tailieunhanh - Aerodynamics of helicopter - part 4

Rộng rãi minh họa trong suốt, mỗi chương bao gồm một tập hợp các vấn đề bài tập về nhà. Sinh viên tiên tiến đại học và sau đại học, kỹ sư thực hành, và các nhà nghiên cứu sẽ tiếp đón các văn bản này triệt để sửa đổi và cập nhật trên các cánh khí động học quay. | 98 AERODYNAMICS OF THE HELICOPTER having the same taper ratio and solidity. Little difference was found to exist between the two rotors. This conclusion is further substantiated by an inspection of Fig. 5-6 which shows that the inflow distribution and consequently the power losses of both the partially and fully tapered rotors operating at the same thrust coefficient is similar. 5i Par tai. fiat ier tap er 0 . .2 .3 .4 .5 .6 .7 .8 .9 IO X Fig. 5-6 Effect of partial instead of full taper on rotor inflow distribution. It should be noted that tip losses have been neglected in all of the preceding calculations. This omission is justifiable in a comparative study because it was found upon investigation that the effects of changes in plan form and twist were not influenced significantly by tip losses. TWIST IN FORWARD FLIGHT. In connection with the use of twist in blade design it will be subsequently shown in Chapters 9 and 10 that negative twist has been found to improve the efficiency of the rotor in forward flight as well as in hovering and may be expected to delay blade stalling at high forward speeds and adverse compressibility effects at high tip speeds and high mean-lift coefficients. Blade stalling is delayed when twist is employed because twist unloads the tips by reducing the tip angles of attack. Compressibility losses are minimized for the same reason in that the critical Mach number of the blade section is increased at reduced lift coefficients. Blade twist has been found experimentally to have little effect on the autorotative performance of rotors in forward flight and in vertical descent. factors affecting hovering and vertical flight 99 Optimum Hovering Rotor The effects of various combinations of blade twist and plan form taper on hovering performance have just been discussed. The question then arises as to what the maximum benefits are that can be obtained by twist and taper or in other words what is the optimum rotor design With respect to induced .