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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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124 <strong>The</strong> <strong>Art</strong> <strong>of</strong> <strong>the</strong> <strong>Helicopter</strong><br />

Fig. 4.5 When viewed along <strong>the</strong> tip path axis, <strong>the</strong> blades are 90 ◦ apart in <strong>the</strong> hover, but if <strong>the</strong> CM is <strong>of</strong>fset,<br />

<strong>the</strong> shaft axis and <strong>the</strong> tip path axis are not aligned. With respect to <strong>the</strong> shaft axis <strong>the</strong> blades are not evenly<br />

spaced. Dragging hinges allow this geometric conflict to be resolved.<br />

is shown in Figure 4.6(a). This, however, does not give constant angular velocity when<br />

it is deflected and this would cause torsional stress in <strong>the</strong> transmission. What is needed<br />

is a true constant velocity joint <strong>of</strong> <strong>the</strong> type used in front-wheel-drive cars. In <strong>the</strong> LZ-5<br />

helicopter designed by Glidden Doman a true constant velocity joint was used (b).<br />

A rotor head having only a constant velocity joint would not be able to transmit any<br />

moments to <strong>the</strong> mast and so would be classified as a zero-<strong>of</strong>fset head. Such heads are<br />

only suitable for lightweight machines. However, it would be possible to fit springs to<br />

<strong>the</strong> head to give it some ability to transmit moments.<br />

Clearly <strong>the</strong> blades are being driven through <strong>the</strong> air by torque supplied to <strong>the</strong> rotor<br />

head. In an articulated head <strong>the</strong> dragging bearings prevent torque being transmitted<br />

to <strong>the</strong> blades. <strong>The</strong> blades actually drag back until <strong>the</strong> blade tension applied to <strong>the</strong> rotor<br />

head is no longer truly radial but is displaced with respect to <strong>the</strong> mast axis. Figure 4.7(a)<br />

shows that this results in <strong>the</strong> torque from <strong>the</strong> transmission being balanced in <strong>the</strong> rotor<br />

head. <strong>The</strong> rotor head will <strong>of</strong>ten be constructed with a typical <strong>of</strong>fset built in as shown<br />

in (b).<br />

Note that <strong>the</strong> blade grip attaches to <strong>the</strong> blade on <strong>the</strong> axis <strong>of</strong> <strong>the</strong> mass centroid <strong>of</strong><br />

<strong>the</strong> blade so that no bending stress is caused at <strong>the</strong> mounting. As <strong>the</strong> mass centroid is

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