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

Fig. 9.15 Yaw control in coaxial helicopters is problematic in autorotation. Changing <strong>the</strong> blade pitch may not<br />

have <strong>the</strong> desired effect. According to where in <strong>the</strong> drag bucket <strong>the</strong> rotor is operating, <strong>the</strong> yaw control may have<br />

no effect or may even reverse.<br />

extended in one or o<strong>the</strong>r rotor. This approach has <strong>the</strong> advantage that <strong>the</strong>re is no<br />

reversal in autorotation.<br />

Ano<strong>the</strong>r suggestion for yaw control is <strong>the</strong> use <strong>of</strong> a friction brake between one or o<strong>the</strong>r<br />

<strong>of</strong> <strong>the</strong> rotor shafts and <strong>the</strong> hull. A moment’s thought will reveal that with a conventional<br />

transmission this would not work. If <strong>the</strong> coaxial helicopter simply has two shafts driven<br />

in opposite directions at <strong>the</strong> same speed by <strong>the</strong> same input shaft, applying a brake to<br />

one <strong>of</strong> <strong>the</strong>se has no yaw effect at all, but simply slows down both rotors. In order to<br />

use brakes, <strong>the</strong> two rotors have to be driven with a differential gearbox that allows one<br />

rotor to speed up as <strong>the</strong> o<strong>the</strong>r slows down. Subject to this complexity, <strong>the</strong> friction brake<br />

system has <strong>the</strong> advantage <strong>of</strong> significant reduction <strong>of</strong> rotating linkage as <strong>the</strong>re is <strong>the</strong>n<br />

no need for differential collective or tip drag brakes. <strong>The</strong>re is also no control reversal.<br />

In forward flight <strong>the</strong> coaxial helicopter has <strong>the</strong> advantage that <strong>the</strong> retreating blade<br />

<strong>of</strong> one rotor is on <strong>the</strong> opposite side to <strong>the</strong> retreating blade <strong>of</strong> <strong>the</strong> o<strong>the</strong>r rotor. This<br />

means that if a suitably rigid rotor head assembly can be built, <strong>the</strong> lift trough due to<br />

a retreating blade can be overcome by <strong>the</strong> lift from <strong>the</strong> advancing blade <strong>of</strong> <strong>the</strong> o<strong>the</strong>r<br />

rotor and higher forward speed becomes possible.<br />

9.8 <strong>The</strong> synchropter<br />

<strong>The</strong> synchropter is a helicopter having two contra-rotating synchronized rotors<br />

mounted side by side that are deeply meshed. Figure 1.17 showed that <strong>the</strong> rotor shafts<br />

are set on pylons and are tilted outwards so that <strong>the</strong> blades <strong>of</strong> one rotor can pass over<br />

<strong>the</strong> head <strong>of</strong> <strong>the</strong> o<strong>the</strong>r. In machines having two-bladed rotors <strong>the</strong> blades are phased 90 ◦<br />

apart so that one blade is safely oriented fore and aft when <strong>the</strong> o<strong>the</strong>r passes overhead.<br />

<strong>The</strong> synchropter principle was invented by Anton Flettner and used on his successful<br />

Kolibri (hummingbird) helicopter.<br />

Although <strong>the</strong>re is no tail rotor, <strong>the</strong>re is no consequent safety advantage. <strong>The</strong> outward<br />

tilt causes <strong>the</strong> rotors to pass close to <strong>the</strong> ground at <strong>the</strong> sides. Synchropters are usually<br />

placarded with warnings to ground personnel to approach from <strong>the</strong> front. Aerodynamically<br />

<strong>the</strong> synchropter is similar to <strong>the</strong> coaxial helicopter in that <strong>the</strong> efficiency loss <strong>of</strong>

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