The 887th Lecture to be given before the Royal Aeronautical Society was held at the Institution of Mechanical Engineers, Storey's Gate, London, S.W.I, on 29th October 1953, and was presided over by Sir William Farren, C.B., M.B.E., M.A., F.R.S., F.R.Ae.S., President of the Society. Sir William said that this was the first main lecture of the session. In one paragraph of his paper Mr. Mercer said that those who were specialists in fields of a highly specialised kind should get together with their friends in other branches of aeronautics, otherwise they might all go on in ignorance of what was being done by others; he hoped therefore, that there were a number of specialists present who would join in the discussion.

Introducing the Lecturer, Sir William said that he had known Mr. Mercer for many years; Mr. Mercer had joined the Blind Landing Experimental Unit at Martlesham Heath when it was formed and it was there that he had done the work described in the paper. The President said he had always felt that one of the things most needed for the real development of aviation was a means of landing in bad weather. He hoped that Mr. Mercer's paper was a contribution towards that end; it was concerned with one phase only of blind landing—approach—but he hoped there would be other papers on the subject. He now called on Mr. Mercer to give his paper.

A simple method of determining local turbulent skin friction on a smooth surface has been developed which utilises a round pitot tube resting on the surface. Assuming the existence of a region near the surface in which conditions are functions only of the skin friction, the relevant physical constants of the fluid and a suitable length, a universal non-dimensional relation is obtained for the difference between the total pressure recorded by the tube and the static pressure at the wall, in terms of the skin friction. This relation, on this assumption, is independent of the pressure gradient. The truth and form of the relation were first established, to a considerable degree of accuracy, in a pipe using four geometrically similar round pitot tubes—the diameter being taken as representative length. These four pitot tubes were then used to determine the local skin friction coefficient at three stations on a wind tunnel wall, under varying conditions of pressure gradient. At each station, within the limits of experimental accuracy, the deduced skin friction coefficient was found to be the same for each pitot tube, thus confirming the basic assumption and leaving little doubt as to the correctness of the skin friction so found. Pitot traverses were then made in the pipe and in the boundary layer on the wind tunnel wall. The results were plotted in two non-dimensional forms on the basis already suggested and they fell close together in a region whose outer limit represented the breakdown of the basic assumption, but close to the wall the results spread out, due to the unknown displacement of the effective centre of a pitot tube near a wall. This again provides further evidence of the existence of a region of local dynamical similarity and of the correctness of the skin friction deduced from measurements with round pitot tubes on the wind tunnel wall. The extent of the region in which the local dynamical similarity may be expected to hold appears to vary from about 1/5 to 1/20 of the boundary-layer thickness for conditions remote from, and close to, separation respectively.

The determination of shear distributions for multi-cell sections can conveniently be considered as two separate problems.

1. (i) the shear flow arising from a pure torsional couple applied in the plane of the section, and

2. (ii) the shear flow arising from a resultant shear force acting on the section so as to produce no resultant twist per unit length of that section, i.e. through the shear centre of that section.

Both problems involve finding the value of one unknown shear flow for each cell, so that in multi-cell construction a set of simultaneous equations is obtained, which must be solved to obtain the shear pattern.

The solution of problem (i) by successive numerical approximations has been dealt with completely by Benscoter. The present paper gives a method for problem (ii).

The usefulness of the method becomes more apparent with increasing numbers of spar webs as may be envisaged for future wing sections of small thickness chord ratios. It is analogous in the procedure used to the Hardy Cross moment distribution method for continuous beams, and has similar advantages. An approximate solution of any required degree of accuracy is quickly obtainable at the design stage and the method is readily adaptable for use by a computer with no knowledge of the structural or mathematical theory involved.

In two recent notes Jacobs and Duncan considered the St. Venant torsion and flexure problems for beams of aerofoil cross section and have given methods for determining the centre of shear once the torsion function for the section has been found. The purpose of the present note is to give results for the location of the centre of shear for a limited range of cross sections bounded by two circular arcs for which the torsion function has long been known. The calculations shed some light on the relative influence of camber and thickness on the location of the centre of shear and are compared with the well known results for thin circular arc sections illustrated in the Structural Data Sheet 00.06.04.

The purpose of this note is to call attention to the convenience of glass cloth/polyester resin laminates as a means of constructing certain parts of experimental apparatus, such as small ducts or nozzles.

The advantages of the material are that it is strong, light in weight, easily worked in that heat and pressure are not required, and a high degree of accuracy and surface finish are easily achieved. For the purposes considered here, convenience in fabrication is the main consideration, since it is easier to shape the outside of a suitable male mould rather than the inside of a small duct.

An electro-mechanical method of simultaneous component separation is suggested as an alternative to mechanical systems. Based on the commonly used weighbeam, employing a motor driven leadscrew, poise weight, selsyn repeater circuit and counter, component separation is proposed by means of the selsyn differential circuit.

The use of the selsyn motor as a direct or differential telemeter mechanism is well known, and as such, has many industrial applications. Such use, applied to the aerodynamic balance, offers the advantage of avoiding many of the difficulties associated with design and in particular, the construction of a mechanically independent and simultaneous measuring instrument.

I wish to support S. W. Greenwood's plea (December 1953 Journal) for defining “specific impulse” as Thrust/Mass flow of propellants.

Apparently some engineers are now using Thrust/weight flow of propellants as the definition of specific impulse, but it would appear that this innovation is merely due to inability to distinguish between mass and weight. The system used at the Royal Naval College, Greenwich, of using the capital letter to denote weight or force (e.g. Lb.) and the lower case to represent mass (e.g. lb.) is a very convenient way of distinguishing between the two.