Question 13 of 13: High-Lift Wing Configurations

Question 13: High-Lift Wing Configurations (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Configuration A shows the wing in its clean, high-speed cruise configuration — essentially the bare airfoil with no leading- or trailing-edge devices deployed, optimised for minimum drag at cruise angle of attack. Configuration B shows a leading-edge slat extended forward and a trailing-edge flap deflected downward, the typical take-off setting: a moderate increase in lift coefficient is needed to get airborne at relatively low speed without an excessive drag penalty that would lengthen the take-off roll or reduce climb performance. Configuration C shows both the slat and a multi-element, more steeply deflected flap fully extended, the landing setting: here the aircraft needs the maximum possible lift coefficient to fly safely at the lowest practical approach speed, and the accompanying large increase in drag is not just tolerated but useful, since it helps the aircraft descend steeply and decelerate for touchdown.

The trailing-edge flap increases lift primarily by increasing the wing's effective camber and chord: cambering the airfoil shifts the pressure distribution so that the upper surface accelerates the flow more and the lower surface pressure rises, increasing the pressure difference (and hence CL) at a given angle of attack, per $F_L=C_L \tfrac12 \rho V^2 A_{wing}$. The leading-edge slat's role is different: opening a slot ahead of the main leading edge allows high-energy air from below the wing to be channelled over the upper surface, re-energising the boundary layer there and delaying flow separation (stall) to a much higher angle of attack than the clean wing could sustain — this is what allows the much larger flap deflections of Configuration C to be used at all without the wing stalling first. Both devices, however, increase parasite drag directly (more wetted area, more exposed gaps and brackets) and induced drag (a higher CL at a given speed increases the lift-induced drag component); this drag penalty is why high-lift devices are retracted for cruise, where the extra fuel burn and reduced top speed they would cause are never worthwhile.