The First Cierva Memorial Lecture was given in the Lecture Theatre of the Society on 16th February 1961 with Dr. E. S. Moult, C.B.E., B.Sc., F.R.Ae.S., M.I.Mech.E., President of the Society, in the Chair. Introducing the lecturer, Dr. Moult said that, beginning in 1948, through the generosity of Wing Commander Graham Douglas, the Helicopter Association of Great Britain awarded a prize annually in commemoration of the great pioneer work of Juan de la Cierva in the field of rotary wing flight. When the Helicopter Association merged with the Royal Aeronautical Society in 1960 it was decided to institute a Cierva Memorial Lecture as a main event in the yearly calendar of the newly-formed Rotorcraft Section of the Society.

This was the first lecture of what he was sure would be an outstanding series and they were honoured by having with them for the occasion Dr. Carlos de la Cierva, son of the great pioneer. Professor Bennett had spent much of his working life associated with rotary wing aircraft and had been closely associated with Senor de la Cierva. He had been with the Cierva Autogiro Company for about 10 years, and for some seven years was head of the Helicopter Branch of Fairey Aviation; in America he was for a period a member of the British Air Commission and later, was Chief of Research of Hiller Aircraft. He was now Deputy Principal and Head of the Department of Aerodynamics at the College of Aeronautics.

The original concept of measuring strains directly from the surface of a structure by coating it with a birefringent material is credited to Mesnager (France, 1930), who was unable to obtain practical results at that time because of the lack of suitably sensitive photo-elastic materials. Subsequent attempts were made by Mabboux (France) in 1932, Oppel (Germany) in 1936 and K. O. Scott (England) in 1949. In March 1953, Col. H. T. Jessop published in the JOURNAL of the Royal Aeronautical Society the results of some research into the feasibility of bonding modern plastics on to structures.

An aspect of structural design which is becoming increasingly more important is the behaviour of a structure when it is subjected to random pressure loading. Random pressure fields can be encountered in several different ways, two of the principal sources of such fields being the noise emitted by a jet, and the fluctuating pressures in a turbulent boundary layer. Many jet-propelled aircraft have suffered secondary structure fatigue failures in those parts of their structures near to the jet stream, and it is not unlikely that certain failures of panelling in engine intakes have arisen because of vibrations set up by the turbulent pressure fluctuations in the boundary layers at the walls.

Solid propellant rockets are now being used in vehicles whose flight paths are mostly outside the atmosphere and need close control. Engines for such vehicles must not simply provide accelerating thrust but should also be capable of having the direction, magnitude and duration of this thrust altered at will. Some devices and methods for accomplishing such control have been described elsewhere, see Refs. 1, 2, 3 and 4. I shall attempt both to discuss these in more detail and to introduce others which I believe have not yet been mentioned.

This note describes our recent attempts to apply the schlieren-interferometer to the study of supersonic flow around axi-symmetric bodies at zero incidence, and in particular shows how the density field distribution for a cone has been derived.

Various optical techniques, e.g. shadowgraph, schlieren and interferometric, have been used in the study of gas flows where density changes are significant. In some instances, however, a qualitative investigation is insufficient and although it is possible to obtain quantitative data from most optical methods, the interferometer has proved, so far, to be the most valuable in this respect.

Natural frequencies of free flexural vibration of rectangular plates may, in many cases, be considerably increased by attaching to the plate one or more elastic stiffening ribs parallel to one edge, or by casting or machining the plate and stiffeners integrally.

Hoppmann has determined by a semi-empirical method the natural frequencies of an integrally stiffened simply-supported square plate, using the concept of a homogeneous orthotropic plate of uniform thickness having elastic compliances which are equivalent to those of the stiffened plate. Filippov has obtained the exact solution for the fundamental frequency of a simply-supported square plate having a number of equally spaced stiffeners and has considered the effect of point loads applied to the stiffeners in a direction perpendicular to the plane of the plate.

It has been recorded in the Annual Reports that String-fellow made a trip to California with Frederick Marriott, who had been the publicity agent for Henson's project of 1843 and the subsequent experiments of Henson and Stringfellow. Marriott, who had become editor of the San Francisco News Letter, began to interest the Californians in a project of a large aeroplane partly lifted by gas, a project which had been suggested in previous years without number.