Airfoil Terminology and Pressure Distribution Lecture 3 Chapter

Airfoil Terminology and Pressure Distribution Lecture 3 Chapter 2

Airfoil Terminology • Review from handout

Typical Airfoil Shape • Cambered, meaning there is more crosssectional area above the chordline than below. (it is not symmetrical) • The amount of camber is a measure of the overall curvature of the airfoil. • Cambered airfoils are normally used to more effectively provide lift in a given direction. (this direction is usually up)

Symmetrical Airfoil • A symmetrical airfoil has no camber. • The meanline and the chordline coincide. • All Airfoils are either cambered or symmetrical.

Upper/Lower Camber • Upper Camber- curvature of the upper surface of the cambered airfoil • Lower Camber- curvature of the lower surface of the cambered airfoil – A symmetrical airfoil has equal curvature of upper and lower surface, yet it technically has no camber.

Lift on Cambered Airfoils • A cambered airfoil at zero degrees angle of attack will produce some lift at this angle because there is more cross-sectional area above the chordline than below. – This causes a greater reduction in the area available for the airflow. – At this angle of attack, the flow will divide near the leading edge.

What if the angle of attack is increased? • The flow no longer divides at the leading edge, but a point farther down the nose of the airfoil. – The stagnation point is the dividing point for the flow to go above or below the airfoil. • It is called the stagnation point because the flow is stagnate at this point. (the flow either goes above or below this point)

The effective upper cross sectional • A cambered airfoil at a moderate angle of attack (fig. 2 -15, p. 22) has increased effective area due to the location of the stagnation point. • This area of the airfoil is therefore increased and the effective lower area is decreased.

Back to Continuity & Bernoulli • Lower area= higher velocity= lower pressure • There is a greater effective upper surface area and leads to a greater lowering of pressure on the top of the surface. • The reverse is true on the lower surface. – The reduction in effective cross-sectional area has reduced the airflow area and resulted in less lowering of pressure on the bottom surface.

Figure 2 -16 p. 23 • An airfoil showing the change in pressure forces of the top and bottom surfaces when the angle of attack is increased above zero. – Angle of attacked increased – Stagnation point moves back – this causes a greater lowering of pressure on top rather than bottom resulting in greater lift.

Symmetrical Airfoils • This airfoil at zero angle of attack have equal upper and lower surfaces (fig. 2 -17, p. 23) • If the angle of attack is increased, the stagnation point moves below the leading edge(just like a cambered airfoil) • The effective upper & lower cross-sectional areas are then different (just like a cambered airfoil)

Symmetrical airfoils • However, a greater angle of attack is required to get the same amount of lift as the cambered airfoil. • Therefore, the symmetrical airfoil is not as efficient in this respect. • So what is the advantage of a symmetrical airfoil?

Advantage of Symmetrical Airfoil • The fact that it can produce an equal amount of lift in either direction at the same positive or negative angle of attack. – Negative lift can also be obtained with a cambered airfoil but at a very great negative angle. (this means you can fly a cambered airfoil inverted)

A Cambered Airfoil Inverted • The inverted angle must be great enough, that the effective lower area of the airfoils (which is now, in reality, the upper)