Wing and Airfoil Selection Questions and Answers

Answer: C

Correct answer: It makes the tip stall before the root

This is the reverse of what washout does. By setting the tip at a lower incidence than the root, washout ensures the root reaches its stalling angle first, so separation begins inboard and spreads outward. The other three statements are correct: the tip incidence is reduced relative to the root, aileron authority is preserved into the stall, and sharply tapered wings need it most because taper itself drives the local lift coefficient up towards the tip.

Answer: D

Correct answer: A shorter field length requirement

Higher wing loading means a smaller wing for the same weight, hence less skin friction and lower cruise drag, and a smaller area over which a gust can act, hence a smoother ride. But stalling speed rises as the square root of the wing loading, so the field length gets longer, not shorter. That is precisely the penalty high-lift systems exist to buy back.

Answer: A

Insects, rain, erosion and manufacturing waviness all trip the boundary layer, and the narrow lift coefficient band over which the benefit exists narrows it further. In-service drag is typically close to a conventional section's.

Answer: B

Correct answer: The optimum is above the value adopted

Aerodynamically, induced drag falls as \( 1/A \) with no limit in sight, so the drag-minimising aspect ratio is always higher than what is built. What caps it is structure and infrastructure: a slender wing is heavier for the same strength, and the span eventually exceeds the airport gate box. The value adopted is therefore a constrained compromise, not the aerodynamic optimum - and higher aspect ratio increases structural weight rather than reducing it.

Answer: C

Correct answer: They prevent supersonic flow from forming on the upper surface

This is false, and it inverts the whole idea. A supercritical section is designed to tolerate a supersonic pocket on the upper surface, not to prevent it: the flat upper surface keeps the flow only weakly supersonic so that the terminating shock is weak and the wave drag small. The other three are true - a flat upper surface, pronounced aft camber to recover the lift, and consequently a strong nose-down pitching moment that the tail must trim out.

Answer: D

Correct answer: Tapering the planform more sharply

Sharper taper makes the problem worse, since it raises the local lift coefficient towards the tip still further - taper is the usual cause of a tip-stall tendency rather than a cure for it. The other three all help: washout lowers the tip incidence, a fence checks the outboard boundary layer drift on a swept wing, and outboard slats raise the stalling angle of exactly the sections that would otherwise go first.

Answer: A

Folding retains the aerodynamic span in flight while meeting the parked limit. Reducing aspect ratio or area gives up induced drag or lift capability permanently, and more sweep would cost low-speed performance for no span benefit.

Answer: B

Correct answer: Fuel contributes to the wing’s lift

Fuel is a weight, not a source of lift. It does help the structure, but by a different mechanism: fuel carried outboard provides bending relief, its weight acting downward against the wing’s lift and reducing the root bending moment. The other three are genuine constraints - tank volume scales roughly with the square of the chord and directly with thickness ratio, so a thin, short-chord wing may simply be unable to hold the mission fuel.

Answer: C

In a sideslip the lower wing meets the flow at a greater effective incidence and lifts more, rolling the aircraft back towards level. Sweep and a high wing position both add to the same effect, so the three are traded together.

Answer: D

Correct answer: Reopen the thickness trade against volume

The thickness ratio was chosen on one criterion alone, and a second requirement has now shown that choice to be infeasible. The proper response is to reopen the trade with both criteria present - perhaps a slightly thicker section with a little more sweep to recover the critical Mach number, or a modest chord increase. Adding a centre tank or cutting the range treats the symptom while leaving an unbalanced trade in place, and more sweep alone does nothing for volume.