Category Aircraft Flight

The wing-bound vortex, together with the trailing vortices, form a kind of horseshoe shape, and this is sometimes called the horseshoe vortex system. The horseshoe system forms three sides of the predicted closed ring. The circuit is completed, as shown in Fig. 2.3, by the starting vortex. In the next chapter, we describe how this starting vortex is formed.

A strong starting vortex is formed and left behind just above the runway when the aircraft rotates at take-off. More starting vorticity is produced and left behind whenever the aircraft produces an increase in wing circulation. An additional starting vortex is thus formed, when an aircraft starts to pull out of a dive.

The counterpart of starting vorticity is stopping vorticity, which rotates in the opposite sense, and is shed every time the circulation is reduced, as on landing.

As we mentioned in Chapter 1, in level flight, the amount of circulation required reduces as the speed increases, so stopping vorticity is shed when an aircraft accelerates in level flight.

Starting and stopping vorticity is left behind the aircraft, and eventually damps out due to the effects of viscosity. It may, however, persist for several minutes, and the rotating masses of air left behind represent a considerable hazard to any following aircraft. It is necessary to leave a safe distance between two aircraft, particularly when landing. This may be several miles if the following aircraft is much smaller than the leading one.

Strong starting and stopping vortices can be generated during violent manoeuvres, and may significantly affect the handling. The formation of starting and stopping vortices is described further in the next chapter.

In recent years there have been major developments in the use of unmanned air­craft. Remotely piloted aircraft were originally used as gunnery target drones, but even before the end of the Second World War, radio-controlled winged glide bombs were used both by the German and American forces, and the V1

flying bomb became the first example of an inertially-guided offensive weapon. Currently, the major use of unmanned air vehicles (UAVs) such as the Predator shown in Fig. 3.19 is for surveillance purposes, both military and civilian. Such aircraft can, however, also be adapted for offensive use by fitting them with air-to-surface missiles for deployment against small targets. There is also con­siderable interest in the potential use of UAVs for air combat. Civilian applica­tions of UAVs not only include surveillance and police work, but also mapping and land resource management. The solar re-charged electrically powered QinetiQ Zephyr shown in Fig. 3.20 is an example of a UAV which is designed for ultra long duration of several weeks at high altitude. Although not small, with a span of 22 metres, its low speed, high altitude and relatively small wing chord mean that it will operate at low Reynolds numbers, so considerable experimental and computational research has been necessary.

Most UAVs are smaller than conventional manned aircraft, and consider­able effort has been expended on research into extremely small aircraft that can be mistaken for birds or even insects. Because they are so small, and fly rela­tively slowly, sometimes at very high altitude, the combination of small length (l), low density p, and low speed v means that the Reynolds numbers (pvl/y) involved are much lower than those encountered on piloted aircraft. As a consequence, the flow over much of the surface will normally be laminar, and wing-sections etc. designed for full-size aircraft are not generally suitable. It has
therefore been necessary to develop new low Reynolds number sections, often drawing on the experience of competition model aircraft.

With wholly or mostly laminar flow, the drag coefficients of such sections may be very low, but the disadvantage is that they often only perform well over a small range of angles of incidence. Flow separations can occur suddenly, pro­ducing increases in drag and the likelihood of stalling.

It is not only the wing-section that has to be suitably designed. Low Reynolds numbers affect the flow on all parts of the vehicle, particularly on propellers and air intakes. As a consequence, there has been a great deal of research activity using both experimental and computational methods.

Recommended further reading

Lachmann, G. V., (editor), Boundary layer and flow control, Vols I & II, Pergamon Press, 1961.

Simons, M., Model aircraft aerodynamics, 4th edn, Nexus Special Interests, UK, 1999,

There are several factors that contribute to the overall drag of an aircraft, and it is convenient to give names to each of them. Some confusion exists in this area because of a lack of standardisation. The British Aeronautical Research Council (ARC) tried to rectify the situation by producing precise definitions (ARC CP 369). Unfortunately, the terms that they chose were long-winded, and as a consequence, the older names are still in general use. In this book we shall use the ARC terms, with the popular equivalent in brackets.

We have already described the origins of surface friction drag and trailing – vortex (induced) drag. In this chapter we shall describe another contribution known as boundary layer normal pressure (form) drag. We shall also describe the various steps that can be taken to reduce each of these contributions.

In high speed flight, a contribution known as wave drag is important, but this will be dealt with later.

Note that drag is really made up from only two basic constituents, a com­ponent of the force due to the pressure distribution, and a force due to viscous shearing. The contributions such as trailing vortex drag act by modifying the pressure distribution or shear forces, and so the contributions are not entirely independent of each other, as is often conveniently supposed.

Figure 6.2 shows schematically, the simplest form of gas-turbine propulsion device; the turbo-jet. The engine consists of three basic components.

1 A compressor is used to increase the pressure (and temperature) of the air at inlet.

2 A combustion chamber, in which fuel is injected into the high-pressure air as a fine spray, and burned, thereby heating the air. The fuel is normally a form of paraffin (kerosene). The air pressure remains constant during combustion, but as the temperature rises, each kilogram of hot air needs to occupy a larger volume than it did when cold. It thus rushes out of the exhaust at a higher speed than at entry. The jet normally emerges at a pressure close to the ambient atmospheric value.

