Incompressible Flow over Airfoils Questions and Answers

Answer: C

Correct answer: Lift

An airfoil is a section shaped so that the flow over it is faster on one side than the other, producing a pressure difference and hence a force perpendicular to the relative wind. Everything else about the shape - the camber, the thickness distribution, the leading edge radius - is a refinement of how well and over what range of conditions it does that.

Answer: D

Correct answer: Front of the section

The leading edge is the forward point of the section, where the flow divides between the upper and lower surfaces, and near which the stagnation point lies. Its radius is one of the more consequential geometric parameters, because it largely decides whether the section stalls gently from the trailing edge or abruptly from the nose.

Answer: A

Correct answer: Leading edge to trailing edge

The chord is the straight line from the leading edge to the trailing edge, and it is the reference length for the whole section: thickness and camber are quoted as percentages of it, angle of attack is measured from it, and Reynolds number is based on it. The distance from root to tip is the span, which belongs to the wing rather than the section.

Answer: B

Correct answer: Perpendicular to the relative wind

Lift and drag are defined relative to the oncoming flow, not to the airframe or to the earth: lift is the component perpendicular to the relative wind and drag the component along it. This is why lift is not vertical in a climb or a bank, and why in a steep turn a large part of it goes into turning the aircraft rather than supporting it.

Answer: C

Correct answer: The upper surface

The flow accelerates over the more curved upper surface and, by Bernoulli’s relation, its pressure falls. The lower surface sees a smaller acceleration or none, so its pressure stays nearer free stream. The difference integrated over the section is the lift, and the upper surface suction typically supplies the larger part of it.

Answer: D

Correct answer: The relative wind

Angle of attack is a purely aerodynamic quantity: the angle between the chord line and the oncoming flow. It is not the pitch attitude, which is measured against the horizon, and the two differ by the flight path angle. An aircraft descending steeply can hold a high angle of attack with the nose below the horizon, which is exactly how an approach-to-stall in a descent catches people out.

Answer: A

Correct answer: Symmetric

A symmetric section has its mean camber line coincident with the chord, so the upper and lower surfaces are mirror images. It produces no lift at zero incidence, has zero pitching moment about the quarter chord, and behaves identically at positive and negative angles - which is why control surfaces and all-moving tailplanes use symmetric sections.

Answer: B

Aspect ratio describes the shape of the wing seen from above and needs the span, whereas the other three are all measured on a single cross-section. Section properties can be quoted without reference to the wing at all.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A finite wing sheds vorticity from its tips, and the trailing vortex system induces a downwash over the wing that reduces the effective angle of attack at every section. Each section therefore works at less incidence than the geometric angle suggests, so the wing’s lift curve slope is lower than the section’s and falls further as aspect ratio decreases.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the whole point. The stall is a flow phenomenon governed by angle of attack, and that angle can be exceeded at any speed given enough load factor - which is what an accelerated stall is. The familiar published stalling speed applies only at one weight, in one configuration, in unaccelerated level flight.