Airfoil Terminology and Pressure Distribution

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# Airfoil Terminology and Pressure Distribution - PowerPoint PPT Presentation

Airfoil Terminology and Pressure Distribution. Lecture 3 Chapter 2. Airfoil Terminology. Review from handout. Typical Airfoil Shape. Cambered, meaning there is more cross-sectional area above the chordline than below. (it is not symmetrical)

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### Airfoil TerminologyandPressure Distribution

Lecture 3

Chapter 2

Airfoil Terminology
• Review from handout
Typical Airfoil Shape
• 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)
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?