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# The Stall, Airfoil development, Wing Lift and Span Effects - PowerPoint PPT Presentation

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, Airfoil development, &Wing Lift and Span Effects

Lecture 4

Chapter 2

• 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.

• 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.

• 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

• Beyond the peak in the negative pressure we find a reversal:

• As the angle of attack increases the center of pressure moves forward and the unfavorable pressure gradient becomes longer and steeper.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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

• 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.

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.

• Four digit airfoils with no camber, or symmetrical would have two zeros in the first two digits.

• 0010, double-oh ten

• 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

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

• 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.

• 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.

• 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.

### Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name

• 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.

• 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.

• 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.

### Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name

Quiz on Lecture 4Chapter 2