The ubiquity of aircraft in being and yet to be, whether civil or military, manned or unmanned, makes them liable to exposure to wide extremes of atmospheric conditions. The range of temperature to which they may be subjected may possibly be from +90° to –90°C. (+194° to –130°F.), that of pressure, from one atmosphere to something approximating to one-tenth of this amount, while the water content (aqueous vapour plus water in suspension; for example, in a very dense tropical cloud), can, on occasion, be as high as 2.5 per cent. by weight and, at stratospheric heights, at least as low as 0.001 per cent. Such variations in ambient conditions are not without chemical and physical repercussions. The engineering problems which arise will be examined, therefore, from both these view points, and attention will be drawn to potential dangers and means suggested for their avoidance.

Under present conditions, the author of any paper on rocket power plant engineering must inevitably apologise for a certain reticence as regards technical details, which is imposed by security requirements. These arise from the important applications of rocket propulsion for guided weapons and other military purposes, and are less stringent in regard to its use as a means for providing take-off assistance. It is with the latter subject that this paper will be mainly concerned when practical applications are discussed.

Most of the fundamentals are common to all uses of the rocket motor and it is hoped that a general survey of these may be of some value, especially since the field of rocketry has been relatively neglected in the technical publications of this country.

Recently published statistics give the impression that an excessively high percentage of non-carrier aircraft accidents investigated during 1948 were primarily due to human error. Further, an examination of available data covering the past decade gives the same impression in respect of all aircraft accidents occurring during that period. The figures quoted convey in fact the unpalatable implication that, although in nearly a half century of aviation great strides have apparently been made in increasing the reliability of the aeroplane, very little has been accomplished in reducing the element of human error in aircraft accidents. Why is this?

It has been claimed that the possibility of accidents in flying is inevitable by reason of the inherent conditions of flying which necessitate, besides reliance on the technical factor, considerable reliance upon the skill, judgment, memory, and physical and psychological conditions of the human being.

The parts played by calculation, and by resonance tests on simplified scale models, in obtaining the normal modes of an aeroplane with wings of unorthodox plan forms are discussed on the basis that the normal modes are the best means of defining the elastic-inertia characteristics of an aircraft for the purpose of flutter investigations.

Methods of simplifying the model for ease and cheapness of construction are put forward.

The radical difference between the effects of shear-lag and of ordinary bending-shear flexibility is described, and an approximate method of allowing for the latter effect is indicated.