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As noted by Orchard and Newman, the interactions between the rotor wake and the propeller present an unknown factor, although effects similar to the tail-rotor–coaxial-rotor wake blade–vortex interaction effect must be expected, but only for certain portions of the flight envelope. For example, a regular tail-installed propeller is strongly affected by the wake of coaxial rotors under certain flight conditions.
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From this point of view, the propeller is a more appropriate choice for off-loading the coaxial rotors of a counter-rotating coaxial-rotor propeller-augmented compound helicopter.Īlthough the propeller is a crucial element that would allow the coaxial compound helicopter to increase its forward flying speed, its utilization as a thrust-augmenting device causes another complication in the aerodynamic characteristics of the helicopter. The turbojet and turbofan also cause power redundancy on the aircraft because the power they produce cannot be used during hovering flight. However, the turbojet and the turbofan were nearly abandoned on compound helicopters because of their inefficiency and noise. In past research development, a pair of turbojets or a turbofan was initially selected to augment the thrust produced by the coaxial rotor. However, the effects of compressibility can be delayed to a higher forward speed, if the coaxial-rotor system is off-loaded by using a suitable means of lift or thrust augmentation, which can reduce rotational speed of the coaxial rotor. With high forward speed, compressibility at the tip of the advancing blade weakens the aerodynamic performance of the rotor regardless of the flapwise stiffness of the system. The propeller is a key part of the coaxial compound helicopter. However, its special configuration of having a coaxial-rotor system and an auxiliary tail propeller leads to a complicated interactional aerodynamics. This accomplishment indicates that the counter-rotating coaxial-rotor propeller-augmented compound helicopter is an important research topic for the development of future helicopters. The X-2 is a representative helicopter that is based on the main technical features of conventional helicopters and simultaneously established a very fast helicopter cruise speed. The common configuration of the coaxial compound helicopter includes coaxial main rotors with a propeller rotor. The counter-rotating coaxial-rotor propeller-augmented compound helicopter is one of the most potential high-speed helicopter configurations. T he low cruise speed of helicopters has motivated research on new configurations of high-speed helicopters.