"Langley Research Center." . . "Rotors (Helicopters) Mathematical models." . . . . "An improved computational procedure for determining helicopter rotor blade natural modes"@en . "An improved computational procedure for determining helicopter rotor blade natural modes" . . . . . . . . . . . . . . . . . . . . . "United States. National Aeronautics and Space Administration. Scientific and Technical Information Office." . . "Army Research and Technology Laboratories (U.S.). Structures Laboratory." . . . .