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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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Presentation Transcript
airfoil terminology
Airfoil Terminology
  • Review from handout
typical airfoil shape
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
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/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
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
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
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
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
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
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 airfoils12
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
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
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)