Question 12 of 13: High-Lift Aircraft Wing Configurations

Question 12 — High-Lift Aircraft 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.

When and why each configuration is used. Configuration A (a single-element slotted flap plus a leading-edge slat) is the lightest, simplest, and lowest-drag high-lift arrangement, so it is typically deployed for TAKEOFF: enough extra lift and stall margin is needed to get airborne at a safely low speed, but the added drag of a fully deployed system would waste runway-limited thrust and climb performance. Configurations B and C add one and two further flap elements respectively, each opening an additional slot; they are progressively more effective at generating lift (at the cost of more drag and complexity) and are reserved for LANDING, where a much steeper approach, a slower touchdown speed, and the additional drag (which helps the aircraft decelerate) are all desirable rather than penalising. Larger, heavier transport aircraft with higher wing loading generally need the more elaborate triple-slotted system (C) to achieve an acceptably low landing speed, while smaller or lighter aircraft can land safely with a single- or double-slotted flap.

Physical phenomena contributing to high lift. A plain wing stalls when the boundary layer on the upper surface separates under the strong adverse pressure gradient created at high angle of attack; high-lift devices work by managing that boundary layer and by increasing the wing's effective size and camber. The leading-edge SLAT opens a slot ahead of the main wing that accelerates a jet of high-energy air from the lower (high-pressure) surface over the upper (low-pressure, separation-prone) surface near the leading edge, re-energising the boundary layer there and delaying leading-edge stall to a much higher angle of attack. Each FLAP slot performs the analogous role at the trailing edge: high-energy air is channelled through the slot onto the upper surface of the next flap element, continually re-energising a boundary layer that would otherwise separate under the ever-steeper adverse pressure gradient created by the large flap deflection. Independently of the slot effect, deploying flaps increases the wing's effective camber and (for slotted/Fowler-type flaps) its effective chord and planform area, both of which directly raise the lift coefficient at a given angle of attack. More slots allow a larger total flap deflection (and hence more camber/area increase) before separation would otherwise occur, which is exactly why the triple-slotted configuration achieves the highest lift coefficient of the three, at the cost of the greatest mechanical complexity and drag.