Of the four forces acting on an aircraft during flight, lift is generally given the most attention, both in aircraft design and engineering discussions. In fact, every external surface of the plane generates lift in varying degrees, with the wings accounting for the highest production. Without lift, the aircraft would never be able to leave the ground, and when this mechanical force is compromised during flight, the results can prove to be catastrophic. In this blog, we will discuss everything you need to know about lift and how it affects flight.
Weight and lift are direct opposites of each other, in that if one were to set the magnitudes of the two forces equal, the airplane would have no net movement on the vertical axis. Therefore, in order for an aircraft to increase in elevation, the sum total of all lift vectors must be greater than those of its weight. The general formula for weight is equal to the mass at any given time during the flight, times the acceleration due to gravity, which is 9.8m/s^2. Conversely, the formula for lift is equal to the lift coefficient times the air density, velocity, and area of wings divided by two. Simply put, aircraft designers and pilots may optimize lift by modulating the two controlled variables, velocity, and wing area.
While the general formulas for lift and weight suffice at a macro scale, the most important concept to understand with regard to lift is what occurs at the wing. Regardless of wing design, which may vary significantly between aircraft, all wings are considered airfoils. An airfoil is designed to have different pressures and fluid velocities to varying points of the structure. Represented algebraically, four variables may be produced from any given airfoil: P1, P2, V1, & V2. P1 and P2 represent pressure which may be measured on top of the airfoil, while V1 and V2 describe fluid velocity which is considered under the structure. Taken together, these variables may be plugged into Bernoulli's equation, that of which eloquently represents how wings produce lift.
Bernoulli's equation is stated as P2-P1=-1/2p(V2-V1)^2. Wings are designed to force air molecules flowing over the top of the structure to travel a further distance than those on the bottom. This causes a higher velocity and lower pressure on top of the airfoil, while the opposite is true for the bottom. The pressure differential created by this dynamic interplay of variables is the primary cause of lift. Another factor playing into the magnitude of lift is the angle of attack, which describes the angle that air takes as it travels from the leading and trailing end of the wing. As the angle of attack increases, so does the lift coefficient.
As is the case with other physical forces in aviation, lift does produce an unintended side effect known as lift-induced drag. This phenomenon is caused by constant vortices forming around the wingtips. These vortices generally tend to gather and orient in a downward direction, causing a perpendicular, backward-facing force to occur. Uniquely, this type of drag has an inverse relation to airspeed, but is directly related to the angle of attack.
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