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

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Cambered, meaning there is more cross-sectional area above the chordline than below. ... The amount of camber is a measure of the overall curvature of the airfoil. ... – PowerPoint PPT presentation

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Title: Airfoil%20Terminology%20and%20Pressure%20Distribution


1
Airfoil TerminologyandPressure Distribution
  • Lecture 3
  • Chapter 2

2
Airfoil Terminology
  • Review from handout

3
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)

4
Symmetrical Airfoil
  • A symmetrical airfoil has no camber.
  • The meanline and the chordline coincide.
  • All Airfoils are either cambered or symmetrical.

5
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.

6
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.

7
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)

8
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.

9
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.

10
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.

11
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)

12
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?

13
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)

14
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)
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