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

two reduction stages as was shown in Figure 6.1. <strong>The</strong> first will reduce engine RPM<br />

to approximately tail rotor RPM, and <strong>the</strong> second will reduce <strong>the</strong> tail rotor RPM to<br />

main rotor RPM. <strong>The</strong> tail rotor gearbox contains a pair <strong>of</strong> bevel gears to turn <strong>the</strong> drive<br />

through a right angle. This has usually a near 1:1 ratio, but some variation is possible.<br />

In development <strong>the</strong> tail rotor RPM may need to be changed to obtain a different<br />

compromise between noise and available thrust, or if <strong>the</strong> solidity is changed. This can<br />

be achieved by changing <strong>the</strong> tail gearbox ratio. In machines with cranked tails a fur<strong>the</strong>r<br />

gearbox may be needed to turn <strong>the</strong> drive at <strong>the</strong> base <strong>of</strong> <strong>the</strong> fin.<br />

Although simple at one level, gearboxes incorporate some subtle detailing. Consider<br />

a gearbox with a ratio <strong>of</strong> 2 : 1. <strong>The</strong> gears have, for example, 40 and 20 teeth. As <strong>the</strong>se<br />

numbers factorize, each tooth on <strong>the</strong> small gear only touches two teeth on <strong>the</strong> large<br />

gear and any irregularities would form a wear pattern. However, if <strong>the</strong> number <strong>of</strong> teeth<br />

were to be relatively prime, every tooth would touch every o<strong>the</strong>r tooth and this would<br />

result in <strong>the</strong> irregularities averaging out. This could be achieved by using 20 and 41<br />

teeth. For this reason gearboxes seldom have simple ratios.<br />

If <strong>the</strong> gear teeth were parallel to <strong>the</strong> shafts, <strong>the</strong> gearbox would make a lot <strong>of</strong> noise and<br />

vibration as <strong>the</strong> drive jumps from one pair <strong>of</strong> teeth to <strong>the</strong> next. In practice <strong>the</strong> teeth are<br />

twisted so that <strong>the</strong> point <strong>of</strong> contact slides along <strong>the</strong> shaft as <strong>the</strong> gears rotate. Contact<br />

between <strong>the</strong> next pair <strong>of</strong> teeth begins just before <strong>the</strong> current pair part company and <strong>the</strong><br />

result is a more even power transmission and less noise and wear. Helical gears produce<br />

end thrust on <strong>the</strong> shafts because <strong>of</strong> <strong>the</strong> angular contact. <strong>The</strong> angled contact faces <strong>of</strong><br />

<strong>the</strong> gear teeth also cause <strong>the</strong> shafts to be forced apart. <strong>The</strong> gear case, <strong>the</strong> shafts and<br />

<strong>the</strong> bearings must be stiff enough to contain <strong>the</strong> forces generated without distorting.<br />

Distortion will reduce <strong>the</strong> accuracy <strong>of</strong> tooth mesh and destroy <strong>the</strong> uniformity <strong>of</strong> <strong>the</strong><br />

tooth loading. Tapered roller bearings resist combinations <strong>of</strong> radial and axial forces.<br />

When used in opposed pairs, <strong>the</strong> outer race <strong>of</strong> one <strong>of</strong> <strong>the</strong> bearings can be adjusted<br />

axially to remove all play in <strong>the</strong> shaft and apply a slight pre-load to <strong>the</strong> rollers to stop<br />

<strong>the</strong>m skidding.<br />

Figure 6.32 shows that in most gear systems both gearwheels have convex teeth<br />

making <strong>the</strong> contact patch between <strong>the</strong> teeth very small. In conformal gearing, one <strong>of</strong><br />

<strong>the</strong> gearwheels has concave teeth designed to rotate around <strong>the</strong> convex teeth <strong>of</strong> <strong>the</strong><br />

o<strong>the</strong>r gear. This allows a much larger contact area so that a given size <strong>of</strong> gear can<br />

transmit more torque. <strong>The</strong> Westland Lynx has conformal gearing.<br />

<strong>The</strong> epicyclic or pla<strong>net</strong>ary reduction gearbox is popular in helicopters because it can<br />

be built with low weight for <strong>the</strong> torque handled. Figure 6.2(a) showed that <strong>the</strong>re is<br />

a fixed internally too<strong>the</strong>d gear which is coaxial with <strong>the</strong> input or sun gear. Between<br />

<strong>the</strong>se gears are three pla<strong>net</strong> gears that are fitted to pins on <strong>the</strong> output shaft. As <strong>the</strong> sun<br />

gear rotates, <strong>the</strong> pla<strong>net</strong>s orbit <strong>the</strong> ring gear and revolve at <strong>the</strong> same time. <strong>The</strong> part <strong>of</strong><br />

<strong>the</strong> pla<strong>net</strong> gear in contact with <strong>the</strong> sun gear is travelling twice as fast as <strong>the</strong> axis <strong>of</strong><br />

<strong>the</strong> pla<strong>net</strong> gear. As a result <strong>the</strong> pla<strong>net</strong>ary mechanism itself gives a 2:1 speed reduction<br />

which may fur<strong>the</strong>r be increased by reducing <strong>the</strong> size <strong>of</strong> <strong>the</strong> sun gear. <strong>The</strong> torque is<br />

shared between <strong>the</strong> three pla<strong>net</strong>s and applied at three different places on <strong>the</strong> sun gear<br />

and <strong>the</strong> ring gear. This makes <strong>the</strong> epicyclic gearbox ideal for <strong>the</strong> final stage <strong>of</strong> rotor<br />

drive where <strong>the</strong> torque is greatest. Figure 6.33 shows a Chinook gearbox, in which <strong>the</strong><br />

final stage epicyclic gear ring can be seen.<br />

<strong>The</strong> gear teeth slide over each o<strong>the</strong>r, and metal-to-metal contact is prevented by<br />

special gear oil known as extreme pressure (EP) oil. <strong>The</strong> viscosity <strong>of</strong> <strong>the</strong> oil means that<br />

it is not squeezed out from between <strong>the</strong> teeth by <strong>the</strong> contact pressure. <strong>The</strong> sliding<br />

<strong>the</strong>n takes place in <strong>the</strong> shearing <strong>of</strong> <strong>the</strong> oil film. Shearing viscous oil produces heat and<br />

<strong>the</strong> gearbox casing may be made with fins to dissipate <strong>the</strong> heat. In large machines an<br />

oil cooler may be fitted, perhaps with a fan and oil pump. In some machines <strong>the</strong> oil

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