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).
High lift devices are identified from a lift curve by asking two questions: does the deployed curve start from a shifted, more negative, zero-lift angle, and does the deployed curve extend the usable range before stall to a noticeably higher angle of attack. The two mechanisms, camber change for flaps and boundary layer control for slats, leave different fingerprints on the curve, which is what this question is really testing.
(A) Figure P is for a flap: In Figure P, the dashed (deployed) curve is essentially the solid (retracted) curve slid up and to the left, crossing into negative $\alpha$ at $C_l = 0$, with $C_{l,max}$ increased but the stall behaviour otherwise similar. A flap increases the effective camber of the section, and increased camber is exactly what produces this upward, leftward shift of the whole curve. So this statement is TRUE.
(B) Figure P is for a slat: A slat's signature is that the low angle-of-attack part of the curve is unchanged, both curves overlap at small $\alpha$, and the curve only diverges near stall, extending to a much higher stalling angle. Figure P does not show this; its curve is shifted from the very start, so it is not a slat. This statement is FALSE.
(C) Figure Q is for a slat: In Figure Q, the dashed curve tracks the solid curve closely at low $\alpha$ and only pulls away near the top, extending well past the retracted curve's stalling angle before reaching a higher $C_{l,max}$. This matches the slat mechanism exactly: it delays separation and lets the section keep generating lift at a higher angle of attack, without shifting the zero-lift line. This statement is TRUE.
(D) Figure Q is for a flap: A flap would shift the whole curve including the low angle-of-attack part, which is not what Figure Q shows, since the curves overlap at low $\alpha$. So this statement is FALSE.
Combining these, Figure P belongs to a flap and Figure Q belongs to a slat, so options (A) and (C) are the correct statements.
Let's summarize:
A flap adds camber: the whole lift curve shifts up and to the left, $C_{l,max}$ rises a little, stall angle barely changes.
A slat delays leading edge separation: the low angle-of-attack curve is unchanged, but the linear region is extended to a much higher angle of attack before stall, raising $C_{l,max}$ by a larger margin.
So the TRUE statements are (A), Figure P is a flap, and (C), Figure Q is a slat.
