Despite the high standard of workmanship employed in its fabrication, a propeller, like any other manufactured article, invariably differs from the ideal product in some respect which may, or may not, be of functional significance according to the nature of the disparity. The component blades of a propeller are no exception to this rule and it is the author's object to examine the consequences of dimensional deviations of the actual blades from those of an assumed standard and to describe apparatus devised to minimise or eliminate the most undesirable of them, namely, the tendency to induce vibration in the aircraft. From the earliest day, aircraft vibration has been attributed to the engine propeller combination and has come to be regarded as an inevitable characteristic. It can be demonstrated, however, that where aerodynamically balanced blades are employed, vibration difficulties are not an inherent property of the propeller. This is a matter of considerable importance to the turbo-propeller in which the prime mover is virtually vibrationless and consequently any unbalance of the propeller would be thrown into prominence.

A summary is presented of information collected on coefficients of friction (rolling and sliding) between rubber tyres and road or runway surfaces. Nearly all the data collected are from tests on automobile tyres and are limited to speeds of about 40 m.p.h. In some cases the primary object of the tests was to distinguish between good and bad roads, and not to determine absolute values of friction coefficients.

The reasons for the importance of the information for aircraft design use are discussed and distinctions are made between coefficients measured in different ways.

The results show the variation of sliding friction coefficient with various parameters at speeds up to 40 m.p.h. There is a set of American tests at speeds up to 110 m.p.h., and some estimates made by the Dunlop Rubber Company up to 120 m.p.h.

It is concluded that for sliding friction coeficients differences in surface texture (as distinct from surface material) are significant, and that information can be given only in the form of limits within which the values can be expected to lie.

Few results are available on the variation of rolling resistance coefficient, but it is felt that the approximate values quoted for different types of surface are sufficiently accurate.

At some stage in the design of every aeroplane it is necessary to estimate or to measure the resonance modes of vibration. This has not always been the case, but the problems of flutter, control reversal and dynamic loads have increased in importance as speeds have risen. Nowadays, it is an airworthiness requirement that these effects be considered and the aircraft made safe for all conditions of flight. A knowledge of the normal modes of vibration is essential for all accurate estimates of these aeroelastic effects.

Taking flutter as an example, the technique of flutter investigations consists of first determining which combinations of the various possible degrees of freedom are liable to excite dangerous oscillations. Typical degrees of freedom for a wing are bending and twist in each normal mode, aileron deflection and tab deflection; for a tailplane and elevator we might consider tailplane bending or twist, elevator deflection, tab deflection, fuselage bending and twist, and pitching of the whole aeroplane.

This paper discusses a few simple ideas which may be supposed to form, consciously or unconsciously, a schematic basis for the servicing of given aircraft in given circumstances. It is not concerned with engineering technique but rather with the assumptions behind it.

The purpose of an aircraft's upkeep, by which is meant its servicing and maintenance in the widest sense, is to enable the machine to furnish the maximum output of flying work of the required kind and intensity. “Intensity” is used to mean an aircraft's rate of output of flying work in a calendar period.

For commercial aircraft the average intensity required is usually near the maximum that can be sustained at reasonable upkeep expense, because this tends to maximise net profit (or to minimise net loss if there is no profit) by lowering the overheads related to the capital cost of aircraft.