Explain the change in the flow pattern pressure distribution and the lift and drag coefficient for an aerofoil as its angle of attack is increased up to the stall. It is assumed that for a given angle of incidence α (angle of attack), the lift and drag forces that act on an aerofoil are proportional to the dynamic pressure and the plan area S.

Formula for lift (L) and Drag (D) is

L = CL ½?v^2S and D = CD½ ?v^2S where CL = lift coefficient,

CD = Drag coefficient

S = Plan Area (m)

v = air speed (m/s)

The graph below present 'variation of lift and drag coefficient versus angle of incidence α' (angle of attack) where plan area is 60m2 and air speed is 500 km/h.

•TASK 2.7:

From the graph above deduce the data for CL and CD (approx.) when angle of attack is 6ᵒ, and 14ᵒ

Your task is to:

• find the lift and drag forces for each case given above

• with aid of sketches / diagrams, explain:

a. how pressure distribution occurs on aerofoil (in each case)

b. how the air speed can be affected by changing the flow pattern pressure distribution, the lift coefficient and drag coefficient.

c. explain from your result under which condition(s) can an aircraft can be stalled