The Stall, Airfoil development, &Wing Lift and Span Effects Lecture 4 Chapter 2.

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

• 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.

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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 airfoil

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

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

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

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.

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.

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.

Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name

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.

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.

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.

Quiz on Lecture 4Chapter 2

Please take out a sheet of paper

Include today’s date and your name

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?