Wing and Airfoil Selection Questions and Answers

Answer: A

Folding roughly a metre per side brings the parked span inside the box while retaining the aerodynamic span in flight. Reducing area or aspect ratio would give up performance, and accepting a larger gate category cuts the aircraft off from part of its market.

Answer: B

Correct answer: Contamination will trip the boundary layer

A six-series section keeps its laminar run only over a smooth, clean, accurately contoured surface. Grass strip operation brings insect strikes, dirt, mud and stone chips, and outdoor parking brings rain, dust and surface degradation, and any one of them trips the boundary layer at the leading edge. The wing then behaves like an ordinary section - a little worse, since the sections are optimised for laminar flow - and the promised drag saving never appears.

Answer: C

Correct answer: aspect ratio; induced

Aspect ratio is \( A = b^2/S \), and induced drag is

so raising it reduces the induced drag in inverse proportion. The penalty is structural: a slender wing has a longer bending arm and less depth, so it weighs more, and it may exceed the gate span. The trade settles most transports between aspect ratios of 8 and 11.

Answer: D

Correct answer: Fowler; plain

The Fowler flap translates aft along tracks as well as deflecting, so it adds planform area as well as camber, and the area term is much of why its lift increment is so large. The plain flap hinges about a fixed point, adding camber alone at unchanged area. This distinction - whether the device moves aft or merely rotates - is the fundamental one among trailing-edge devices.

Answer: A

Correct answer: sweep; normal

The compressible behaviour of a section is set by the velocity component perpendicular to its leading edge, \( M\cos\Lambda \), the spanwise component being carried along without affecting the pressure distribution over the section. Sweeping the wing therefore lets the aircraft fly faster before the section itself reaches its critical condition, which is why every transport designed for Mach 0.8 and above is swept.

Answer: B

Washout lowers the tip incidence so the root reaches its stalling angle earlier and the separation spreads outward. Washin would do the opposite, and the dihedral terms concern lateral stability rather than stall pattern.

Answer: C

Correct answer: supercritical; aft

A supercritical section has a comparatively flat upper surface, so the flow accelerates less and the supersonic pocket is terminated by a weaker shock with less wave drag; the lift lost from the flattened upper surface is recovered by pronounced camber near the trailing edge. The result is that the aircraft can cruise closer to Mach 1, or use a thicker wing at the same Mach number - which is why almost every modern transport wing is supercritical.

Answer: D

Only P and R fall inside 36 metres, and of those R has the greater sweep, giving a normal Mach number of \( 0.84 \cos 35^\circ = 0.69 \) against P's 0.76. R is therefore the only candidate comfortable at Mach 0.84 within the span limit.

Answer: A

Correct answer: 1.43

Stalling speed varies as the inverse square root of the maximum lift coefficient, so the reduction factor is

The lift coefficient has been more than doubled, but the stalling speed falls only by a factor of 1.43, that is by about 30 per cent. The 2.04 answer is the lift coefficient ratio itself, forgetting the square root that stands between lift and speed.

Answer: B

Section B's design lift coefficient matches the cruise value exactly and its critical Mach number of 0.74 leaves margin above 0.72. A is too thin in design lift, C's critical Mach is too low, and D is designed for a higher lift coefficient than needed.