Several recent theoretical advances have been incorporated in computer programs which find the initial buckling loads of prismatic assemblies of thin plates “exactly”. Any such assembly of plates which are connected together and/or supported along their longitudinal edges, while being simply supported at their ends, can be analysed. Furthermore, curved plates and anistropic materials are allowed. Thus plate assemblies with many practically important types of cross-section can be analysed. Consequently, published results mainly relate to a few crosssections of each of several possible types. However, the designer needs comprehensive design curves, like those in the Structures Data Sheets of the Royal Aeronautical Society, for as many common cross-section types as possible. Unfortunately, production of such curves using the programs mentioned is unlikely to be achieved for all desired types of cross-section, at least for some time, because each type involves plotting curves as several parameters are varied. Nevertheless, a range of results for one type of cross-section is obviously valuable. Thus the lipped channel strut made from isotropic material is considered in this Note (see Fig. 1).

It is a great honour for me to be invited to give this lecture but it has been rather a problem to know how best to tackle it. It is a great temptation when you are my age, and you have been in the aerospace business as long as I have, to reminisce about the things that you have done in the past—and it is one that, in part, I could not resist. If by any chance any of the younger members of the community that still have their working lifetime in front of them can discern any sort of a message here and there in what I say, then I hope they will do so and that they will have the courage to do something about it.

The transport of thermal energy from one location to another, for example from a heat source to a remote heat sink, has been a major topic of study over the past decade.

Most of the work has been directed at improving the efficiency of heat transport, while retaining the simplicity and reliability of what is, in general, considered as a secondary part of any system.

One device which has fulfilled most of the requirements of such a heat transport system is the heat pipe. The heat pipe was first proposed by Gaugler of the General Motors Corporation (US) in 1942, and was the subject of a patent published in 1944. Because the heat transfer problems of that time could be solved by more conventional methods, Gaugler's ideas remained unexploited for approximately 20 years. It was not until the early 1960s that Grover, without knowledge of Gaugler's proposals, in effect re-invented the heat pipe while working at Los Alamos Scientific Laboratory. Grover and his colleagues were working on spacecraft power generation systems at the time.

In many branches of engineering there is an increasing demand for light materials which will operate in conditions of elevated temperature and /or under severe temperature gradients. Magnesium alloys have found extensive use in the field of aviation, due to their lightness and retention of structural strength at elevated temperatures. When this type of structure is subjected to service conditions, it is likely that the general temperature level will change, in addition to the establishment of temperature gradients. A detailed analysis of the structural displacement, loading and stress under such conditions would undoubtedly be aided by a knowledge of the mathematical “law” describing the coefficient of linear thermal expansion as a function of temperature. A single integration of this equation over the limits of temperature will give the displacement per unit length. Integration of the product of this equation and Young's modulus of elasticity, ie integration of the thermal modulus, provides an accurate determination of the thermal stresses promoted by internal restraint, resulting from temperature gradients in the structure.

Although this paper is entitled “Design Philosophy” it is my intention to show how this philosophy has influenced the design and construction of the Westland Lynx helicopter, with particular attention to those aspects which directly relate to all weather operation.

To set the scene I would like to give you a very brief description of the aircraft.

Most service failures in fatigue start on a free surface, as in many cases the most severe stress conditions exist there and are of a bi-axial stress nature, such as a drive shaft under combined bending and twisting. This particular case of multi-axial stress fatigue, because of its importance in design, has seen considerable investigation, although several aspects of the stress situation complicate the analysis of the test results with regard to the development of a general criterion of fatigue failure under multi-axial stress. The principal stress axes rotate with variations in the ratio of bending to twisting and the shaft is subjected to a stress gradient. Only the extreme portions of the shaft in the plane of bending are under the maximum bending stress, whereas the complete surface of the shaft is subjected to the maximum torsion stress.

When a rectangular beam is bent by two end moments about a transverse axis, it is well known that an “anti-clastic” curvature about a longitudinal axis also tends to occur. The theory for this phenomenon in isotropic beams has been previously discussed. For some recent work on the elastic properties of oriented silicon-iron transformer sheet, which is not only anisotropic but also has a non-linear stress-strain curve due to magnetic domain effects, a theory for anticlastic bending was necessary. The experimental results are reported elsewhere, but the theory used for anticlastic bending may find wider application. An expression is derived for the effective Young's modulus for a wide sheet; and its significance is demonstrated for transformer sheets of different width, ignoring the magnetic effects.

There is much current interest in numerical methods of solution of the laminar boundary-layer equations. Methods used to date may generally be classified as

• (i) explicit finite-difference methods (e.g. Refs. 1, 2, 3);

• (ii) implicit finite-difference methods (e.g. Refs. 4, 5, 6, 7);

• (iii) difference-differential methods (e.g. Ref. 8).

