The Stall, Airfoil development,

1
The Stall, Airfoil development, Wing Lift and
Span Effects

• Lecture 4
• Chapter 2

2
The Stall

• What happens when we increase the angle of
  attack?
• Can we increase our angle of attack too much?
• A practical limit to the angle of attack is the
  stalling point.

3
Factors that contribute to a stall

• Angle of attack increases the stagnation point
  moves farther down on the forward part of the
  airfoil-making a longer effective upper surface.
• This creates friction that increases with travel
  distance.

4
Factors that contribute to a stall

• Pressure gradient (pressure change)
• There is a decrease of pressure from the leading
  edge back that pressure decreases with distance.
• This decreasing pressure tends to induce the flow
  to move along the surface, promoting the flow in
  the direction we want.
• We call this favorable pressure gradient

5
Factors that contribute to a stall

• Beyond the peak in the negative pressure we find
  a reversal
• An unfavorable pressure gradient
• As the angle of attack increases the center of
  pressure moves forward and the unfavorable
  pressure gradient becomes longer and steeper.

6
Factors that contribute to a stall

• Eventually, the combined effect of the
  unfavorable pressure gradient and the surface
  friction become greater than the energy available
  in the airflow to overcome them.
• At this point the flow will detach itself from
  the surface.

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Figure 2-25, p. 29

• With no flow over the top surface, there is no
  mechanism to reduce the pressure over the surface
  and lift decreases drastically.
• The upper surface separation causes a great loss
  in lift production and stalls.

8
The Stall

• The lift does not go to zero because there is
  still flow over the surface and at this angle of
  attack is normally exerting positive pressure.
• The upper surface separation causes a great loss
  of lift.
• The result on an aircraft in flight is a sudden
  loss of lift it will drop due to weight now
  being greater than lift.

9
Reducing the abruptness of the stall

• The roundness of the leading edge
• A very sharp leading edge can act as a barrier to
  the flow at a high angle of attack.
• A stall Strip
• A stall strip causes the flow to separate at the
  leading edge at an angle of attack somewhat below
  the normal stall angle.

10
Stall Warning Devices

• Vane-type- which takes advantage of the relation
  between the stall angle of attack and stagnation
  point.
• There is a distinct stagnation point for each
  angle of attack.
• The vane is positioned so that the stagnation
  point is above it in normal flight.

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Figure 2-27a p. 30

• The air stream hitting the vane is, then that
  going over the lower surface, which holds the
  vane down.
• The vane is connected to an electrical
  switch-which is open when the vane is down.
• As the angle of attack is increased the
  stagnation point moves below the vane.

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Airfoil Development and Designation

• What is the typical airfoil?
• What is the simplest?
• The Flat plate
• It is not efficient because it creates quite a
  bit of drag.
• The sharp leading edge also promotes stall at a
  very small angle of attack severely limits lift
  producing ability.
• Figure 2-28 p.32

13
The National Advisory Committee for Aeronautics

• NACA, the forerunner of NASA looked at
  aerodynamic characteristics of airfoils in wind
  tunnels
• They looked at the thickness form and meanline
  form
• They then proceeded to identify these
  characteristics in the numbering systems for
  airfoils.

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NACA 2412 twenty-four twelve

• The first number (2) is the max camber in of
  the chord length.
• The second number (4) is the location of the max
  camber point in tenths of chord.
• The last two numbers (12) identify the maximum
  thickness in of the chord.

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Four digit airfoil

• Four digit airfoils with no camber, or
  symmetrical would have two zeros in the first two
  digits.
• 0010, double-oh ten

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The six series airfoil

• NACA 652-415
• The first digit is the series number (6)
• The second number is the location of the minimum
  pressure in tenths of a chord (5)
• The subscript (2) indicates the range of lift
  coefficients above below the design lift
  coefficient where low drag can be maintained

17
NACA 652-415

• The next number (4) indicated the design lift
  coefficient of .04
• The last two digits (15) represent the max
  thickness in of the chord.
• The 6-series airfoils were first used in the wing
  of the P-51 Mustang for their low drag qualities

18
Richard Whitcomb

• NASA research engineer
• Developed the supercritical airfoil
• The airfoil was intended to improve drag at
  speeds near Mach 1, but the methodology was also
  used to for low-speed airfoils.
• The general aviation GA(W) was incorporated
  into Piper Tomahawk p. 36.

19
Wing Span

• The profile shape has a great deal to do with the
  aerodynamic characteristics of a wing.
• The length of a wing or span, and the planform of
  the wing also affect the aerodynamic
  characteristics.
• Planform is the shape of the wing as viewed from
  directly above or below.

20
Figure 2-34 p. 37

• 2-34A- Along the span of the wing the pressure
  force exerted against the wing, except at the
  wing tips
• 2-34B-Wing tip vortices, more commonly called
  wake turbulence.
• 2-34C- Downwash results in a change of direction
  of the incoming air stream in the vicinity of the
  wing.

21
Quiz on Lecture 4Chapter 2

• Please take out a sheet of paper
• Include todays date and your name

22
Downwash effect

• Downwash- pushing downward on air stream causing
  a rearward tilted lift vector.
• The downwash effect is greatest at the wing tip,
  but is experienced across the span.
• When the lift vector is tilted backward, not all
  of the lift is acting perpendicular to the
  incoming stream.

23
Downwash effect

• Because of the downwash a little more angle of
  attack is needed to make up for this loss of lift
  downwash creates.
• This additional angle of attack is called the
  induced angle of attack.
• This angle is necessary because of the flow
  induced by the downwash.

24
Aspect Ratio

• Aspect ratio is the span divided by the average
  chord.
• Figure 2-37 p. 40 shows two wings of different
  aspect ratios, but have the same area.

25
Quiz on Lecture 4Chapter 2

• Please take out a sheet of paper
• Include todays date and your name

26
Quiz on Lecture 4Chapter 2

• Explain favorable pressure gradient.
• List and explain two things that can affect the
  abruptness of a stall.
• Explain NACA 2413.
• What is planform?