In the spring of 1914, in view of the tenth anniversary of the Aerodynamic Institute at Koutchino, near Moscow, I published a booklet in which I gave a survey of the Institute's activities since its foundation and concluded it as follows: “The problem of aerodynamical flying is solved, but, after the conquest of air space, another conquest, more difficult and of higher import, is offered to the ambition of man— the conquest of interplanetary spaces. “The brilliant progress of science gives hope that, in a more or less distant future, this problem will be solved in its turn through the patient and co-ordinated efforts of searchers impassioned by the greatness of this idea.

“The brilliant progress of science gives hope that, in a more or less distant future, this problem will be solved in its turn through the patient and co-ordinated efforts of searchers impassioned by the greatness of this idea.

In addressing the Historical Group it is desirable to let technical detail fall into the background so that the more general consideration of personal problems, policy and organisation can be clearly examined.

Technical detail changes so much more with time than do the human and administrative considerations, that the greatest value for history comes perhaps from the study of the latter.

I should like to express my gratitude to the Society, on behalf of the Federal Aviation Agency of the United States, for the opportunity of participating in this meeting.

Before speaking of the lessons gained in Helicopter Air Traffiic Control some background information is necessary on the types of helicopter operations which are being conducted in the United States.

As in many countries, helicopter operations are conducted by both the military services and civil operators. The military operations are primarily those of a tactical training nature and search and rescue work. Civil helicopter operations are conducted mainly for transport and utility purposes.

In addition to numerous civil air taxi operators providing transport by helicopter, we have three commercial scheduled helicopter airlines; New York Airways, Chicago Airways, and Los Angeles Airways. These three helicopter airlines serve the aerodromes and residential communities in the metropolitan areas of the cities from which they derive their names. Utility helicopter flying includes such services as forest fire fighting, insecticide spraying of farm crops, power line inspection, and construction work, just to name a few.

The operations of Chicago Helicopter Airways represent a portion of the total Federal effort within the United States on behalf of helicopter development. This effort has been an outgrowth of the interest of the Civil Aeronautics Board and the U.S. Post Office Department which has a long tradition of looking for new developments in transport and of experimenting in new ways to move mail. Post Office interest in the aeroplane was the chief stimulus to the early development of U.S. airlines and dates back to the first scheduled air mail route authorised between New York and Washington in August 1918. It was natural, then, that the Post Office Department should have interest in the helicopter as it emerged in usable form for civil use after the Second World War.

After many years of helicopter design, development, and operations, a practical flying crane helicopter is under construction whose particular features promise an unprecedented utility. The steps leading to this new helicopter configuration were evolutionary in nature dating back as far as 1922 when the deBothezat quadrotor machine hovered for one minute and forty-two seconds.

Early military interests in improved mobility of the armed forces and the experiences gained with helicopters in the Korean War gave impetus to the development of vertical lift capability. In addition to the basic helicopter developments specific crane flying test beds and parametric studies were sponsored by the military. These projects included the Hughes XH-17 flying crane, the McDonnell XHCH-1 rotor test programme and the Sikorsky HR2S-1 (H-37) helicopters. In addition design study contracts were sponsored which determined the feasibility of constructing crane-type helicopters of 8 to 16 ton payload capability.

The Belvedere, previously known as the Bristol 192 helicopter, was designed and developed to an Air Staff Requirement for a General Purpose helicopter for the Royal Air Force.

While the design embodied certain principles which had been well proved in the Bristol Sycamore and, while useful information had been learned about the tandem configuration from the research 173 helicopter, the design of the Belvedere was new in all its engineering aspects. The machine was to be powered by turbine engines with more than twice the installed power of the 173; it was to be approaching twice the weight of the 173. The choice was the Napier Gazelle engine—the first turbine designed primarily for helicopter use. Hence, the helicopter was of new and larger design than that which had preceded it, and the engine was also of new design and type.

It is well known that the main purpose of the by-pass principle is to improve the propulsive efficiency of a simple jet engine by removing some of the energy left in the jet gases and using this to compress an extra quantity of air, known as the by-pass air, this air being ejected rearwards with the jet gases. In this way a greater mass of air is ejected rearwards at a lower jet velocity and thus a better propulsive efficiency is obtained. This is an extremely simplified view of the advantages of the by-pass engine, however, since an equivalent result of obtaining a lower jet velocity can be obtained by designing the jet engine for a lower combustion temperature. The by-pass principle is of advantage because it enables a higher propulsive efficiency to be obtained at the same time as employing a high combustion temperature and therefore a high basic cycle efficiency. If the component efficiencies of a gas turbine were 100 per cent, cycle efficiency would not depend upon combustion temperature at all, and there would thus be no advantage in principle in using the by-pass engine. In practice there would probably be some residual advantage left in that for a given thrust a lower engine weight could be obtained.

Figure 1 illustrates a load diffusion problem which occurs in ship structural design, and which may also be relevant to aircraft structures. A load applied to one end of a stringer is diffused into the attached sheet and eventually reacted by shearing forces at the longitudinal edges. The transverse edges of the sheet and the other end of the stringer are not loaded.

Various methods, which differ in their assumptions regarding stresses and displacements in the sheet, are available for tackling this problem. For calculating the distribution of load or stress in the stringer it makes little difference which method is used. The “finite stringer” method of reference (1) is the simplest and should be sufficiently accurate for most cases.