Control is a fundamental part of the aeronautical sciences playing a necessary role in the design of all air-vehicles whether they are aircraft or balloons, missiles or kites. The early years of aircraft research were characterised by the amount of effort devoted to the control of the aircraft, and the shortcomings of early aircraft designs were frequently due to instability and to poor handling characteristics. Indeed the triumph of the Wright Brothers in 1903 when their first sustained powered flight was made at Kitty Hawk was due as much to their mastery of the problems of control as to their work on aerodynamics, structure or powerplant, vital though these latter were.

There are inherently two ways of obtaining an aircraft design having the desired control characteristics, either by so designing the structure and the aerodynamics of the aircraft itself or by providing some ‘artificial’ or ‘automatic’ control to change its characteristics. The latter method uses the signal from some sensor in essentially the same way that the human being uses signals from his vestibular organs to stabilise himself, and the crane fly similarly stabilises his flight using signals from his vestibular organs to stabilise himself, and the crane fly similarly stabilises his flight using signals from his halteres. The technique was tried even before 1903, and Sir Hiram Maxim in 1891 had produced a patent for an aircraft using a pendulous gyroscope for pitch stabilisation (Fig. 1). For various reasons most of these early efforts were rather unsuccessful, and in the period up to the Second World War the principal advances in the science of aircraft stability and control came largely from the refinement of the basic aerodynamics of the vehicle.

Pesticides help to protect man, plants, animals, structures and various goods from injury, disease or destruction. They maintain and improve our food supply and public health. In some cases, actual economic survival is dependent upon these chemicals.

Although biological and integrated control play an important role in our overall control of pests, we must still rely, to a large extent, upon chemicals to control or suppress the majority of pests. Although there are other methods of controlling insects and pests by biological and physical means, the object is to prevent pest damage of economic significance by the most compatible, acceptable, efficient and economic method.

At the present time, this means chemicals, and in many cases, the rapid application to match an insect invasion, a vector-borne disease outbreak or application in inaccessable areas requires aircraft to apply them.

The dispersal of droplets sprayed from an aircraft, be it fixed or rotary winged, is affected by influences which can be placed in four categories:

• 1. Particle characteristics e.g. terminal velocity.

• 2. Aircraft wake.

• 3. Meteorology.

• 4. Nozzle or atomiser position.

Although all four categories combine to produce a particular application the relative importance of each influence varies with particular circumstances. Large particles for instance, are little influenced by air movement and so aircraft wake and meteorology are unimportant. Small droplets, on the other hand, have low terminal velocities and are more easily affected by air movements. Situations where wake predominates over meteorology are usually those where flying heights and wind speeds are low. For example, in Ultra Low Volume spraying, Lawson and Uk (1979) found wake effects at low flying heights.

Fatigue is a problem which all designers have to face. If they are starting on the design of an aeroplane, they will work out all the high loading cases first, make sure that the design will be good enough to meet them, then they will turn their thoughts to the survival of the proposed structure within the lower stress levels of everyday operations.

Some parts may see one cycle, some hundreds, many will see thousands, some millions and millions. All should function without cracking from fatigue.

If fatigue cracks do appear after some time, this indicates that while the design was reasonably close to optimum, it needs a little more material adding locally. The operator will probably groan at the cost and disruption the beefing-up operation will require, and the designer will be tempted to overdesign in the first place to avoid this situation.

A serious disadvantage of using a lattice of vortex rings to model a surface of vorticity, is that points close to the lattice surface may see large ‘holes’ in what from a larger distance appears as a smooth vorticity distribution. This failing is particularly important when two surfaces approach each other, such as occurs at the trailing edge of a wing, or when a free vortex passes near to a solid boundary. In these cases the error in induced velocity can adversely affect the overall flow computation. A relaxation process to find the force-free position of a free vortex close to a surface would obviously fail, for example. The obvious way of overcoming this problem is to improve the accuracy of the vorticity model whenever a point draws critically near to the vortex lattice.

Maskew suggested a way of overcoming this problem in two dimensions and his ideas have been extended in this note to three dimensions.