Engineering topics

Johnson, W.

Publications and source records attributed to Johnson, W..

97 records · Page 6

NASA NTRS1974

Theory and comparison with tests of two full-scale proprotors

A nine-degrees-of-freedom theoretical model has been developed for investigations of the dynamics of a prop rotor operating in high inflow axial flight on a cantilever wing. The theory is described, and the results of the analysis are presented for two prop rotor configurations: a gimbaled, stiff in-plane rotor, and a hingeless, soft in-plane rotor. The influence of various elements of the theory is discussed, including the modeling used for the blade and wing aerodynamics and the influence of the rotor lag degree of freedom. The results from full-scale tests of these two prop rotors are presented and compared with the theoretical results.

Johnson, W.↗

NASA NTRS1972

On the mechanism of dynamic stall.

The sequence of events comprising dynamic stall of an airfoil is discussed, with emphasis on the role of the leading edge laminar separation bubble and shed vortex. A simple bubble model, based on a combination of theoretical and experimental investigations, is used to discuss the events prior to the shedding of the vortex, and provides the basis for a heuristic estimate of the delay in the occurrence of dynamic stall on a pitching airfoil. The evidence for the existence and dominant effect of the leading edge vortex on the dynamic stall required (but in most cases not presently available) for the prediction of the effects of stall on helicopter rotor blades are discussed. It is the intention of this paper to focus attention on the laminar separation bubble and the shed leading edge vortex as the dominant features of the dynamic stall mechanism in the hope of stimulating greater emphasis on these features in future dynamic stall research.

Johnson, W.↗

NASA NTRS1972

A pertinent solution of helicopter rotor flapping stability

The stability of the flapping motion of a single blade of a helicopter rotor is examined using the techniques of perturbation theory. The equation of motion studied is linear, with periodic aerodynamic coefficients due to the forward speed of the rotor. Solutions are found for four cases: small and large advance ratio and small and large Lock number. The perturbation techniques appropriate to each case are discussed and illustrated in the course of the analysis. The application of perturbation techniques to other problems in rotor dynamics is discussed. It is concluded that perturbation theory is a powerful mathematical technique which should prove very useful in analyzing some of the problems of helicopter dynamics.

Johnson, W.↗

NASA NTRS1971