On June 5th 1783, one hundred and seventy years ago, almost to the very day of the Coronation of Queen Elizabeth II, the first public “aerostatic experiments” were made which were to herald the dawn of a diuturnity of human endeavour.

In the Hodgson-Cuthbert Aeronautical Collection the Royal Aeronautical Society possesses a direct link spanning those one hundred and seventy years, a link which joins those wondrous days of long ago when people cried “What on Earth in Heaven is that?” with the present welfare days in which many have learnt how to move Heaven and Earth simultaneously atomically.

Most sections of the Aircraft Industry with something to make, something to describe or something to sell will doubtless find much to take issue with in what is about to be said. No apologies are forthcoming, however, since it is proposed to speak about, and of, aircraft as found and experienced by pilots and operators.

Along with Lease-Lend the North American idea of high pressure salesmanship appears to have permeated British aeronautical matters together with all the superlatives ending in " ER." Of all mankind's ingenious devices probably none are more full of compromises than aeroplanes, therefore adjectives describing one or more of their aspects may be noteworthy but, when they tend to overflow, a sceptical eyebrow may well be raised.

The practice of removing aircraft items for overhaul at specified periods has gradually extended in recent years, so that most types of equipment and instruments have come to be dealt with in this way. The period between overhauls is often called–somewhat inappropriately–the life of the item and, for the sake of brevity, this is the word used in this paper.

The primary object of this procedure is to remove the items for overhaul before defects occur. If this is achieved, two benefits accrue:–

1. (a) the defect rate is minimised, with consequent improvement to safety.

2. (b) the operator does the work at convenient times at the overhaul base instead of being driven to do work when and where defects occur.

The noise problem associated with an aircraft flying at supersonic speeds is shown to depend primarily on the shock wave pattern formed by the aircraft. The noise intensity received by a ground observer from a supersonic aircraft flying at high as well as low altitudes, is shown to be high, although it is of a transient nature.

A study of the shock wave patterns around an aircraft in accelerated and retarded flight is shown to lead to an explanation of the one or more booms, of short duration, heard by ground observers after an aircraft has dived at supersonic speeds.

The shock wave patterns associated with an aircraft flying in accelerated or retarded flight at transonic speeds are shown in certain cases to be very different from the corresponding patterns observed in steady flight. The significance of these results, with reference to problems of flight at supersonic speeds is briefly discussed.

The problems discussed in this note refer to the simplest case of the use of linear accelerometers, in which the flight path is in the vertical plane of symmetry of the aeroplane, and the acceleration normal to the path is to be determined.

Generally two facts are neglected:–

1. (i) That the accelerometer senses along an arbitrary body axis ZB (Fig. 1), instead of along the wind axis Z (normal to the path).

2. (ii) That the accelerometer is not placed exactly at the e.g. of the aeroplane but at some point A, having co-ordinates xA and zA in the body axes.

This note describes apparatus which has been used for remote measurements of forces and moments on models on the Whirling Arm at the National Physical Laboratory. Good short-term stability, independent of mains frequency and voltage variations, has been obtained with simple apparatus. Descriptions are included of the differential inductance bridge, a stable 1 kc./sec. oscillator used therewith, and a regulated power supply for the oscillator. General comments are made on problems involved in the force and moment measurements.

Aerodynamic theory may be used with reasonable confidence to estimate the low-speed value of the stability derivative Ip for wings of most plan forms under quasi-steady conditions so long as viscous effects are unimportant. Wind tunnels tests are made to cover other cases such as the partially stalled swept wing, but the writer has heard of a difficulty encountered by a firm when using the free-rolling method. A variant of that technique in which this difficulty is avoided and which has been used at Imperial College for some years may be of interest.

The problem of deducing resonance modes of vibration of an aircraft in free space is a concomitant of flutter calculations if the number of degrees of freedom used is to be small. When the structure is complex in that it involves wings, fuselage and tailplane, each of which possesses infinitely many normal modes, it becomes apparent that the number of point masses which must be considered, in constructing a dynamical equivalent to give a sufficient coverage of the frequency range in which flutter is likely, is very large. For example, it may be necessary to use four rods per wing and per tailplane and four point masses on the fuselage. This would involve 8 + 8 + 4 degrees of freedom, and if the usual technique of characteristic roots is used it would be necessary to consider a characteristic root matrix of order 20 × 20.