Incompressible Flow over Airfoils Questions and Answers

Answer: A

Correct answer: Begins at the rear and moves forward as incidence increases

On a thick, well-rounded section the adverse pressure gradient first defeats the boundary layer near the trailing edge, and the separation point creeps forward as incidence rises. The lift curve therefore rounds over gradually and the stall is gentle and preceded by buffet - the behaviour a designer wants, because it gives the pilot warning.

Answer: B

Correct answer: A thin section with a small leading edge radius

A sharp nose forces the flow to turn through a large angle in a short distance, producing an intense suction peak and a violent adverse gradient just behind it. The boundary layer separates at the leading edge and the whole upper surface unloads at once, so lift collapses abruptly with little warning. A generous leading edge radius produces the gentle trailing edge stall instead.

Answer: C

Correct answer: Feeding high-energy air over the upper surface through the slot

The slot between the slat and the main element channels air from the high pressure lower surface onto the upper surface, adding momentum to the boundary layer just where it is about to give up against the adverse gradient. The energised layer resists separation to a higher incidence, so the stall is delayed and the maximum lift coefficient rises.

Answer: D

Correct answer: Increasing the wing area as it extends rearwards

A plain flap only adds camber by deflecting the trailing edge. A Fowler flap translates aft on tracks as it deflects, so it adds camber and enlarges the reference area at the same time - and since lift is proportional to area, that second effect is substantial. It is why Fowler flaps give the largest lift increment of the common trailing edge devices.

Answer: A

Correct answer: A nose-down pitching moment

Deflecting a trailing edge flap loads the rear of the section, which moves the centre of pressure aft and generates a nose-down couple. The aircraft must be retrimmed on flap selection, and the tail has to supply the balancing load - which is one reason tailplane authority is checked at the most adverse flap and centre of gravity combination.

Answer: B

Correct answer: The lift curve slope predicted by thin airfoil theory

Thin airfoil theory is inviscid, and Reynolds number is a ratio of inertial to viscous forces, so it cannot appear in the result - the theory gives a slope of \( 2\pi \) per radian regardless. The other three all depend on viscosity: transition, maximum lift, and skin friction are all boundary layer phenomena.

Answer: C

Once the starting vortex has been shed and washed downstream, the bound circulation settles at the value the Kutta condition requires and stays there. Kelvin's theorem is satisfied because the two together still sum to zero.

Answer: D

Lift depends on the pressure DIFFERENCE around the section, not on the absolute level, so raising the ambient pressure while holding density and speed constant changes nothing. Angle, camber and density all enter directly.

Answer: A

The section is first replaced by a distribution of vorticity, the Kutta condition then fixes the otherwise arbitrary strength, and only with the circulation known can the lift be obtained. Reversing any step leaves the problem undetermined.

Answer: B

Correct answer: Profile drag from friction and form effects

At the zero-lift angle the section produces no lift, but the air still flows over it, so the boundary layer still exerts skin friction and the displacement effect still alters the pressure distribution. Induced drag, by contrast, is a consequence of lift and of trailing vortices, so a two-dimensional section at zero lift has none.