More than sixty years ago Theodore von Kármán showed in a paper entitled Über die Turbulenzreibung verschiedener Flüssigkeiten (On turbulent friction of various fluids) how the pressure drop of various fluid flows through pipes could be correlated in a single curve. The correlation parameter he used is now known as Reynolds number. It had been named after Osborne Reynolds only three years earlier in 1908 by A. Sommerfeld, shordy after he had left Aachen, in a note at the Internationaler Mathematiker Kongress in Rome. Sommerfeld had recognised the importance of Reynolds' similarity law and was deeply persuaded by his experimental and theoretical work.

The Remotely Piloted Vehicle, or RPV, is here as a viable element in the arsenal of aerospace power. The astute student of air power, the United States Air Force planner, and we in the research and development community are well aware of the current and potential applications of the RPV in fulfilling aerospace missions.

The need to provide members of a population with acceptable standards was clearly recognised by Juan de la Cierva. In a paper published in the Cambridge University Engineering and Aeronautical Society's Journal of 1934 under the title “Rotary Wing Aircraft”, he discusses the relative merits of autogyros, helicopters and fixed wing aircraft. It is interesting to note that his section headings include efficiency, utilisation, piloting, safety, simplicity and comfort, which show that he was well aware of the human and economic factors as well as the aerodynamic and structural developments for which he is rightly remembered. I have little doubt that if Cierva were to lecture today on choice, he would review not only the topics listed above for the occupants of the rotary winged aircraft but also he would examine the environmental advantages and possible difficulties which the use of such aircraft can confer today and in the immediate future. This lecture, which it is an honour to be invited to give, is an attempt to perform such an exercise.

Despite the wide use of Alclad aluminium alloy sheets in the aircraft industry, information on the effect of cladding on the mechanical strength of the strong alloy core is mainly limited to comparative fatigue testing of clad and unclad sheets, tested as plain specimens and subjected to constant-amplitude loadings. S/N curves obtained under such conditions were published by Smith, Brueggeman and Harwell for 24S-T3 material and by Shabalin for the D16T alloy (Russian specification). Both sets of data show that the soft cladding had a detrimental effect upon the fatigue strength of the strong alloy. Forrest has presented several comparative S/N curves which also show a reduction in the fatigue strength of the Alclad unnotched specimens compared with the bare alloy. Karlashov and Batov investigated the behaviour of cyclically-strained D16AT alloy (Russian specification), with particular reference to the nature of the mechanism of protection offered by pure aluminium. Although the core was electrochemi-cally protected against corrosion, intense damage occurred in the cladding layer.

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

The plane free jet is a difficult flow in which to make accurate measurements because of the high intensity of the turbulence. Nevertheless, a reasonable agreement might be expected between comparable measurements of a quantity such as growth rate.

As can be seen in Table I, however, a variation of 10% exists in measurements made by various experimenters in fully-developed jets (x/b>100). If the results are grouped according to the value of (x/c)max,where c is the width of the wall in the x = 0 plane (see Fig. 1), a consistent variation can be seen, with the rate of growth for x/c<1, being 10% higher than for x/c>l.