Robert W. Mellette
LIFT is a force that opposes the force of GRAVITY. LIFT allows an aircraft to leave the earth and to remain in the air. LIFT is a resultant force caused by the movement of air over the wings of the aircraft. A small amount of LIFT is also produced by the body and tail surfaces of the aircraft.
The scientific explanation of LIFT rest in a large part on a theorem first proposed by a gifted Swiss mathematician Daniel Bernoulli. In 1738, Bernoulli published a paper entitled HYRODYNAMICA. The subject of his paper was a discussion of pure and applied fluid motion. One very important concept in this paper was the principle that, “The higher the speed of a flowing liquid or gas, the lower the pressure”. This theorem used in conjunction with the concept of a “boundary layer” advanced by Ludwig Prandtl in 1904 can be used to explain the phenomenon of LIFT. It is interesting to note that at the time Bernoulli lived (1700-1782) no airplanes existed.
There are several factors that determine the amount of LIFT generated by an aircraft. The primary factor is the surface area of the wing. Generally, the larger the surface area the greater the LIFT. Aware of this fact, the designer of “The Spirit of St. Louis” increased the wingspan from 42 feet to 46 feet to gain additional LIFT to help carry the heavy load of fuel necessary for the journey.
A second factor that determines the amount of LIFT is the relative speed of the airstream. In accordance with Daniel Bernoulli’s principle, the faster the airstream passes over the curved surface of the airfoil, the greater will be the pressure
differential
between the upper and lower surfaces of the wing. It is important to keep in mind that LIFT is produced by a difference in pressures on the upper and lower surfaces of the aircraft. The lower air pressure created on the upper surface of the wings results in an upward force from below-LIFT. This relative increase in pressure or LIFT on the bottom of the wing can counter the downward force of GRAVITY and support the WEIGHT of the aircraft and crew.
A third factor is “THE ANGLE OF ATTACK” or the angle formed by the chord of the airfoil and the direction of the relative wind. A high angle of attack will increase the amount of LIFT, there is a point at which increasing the angle of attack will have a negative if not disastrous result. At a certain point, a high angle of attack will cause a separation of the “boundary layer” of air that surrounds the airfoil. Normally, the boundary layer forms a thin stratum of air immediately adjacent to the surface of the airfoil. The air flow right at the surface of the airfoil actually adheres to the skin of the wing. This is due to the frictional forces involved between the two surfaces. At this interface the velocity of the airstream is actually zero. When this boundary layer is disturbed,the flow of air may separate from the wing and form a turbulent pattern. This turbulence can increase to a point where there is a loss of LIFT where it is taking place. The angle at which the airflow begins to “burple” or become turbulent is called the “CRITICAL ANGLE OF ATTACK”. When the air moving over the entire surface of the wing is turbulent, the stall is complete and there is no longer “LIFT” to support the WEIGHT of the aircraft. There are maneuvers a pilot can execute to recover from a stall. One way is to drop the nose of the aircraft to decrease the angle of attack. Another is to accelerate the speed of the aircraft. If the aircraft is already moving at full throttle, the only way to regain LIFT is to drop the nose. A stall condition during landing is very serious and pilots need to guard against it.