Concise - Self Test 2

Question 1

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

Answer 1

• Stall occurs when the angle of attack increases
• Causes boundary layer separation from the upper surface
• Results in reduction of lift
• Leads to 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 between the freestream velocity direction and the chordline
• At a positive angle of attack, the leading edge is up (above the trailing edge)

Question 3

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

Answer 3

• Incompressibility conditions:
  
  • Mach number squared (M²) much less than 1
  • (M/Fr)² much less than 1
  • M²/Re much less than 1
• Where:
  
  • M = Mach number
  • Fr = Froude number
  • Re = Reynolds number

Question 4

Define the aerodynamic center.

Answer 4

• The aerodynamic center is the point where the pitching moment (Cm) remains constant regardless of the angle of attack
• This is valid below the stall angle

Question 5

Define the Center of Pressure on an aerfoil.

Answer 5

• The center of pressure is the point where the pitching moment is zero
• The pressure distribution over the aerofoil can be reduced to Lift and Drag at this point

Question 6

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

Answer 6

• Camber (Camber height) is the maximum deviation of the camber line from the chordline
• It measures the curvature of the aerofoil

Question 7

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

Answer 7

• The maximum allowable Mach number for the incompressible flow assumption is Ma = 0.3