Aerodynamics: Angle of Attack

A key factor in the amount of lift generated by an aircraft's wing is the angle of attack, the angle at which the wing meets the air flow. Exceeding the maximum angle of attack results in a stall and, potentially, complete loss of control of the aircraft.

 

1. Ideal lift

As might be expected, the lift provided by a wing falls off towards the tips. When seen from the front. the lift from a wing ideally has an elliptical form (illustrated by the broken line in the figure below) but, in practice, the presence of a fuselage reduces the lift from the inner parts of the wing, as shown.

 

2. 'Upside-down' wing

Quite apart from this, sometimes the camber (curvature) of the aerofoil of the inner part of the wing of large, fast, jet aircraft may have to be made negative in order to avoid the supersonic air flow caused by the presence of the fuselage. This was a feature of the DC8. At first glance an 'upside-down' wing seems ridiculous!

 

3. Angle of incidence

So far, we have only considered wings in normal flight. Normal flight usually involves an angle of attack (AoA) of about 3-4º because, at such angles, the L/D (lift/drag) ratio is at a maximum. Although this is the angle at which the wing meets the air, it does not follow that the fuselage is at the same angle. The wing is fixed to the aircrait at an angle called the angle of incidence. This is generally not far from 4º so that, in cruising flight, the fuselage is horizontal, for minimum drag. Sometimes, however, the angle of incidence is much higher. The Armstrong Whitworth Whitley bomber was designed in 1934, before it was common to fit wings with flaps, so the wing was set at a large angle of incidence. Like any other aircraft, the faster the Whitley flew, the lower the AoA. Illustrated below is a Whitley in high-speed flight, note the amazing attitude of the fuselage. A later example of an aircraft with a large angle of incidence is the B52. In this case, the unusual configuration was chosen because the landing gear makes it impossible to 'rotate' on take-off. Accordingly, on take-off the aircraft rises like a lift while the fuselage points downwards!

 

4. Varying angle of attack

What happens when we vary the AoA? Illustrated below is a graph showing the L/D ratio plotted against the AoA for a typical wing. We can see that, at the best AoA, the lift is about 24 times as great as the drag (A supersonic fighter is unlikely to reach this ratio, while a competition sailplane should do better.)

 

5. The stall

It is not easy to fly with a sustained AoA less than the optimum but, in a wind tunnel, any angle can be tested, and it is at once evident that, as the AoA is decreased, the lift falls very rapidly to zero. At an AoA lower than -2º, the wing would actually push downwards. On the other hand, it is easy to fly with an AoA greater than the optimum figure; all that must be done is to reduce the airspeed. The lift from a wing is mathematically given by the formula , where L is lift, C, is lift coefficient (a function of the wing profile shape and AoA), p is air density, V is airspeed and S is wing area. In order to maintain height as V is reduced, something else must be increased. Neither p nor S may be increased, so C, must be increased and the only way this can be done is to pull back on the aircraft's control column and increase the AoA. As airspeed (V) is reduced, the pilot must keep pulling back on the control column to increase the AoA until, quite suddenly, at an AoA of around 16º, the wing stalls. The aircraft typically drops like a stone. The stall occurs when the air can no longer remain 'attached', i.e. flowing smoothly over the top of the wing (a). Instead, it suddenly breaks away into a mass of turbulent eddies (b). Lift very suddenly falls close to zero.

 

6. Varying stall speed

Early aviators were terrified by the stall. On finding that they were falling out of the sky, it seemed natural to haul back on the control column, but that just made things worse. The correct action is to push forward firmly, putting the aircraft as quickly as possible into a dive. Airspeed increases, and the AoA returns below the stalling angle. Note: the wing always stalls at a particular AoA, not at a particular speed. For example, if the required lift L is doubled (either by adding payload or by 'pulling g in a tight turn or dive pullout), then the stall will occur at a much higher V. Likewise, if p is halved by climbing from sea level to 22,000 ft (6700 m), then the stalling speed is again found to be much higher (actually 1.41 times higher, the square root of 2). However, the stall will still be at the same AoA throughout. This illustration shows an aircraft which, in Ig(straight and level) flight, stalls at 60 kt 169 mph; ill km/h). It is shown in a tight turn with an angle of bank of 75º. Its effective weight is multiplied by 4 (i.e., the acceleration due to the turn is 4g). In this turn, the stalling speed is increased by the square root of 4 which is 2, in other words, the stall will now occur at twice the previous staliing speed, i.e. 120 kt (138 mob, 222 km/h).

 

7. Frederick Handley Page and his slat

In 1919, a young British designer, Frederick Handley Page, invented a way of postponing the stall. He tested models in a wind tunnel and found that, if he added a small curved strip, almost like a very narrow cambered wing, just above the leading edge, then the air would keep on flowing smoothly over the top of the wing to a higher AoA than the stalling angle for the plain wing. He called the extra piece a slat. It worked by speeding up the flow of air through the gap between the slat and the wing.

An early application of Handley Page's slat was on the H.P.34 Hare high-altitude day bomber project. The slat is clearly illustrated here, mounted above and slightly ahead of the Hare's upper wing. The Hare did not prove to be a successful aircraft, however.

 

This page was borrowed from the World Aircraft Information Files, which is produced by Areospace Publishing Ltd. and published by Bright Star Publishing plc. www.airpower.co.uk