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Lift is proportional to the density of the air and approximately proportional to the square of the flow speed. Lift also depends on the size of the wing, being generally proportional to the wing's area projected in the lift direction. In calculations it is convenient to quantify lift in terms of a lift coefficient based on these factors.
The section lift coefficient is based on two-dimensional flow over a wing of infinite span and non-varying cross-section so the lift is independent of spanwise effects and is defined in terms of ′, the lift force per unit span of the wing. The definition becomes
Lifting line theory supposes wings that are long and thin with negligible fuselage, akin to a thin bar (the eponymous "lifting line") of span 2s driven through the fluid. . From the Kutta–Joukowski theorem, the lift L(y) on a 2-dimensional segment of the wing at distance y from the fuselage is proportional to the circulation Γ(y) about the bar a
Wing loading is a useful measure of the stalling speed of an aircraft. Wings generate lift owing to the motion of air around the wing. Larger wings move more air, so an aircraft with a large wing area relative to its mass (i.e., low wing loading) will have a lower stalling speed.
In this case the initial lift is one half of the final lift given by the Kutta–Joukowski formula. [10] The lift attains 90% of its steady state value when the wing has traveled a distance of about seven chord lengths. Impulsively started flow at large angle of attack
The distribution of forces on a wing in flight are both complex and varying. This image shows the forces for two typical airfoils, a symmetrical design on the left, and an asymmetrical design more typical of low-speed designs on the right. This diagram shows only the lift components; the similar drag considerations are not illustrated.
Most importantly, the maximum lift-to-drag ratio is independent of the weight of the aircraft, the area of the wing, or the wing loading. It can be shown that two main drivers of maximum lift-to-drag ratio for a fixed wing aircraft are wingspan and total wetted area. One method for estimating the zero-lift drag coefficient of an aircraft is the ...
The red vector labeled "L eff" is the actual lift on the wing; it is perpendicular to the effective relative airflow in the vicinity of the wing. The lift generated by the wing has been tilted rearwards through an angle equal to the downwash angle in three-dimensional flow.