<p> Prototypes of the recently developed periscopter, a flying platform tethered to a ground station, are presently extremely difficult to fly. Tests conducted by the Defence Research Board of Canada in Valcartier, Quebec, and by Westinghouse of Canada near Hamilton, have led to several crashes. Possible causes for the lack of flying qualities are: unbalanced aerodynamic forces and moments; inadequate controls; and poor inherent stability characteristics. </p> <p> In this investigation, the system of counter-rotating lifting rotors used in the present periscopters is examined with a view to improvement of the flying qualities. The aerodynamic theory of helicopter rotors is considered as a background. </p> <p> The blades of the present periscopter rotors neither flap nor feather. The feasibility of using either articulated (flapping) blades or rigid feathering blades is examined. It is found that flapping blades are not feasible mainly because of associated stability and control problems. Also the two counter-rotating rotors would tend to strike against each other. A rigid rotor system featuring feathering blades is found to be feasible. Such a system is therefore examined in detail by computing all relevant aerodynamic parameters. It is shown that the feathering system can provide all required control moments. Its introduction would therefore eliminate the present bail mechanism. </p> <p> An analysis of the stability characteristics of a periscopter featuring a rigid feathering rotor system is developed. However, when hovering in still air, such a periscopter is shown to be unstable. The possibility of rendering it stable by the use of rotor controls is demonstrated. No attempt is made to suggest a specific design for the control system to be used. </p> <p> The effect of various operational parameters on the flying qualities of the periscopter is investigated. </p> / Thesis / Master of Engineering (ME)