T E National Advisory Committee for Aeronautics has for some time been directing its efforts toward the improvement of safety in flight, and with that purpose in view has done extensive research on rotating-wing aircraft at the Langley Memorial Aeronautical Laboratory. At present an extensive program of research on rotating wings is under way in the propellerresearch wind tunnel, where autogiro and gyroplane models 10 feet in diameter are to be tested to determine the influence of varying such basic properties of the rotor as the pitch angle, the airfoil section, the plan form, and the blade mass. Similar tests are being made on a model of a cyclogiro rotor 8 feet in diameter. The following analysis of the choice of airfoils for these rotors is supported by work that has already been completed; much valuable information concerning this and related subjects is anticipated from the current program. Rotating-wing aircraft may be divided into two main classes: those normally employing power-driven rotors for sustentation, such as the helicopter, helicogyre, and cyclogiro; and those employing autorotating wings for sustentation, such as the autogiro and gyroplane. The autogiro and helicopter are familiar types. The helicogyre is an aircraft supported by a power-driven lifting propeller which is rotated by means of an additional propeller on each blade of the main screw. The cyclogiro is supported by a paddlewheel rotor on each side of the fuselage, rotating about the lateral axis. The gyroplane is quite similar aerodynamically to the autogiro except that the blades on the gyroplane rotor are free to rotate only about the span axis, and the opposite blades are rigidly joined; lift on opposing sides of the rotor is equalized by rotation of a blade pair about its span axis. The aerodynamic characteristics of an airfoil section required for the optimum performance of a rotating wing are the same regardless of the type under consideration. I t is true, however, that the choice of the proper airfoil is more critical for the autorotating systems than for the power-driven; for this reason, the following discussion will be concerned with the autogiro and gyroplane rotors. The aerodynamic similarity between these two types is so complete that the analysis applies with equal validity to both systems. The autogiro is a windmill of low pitch operating at large angles of yaw and maintaining a condition of zero aerodynamic torque during a state of steady autorotation. This condition means essentially that the average moment, during one revolution, of the blade element chord forces about the axis of rotation is zero, the chord force on any element being the sum of two components of opposite sign, and being derived from the lift and drag of the element. The torque variation with rotor speed is such that a decrease in speed in any state of operation generates an accelerating torque, and conversely; the rotation is therefore stable. The optimum airfoil for a rotor of given geometrical form is defined as the airfoil that gives the largest maximum lift-drag ratio of the rotor. I t has been shown by Lock (British R. & M. 1127) that by consideration of the energy losses in the rotor, the ratio of drag to lift can be expressed as