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A study has been conducted in an effort to gain basic knowledge about the boundary layer of rotors in the hovering condition. The results were obtained using evaporative chemical films, pressure probes and hot wire anemometers. Tuft and oil film studies were also conducted for comparison with the chemical film technique. Information is presented on the location of transition from laminar to turbulent flow, on the forming of tip vortices and other aerodynamic features. A slightly inward flow of the boundary layer over a large part of the airfoil surface was noted, indicating that, for the hovering rotor, the pressure forces along the blade and the effects of the contraction of the wake have to be taken into account besides the centrifugal acceleration. Also presented are preliminary velocity measurements in the boundary. Results using the chemical film and pressure probe measurement techniques on rotating rotors are compared to non‐rotating tests conducted in a wind tunnel.

Projections of future trends in any field must be somewhat subjective; this assessment of future research on rotorcraft is no exception. However, an effort has been made at objectivity by presenting results of past and current research efforts as a foundation from which the projections are made. These projected trends are solely the opinion of the author and do not represent the policy of any agency within Government or industry, nor are they committal in any sense. Although some illustrations are derived from specific systems under investigation, the general intent of the paper is to avoid discussion of systems and to present illustrations representative of basic problems in the field of rotorcraft for which research effort can be expected to yield significant advance in performance and design.