Self Test 2 - Basic calculations and definitions

Question 1

What is meant by stall for an aerofoil? How does it affect the performance of the aerofoil?

Answer 1

Stall occurs as the angle of attack increases, causing boundary layer separation from the upper surface. It results in the reduction of lift and a dramatic increase in drag.

Question 2

Define the angle of attack of an aerfoil. Describe what orientation the aerofoil would be in at a positive angle of attack.

Answer 2

Angle of attack is the angle made between the freestream velocity direction and the chordline.

Leading edge up (i.e. above the trailing edge) is a positive angle of attack.

Question 3

Specify what the conditions (in terms of non-dimensional quantifies) are necessary for an assumption of incompressibility in the flow to be valid.

Answer 3

M2 &laquo_space;1

(M/Fr)2 &laquo_space;1

M2/Re &laquo_space;1

where M is Mach number, Fr is Froude number and Re is Reynolds number

Question 4

Define the aerodynamic center.

Answer 4

The aerodynamic center is the point at which the pitching moment Cm remains constant regardless of angle of attack (below the stall angle).

Question 5

Define the Center of Pressure on an aerfoil.

Answer 5

The center of pressure is that point at which the pitching moment is zero and the pressure distribution over the aerofoil can be reduced to just Lift and Drag.

Question 6

What is the camber (or camber height) of an aerofoil?

Answer 6

The camber is the maximum deviation of the camber line from the chordline. It is a measure of the curvature of the aerofoil.

Question 7

Practically, what is the maximum allowable Mach number for the incompressible assumption to be valid?

Answer 7

Ma = 0.3