It is many years since aircraft were first A operated under extreme low temperature conditions, mainly by bush pilots pioneering the northern latitudes of Canada.The aircraft used were of the light and semi—medium type fitted with simple, low—powered engines. Consequently, crude methods could be adopted to keep them operating.

With the advent of high—powered boosted—engined aircraft, with their complexity and numerous complicated installations, together with other associated equipments, the difficulties became manifold and the earlier methods used were either impracticable or inadequate.

For this reason, some intensive testing and research has been done on the latest type of aircraft, engines and equipment, in an endeavour to solve the problems and, if not entirely successful, find what medium of success can be achieved with the least amount of subsidiary aid.

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 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.