Shown below are qualitative illustrations of the lift curve for an airfoil when two different control surfaces are in their respective retracted and deployed configurations. Which of the following is/are TRUE?
Show Hint
A flap shifts the whole lift curve up-left (added camber); a slat leaves the low angle-of-attack curve unchanged but extends it to a higher stall angle (delayed separation).
Step 1: Recall What a Flap Does to the Lift Curve.
A trailing edge flap works mainly by adding camber to the airfoil. Adding camber shifts the whole lift curve up and to the left: the zero-lift angle of attack becomes more negative, the curve now crosses \(C_l = 0\) at a negative \(\alpha\), and \(C_{l,max}\) increases, but the slope of the linear part and the stalling angle stay roughly the same, sometimes even a little lower, because a flap does not fix the flow separation problem at the leading edge.
Step 2: Match This to Figure P.
In Figure P, the deployed (dashed) curve is shifted up and to the left of the retracted (solid) curve. The dashed curve starts from a negative angle of attack, it is already above zero lift at \(\alpha=0\), reaches a higher \(C_{l,max}\), and both curves have a broadly similar shape and stall behaviour, just parallel shifted. This upward left shift with an unchanged basic shape is exactly what a camber-adding device, a flap, produces. So Figure P is for a flap, which makes option (A) TRUE and option (B), Figure P is a slat, FALSE.
Step 3: Recall What a Slat Does to the Lift Curve.
A leading edge slat works differently: it re-energises the boundary layer near the leading edge, by letting a jet of high energy air from the lower surface flow through the slot onto the upper surface, and delays flow separation. It does not add camber the same way, so the low angle of attack, linear part of the lift curve is nearly unchanged. What it does is extend that linear region to a much higher angle of attack before stall happens, giving a large increase in both \(C_{l,max}\) and the stalling angle \(\alpha_{stall}\).
Step 4: Match This to Figure Q.
In Figure Q, the deployed (dashed) curve follows the same line as the retracted (solid) curve for the initial linear part, then continues climbing well past the point where the retracted curve stalls, reaching a higher \(C_{l,max}\) at a higher angle of attack. This is exactly the slat signature: same low angle-of-attack behaviour, extended range before stall. So Figure Q is for a slat, which makes option (C) TRUE and option (D), Figure Q is a flap, FALSE.
Final Answer:
\[ \boxed{\text{Figure P is a flap and Figure Q is a slat, i.e. options (A) and (C)}} \]
