Category Aircraft Flight

The aerofoil section

Although wings consisting of thin flat or curved plates can produce adequate lift, it is difficult to give them the necessary strength and stiffness to resist bend­ing. Early aircraft that used plate-like wings with a thin cross-section, employed a complex arrangement of external wires and struts to support them, as seen in Figs 1.7(a) and 1.7(b). Later, to reduce the drag, the external wires were

The aerofoil section

Fig. 1.7(a) Curved plate wing

The thin cambered-plate wing section is evident on this 1910 Deperdussin monoplane (Photographed at Old Warden, Shuttleworth collection)

The aerofoil section

 

(b) Some early aircraft had almost flat plate-like wings

(Photographed at Duxford museum)

 

The aerofoil section

Fig. 1.8 Cambered aerofoil

The degree of camber is usually expressed as a percentage of the chord. (e/c) x 100%

removed, and the wings were supported by internal spars or box-like struc­tures which required a much thicker wing section. By this time, it had in any case been found that thick ‘aerofoil section’ shapes, similar to that shown in Fig. 1.5(a), had a number of aerodynamic advantages, as will be described later.

The angle at which the wing is inclined relative to the air flow is known as the angle of attack. The term incidence is commonly used in Britain instead, but in American usage (and in earlier British texts), incidence means the angle at which the wing is set relative to the fuselage (the main body). We shall use the term angle of attack for the angle of inclination to the air flow, since it is unambiguous.

The camber line or mean line is an imaginary line drawn between the lead­ing and trailing edges, being at all points mid-way between the upper and lower surfaces, as illustrated in Fig. 1.8. The maximum deviation of this line from a straight line joining the leading and trailing edges, called the chord line, gives a measure of the amount of camber. The camber is normally expressed as a per­centage of the wing chord. Figure 1.5 shows examples of cambered sections. When the aerofoil is thick and only a modest amount of camber is used, both upper and lower surfaces may be convex, as in Fig. 1.8.

A typical thick cambered section may be seen on the propeller-driven transport aircraft of the early postwar period, in Fig. 1.9. Nowadays, various forms of wing section shape are used to suit particular purposes. Interestingly, interceptor aircraft use very thin plate-like wings, with sections that are con­siderably thinner than those of the early biplanes.

Before we can continue with a more detailed description of the principles of lifting surfaces, we need to outline briefly some important features of air and air flow.

Delta wings

The amount of sweep required to maintain low speed flow patterns on a wing depends on the maximum flight Mach number required (the ratio of the max­imum speed of the aircraft to the speed of sound). Subsonic airliners which are designed to cruise at Mach numbers less than 0.9 (90 per cent of the speed of sound) require a sweep angle of around 25 to 30 degrees measured along the – chord line (a line drawn along the span – of a chord back from the leading edge). An aircraft designed to travel at twice the speed of sound would require a sweep angle in excess of 60 degrees. The BAe Lightning shown in Fig. 8.1 is an early example of a swept-wing aircraft designed to fly at twice the speed of sound (Mach 2).

With such large angles of sweep, it becomes difficult to build a wing with sufficient bending stiffness or strength. The wing is long in relation to the over­all span, and for reasons given in Chapter 9, it may also need to be very thin.

Delta wings

Fig. 2.21 Swept and delta wings compared

Relatively short straight spars can be used in the delta wing. The delta wing is also thicker at the root for a given thickness-to-chord ratio

On the Lightning, the thickness to chord ratio was only around 6 per cent, and even this is quite thick by modern standards!

The delta planform allows the spars to run straight across, as illustrated in Fig. 2.21(b), instead of along the wing, as in 2.21(a). The delta wing was yet another feature developed by German engineers during the Second World War.

It should be noted that there are two types of delta wing. They are charac­terised by the type of flow regime employed rather than their shape, but for convenience we may classify them as broad and slender deltas. The older broad type was introduced first, and is typified by the Avro (BAe) Vulcan bomber seen in Fig. 2.22. This type of delta wing is essentially a form of swept wing with a large degree of taper. Wings of this form are designed to operate with attached flow for most flight conditions, but separated conical-vortex flow will occur at high angles of attack.

For low speed flight, the low aspect ratio, high taper, and sweepback of the delta planform result in a poor lift-to-drag ratio. This is offset by the structural advantages, and by the large wing volume that results; useful if a high fuel load is required. However, it is in high speed flight, where large amounts of sweep – back are required, that the delta shows its main advantages.

Choice of section

The choice of section shape depends partly on the range of CL values for which efficient low-drag cruising is required. A wide low-drag bucket will be required, if the aircraft is designed to fly efficiently for a large range of speed, weight and altitudes. For steady level flight, CL is equal to weight/(dynamic pressure x wing area), so it is the range of values of weight/dynamic pressure that matters. Note that the weight changes considerably during flight as the fuel is consumed.

The choice of section is also dependent on the maximum value of CL needed. This in turn depends on the weight, the wing area and the stalling speed that can be tolerated without flaps deployed.

The choice of section may be a lengthy iterative process, and at the end, the aerodynamic designer may well be told to go away and think again by the structural designer, who needs a thicker section, with plenty of depth at the rear to accommodate the flap mechanism. The characteristics of typical aerofoil sections are given in the Appendix, p. 361.

Feathering and thrust reversal

In addition to its use as a kind of gearing, the variable pitch mechanism can be used to reduce drag if one engine has stopped. This is done by feathering the propeller blades; turning them edge-on to the wind so that they stop rotating, as seen in Fig. 6.6.

Feathering may be automatic on some multi-engined aircraft to prevent adverse handling problems if one engine fails.

After touch-down, the variable pitch mechanism can be used to set the pitch to a negative angle, so that negative thrust or drag is produced. This reversed thrust feature can shorten the landing run considerably, and is almost invari­ably used on propeller-driven transport aircraft. It also conveniently enables the aircraft to be backed in or out of parking spaces.

Cruising flight

For the most part, the flight of an aircraft can be divided into at least three dis­tinct phases – take-off and climb, cruise, descent and landing. In this chapter we will be primarily concerned with the cruise performance of the aircraft. Landing and take-off will be discussed later in Chapter 13.

The nature of the cruise will change depending on the use to which the aircraft is to be put. For example, a commercial airliner must operate as eco­nomically as possible, and so reducing fuel usage over a given route is of prime importance. However, as we shall see later, this is not the only factor that matters as far as the operator is concerned. For a patrol aircraft, such as the airborne radar system, AWACS, or a Police observation aircraft (Figs 7.3 and 4.9) endurance is likely to be the overriding consideration. For a fighter it may well be a combination of high speed, in order to make an interception, coupled with a need for either range or endurance, depending on the particular mission undertaken. In this case the ‘cruise’ phase of the flight can be subdivided. This is also true of other aircraft types. For instance, a commercial airliner must frequently spend some time in waiting its turn to land at a busy airport, and so an important ‘stand off’ phase is introduced, which is required purely for organisational purposes.