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.