The Fifth British Commonwealth and Empire Lecture was given before the Royal Aeronautical Society by Mr. E. H. Atkin, B.Sc, F.R.Ae.S., on the 15th September 1949 in the Lecture Hall of the Institution of Civil Engineers, Great George Street, S.W.I. Sir John Buchanan, C.B.E., F.R.Ae.S., President of the Society, presided.

The primary structure of an aeroplane usually consists basically of a set of tubular beams. The main structural box of the wing or tailplane is a well-known example: a semi-monocoque fuselage is another. For any given loading condition of the aircraft the material in the tube is stressed mainly in tension, in shear, or in compression, depending on its location in the tube cross section.

The aim of the designer is to make the material fulfil these three functions in the most economical manner. In tension, he is limited only by the quality of material available. In shear, this is again substantially the case, although it is well known that very light shear webs over great depths do not develop as high an effective failing stress as do more sturdy webs. This property of dependence on the intensity of loading is much more marked in the case of the compression structure, which is liable to instability in various ways.

An approximate theory of parachute opening is suggested. A formula is derived for the critical opening speed (the highest speed at which the canopy develops fully) which indicates variation of the critical speed with fabric porosity, rigging line length and so forth, of the order observed in wind tunnel tests. Assuming a simple form for the air flow about the parachute, a formula is obtained also for the rate of opening of a canopy. This enables an analysis to be made of the motion of a store-parachute system during canopy development. The theory confirms the possibility of a large increase with altitude, found experimentally, in the maximum parachute force on the store.

This review is an attempt to examine, very briefly, the existing methods by which the airframe, engine and accessory manufacturers are informed of the operational troubles which are experienced by the users of their equipment, and how these manufacturers thus come to possess the knowledge which alone can enable them to avoid the recurrence of similar faults in the future. The importance of this knowledge can hardly be over-emphasised, but it is suggested that, in the past, there has been an abundance of such information which has failed to reach the right people at the right time.

For the purpose of this review it is proposed to divide the troubles to which the aeroplane is heir, into three main categories:—

1. 1 Aerodynamic or handling faults.

2. 2 Structural or design failures.

3. 3 The general unreliability of components which causes low serviceability and utilisation rates.