Rotor Mechanisms for Forward Flight
The edgewise rotor
In level forward flight the rotor is edgewise on to the airstream,a basically unnatural state for propeller functioning. This is shown in Fig. 4.1. Practical complications which arise from this have been resolved by the introduction of mechanical devices, the functioning of which in turn adds to the complexity of the aerodynamics. Figure 4.2 pictures the rotor disc as seen from above. Blade rotation is in a counter-clockwise sense (the standard adopted for all helicopters of the Western countries) with rotational speed W. Forward flight velocity is V and the ratio V/WR, R being the blade radius, is known as the advance ratiosymbol m,and has a value normally within the range zero to 0.5. Azimuth angle y is measured from the downstream blade position: the range y=0°–180° defines the advancing side and that from 180°–360° (or 0°) the retreating side. A blade is shown in Fig 4.2 at 90° and again at 270°. These are the positions of maximum and minimum relative air velocity normal to the blade, the velocities at the tip being (WR + V) and (WR - V), respectively. If the blade were to rotate at fixed incidence, then owing to this velocity differential, much more lift would be generated on the advancing side than on the retreating side. Calculated pressure contours for a fixed-incidence rotation with m=0.3 are shown in Fig 4.3. About four-fifths of the total lift is produced on the advancing side. The consequences of this imbalance would be large oscillatory bending stresses at the blade roots and a large rolling moment on the vehicle. Both structurally and dynamically the helicopter would be unflyable.
The edgewise rotor
In level forward flight the rotor is edgewise on to the airstream,a basically unnatural state for propeller functioning. This is shown in Fig. 4.1. Practical complications which arise from this have been resolved by the introduction of mechanical devices, the functioning of which in turn adds to the complexity of the aerodynamics. Figure 4.2 pictures the rotor disc as seen from above. Blade rotation is in a counter-clockwise sense (the standard adopted for all helicopters of the Western countries) with rotational speed W. Forward flight velocity is V and the ratio V/WR, R being the blade radius, is known as the advance ratiosymbol m,and has a value normally within the range zero to 0.5. Azimuth angle y is measured from the downstream blade position: the range y=0°–180° defines the advancing side and that from 180°–360° (or 0°) the retreating side. A blade is shown in Fig 4.2 at 90° and again at 270°. These are the positions of maximum and minimum relative air velocity normal to the blade, the velocities at the tip being (WR + V) and (WR - V), respectively. If the blade were to rotate at fixed incidence, then owing to this velocity differential, much more lift would be generated on the advancing side than on the retreating side. Calculated pressure contours for a fixed-incidence rotation with m=0.3 are shown in Fig 4.3. About four-fifths of the total lift is produced on the advancing side. The consequences of this imbalance would be large oscillatory bending stresses at the blade roots and a large rolling moment on the vehicle. Both structurally and dynamically the helicopter would be unflyable.
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