It is not only a great honour but also a great pleasure to have been asked to deliver this Sixty-Third Wright Brothers Lecture to the Royal Aeronautical Society. My own associations with the Society and its members have almost always been highly edifying, stimulating and productive in both the personal and professional spheres. I believe that the relationships between our two nations which I have had the opportunity to observe from the government, industrial and scientific standpoints have greatly enhanced the qualities of our institutions and the extent of our progress in aeronautics as in many other areas. We have often been competitive, sometimes fiercely so, but almost never in a destructive way. I still remember from my youth the vicarious thrills of the editorial battles of the 1930s between The Aeroplane and Aero Digest over the merits of British and American air transports and the genuine delight I experienced when I actually met C. G. Grey, Editor of The Aeroplane in that era, at a Society garden party years later. The challenge of worthy adversaries, the hard but fair rules of engagement, and the sportsman’s appreciation of the game well played, in victory or defeat, are as potent sources for excellence in science, technology, business and the professions as they are in contests of skill and daring.

The designer of a strike/fighter wing must bear in mind the many manoeuvres which the aircraft must perform. It is not sufficient to consider a single altitude cruise design point. Instead one must aim for:

(a) An economic altitude cruise, i.e. a moderately high Mach number and lift coefficient without drag rise.

(b) A sea level dash capability, i.e. a high subsonic Mach number and a low lift coefficient without drag rise.

(c) Good manoeuvrability at intermediate and high subsonic Mach numbers, requiring high maximum usable lift coefficients.

(d) Good field performance with very high lift coefficients in both take-off and landing configurations.

(e) Some supersonic capability.

Such a wide range of performance inevitably leads to some incompatability in the design. The high speed requirements are necessarily in conflict with those at low speed and similarly there is an inconsistency between the needs for altitude cruise and sea level dash. Reference 1 develops a design procedure based on fixed geometry sections, which attempts to satisfy all the design requirements in a reasonable manner. This paper is restricted to consideration of a method in which flap deflections may be incorporated into the design conditions so that the general performance can be improved without seriously degrading any of the individual requirements.

Increasingly stringent attitude stabilisation requirements are the current trend in both experimental and commercial satellites as is seen in the current Intelsat, ESRO and UK communication studies. These craft must be lightweight, compact and rugged during the launch phase but after mission capture such requirements no longer apply. The increasingly high power requirements of such craft are met by the use of large flexible solar arrays which are packed away during launch and unfurl when the craft becomes operational. For the three-axis stabilised craft being studied reaction jets are used to achieve the high pointing accuracy required. Such actuators may be hot or cold gas systems or in the future may be electric engines’. The broad spectral content of these actuators will inevitably excite modes of vibration in a wide range of frequencies. The influence of these highly resonant modes on the performance of on-board controllers, needing a relatively high bandwidth of 0-10 Hz, to achieve high pointing accuracy of up to a few seconds of arc may lead to design difficulties.

Three theories of bending are available to the designer for the determination of bending stresses in curved beams. (1) The Euler-Bernoulli hypothesis (often referred to as the simple theory of bending), which assumes a linear distribution of bending stress, having a zero stress value at the mid-plane, i.e. the centroidal and neutral axes are coincident. (2) The Winkler theory, which predicts a depression of the neutral axis away from the centroidal axis and towards the centre of curvature. This theory produces a hyperbolic distribution of bending stress across the section, having a maximum amplitude at the fibre nearest the centre of curvature. (3) The Golovin theory, which is exact.

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.