Airplane Lift Flying

Posted by Admin on

Lift may be defined by the same Newtonian laws that refer to gravity. That is, a force is the product of a mass and acceleration. In this case, the air is the mass, and the wing provides the acceleration. The wing basically displaces air downward, and the resulting force is upward. The amount of lift is strongly affected by changing either part of the formula, that is mass, or acceleration. The mass of air that may be affected by the wing is increased or decreased by corresponding changes in the aircraft’s velocity. A faster plane encounters more air, enlarging the mass part of the formula, and therefore may produce more lift. An airplane flying relatively slow may have to compensate for the low air mass by imparting a correspondingly large acceleration to the air, which may be directly accomplished by increasing the angle of attack.

Figure depicts the angle of attack of a wing. By definition, the angle of attack is the angle created between the chord line of the wing and the relative wind. The chord line is an imaginary straight line that extends from the trailing edge of the wing through the leading edge. The relative wind is the direction of the airflow caused by the forward velocity of the airplane.

As the angle of attack increases, the lift generated by the wing also increases, up to a point. As the angle of attack increases, the smooth airflow present at lower angles of attack begins to become turbulent. Beyond a certain angle of attack, known as the critical angle of attack, there is so much turbulence created in the airflow over the wing that the amount of lift generated may be sharply reduced.

Figure depicts a wing at progressively higher angles of attack, up to the critical angle of attack, and subsequent disruption of smooth airflow. Although highly specific to a particular airplane and configuration, for most general-aviation airplanes, the critical angle of attack is approximately 17 degrees.

The disruption of an otherwise smooth airflow over the wing due to excessive angle of attack is called a stall. Wings generally do not stall all at once. Instead, the turbulent flow of air works its way across the wing, in patterns specific to a given wing shape. Aircraft engineers pick a wing shape, or planform, that results in characteristics best suited for the plane.

Figure shows six different wing planforms and the pattern of stall progression on each of them at critical angles of attack. Notice that the stall normally begins on the aft portion of the wing and moves forward toward the wing’s leading edge. On some planforms, the stall occurs first near the wingtips and moves inward. 

Manufacturers of light general-aviation aircraft normally try to design the wing so that the stall begins near the fuselage and progresses outwards toward the wing-tips, ensuring that the ailerons remain effective as the stall develops. A wing planform that possesses undesirable stalling characteristics may be altered by twisting or other aerodynamic modifications until its stall behavior is acceptable.

« Prev Post