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.

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.

With all transport vehicles a light structure weight is of prime importance. Light weight vehicles require less power to accelerate, less braking to stop, less power to climb hills. The power /weight ratio broadly determines the ability of a vehicle to get out of difficulties, avoid collision and, generally speaking, settles the vehicle's performance characteristics.

This is particularly true of the aeroplane where, with a third dimension of travel, the effect of gravity is present to a degree unknown to sea or land travel, even in mountainous countries.

Weight saving thus becomes of paramount importance in aircraft design and leads inevitably to the conclusion that nothing should be carried in the aeroplane to fulfil a service which can be satisfactorily and safely performed on the ground.

A further result, and one that most concerns us here, arises from this weight factor. In many trades a great deal of the technical execution of the work in detail is carried out in accordance with the superintendent's or foreman's knowledge and craftsmanship, and whether there is a little more—or less— material used is a matter for commercial considerations of cost, rather than one of necessity to save weight in order to fulfil performance.

The Aerodrome, as the point of contact between the aircraft and the world of reality, has to a large extent been taken for granted. The designer regards it as the base for prototype tests and the airline operator as his aerial railway station. It is my intention to show that aerodrome development is at a critical stage of evolution on this side of the Atlantic, that many of the related problems do concern the aircraft designer and operator, and that if these problems are not given consideration, difficulties will be encountered in civil operations.

To quote an example; the optimum aircraft type to serve a system of air routes can be worked out taking into consideration the range required, payload and speed, but if the aircraft ultimately ordered and produced cannot safely operate from the aerodromes to be served because of the runway length and approaches, then the operations will either have to be reorganised or continued without due regard for safety.

The purpose of this paper is to indicate in a simple but quantitative form the primary aerodynamic factors governing the performance of a hovering helicopter (since the ability to hover is its prime virtue) and to show how the necessity to operate at a reasonable forward speed restricts that hovering performance. The paper is mainly of interest to potential users of helicopters and to others not actively engaged on helicopter design.

The standard power relationships for hovering flight, as developed by Glauert and Squire, are interpreted in graphical form, and the effect of forward flight in limiting the choice of aerodynamic and operational parameters, due to the onset of blade stalling and compressibility, is illustrated by means of boundaries on these graphs. The power requirements in forward flight are not considered.

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.

In the course of investigation into the operating economics of transport aircraft, the cruising speed arises as a fundamental variable and a rapid method of correction of speed for variation of power, height and weight is frequently required. Other problems present themselves, such as the estimate of cruising speeds in the course of a new type design study, or the correction of a “one–point” published performance of a new aircraft to a standard operational condition differing in specified weight, altitude or power output for cruising.

In this note no account will be taken of the block speed correction arising because of the modification of true air speed by the various factors, such as wind, climb and glide which distort the optimum flight path. The principle of the method here suggested involves the unique relationship of p, the ratio of thrust power required at any speed (V) to the thrust power at the minimum drag speed (u) at the same altitude, and n the wellknown speed ratio V/u.

From the start of the development of helicopters at the Bristol Aeroplane Company the necessity was appreciated of providing means of testing, independently of the aircraft, rotor hubs and blades. The usual method of testing on the helicopter suffers from several disadvantages, the most important being that proximity of the ground beneath the rotor greatly affects the flow through it and makes such performance figures as can be obtained unrepresentative. Others are the probable inconvenience of having to wait for the completion of the aircraft before rotor-testing can start, the comparative difficulty of instrumentation, and the risk of damage to the aircraft by failure or misbehaviour of the rotor.

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.

As stated in the introduction to Section 6.5 the practically important case of an open section tube with no St. Venant torsional stiffness (see Fig. 38) will be treated as the limiting case of the open tube with J as J→0.It will be shown that the stress system g 1 h 1over the open tube degenerates into a stress system gгhг which is statically equivalent to the torque TBS about the shear centre. This stress system is for a uniform cylindrical tube with no J identical to the Wagner-Kappus torsion-bending stress system in open tubes.

Before I present my written paper I would like to refer to two aspects of my subject which have been omitted from the paper, icing and static discharge.

I have no records of icing being a direct cause of accidents but it is important to appreciate the effect that it has on a pilot when flying an aircraft which picks up a lot of ice which he knows he cannot get rid of when he wishes. I look forward to the time when there will be installed on aircraft a de-icing system which will enable a pilot to get rid of ice at the right time.

The 767th Lecture was read before the Royal Aeronutical Society on Thursday, 17th February 1949, at the Institution of Civil Engineers, Great Georgr Street, London, S.W.1. Dr. H. Roxbee cox, D.I.C., F.R.A.e.S., President of the Society introduced the Lecture Squadron Leader E. A. Harrop, O.B.E., A.F.R.Ae.S., R.A.F., of the Department ot the Air Member for Technical Services, Air Ministry, and a serving engineer officer of long experience in the Royal Air force.

The paper discusses the application of wind tunnels to aircraft design and consideration is given to the potentialities and limitations of types designed specifically for industrial use. Arguments affecting the choice of type, size and speed are stated and a detailed description is given of a particular tunnel which has very steady and even flow with a reasonable turbulence level. This description covers the aerodynamic and engineering design, the electrical driving arrangements and the six-component automatic balance.

A description is also given of a new type of high speed tunnel which has recently been put into operation. This is of the simple straight-through type and is driven by a jet engine acting as an ejector pump; the exhaust heat is used to control humidity and prevent condensation in the working section. This tunnel reaches a Mach No. of 0.9 when empty, and over 0.85 with a model in place. The Reynolds No. is 1.6 x 106 for a three-dimensional half model and 5 x 106 for two-dimensional wings.

The paper “Aircraft Servicing” by Norman and Wilkinson published in the November issue of the Journal demands the close attention of both operators and aircraft manufacturers. The suggestions made can be further developed.

There is little doubt that the attainment of higher utilisation rates is one, if not the ultimate, solution of the economic problems that face air transport. This is even more evident when the present–day capital cost per pound of complete but unladen weight of varying transport vehicles is compared.

With the return of British civil aviation to normal operating conditions after the war it was found that several problems which had previously given little cause for concern had become acute. More especially, the increase in the volume of traffic, particularly under instrument flying conditions, together with certain indirect consequences of the greater sizes and weights of aircraft, had brought about greatly increased congestion around major airports, and there was, therefore, an urgent requirement for an effective system of close air traffic control to ensure the safety and rapid movement of the traffic.

The Ministry of Civil Aviation immediately gave much attention to this problem of air traffic control, but at once found itself in serious difficulties on account of lack of sufficiently precise knowledge of the traffic and the traffic pattern. The then Controller of Technical and. Operational Services, Sir Conrad Collier, realised that the obtaining of the necessary data concerning air traffic control and other matters of technical interest was essentially a matter of operational research, and in 1947 a small operational research section was established which began work in the autumn of that year.

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.

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.