Combustion

Compressor 1.8 7" chamber З 7 Turbine 3.7 7

— Outlet

nozzle 0.5 Г

Net thrust 7

Fig. 6.2 A simple turbo-jet engine with axial compressor and turbine stages

Approximate contributions to the net thrust T are shown for a typical engine

3 A turbine which extracts some of the energy available in the exhaust jet in order to drive the compressor.

The exhaust gases leave the turbine at subsonic speed, and for subsonic aircraft they are normally accelerated by means of a simple fixed converging nozzle. The maximum Mach number that can be obtained in such a nozzle is 1, but as the gases are hot, the speed of sound in the exhaust is faster than that in the surrounding atmosphere. Thus, a converging nozzle can still the­oretically be used at supersonic speeds. In practice, aircraft designed for super­sonic flight normally require a variable geometry nozzle that can be adjusted to produce a convergent-divergent configuration for high speed flight. In a convergent-divergent nozzle, the jet can be accelerated to Mach numbers greater than 1.

Supersonic aircraft invariably use reheat, which also requires the use of a variable geometry nozzle. The designs are often complicated, involving a large number of moving parts, all of which have to stand up to very high tempera­tures. The complex interleaved plates of the Concorde variable geometry final outlet nozzle may be seen in Fig. 6.32.

The complexity of the nozzle mechanism may be reduced if a two­dimensional design is used instead of the conventional axi-symmetric arrange­ment. The two-dimensional nozzle takes the form of a variable-geometry slot, as illustrated in Fig. 6.36, and can be arranged to produce thrust vectoring for control purposes, and STOL (short take-off and landing).

We have made very little reference to the role of the boundary layer in the development of the flow over the swept wing. The spanwise component of velocity, which we have so far assumed to have no effect on the flow will, in fact, modify the way in which the boundary layer forms. In Chapter 3 we saw that, because the flow due to this velocity component is directed towards the tips on a swept back wing, the boundary layer will tend to be thicker at the tips than it is at the centre section.

In designing a swept wing we must therefore bear in mind the various difficulties outlined in Chapter 3. The tip region will be most prone to bound­ary layer separation leading to local stalling of the wing, which will be of par­ticular concern in highly loaded manoeuvres and in the approach to landing at low speed. The tip is a particularly bad location for this to happen first because a change here will produce the maximum change both in pitching moment and also in rolling moment if one tip stalls before the other. Worse still a flow separation in this region is likely to severely affect the aileron effectiveness so roll control will be lost.

Spoilers are small surfaces which are designed to spoil the flow over a wing and thus reduce its lift. They normally take the form of small hinged plates which, when deployed, project up into the flow on the top surface of the wing.

ailerons

Fig. 10.10 Spoilers, ailerons and flaps on a Boeing 747

Spoilers were originally used as a means of producing drag to slow an air­craft down. They were also fitted to gliders, in order to control descent rate on approach, shorten the landing run and to ensure that once landed, the aircraft stayed down. Nowadays, spoilers are fitted to most large aircraft, being used differentially (deployed on one wing and retracted on the other) to provide roll control, or collectively (deployed simultaneously on both wings) to provide a means of increasing drag and reducing lift. Figure 10.10 shows the spoiler and aileron locations on a Boeing 747 ‘Jumbo’.

Since spoilers are often used in combination with ailerons in a complicated way, and may only operate under certain flight conditions, some degree of automation is necessary in the spoiler control mechanisms. A good description of the use of spoilers on large aircraft is contained in Davies (1971).

If an aircraft rolls slightly from its level-wing position, then the lift force will have a sideways component (as shown in Fig. 10.11). This results in a sideslip, and we can use the sideslip to produce a restoring roll moment by a number of means. The traditional method is to crank the wings upwards to give lateral dihedral, as shown in Fig. 11.14. Figure 11.15 shows an aircraft with wing dihedral viewed from the direction of the approaching air. As the aircraft is sideslipping, the air approaches from the front quarter. From this view, we can see that the near wing presents a greater angle of attack than the far wing. The near wing will, therefore, generate more lift, rolling the aircraft back towards the horizontal.

The quantity – pV2 is usually referred to as the dynamic pressure. There is a more precise definition of dynamic pressure, but this need not concern us now. Although it has the same units as pressure, dynamic pressure actually represents the kinetic energy of a unit volume (e. g. 1 cubic metre) of air.

Aerodynamic forces such as lift and drag are directly dependent on the dynamic pressure. It is, therefore, a factor that crops up frequently, and for simplicity, it is often denoted by the letter q. Pilots sometimes talk of flying at ‘high q’, meaning high dynamic pressure.

The other term in the expression, the pressure p, is often referred to as the static pressure.

It is tempting to ignore biplanes and multiplanes as being of purely historical interest, but old ideas have a habit of returning, and small biplanes have once again become popular for aerobatic and sport flying.

The wings of early aircraft had little or no bending stiffness, and had to be supported by external wires and struts. The biplane configuration provided a simple convenient and light structural arrangement, which was originally its main attraction.

A biplane produces virtually the same amount of lift as a monoplane with the same total wing area and aspect ratio. The biplane, however, has the smaller overall span, which makes it more manoeuvrable. The highly aerobatic Pitts Special, shown in Fig. 2.26, is an example of a modern biplane which is not merely a gimmick. The manoeuvrability of biplanes was one factor that led to their retention even when improvements in structural design had removed the necessity for external bracing.

Reductions in drag can also be obtained by careful attention to the shape of the wing tip. This is particularly true in the case of aircraft with untapered wings. Although untapered wings are not the best shape in terms of minimising drag, they are often used on light aircraft because of their relative simplicity of con­struction, and their docile handling characteristics. (The inboard section tends to stall first.)

Two simple approaches; the bent and the straight-cut tip are illustrated in Fig. 4.13. Both of these tip designs are said to reduce drag by producing

separation of the spanwise flow at the tip, resulting in a beneficial modification of the tip flow-field. It should be noted, however, that unusual tip shapes are often intended primarily to inhibit tip stall, rather than reduce drag. Upward bent tips are evident on the Aerospatiale Robin shown in Fig. 4.14.