A method based on converting the boundary-layer problem into an integral equation and solving the latter by iteration has been developed by one of the present authors.

No one who has served in the Royal Air Force for any length of time can be anything but vividly aware of the intimate connection of Sir Sydney Camm with the Service, and the profound effect that he and his aircraft have had upon us. The list of military aircraft for which he was responsible from 1925 onwards reads almost like a history of the Royal Air Force: Hornbill, Hart, Fury, Hurricane, Typhoon, Tempest, Hunter, P.1127 and the Hawk.

Foppl’s stability analysis (1913) of the equilibrium positions of a line vortex pair in the downstream neighbourhood of a circular cylinder in the two-dimensional potential flow of an incompressible fluid moving with uniform speed at infinity has been corrected recently by Smith (1973). Smith shows that it is only for small symmetric displacements from any one pair of points on the locus of possible rest positions which is known as the Foppl curve there is neutral spatial stability, all other displacements being unstable. This correction now makes the results of the stability analysis consistent with the work of Dolaptchiev and Sendov (1959) who determined the trajectories followed by such a vortex pair when constrained so as to move in a wholly symmetric motion. Dolaptchiev and Sendov obtained the analytical solution of the differential equations for the completely symmetric motion, but for motion other than symmetric no simple solutions of this sort appear to exist although the vortex trajectories for any case can be deduced numerically without difficulty.

Numerical nominally-exact solutions of the full equations describing the inviscid compressible flow about two-dimensional aerofoils have been given by Nieuwland and Sells. However, these methods are not readily extendible to include such effects as viscosity and finite aspect ratio. Thus there is a general need for rapid approximate solutions. approximate solutions.

One such method is described in Ref. 3. The inviscid first-order (or pseudo first-order) solution given in Ref. 3 forms the basis of a standard, widely used, approximate method. It combines the small perturbation theory of Ref. 4 with an extended semi-empirical compressibility law originally derived for symmetrical aerofoils at zero incidence by Wilby. A detailed evaluation of the method is given in Ref. 6

One of the early triumphs of the Lanchester-Prandtl wing theory was that it explained the effects of wind tunnel boundaries and provided explicit and simple formulae to correct wind tunnel measurements for these effects. As every aerodynamicist knows, this theory tells us, with the help of “images”, that the boundaries, in the presence of a lifting wing, produce extraneous velocity components in the flow. At least two generations of experimenters have routinely corrected their angles of attack and their measured drag values for the presence of the vertical component.

Due to their high strength to weight ratio, sandwich panels have become increasingly popular in various areas of structural design. This is particularly true with the aerospace industry where high strength low density materials play an important role in the fabrication of major structural components. Along with the continual quest for strong lightweight structures, recent optimisation methods in structural design have led to a re-examination of the validity of the traditional conservative designs based on the usual linear assumptions of structural analysis and tend to adopt more realistic approaches incorporating geometric nonlinearity of the structure by using more exact finite-displacement, strain-displacement equations.

A problem of some practical importance concerns the interference between a wing surface in a free stream and a nearby streamwise vortex. This situation can arise for example, either in a helicopter rotor system, or when concentrated vortices, which are shed from a fuselage nose or from forward canard controls, interfere with an aft lifting surface.

An approximate non-linear analytical study is described in Ref. 1 in which the three-dimensional loading on a two-dimensional non-swept wing underneath a streamwise line vortex in low speed flow (as shown in Fig. 1) is deduced. Since publication of Ref. 1 a number of linearised theoretical studies have come to light.

The effect of imperfections on the behaviour of a thin-walled strut designed for simultaneous local and overall buckling, the theoretical optimum design, has been illustrated by van der Neut. Local deviations from flatness remove the discontinuity between the pre- and post-buckled states, and loss of stiffness prior to local buckling may initiate premature failure in an overall mode. Imperfection sensitivity of this kind is most severe when the two modes are coincident. Thompson and Lewis have examined the optimum design of a strut with imperfections and show, in a simple example, that maximum efficiency is achieved by ensuring that overall buckling occurs before local buckling and that the actual failing load may be significantly reduced. For larger imperfections the optimum may be virtually eliminated.

After the merging of the British helicopter industry under the Westland banner in early 1960, the three larger constituent parts continued on a semi-autonomous basis putting into service such aircraft as the Wessex, Belvedere and Wasp. Future thinking was still contained within the separate groups with activities related principally to the Westminster, Bristol 194 and the Rotodyne. All three of these projects had merit but were in competition and obviously could not all continue. In the event, however, all were unfortunately abandoned and in 1963 the Group's project activities were centralised at Yeovil. The first task of this new Project Group was to attempt to make proposals for a rational range of future products particularly with respect to the future requirements of our major customer—the British Armed Forces.