Saturday, October 24, 2009

Stability (Rolling)

About the aircraft’s longitudinal axis, which extends nose of the aircraft to its tail, is called lateral This helps to stabilize the lateral or “rolling effect” wing gets lower than the wing on the opposite side aircraft. There are four main design factors that make laterally stable: dihedral, sweepback, keel effect, weight distribution.

Most common procedure for producing lateral stability builds the wings with an angle of one to three degrees perpendicular to the longitudinal axis. The wings on side of the aircraft join the fuselage to form a slight V or called “dihedral.” The amount of dihedral is measured angle made by each wing above a line parallel to the axis.

Involves a balance of lift created by the wings’ AOA side of the aircraft’s longitudinal axis. If a momentary wind forces one wing to rise and the other to lower, the banks. When the aircraft is banked without turning, tendency to sideslip or slide downward toward the lowered occurs. [Figure 4-25] Since the wings have dihedral, strikes the lower wing at a much greater AOA than the wing. The increased AOA on the lower wing creates than the higher wing. Increased lift causes the lower begin to rise upward. As the wings approach the position, the AOA on both wings once again are equal, the rolling tendency to subside. The effect of dihedral produce a rolling tendency to return the aircraft to a laterally balanced flight condition when a sideslip occurs.

The restoring force may move the low wing up too far, so that the opposite wing now goes down. If so, the process is repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached.

Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. The aircraft may be so stable laterally that it resists an intentional rolling motion. For this reason, aircraft that require fast roll or banking characteristics usually have less dihedral than those designed for less maneuverability.

Sweepback is an addition to the dihedral that increases the lift created when a wing drops from the level position. A sweptback wing is one in which the leading edge slopes backward. When a disturbance causes an aircraft with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises, and the aircraft is restored to its original flight attitude.

Sweepback also contributes to directional stability. When turbulence or rudder application causes the aircraft to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the right wing increases and it acquires more drag than the left wing. The additional drag on the right wing pulls it back, turning the aircraft back to its original path.

Keel Effect and Weight Distribution
An aircraft always has the tendency to turn the longitudinal axis of the aircraft into the relative wind. This “weather vane” tendency is similar to the keel of a ship and exerts a steadying influence on the aircraft laterally about the longitudinal axis. When the aircraft is disturbed and one wing dips, the fuselage weight acts like a pendulum returning the airplane to its original attitude.

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