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

Figure 4.35(e) shows a head developed by Aerospatiale. This uses a spherical laminated<br />

elastomeric bearing which takes <strong>the</strong> axial blade thrust whilst allowing fea<strong>the</strong>ring,<br />

flapping and dragging. Elastomeric blocks are fitted inside <strong>the</strong> blade grip and <strong>the</strong>se<br />

couple with extension arms on <strong>the</strong> hub. <strong>The</strong> arms are stiff in <strong>the</strong> dragging plane, but<br />

<strong>the</strong> elastomeric block is in shear for dragging movements, giving a s<strong>of</strong>t-in-plane characteristic<br />

and providing <strong>the</strong> drag damping. In flapping <strong>the</strong> blocks are in compression<br />

where <strong>the</strong>y are quite stiff. Flapping flexibility is provided through a virtual hinge in <strong>the</strong><br />

arms. <strong>The</strong> head arms also twist to allow fea<strong>the</strong>ring.<br />

Figure 4.35(f) shows <strong>the</strong> head <strong>of</strong> <strong>the</strong> Bell 680. Pairs <strong>of</strong> blades are joined by long<br />

composite yokes that divide to pass clear <strong>of</strong> <strong>the</strong> mast. <strong>The</strong> yoke bends to allow flapping<br />

and dragging and twists to allow fea<strong>the</strong>ring. <strong>The</strong> yoke is long so that <strong>the</strong> strain due to<br />

fea<strong>the</strong>ring is moderate. <strong>The</strong> blade is attached at <strong>the</strong> end <strong>of</strong> <strong>the</strong> yoke, at some considerable<br />

radius from <strong>the</strong> mast. <strong>The</strong> blade is hollow at <strong>the</strong> root forming a cuff so that much <strong>of</strong><br />

<strong>the</strong> yoke is inside <strong>the</strong> cuff. <strong>The</strong> inboard end <strong>of</strong> <strong>the</strong> cuff carries <strong>the</strong> pitch arm connected<br />

Fig. 4.35 (a) Hingeless head <strong>of</strong> <strong>the</strong> Bo-105. Flapping and dragging are permitted by <strong>the</strong> flexibility <strong>of</strong> <strong>the</strong> blade<br />

shank. (b) Lockheed Cheyenne head has virtual flapping hinge but is very stiff in dragging. (c) Westland Lynx<br />

head has flexible members to allow flapping and dragging. Virtual flapping hinge is stiff. Dragging hinge is s<strong>of</strong>t<br />

and drag dampers are needed.

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