Aerofoil Lift and Drag

Question 1

What are the lift, drag and moment coefficients for an aerofoil (Cl, Cd & Cm) independent and dependent on?

Answer 1

Independent of: b & c and the air velocity
Dependent on: AoA & Airfoil geometry, most notably the camber

Also at the aerodynamic centre the coefficient of moment is independent of the AoA

Question 2

Lift Curves - Graph of Cl vs α

Answer 2

For a symmetrical airfoil the αL=0 =0 (zero lift angle of attack) equals zero and therefore the lift curve intercepts the origin
For cambered airfoil the zero lift angle of attack ≠ 0 and so the x intercept will be at an angle to the left of the y axis

Lift ‘curves’ are a linear straight line terminating in a drooping curve.
As the Re increases the linear part of the line increases in length before terminating in a drooping curve

The drooping curve shows an abrupt drop in lift at the stall angle

Question 3

What causes stall?

Answer 3

Stall is due to flow separation that occurs when the angle of attack is too large to allow for attached flow

Question 4

How can the stall angle and max CL be increased?

Answer 4

By providing a more turbulent boundary layer, as there is more KE in a turbulent flow, attached flow is maintained for larger AoA

Question 5

Airfoil Drag Polars
How to plot, symmetric/cambered airfoil

Answer 5

Cd vs. Cl graph - plotted as a +ve parabola
For a symmetric airfoil it is centred at the y-axis and is symmetrical about it.
For a cambered airfoil the parabola is shifted to the right ∴ the minimum drag for a cambered airfoil is associated with a non-zero lift

Question 6

Eq for Cl of a non-flapped aerofoil

Answer 6

Cl = Clα * (α - αL=0)