Thin airfoil theory gives the lift coefficient of a symmetric section as
Answer: A
For a symmetric section the zero-lift angle is itself zero, so the slope of two pi per radian applies directly to the geometric angle. Expressed per degree this is about 0.11, and real sections come close to it until viscous effects intervene near the stall.
Please log in or register to report this question.
The Kutta condition is imposed in airfoil theory in order to
Answer: B
Potential flow admits infinitely many circulations around a given shape, and all but one give an infinite velocity round the sharp trailing edge. Requiring smooth departure there picks the value nature adopts and so fixes the lift.
Please log in or register to report this question.
The starting vortex shed when an airfoil begins to move is explained by
Answer: C
Kelvin's circulation theorem states that the circulation around a material contour enclosing the whole field cannot change, so the bound circulation that produces lift must be matched by an equal and opposite vortex left behind in the fluid.
Please log in or register to report this question.
In the NACA four-digit designation 2412, the digit 2 represents
Answer: D
The first digit gives maximum camber as a percentage of chord, the second its position in tenths of chord, and the last two the thickness as a percentage. So 2412 has two per cent camber at four tenths chord and is twelve per cent thick.
Please log in or register to report this question.
In the NACA designation 2412, the final two digits 12 represent
Answer: A
The last pair always gives thickness as a percentage of chord, so this section is twelve per cent thick. A NACA 0012 has the same thickness with no camber at all, which is why it is the standard symmetric test section.
Please log in or register to report this question.
A NACA 0009 section is
Answer: B
Leading zeros mean no camber and no camber position, so the section is symmetric, and the final pair gives the thickness. Thin symmetric sections of this kind are common on tailplanes and fins, which must work equally in both directions.
Please log in or register to report this question.
Which of the following is NOT a prediction of thin airfoil theory?
Answer: C
Thin airfoil theory is inviscid, so it gives no drag whatever, which is d'Alembert's paradox in another form. The other three results are exactly what the theory does provide, and all three agree well with measurement at low incidence.
Please log in or register to report this question.
All of the following raise the maximum lift coefficient of a section EXCEPT
Answer: D
Roughness at the leading edge trips the flow and promotes early separation, cutting the maximum coefficient, which is why icing and insect contamination matter so much. The other three are all recognised high-lift measures.
Please log in or register to report this question.
Assertion (A): A symmetric airfoil produces no lift at zero angle of attack.
Reason (R): A symmetric airfoil has its mean camber line coincident with the chord line.
Select the correct answer from the options below.
Answer: A
Both statements are true and the reason is the explanation: with the mean line on the chord the upper and lower surfaces are mirror images, the pressure distributions cancel, and no lift results. Introducing camber breaks that symmetry and produces lift at zero incidence.
Please log in or register to report this question.
Assertion (A): The aerodynamic centre is preferred to the centre of pressure as a reference point.
Reason (R): The pitching moment about the aerodynamic centre does not vary with angle of attack.
Select the correct answer from the options below.
Answer: B
Both are true and the second explains the first. The centre of pressure moves with incidence and recedes towards infinity as lift approaches zero, whereas a fixed point carrying a constant moment is far more convenient for analysis.
Please log in or register to report this question.
