Airfoil%20Terminology%20and%20Pressure%20Distribution

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Airfoil TerminologyandPressure Distribution

• Lecture 3
• Chapter 2

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Airfoil Terminology

• Review from handout

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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)

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Symmetrical Airfoil

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

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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.

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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.

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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)

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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.

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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.

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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.

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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)

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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?

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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)

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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)