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
Fig. 3.19 Unmanned air vehicles (UAVs) are increasingly finding both military and civilian uses
Their relatively small size means that low Reynolds number effects have to be taken into account. Specially designed wing-sections are required
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.