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THE fact that the anti‐knock rating of aviation fuels in octane numbers presents difficulties, especially above 80 octane, has led us to develop a more suitable test method for aviation fuels, which might be called the “Allowable Boost Ratio ” method.

THE problem of undercarriage retraction has now been completely solved, and difficulties may occur only when the wheel has to be stowed flat under the wing as the motion generally requires a rotation about the centre line of the shock absorber.

SINCE THE INTRODUCTION of the hypoid gear as the right‐angled drive in automotive rear axles there has been a constant demand for specialised lubricants, particularly for running‐in purposes. Requirements are gradually and constantly becoming more severe and standards of performance adequate 5 or 10 years ago may be marginal today and unacceptable in the future.

ACTUALLY more by accident than design, this issue is predominantly one of our aero‐engine. We had arranged to publish another of the series of articles dealing with machining operations in various aero‐engine factories, in this instance describing production at the magnificently equipped works of the Bristol Company at Filton. We also had available particulars of two interesting French engines. As it chanced, details of two types of British engine happened to be released for publication at the same time, so that this issue is of particular interest in regard to power plants.

THE original work of Mr. H. R. Ricardo in 1919 showed some of the technical advantages of using fuels of high anti‐knock value. Since this time the development of aero‐engines in respect of power output and economy has depended more upon the fuels available than upon any other factor. Considerable progress has been made in the direction of improving the anti‐knock value of petroleum spirits, which constitute the bulk of the fuel used in aviation throughout the world. However, until very recently this progress had not been very rapid, and as Mr. Ricardo showed, far greater improvements in anti‐knock value could be achieved by the use of other fuels such as benzol, toluol and alcohol. On the octane scale benzol and toluol have a blending value by the C.F.R. motor method of about 90 in concentrations up to 50 per cent; that of alcohol being about 105. This is shown in Fig. 1, from which it will be observed that two curves are given for ethyl alcohol, one being obtained under motor method conditions (mixture temperature 149 deg. C.) and the other with the same heat input as required for a normal aviation gasoline. By this means the additional advantage of the higher latent heat of the alcohol blends is shown. Neither benzol, toluol nor alcohol is produced in very large quantities, and moreover, in times of national emergency they are likely to be required for purposes other than for use as aviation fuels.

We welcome queries from readers on any subject allied to Lubrication, whether it be from manufacturers or users of oils and greases. Technical and practical problems will be answered by experts. Questioners' names and addresses will not be published if a nomme‐de‐plume is given, but they must be included with questions, for our own information and as a mark of good faith; such information will be treated in strict confidence.

An explanation is given of the physical factors which govern the influence of the speed of sound on the drag, of an aeroplane, and on its performance. Observing the processes at the point of stagnation, the criterion for the compressibility influence is the Machs number, the introduction of which is also explained.

There can be few scientific and engineering subjects which have progressed as rapidly as the theory and practice of high‐speed aerodynamics and jet propulsion during recent years. The number and scope of papers that have poured out from research establishments, universities and the industry are such that it has been impossible for all but the very gifted to keep ahead of developments in more than a few limited aspects of the subjects.

IN the year 1890, Herbert Akroyd Stuart took out a British patent in which, for the first time, mention is made of an engine which may be said to bear some resemblance to the modern compression‐ignition engine.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Committee, Reports and Technical Notes of the U.S. National Advisory Committee for Aeronautics and publications of other similar research bodies as issued