Question

1. Define/explain following terminologies:

   1. (a) Sketch a positive cambered airfoil and identify upper surface, lower surface, leading edge, trailing edge, camber line, camber, chord line and airfoil thickness on your sketch

   2. (b) Sketch and explain the lift curve (lift coefficient versus angle of attack) for a positive cambered airfoil. Then compare it with a symmetrical airfoil lift curve.

Answer 1

a)

Mean camber line- The locus of points halfway between upper and lower surfaces

Leading edge- The most forward point

Trailing edge- The most rearward point

Chord line- The straight line connecting leading and trailing edges

Camber- The maximum distance between mean camber line and chord line

b)

• Experimental data indicate that c_l varies linearly with over a large range of angle of attack
• The slope of the linear portion of the lift curve is given by

• When =0, there is still a positive value of c_l; that is, there is still some lift at zero angle of attack. This value of is called the zero-lift angle of attack.
• For large values of , the linearity is broken. As is increased beyond a certain value, c_l reaches some maximum value and then drops as the value of is further increased. This situation in which lift is rapidly decreasing, the aerofoil is stalled.

For symmetric aerofoil, it is observed that lift curve goes through the origin. That is, when =0, lift is also zero