AIRPLANE AXESAn aircraft in flight is free to rotate about three axes, and its flight controls are designed to allow the pilot to control its rotation about each axis. The three axes pass through a common reference point called the center of gravity (CG), which is the theoretical point where the entire weight of the aircraft is considered to be concentrated. Since all three axes pass through the CG, the airplane always moves about its CG, regardless of which axis is involved.

LONGITUDINAL AXISThe longitudinal axis is a straight line passing through the fuselage from nose to tail. Motion about the longitudinal axis is called roll, and this axis is often referred to as the roll axis.

The ailerons cause an airplane to roll about the longitudinal axis. The primary purpose of the ailerons is to bank the wing, causing the airplane to turn.

LATERAL AXISThe lateral axis is a straight line extending parallel to the wing span at right angles to the longitudinal axis. You can think of it as extending from wingtip to wingtip. Motion about the lateral axis is called pitch and this axis is referred to as the pitch axis.

The elevators cause an airplane to pitch about the lateral axis. The primary purpose of the elevators is to change the angle of attack, and thereby control airspeed.

VERTICAL AXISThe vertical axis is the third axis, and is perpendicular to the other two. It can be envisioned as a straight line from the top to the bottom of the airplane. Motion about the vertical axis is called yaw, and so this axis is sometimes called the yaw axis.

The rudder causes an airplane to yaw about the vertical axis. The primary purpose of the rudder is to counteract aileron drag and keep the fuselage streamlined with the relative wind. This improves the quality of turns and reduces drag.

STABILITY AND CONTROL

Stability, maneuverability and controllability all refer to movement of the aircraft about one or more of the three axes of rotation.

Stability is the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed. Maneuverability is the characteristic of an airplane that permits the pilot to easily move the airplane about its axes and to withstand the stress resulting from these maneuvers. Controllability is the capability of an airplane to respond to the pilot's control inputs. Aircraft are not designed to be stable in their attitude with respect to the earth, but they are stable with respect to the relative wind. An aircraft that is stable in pitch will return to the angle of attack for which it is trimmed any time it is disturbed from this angle. An aircraft that is stable in roll will tend to return to a wings-level attitude. An aircraft that is directionally stable will tend to weather-vane, so as to align its fuselage with the relative wind.

TYPES OF STABILITY

STATIC STABILITYAn airplane is in equilibrium when there are no forces trying to disturb its condition of steady flight. If the plane is disturbed from steady flight, static stability will try to return it to its original attitude.

DYNAMIC STABILITYWhile static stability creates a force that tends to return the aircraft to its original attitude, dynamic stability determines how it will return. Dynamic stability is concerned with the way the restorative forces act with regard to time.

CONDITIONS OF STABILITY

Both static and dynamic stability can be designed in one of three states; positive, negative or neutral. At times, turbulence or erratic movement causes buffeting in an airplane. If an aircraft is designed with positive stability, it will return to its original flight condition when turbulence ceases. Positive stability is desirable for most aircraft but advanced fighter aircraft with computer augmented flight controls may employ neutral or negative stability to enhance combat maneuverability.

POSITIVE STABILITYPositive stability can be illustrated by considering the action of a ball in a U-shaped trough. If the ball is rolled up to one edge of the trough and released, positive static stability will cause it to roll back down towards its original

position.

Positive static and dynamic stability, as illustrated by the ball in a trough, is a desirable characteristic for most airplanes. Most airplanes are designed to exhibit the damped oscillation form of stability when disturbed from pitch equilibrium.

When the ball returns to its original position at the bottom of the trough, it will probably overshoot its position of equilibrium and start up the opposite side. As soon as it starts up the opposite slope, positive static stability will tend to return it to the bottom. The ball will rock back and forth, each time moving a shorter distance up the slope, until it finally stops at the bottom a demonstration of positive dynamic stability.

NEGATIVE STABILITYIf the ball is released from the top of a hill, it will not tend to return to its original position. In this condition, the ball is said to have negative static stability, or to be statically unstable. If the corrective forces increase with time, the body has negative dynamic stability. Depending on the static stability of the body, the corrective forces can either diverge or set up an oscillation that grows larger over time. Neither condition is desirable in most airplanes.

Negative stability, as illustrated by a ball rolling off the crest of a hill, is an undesirable characteristic in airplanes. A pilot would be very likely to lose control of an airplane with negative stability.

NEUTRAL STABILITYIf the ball is moved across a perfectly level surface, it will not tend to return to its original position, nor will it tend to move further away. This illustrates neutral static stability. The corrective forces of a body with neutral dynamic stability neither increase nor decrease with time. Energy is not added to the body or taken away by any form of damping.

An object that has neutral stability remains displaced from its original state whenever a force is applied. A neutrally stable airplane would be difficult to control and would probably require computer-augmented flight controls.

STABILITY ABOUT THE AXESThe longitudinal, or pitch, stability of an airplane determines its ability to be trimmed to fly hands-off at any airspeed. Because the wing's center of lift is behind the center of gravity, the wing produces a nose-down pitching moment. This pitching moment is counter-acted by a down load produced by the horizontal tail surfaces. Elevator trim can be adjusted by the pilot to produce the required down load at any speed, thereby balancing

the plane so that it will maintain level flight with little or no control input.

With the center of pressure aft of the center of gravity, an airplane wing produces a nose-down pitching moment. To counter-balance this moment, the tail down load can be varied to make the airplane longitudinally stable over a

wide range of airspeeds.

Lateral, or roll, stability is provided primarily by dihedral in the wings. Dihedral is the upward angle between the wing and the lateral axis of the airplane. The dihedral angle of most airplanes is usually just a few degrees. When an airplane enters a downward sideslip toward the low wing, the direction of relative wind changes. The

low wing experiences a higher angle of attack while the angle of attack on the high wing is reduced. The changes in angle of attack cause the low wing to generate more lift, at the same time the high wing generates less lift, and the combined forces tend to roll the aircraft back to a wings-level attitude.

Wing dihedral is a major contributor to lateral stability. Airplanes generally have less roll stability than pitch stability.

Because they are inherently more stable laterally, high-wing aircraft such as the Cessna 172 on the left are designed with less dihedral than the typical low-wing aircraft, as exhibited by the Beechcraft Bonanza on the right.

Directional stability is stability about the vertical axis and is provided primarily by the vertical tail, which causes the airplane to act much like a weather vane. Most airplanes are designed with more fuselage surface area behind the CG than forward of it.

When the airplane enters a sideslip, the relative wind strikes the side of the fuselage and the vertical tail. Since the force exerted on the airplane aft of the CG tends to cause the nose to turn towards the sideslip, this aligns the fuselage with the relative wind.

To be directionally stable, an airplane must have more surface area behind the CG than in front of it. When an airplaneenters a sideslip, the greater surface area behind the CG helps keep the airplane aligned with the relative wind.