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The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

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4.1 Introduction<br />

4<br />

Rotors in practice<br />

<strong>The</strong> loads on <strong>the</strong> rotor blades are large and time variant and <strong>the</strong> articulated head was<br />

developed to minimize <strong>the</strong>m. <strong>Art</strong>iculation also reduces <strong>the</strong> rate at which <strong>the</strong> aircraft<br />

responds to <strong>the</strong> controls. In this chapter <strong>the</strong> dynamics and control <strong>of</strong> <strong>the</strong> articulated<br />

rotor will be introduced, along with later developments such as hingeless rotors that<br />

became possible with <strong>the</strong> development <strong>of</strong> modern materials.<br />

<strong>The</strong> essential functions <strong>of</strong> <strong>the</strong> rotor head are to transfer shaft power to <strong>the</strong> blades, to<br />

transfer <strong>the</strong> resulting rotor thrust to <strong>the</strong> mast, to resist <strong>the</strong> blade tension and yet to allow<br />

<strong>the</strong> blades to move on <strong>the</strong>ir fea<strong>the</strong>ring axes so that cyclic and collective pitch control<br />

is possible. This simple and elegant arrangement was initially not possible because it<br />

requires extremely strong materials. Thus practical rotor heads are <strong>of</strong>ten much more<br />

complex.<br />

Figure 4.1demonstrates that <strong>the</strong> rotor blade <strong>of</strong> a typical helicopter pulls outwards<br />

with a force roughly equal to <strong>the</strong> weight <strong>of</strong> a bus. It’s no wonder those floppy blades<br />

straighten out. It requires little imagination to predict what would happen if a blade<br />

attachment failed. Clearly <strong>the</strong> rotor head has to be extremely strong and reliable and<br />

as a result it is one <strong>of</strong> <strong>the</strong> heaviest individual components in <strong>the</strong> machine.<br />

Fig. 4.1 Given <strong>the</strong> coning angle and <strong>the</strong> weight <strong>of</strong> <strong>the</strong> helicopter, <strong>the</strong> blade root tension follows. Here it will<br />

be seen that <strong>the</strong> same coning angle could be obtained by suspending <strong>the</strong> helicopter on cables with a bus on <strong>the</strong><br />

o<strong>the</strong>r end. Blade attachments have to withstand tremendous loads yet allow <strong>the</strong> blades to fea<strong>the</strong>r.

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