Theoretical And Experimental Research On Aerodynamic Interaction Characteristics For Tandem Twin-rotor Helicopters

In this thesis, an iterative free-wake computational method has been developed for the prediction of aerodynamic interactional characteristics between the twin rotors of a tandem helicopter both in hover and in forward flight. By using this method, the calculations and analyses are given on the rotor wake and aerodynamic characteristics of a tandem twin-rotor and a single rotor. Meanwhile, the experiments on the aerodynamic interaction of the tandem twin rotors have been performed by using the model rotor test rig and wind tunnel. The major contributions of the author's research work are as follows:As the background of present work, the objectives of the research are described in the first chapter. The investigations and developments in the field of helicopter rotor free-wake research as well as the theoretical and experimental researches on the aerodynamic interaction of the tandem twin-rotor are introduced. The difficulties in the current research on the interactional aerodynamics of a tandem helicopter are pointed out. Also, the methods used in the present research are briefly introduced.In Chapter 2, according to the feature of vortex governing equations for a tandem rotor, the Method of Line (MOL) is adopted to help change the vortex partial differential equations into the ordinary differential equations in order to improve the solution stability. Based on the MOL, a new numerical algorithm is derived to solve the rotor vortex governing equations. A free-wake computational method is established for a tandem rotor in Chapter 3, based on the MOL in Chapter 2. In this chapter, an iterative computational method has been developed for the prediction of aerodynamic interaction characteristics on the twin rotors of a tandem helicopter. In this method, the rotor free wake model, blade aerodynamic model, blade flap equations and twin-rotor trim model are coupled, and the effects of flow separation and compressibility are also included. Moreover, the aerodynamic correction at the large angle-of-attack condition is considered. Then, the induced-velocity distribution, the blade aerodynamic loading, and rotor performance on a tandem twin-rotor are calculated for both hover and forward flight, and the comparisons with the available experimental data are also made to show the capability of the present method.In Chapter 4, by the method developed, rotor wake geometries, induced velocities, flow field characteristics of tandem rotors both in hover and in forward flight are calculated, the differences of rotor wake geometries and flow field between overlapping and un-overlapping area are discussed emphatically. On the basis of the above investigation, some new conclusions are obtained.In Chapter 5, calculations and comparisons of the blade aerodynamic loading and rotor performance between a single rotor and a tandem rotor for both hover and forward flight are carried out. A set of Interaction Factors for twin rotors and Additional Power Factor for the interaction are defined. Based on the factors defined, the effects of such parameters as the longitudinal separation and axial separation between twin rotors on the rotor thrust and power are analyzed. As a result, some meaningful conclusions are obtained.By the PIV technique, experimental investigations on the rotor blade tip vortices, vortex core dissipation and wake boundary for a tandem rotor model at the condition of interactions are studied in Chapter 6. The interactions between the twin rotors and their influences on blade tip vortices are discussed by altering the longitudinal and axial distances between twin rotors and thus changing the overlapping area. Meanwhile, the variations of vortex core radius with wake age for the single-rotor and dual-rotor at different blade collective-pitch angles are studied. In addition, the comparisons between experimental and calculated results by the present method on wake boundary are also made.In Chapter 7, experimental investigations on the rotor flow fields and rotor performance of a tandem twin-rotor both in hover and in forward flight are carried out on the re-built tandem rotor model rig in the wind tunnel at the Nanjing University of Aeronautics and Astronautics. The results of the average wake velocity field, transient velocity field and vorticity distribution both for a tandem rotor and a single rotor are obtained. And the rotor performance variations with such parameters as the longitudinal separation and axial separation between twin rotors are also given. At the meantime, by applying the method developed in the previous chapters, computations of rotor flow field and rotor performance for a single rotor and a tandem rotor at typical measured conditions are performed, and then the computational results are compared with the present experimental data.