During the war period, effort has been concentrated on the design of military aircraft, in the performance of which operational efficiency has been an overriding consideration. Speeds and powers have increased, resulting in a greater production of noise; but since noise reduction usually involves some loss of performance, perhaps from the additional weight of a soundproofing treatment, or the slight loss of engine efficiency which may accompany the use of an exhaust collector system, noise has had to be largely ignored. In the military machine, moreover, the incidence of noise on the ears of the crew is mitigated to an appreciable extent by the use of flying helmets incorporating well fitting earphones–a measure which it is hardly practicable to apply to civil machines.

When first I was approached to lecture to the Society it was suggested that I might speak to you on “Modern Aircraft Materials.” I did not choose this subject, firstly because I felt it was much too comprehensive to be susceptible of compression within the period of any ordinary paper; and secondly, because the relevant data (in so far as it is known) is enshrined in the official specifications, in the catalogues published by the metal manufacturers, and in a large number of papers presented to this and other learned Societies.

A Stress analysis of screw threads of Whitworth form has been performed.

The critical stress in the threads is a shear stress, and this may be expressed simply in the form:–

where

Smax=shear stress.

f=distribution factor.

= 1.0 when screw and nut are both in tension or compression.

= 2.0 when screw and nut are in tension and compression respectively, unless specially designed for an improved distribution of load.

k = coefficient, depending mainly on fit, but also on pitch of threads, L and D.

<0.8 for close fit.

<0.9 for medium fit.

<1.0 for free fit.

P=axial load.

L=length of engagement.

D=nominal diameter.