Airfoil Terminology and Pressure Distribution - PowerPoint PPT Presentation

Airfoil TerminologyandPressure Distribution

Lecture 3

Chapter 2

• Review from handout

• Cambered, meaning there is more cross-sectional 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)

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

• 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.

• 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.

• 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)

• 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.

• 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.

• 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.

• 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)

• 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?

• 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)

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