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The Stall, Airfoil development, &Wing Lift and Span Effects. Lecture 4 Chapter 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. Factors that contribute to a stall.

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The stall l.jpg
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.


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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.


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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


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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.


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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.


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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.


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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.


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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


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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


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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


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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.


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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.


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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.


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Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name


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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.


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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.


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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.


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Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name


Quiz on lecture 4 chapter 226 l.jpg
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?


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