3.4.1: Necessity of high-lift devices

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As shown in previous sections, there is a maximum coefficient of lift, \(C_{L_{\max}}\), that can not be exceeded by increasing the angle of attack. Consider the uniform horizontal flight, where the weight of the aircraft (\(W = mg\)) must be balanced by the lift force, i.e.:

\[W = L = \dfrac{1}{2} \rho V^2 S_w C_L.\]

Therefore, the existence of the maximum coefficient of lift, \(C_{L_{\max}}\), implies that the aircraft can not fly below a minimum velocity, the stall speed, \(V_S\):

\[V_S = \sqrt{\dfrac{W}{\tfrac{1}{2} \rho S_w C_{L_{\max}}}}.\label{eq3.4.1.2}\]

Looking at equation (\(\ref{eq3.4.1.2}\)), it can be deduced that increasing the area (\(S_w\)) and the maximum coefficient of lift (\(C_{L_{\max}}\)) allows to fly at a lower airspeed since the minimum speed (\(V_S\)) decreases.

Deploying high-lift devices also increases the drag coefficient of the aircraft. Therefore, for any given weight and airspeed, deflected flaps increase the drag force. Flaps increase the drag coefficient of an aircraft because of higher induced drag caused by the distorted span-wise lift distribution on the wing with flaps extended. Some devices increase the planform area of the wing and, for any given speed, this also increases the parasitic drag component of total drag.

By decreasing operating speed and increasing drag, high-lift device shorten takeoff and landing distances as well as improve climb rate. Therefore, these devices are fundamental during take-off (reduce the velocity at which the aircraft’s lifts equals aircraft’s weight), during the initial phase of climb (increases the rate of climb so that obstacles can be avoided) and landing (decrease the impact velocity and help braking the aircraft).