The Stall Airfoil development Wing Lift and Span

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 -27 a 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 -34 A- Along the span of the wing the pressure force exerted against the wing, except at the wing tips • 2 -34 B-Wing tip vortices, more commonly called wake turbulence. • 2 -34 C- Downwash results in a change of direction of the incoming air stream in the vicinity of the wing.

Quiz on Lecture 4 Chapter 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 4 Chapter 2 Please take out a sheet of paper Include today’s date and your name

Quiz on Lecture 4 Chapter 2 • Explain favorable pressure gradient. • List and explain two things that can affect the abruptness of a stall. • Explain NACA 2413. • What is planform?