Main Rotor

Figure 6-42. Helical four fluted gate blower rotors/two lobes main rotor intermeshing. See Figure 6-44. By permission, Gardner-Denver Industrial Machinery-Cooper Industries.

The most important part of the transmission is the main gearbox, which contains the most heavily loaded reduction gears transmitting power to the main rotor blades. It is always oil lubricated but a variety of different oils are used in different helicopters. 

If flight forward speeds are low, the main rotor may contribute to greater crop penetration... 

References 7 and 8 describes these commutation process in relation to the use of diode bridges in the main rotor circuits of synchronous generators. 

Fig. 23 shows the mass distributions of the HHV rotor shaft and indicates the measurement points. Fig. 24 shows the dynamical behaviour of the main rotor shaft at the runup, measured at point A. The figure shows clearly, that the rotor shaft was excellently balanced. Fig. 25 shows the dynamical behaviour of the main rotor shaft at rundown, also measured at point A. The figure indicates, that there had been some slight thermal distortions of the rotor shaft. Fig. 24 and 25 show moreover that the first resonance frequency is at 1700 - 2000 rpm. 

Talbot, P.D., Tinling, B.E., Decker, W.A. and Chen, R.T.N., A Mathematical Model of a Single Main Rotor Helicopter for Piloted Simulation, NASA... 

The lateral thrust of the tail rotor of a helicopter has to compensate for the main rotor torque. For maximum effectiveness, it is operated with a rotational speed as high as permitted by the blade tip velocity, but well below the speed of sound. In general, the noise- and vibration-generating mechanisms are similar to those of the main rotor. While there is no cyclic blade pitch, the interaction with the main rotor outflow has to be taken into account. Due to the comparatively small diameter of the tail rotor, its rotational speed is significantly higher than that of the main rotor and thus the emitted noise and vibrations have higher frequencies, see Staufenbiel et al. [168]. 

The soimd radiated by the rotorcraft into its environment is what the critical observer perceives as noise pollution and what delivers a characteristic acoustic signature for aircraft detection and classification. The tjq)ical rotor-craft sound is composed of several components with destinctive directivity and intensity, depending on the flight conditions. In general, at a distance, the main rotor noise is dominant, the high frequency emissions of the tail rotor have some relevance, and the engine noise is secondary. 

As has been shown in the previous section, the main rotor plays the decisive role for many noise and vibration problems. It is best to tackle such a problem at the source before diffusion and diversification. Figure 2.2 shows the excitation mechanisms at the main rotor. Their characteristics are discussed in detail based on the investigations of Brentner and Farassat [25], Schmitz and Yu [161], Edwards and Cox [71]. 

Fig. 2.2. Aerodynamic sources of noise and vibration at the helicopter main rotor original photograph by Eurocopter.

The examination of the helicopter main rotor has yielded a multitude of excitation mechanisms for noise and vibration. Dealing with such a complicated system, it is unlikely that a single solution exists to produce relief in all aspects. Thus, a variety of partially very different approaches has been discussed and developed. In this section, the major ideas involving non-active elements will be presented for the classical helicopter configuration. Details on these elements are given by Bielawa [21], Brarnwell et al. [23]. 

Fig. 2.7. Main rotor blade tip shapes original photographs by Domke [63].

Table B.l. Stiffness and geometry properties of the BO f05 main rotor system, Ref. [f70].

Rotor Blade Construction Fig. 74 shows the general design of a typical main rotor blade tail rotor blades are, typically, very similar. Fig. 75 shows the typical bonding tool for the main rotor blade. 

Figure 74 Schematic of the main rotor blade of Westlands Sea King Helicopter (courtesy of Westland Helicopters Limited), (A) BIM manifold, (B) Root doublers, (C) Erosion shield, (D) Packing piece, (E) Heater mat, (F) Outer wraps, (G) Uni-directional nose moulding, (H) Inner wraps, (J) Uni-directional sidewall slab, (K) Uni-directional backwall slab, (L) Skin, (M) Uni-directional sidewall slab, (N) Honeycomb, (O) Closing Channel, (P) CauUc, (R) Dummy skin, (S) Cuff, (colour version of this figure appears on p. xviLi).

Figure 75 Typical bonding tool for the EHlOl main rotor blade (courtesy of Westland...

In, for example, the EHlOl, apart from the main rotor structure, many sub-assemblies are adhesively bonded this gives both structural and non-structural components. Thus ... 

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