes have their "tail feathers" in the form of horizontal and vertical surfaces located at the rear of the fuselage. These surfaces are the horizontal stabilizer and the vertical stabilizer or fin.
If all the upward lift forces on the wing were concentrated in one place, there would be established a center of lift, which is usually called center of pressure (CP). In addition, if all the weight of the airplane were concentrated in one place, there would
be a center of weight, or as it is termed, center of gravity (CG). Rarely, though, are the CP and the CG located at the same point.

The locations of these centers in relation to each other have a significant effect on the stability of the airplane. If the center of the wing's lifting force (CP) is forward of the airplane's center of gravity (CG) - the airplane would always have a tendency to nose up and would have an inherent tendency to enter a stalled condition. Therefore, most airplanes are designed to have their CG located slightly forward of the CP, to create a nose down tendency so the airplane will have a natural tendency to pitch downward away from a stalling condition (Fig. 3-9). This provides a safety feature in the characteristics of the airplane.

While the airplane is flying within its range of normal speeds, the airflow exerts a downward force on the horizontal stabilizer; thus, at normal cruise speed it partially offsets the inherent nose heaviness of the airplane. In addition, many airplanes have the line of thrust located lower than the CG. In this situation the propeller's thrust provides a nose up pitching force to help overcome the inherent nose heaviness. With this balanced condition, the airplane characteristically will remain in level flight. However, when the power is reduced and the airspeed is decreased, the airflow exerts less downward force on the horizontal stabilizer. At the same time the nose up force of thrust is also decreased (Fig. 3-10). Due to this unbalanced condition the airplane's nose will tend to lower and the airplane will enter a descent of its own accord.

During the descent the airspeed will begin increasing. As a result, the downward force increases on the horizontal stabilizer, causing the nose to rise. This process will continue again and again if the airplane is dynamically stable (and if the pilot takes no action to stop it), but with each oscillation the nose up and nose down motion becomes less and less. Eventually, the airplane's descent attitude and airspeed will stabilize.

 Like the feathered arrow, the most important factor producing directional stability is the weathervaning effect created by the fuselage and vertical fin of the airplane (Fig. 3-11). It keeps the airplane headed into the relative wind. If the airplane yaws, or skids, the sudden rush of air against the surface of the fuselage and fin quickly forces the airplane back to its original direction of flight.

Generally, in straight and level flight, the wings on each side of the airplane have identical angles of attack and are developing the same amount of lift. This laterally balanced condition normally keeps the airplane level. Occasionally, though, a gust of air will upset this balance by increasing the lift on one wing and cause the airplane to roll around its longitudinal axis. A well designed airplane has certain design features to counteract this momentary unbalanced condition and return the airplane to a wings level attitude.

Most airplanes are designed so that the outer tips of the wings are higher than the wing roots attached to the fuselage. The upward angle thus formed by the wings is called the dihedral, and is usually only a few degrees (Fig. 3-12).

   The rolling action of an airplane caused by gusts is constantly being corrected by the dihedral of the wings. If one wing gets lower than the other when the airplane is flying straight, it will have a different attitude in relation to the oncoming air. The result is that the lowered wing has a greater angle of attack and thus more lift than the raised wing and consequently will rise.

If this rising action causes the wing to go past the level attitude, the opposite wing will then have a greater angle of attack and more lift. A dynamically stable airplane will oscillate less and less and eventually will return to its original position as the oscillation dampens.

   Although stability in an airplane is desirable, it must not be so strong that the pilot cannot overcome the inherent stability. The pilot must be able to control or maneuver the airplane at will about the airplane's three axes.

   Roll, pitch, and yaw, the motions of an airplane about its longitudinal, lateral, and vertical axes, are controlled by the three control surfaces. This will be discussed in the chapter on The Effect and Use of Controls.