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THE “escalator” method of solving frequency equations, developed by Morris and Head (Ref. 1), and the natural extension of a Lemma by Professor Temple, together enable simple proofs to be given of two important fundamental theorems:

THE majority of troublesome vibrations in aircraft arc forced vibrations, due to periodically varying forces originating in or around the power plant; the only important type which is not due to such a cause is flutter. Flutter is a vibration in a part of the aircraft structure set‐up by turbulence in the slipstream passing over the part, and has been the subject of a good deal of investigation from the early days of flying up to the present time. Control surfaces appear to be particularly susceptible to flutter, but a really spectacular example of mechanical failure from this cause is to be found in another branch of engineering—the failure of the Tacoma Bridge in America. In this case a suspension bridge, with a half‐mile central span, was completely wrecked by the effects of turbulent flow. The methods whereby aircraft designers avoid.the occurrence of similar failures are too specialized and intricate to be considered in this elementary study, and the reader is referred to the many reports on flutter included in the Air Ministry publication list.

THE practical importance of preventing large vibrations in aircraft results from the two main effects of vibration, discomfort and danger. The discomfort aspect should not be disregarded, or dismissed as unimportant in comparison with the danger aspect. Both in military and in civil machines, discomfort from any cause is to be avoided if at all possible, as it can very seriously impair the operational efficiency of the air‐crew and lessen the attractiveness of flying as a means of transport. The provision of an adequate margin of safety is, however, essential, and this aspect is therefore of more immediate interest and importance to the engineer.

THE general properties of the resonance phenomenon, described in Part I, in connexion with the simple system consisting of a concentrated mass on a cantilever spring, pertain also to all the types of vibration possible in complicated structures.

CASTORING stability requires automatic return, after displacement, of the castoring wheel to the central position. A hinged wheel may be automatically stable under static conditions, but dynamically unstable, or vice versa. Dynamic stability may be defined as the condition in which the forces on the wheel in motion secure its return to the path of direction of motion.

THE present series of articles is intended to cover in a general manner the whole subject of vibration in aircraft. The study of aircraft vibration is a specialized branch of engineering science, calling for considerable practical skill and experience in the design and use of measuring equipment, as well as the ability to carry out the mathematical analyses which render test results intelligible and lead to the successful solution of vibration problems. The leading features of all mechanical vibration problems are, however, of such a nature as to be readily understood, and the aim of this series is to describe these leading features so as to facilitate the appreciation of the broad principles governing the avoidance of unpleasant or dangerous vibrations.

NETWORKS of the type considered are of practical importance as components of automatic control systems and similar devices. The electrical variety is exemplified by the well‐known ‘phase‐advance’ network, while analogous arrangements of dashpots and springs arc often used to obtain the same effect.

IN its history, the aeroplane has rarely been considered prosaically as a transport vehicle, with the absolute acceptance of the implied requirements of public service, viz. safety, comfort, speed and punctuality.

Early aircraft engines were usually bolted direct to the aircraft structure and no attempt was made to prevent the vibrations which they set up from being transmitted to the airframe. With increasing engine powers and the use of larger airscrews these vibrations eventually become of sufficient magnitude in some cases to cause annoyance to the occupants of the aircraft and also failure by fatigue of parts of the structure. Various attempts were made both to analyse the source of the vibrations with a view to eliminating them or reducing them to an acceptable magnitude. Where this was not possible attempts were made to isolate the disturbances from the airframe and its occupants. This paper presents the basic theory of vibration isolation and gives an account of the various sources of vibration met with in reciprocating, turbo‐propeller and pure‐jet installations. The loads acting on the engine during various conditions of flight are then examined as a knowledge of these is required in order to determine the strength of the supporting units. Various practical engine mounting configurations are then considered which will give vibration isolation together with adequate support of the engine under all conditions of flight. Some account is given of the properties of rubber and the design and testing of rubber vibration isolators, and some installation problems are examined. Finally, the complete programme of testing an installation both on the test‐bed and inflight to evaluate the degree of vibration isolation achieved is described, together with various criteria of acceptability both from a structural and physiological standpoint. A bibliography covering the various sections is included.

DURING the expansion of the aircraft industry it is very easy for us to forget the early struggles and hard‐won lessons of the past. We are now immersed in long and elaborate discussions of the merits of this or that manner of making some piece of accessory equipment and never seem to question the advisability of doing away with the gadget entirely. The writer, through his contacts with young engineers and students, has noticed that they frequently lose sight of the principal objectives in aircraft design and construction. This has left him with the feeling that these objectives and the hard underlying principles of sound engineering need a restatement.