The pattern of the lift and pressure distribution around a wing of typical aerofoil cross section changes with angle of attack. This change is shown in Figure 3.20, overleaf, where we assume that the velocity of the free-stream airflow is constant for all angles of attack.
We have already discussed the reason why aircraft have wings of aerofoil cross section. Typically, the wing of a light training aircraft will have an upper surface of pronounced positive camber and an under surface which is straighter. (Aerofoil terminology is covered in the next chapter.) The angle of attack is the angle between the aerofoil’s chord line and the relative (free stream) airflow. Angle of attack is usually represented by the symbol a (the Greek letter “alpha”). The relative airflow is not shown in Figure 3.20, but the angle of attack is indicated.
In Figure 3.20, the relative pressures are represented by arrows. A higher relative pressure is depicted by arrows pointing into the surface of the aerofoil, while arrows pointing outwards from the surface represent a lower relative pressure. With an angle of attack of about -5° the stagnation point in the airflow is on the upper surface of the wing, and the lift is negative.
-2° is a typical angle of attack for zero-lift, that is, when the upwards-acting force is equal to the downward acting force. The stagnation point remains on the upper surface of the wing. Note that even though the upward and downward acting forces are equal, the distribution of the lines of action of the forces gives the wing a nose- down pitching moment.
At 0° angle of attack, there is a net upwards-acting lift force but the distribution of the forces still causes a nose-down pitching moment on the aerofoil (though not necessarily on the aircraft a whole).
At 2° angle of attack, the stagnation point is on the lower surface of the wing. Lift is positive and we show the lift component of the total reaction force acting through the Centre of Pressure which is at a typical position on the aerofoil, forward of the aerofoil’s geometric centre.
Figure 3.20 variation of lift and pressure distribution with angle of attack.
Lift can be considered as acting through the Centre of Pressure.
During flight, the angle of attack is usually between 2° and 8°. During your flying training, though, you will experience angles of attack of 16° and greater when learning to recognize and recover from a stall. Therefore, we have shown representative angles of attack up to 20°. The diagrams of the aerofoils between 2° and 20° angle of attack include a depiction of the lift force.
As the angle of attack increases from 2° to 15° the Centre of Pressure gradually moves forwards and the resultant lift force increases in magnitude, until reaching about 16° (this is a typical stalling angle of attack for many light training aircraft). Beyond this angle of attack, lift force decreases abruptly and the Centre of Pressure moves rearwards again. This abrupt decrease in lift and rearwards movement of the Centre of Pressure is due to the separation of the airflow from the wing’s upper surface. You will learn about separation in the Chapter on Stalling.
Notice that the gradual forwards movement of the Centre of Pressure with increasing angle of attack (up to the stalling angle of attack) tends to cause the angle of attack to increase even more, which, in turn, will cause the Centre of Pressure to move further forwards, and so on to the stall. This phenomenon is called instability and is one of the problems that aircraft designers have to deal with. Instability of this nature is, of course, why conventional aircraft have tailplanes (horizontal stabilisers).
One final point that you should note from the pressure distribution patterns in Figure
3.20 is that where the lift force is relatively large (angles of attack from 2° to 15°), the
greater contribution to the lift is made by the upper surface of the wing. Notice that the value of the higher pressure on the under surface of the wing changes relatively little between 2° and 20° angle of attack. You will not be surprised, therefore, to learn from your flying instructor that keeping the upper surface of an aircraft wing free from contamination (for example, through accumulations of ice and/or water) is critical.
Angle of Attack.
As we defined earlier, the angle of attack is the angle between the aerofoil’s chord line and the relative (free stream) airflow. See Figure 3.21. Do not confuse the angle of attack with the pitch attitude of the aircraft. Your flying instructor will have a lot to say to you about pitch attitude and will define attitude for you precisely. As an approximate definition, we may say that pitch attitude is the angle of the aircraft’s nose relative to the horizon.
Figure 3.21 Angle of Attack.
Lift increases with increasing angle of attack until reaching
the stalling angle of attack of around 16°, at which point lift decreases abruptly.
Angle of attack is the angle between an aerofoil’s chord
An aircraft is rarely following the line of flight in which its nose is pointing and, therefore, pitch attitude is mostly not a good indication of angle of attack. Certainly, in maintaining level flight at different airspeeds and power settings, the angle of attack is increased (to fly slower) or decreased (to fly faster) by raising or lowering the nose of the aircraft, but this relationship between angle of attack and pitch attitude can not be assumed in all cases.
At a given aircraft weight, a given angle of attack corresponds to a particular airspeed. The pilot of an aircraft may elect to descend at, say, 80 knots, fly straight and level at 80 knots, or climb at 80 knots (see Figure 3.22). Because the airspeed remains the same, the angle of attack will be the same in all cases. Pitch attitude (and power) will, however, not be the same in all cases because the aircraft is, respectively, descending, flying level or climbing.
Remember, then, you must not treat the pitch attitude of the aircraft as an indication of the angle of attack between the wing and the relative airflow.