Three-dimensional unsteady Euler equations are numerically solved to simulate the unsteady flows around forward flight helicopter with coaxial rotors based on unstructured dynamic overset grids. The performances of the two coaxial rotors both become worse because of the aerodynamic interaction between them, and the influence of the top rotor on the bottom rotor is greater than that of the bottom rotor on the top rotor. The downwash velocity at the bottom rotor plane is much larger than that at the top rotor plane, and the downwash velocity at the top rotor plane is a little larger than that at an individual rotor plane. The downwash velocity and thrust coefficient both become larger when the collective angle of blades is added. When the spacing between the two coaxial rotors increases, the thrust coefficient of the top rotor increases, but the total thrust coefficient reduces a little, because the decrease of the bottom rotor thrust coefficient is larger than the increase of the top rotor thrust coefficient.
1. Introduction
Coaxial rotors configuration is one of the technological solutions for increasing helicopter load-carrying ability. Since two rotors produce the net thrust instead of a single rotor in the conventional design, the diameter of the rotors can be reduced to carry the same amount of weight. The most attractive feature of a coaxial design is the resulting compactness and safety of the vehicle. Secondly, the converse torsional moments generated by the two rotors would be canceled due to the opposite rotational directions, and the tail rotor and tail boom can be eliminated, resulting in a smaller and lighter vehicle.
5. Conclusions
In the present article, the forward flight flows around the coaxial and individual rotor helicopters have been simulated by solving the three-dimensional unsteady Euler equations based on the unstructured dynamic overset grids, and some conclusions are obtained as follows.
(1) Compared to the individual rotor helicopter, the thrusts of both coaxial rotors are smaller; especially the thrust of bottom rotor is much smaller, which demonstrates that the interaction between the coaxial rotors leads to decreasing the performance of the coaxial rotors, especially the bottom rotor.
(2) The interaction between the coaxial rotors makes the downwash velocity at the bottom rotor plane much larger than that at the individual rotor plane, and the downwash velocity at the top rotor plane is a little larger than that at the individual rotor plane.
(3) There are upwash vortex flows on the top of the fuselages of both helicopter models, and the strength of the coaxial rotor helicopter is weaker than that of the individual rotor helicopter.
(4) The downwash velocity and thrust coefficient both increase when the collective angle of blade becomes larger.
(5) When the rotor spacing increases, the thrust coefficients will increase for the top rotor and decrease for the bottom rotor, and the total thrust reduces a little.
It is demonstrated that the present method of unstructured dynamic overset grids is efficient and robust for simulating unsteady flows around helicopter with coaxial rotors, and new understandings of coaxial rotor systems and rotor/fuselage interactions will be studied in the future to uncover more aerodynamic problems of coaxial rotor systems.
