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THE increasing speed of modern aircraft has brought to the forefront the necessity for making a careful drag analysis of all aircraft in order to separate out the essential drag, that is to say the drag that is unavoidable, from the non‐essential drag. Most designers, we believe, now do this in order to see what progress is being made in the streamlining of their products. By this means we are enabled to see the relative importance of the drag terms and to arrive at a figure of merit. The ideally‐streamline aeroplane, though not at present a precise proposition, is like other ideals unattainable. It is the standard to which designers may aspire, but which they cannot achieve.

EVER since I first began to study Aeronautics I have been annoyed by the vast gap which has existed between the power actually expended on mechanical flight and the power ultimately necessary for flight in a correctly‐shaped aeroplane. Every year, during my summer holiday, this annoyance is aggravated by contemplating the effortless flight of the sea birds and the correlated phenomenon of the beauty and grace of their forms.

THE authorities of the Institute of the Aeronautical Sciences have decided, so I am instructed, that the Wright Brothers' Lecture should deal with subjects upon which the lecturer is engaged at the time, rather than with a general survey of some wide branch of aeronautical knowledge. This decision has the advantage that the lecturer is actively interested in the subject about which he talks, but it leaves to chance the question whether he is in a position to end his lecture with simple and clear cut conclusions. I mention this because the problem upon which we are working at Cambridge, and about which I shall speak, is not yet solved and my lecture must, perforce, be confined to a discussion of aims and methods and of results so far obtained; it does not contain that simple statement of conclusions which is the ultimate aim of all good research. After this explanation you will not, I hope, be disappointed when the lecture ends on a note of interrogation.

This volume is the fifth of a series on the general subject of Aerodynamic Theory, prepared under a grant from the Guggenheim Fund for the Promotion of Aeronautics. The earlier volumes were reviewed in AIRCRAFT ENGINEERING, September,. 1934, p. 249; April, 1935, p. 99; September, 1935, p. 233; and December, 1935, p. 308.

The third term has been expressed as but in wind tunnel work it is often more convenient to measure were the omission of the dash signifies that the moment is now measured about a wind axis. The two quantities are very closely related and the measurement of one tells us almost as much as if the two were known. The latter, however, tells us either directly or indirectly what effect the addition of fin and rudder will have on the autorotation properties of the wings alone. The damping of fin and rudder being due essentially to the air flow meeting them at an angle on account of the rotation it should theoretically be possible to deduce this dynamic quantity from a simple static test of moment due to yaw angle. An experiment to test this was carried out several years ago but the static test did not give any approximation to the truth. This was ascribed at the time to the shielding of fin and rudder by the tail plane in the rotative experiment and subsequent work has amply confirmed this view. It is now known that shielding by the tail plane is by far the most important factor in determining the efficiency of the vertical surfaces at high angles of attack.

The following series of articles presents a new geometrical system of determining the lateral stability of aeroplanes. The method is intended to appeal particularly to engineers on account of two advantages: it is simple and rapid in operation, and gives a clear insight into the several factors governing the stability. Thus, whereas in the classical method stability calculations entail the drawing and analysis of quartic curves, the results are here obtained, and with greater generality, merely by the use of curves of the second degree. Furthermore, the effects of typical changes in design characteristics may easily be assessed with the minimum of effort. The fundamental analysis is essentially mathematical and follows the treatment first laid down by G. H. Bryan in 1911 and since developed by Bairstow, Glauert, Jones and Bryant. Physical explanations are included where possible to amplify the underlying principles.

NEW COUNCIL The Council for the year 1948/49 is as follows:

IN high‐speed level flight in the compressibility region an entirely new factor makes its appearance, viz: small variations of atmospheric density and speed of sound with height. This factor affects dynamic stability due to continuous changes of height during longitudinal disturbances; there is no effect in lateral disturbances. The affects are very small in low‐speed flight but they increase steadily with Mach number. The short‐period oscillations are not affected but the corrections to phugoid motion become appreciable in high subcritical flight, larger in supercritical (transonic) range, and very important in supersonic flight. The effects of compressibility are of paramount significance but they should be considered in conjunction with varying height effects. Another result of the investigation is the appearance of a new mode of disturbance, due to the stability quartic being converted into a quintic. The fifth (real) root is often small, it may vary in sign according to aerodynamic properties of the aircraft and characteristics of the power unit. The new mode is a subsidence or a divergence, and it determines height stability or instability, hence it may show to what extent an aircraft is able to keep constant altitude over long stretches of time.

ENGINE starting at low temperatures is difficult for the following reasons: (a) A cold engine requires higher torque to start it and thus imposes a greater load on the accumulator. This is not so marked with ball or roller bearings.

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