When a continuum is represented in lumped terms as a set of nodes which are regularly connected in a specified sense, the state of stress at each interior node may be inferred from the forces which act on it. These ideas require some extension for plates and shells, whose flexure and twisting is basically governed by the Love-Kirchhoff hypothesis. The latter requires that the mid-plane stresses be combined with bending and torsional stress couples acting on interfaces perpendicular to the mid-plane.

It is commonly recognised among operators of modern commercial aircraft that maintenance is a significant and costly part of their operation. In fact, it is estimated that 28% of all aircraft operating costs can be attributed to maintenance-related activities. A major portion of this activity can be classified as unscheduled, that is, maintenance other than regular servicing and periodic overhauls. Much of the cost of unscheduled maintenance derives from the actual man-hours and materials involved in isolating and replacing faulty components. However, additional consideration must be given to the fact that an aircraft which is detained beyond its scheduled departure time is neither producing revenue for the operating airline during that period, nor promoting goodwill among passengers inconvenienced by delays or cancellations.

The words “space station” mean many things to many people. In fact, a very wide range of craft have been studied under this name over the years. The range of such studies has included large rotating craft providing an artificial gravity environment and accommodating hundreds of people (the type envisaged by Arthur Clarke in his book, 2001: A Space Odyssey) and small two or three-man craft designed for mission durations of 30 days. The one thing that all these programmes have in common is that they emphasise the use of spacecraft for the conduct of meaningful and beneficial investigations using the vantage point or the environmental features of space. In addition, they have involved the presence of man as a means of providing more effective conduct of the assigned tasks. This approach differs somewhat from activities that are strongly aimed at specific and demanding mission accomplishments, missions which place great emphasis on reliable and safe operation of the flight hardware and the demonstration of new operational techniques. Nevertheless, past programmes such as Mercury, Gemini, and Apollo have included many of the basic space station ideas.

Many engineering problems are unsuitable for either pure digital or pure analogue computation. An example is provided by the helicopter. Owing to the size of the problem, a digital solution takes too long, and far more equipment is required than is generally available on analogue computers. In addition, there is the difficulty of including important non-linear effects.

A hybrid computer is a combined digital and analogue computer and can be used to advantage in these circumstances. I shall mainly be concerned with the integration difficulties which arise when applying this form of computation to helicopter simulations. The concepts developed are not restricted to the helicopter problem and an integration technique will be explained which has wide application in the study of other engineering systems.

In my paper “Mediaeval Kites and Windsocks”, I attempted to throw some new light on the curious dragon-shaped objects shown flying on the ends of lines, or fixed to the tops of poles, in a number of mediaeval and Renaissance books of military technology. Until recently these have usually been interpreted as primitive aerostats whose lift is supplemented by wind-forces similar to those which raise a kite.

A further illustration of the flying dragon is reproduced here (Fig 1). As it does not belong to any of the general categories or groups into which the other illustrations fall, it is worth considering in detail. Its three-part tail and general outline indicate that it bears some relationship to the dragons depicted in Figs. 6 and 7 of my paper.

The technical note “Errors due to Pulsation in Orifice Meters” by J. Grimson and N. Hay in the April 1971 issue of the Aeronautical Journal adds some welcome data to a field in which there is insufficient experimental evidence from which an explanation of the characteristics of orifice meters operating under pulsating flow conditions might be obtained.