Biaxial means two rotors located some distance apart from each other. There are four types of biaxial helicopter configurations:
i. Conventional (main rotor and tail rotor). In this configuration, the tail rotor axis is perpendicular to the main rotor axis.
ii. NOTAR (no tail rotor). Notwithstanding the nomenclature, there is a tail rotor situated closer to the main rotor gearbox that compresses air into an ejector tube and performs the same functions.
iii. Tandem. In this configuration, there are two main rotors inline to the flight path.
iv. Synchropters. In this arrangement, there are two main rotor axes—intermeshing under a small angle and a side-by-side configuration, in which two main rotor axes are perpendicular to the line of flight.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved