The Art of the Helicopter John Watkinson - Karatunov.net

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Other types of rotorcraft 363 Fig. 9.14 In powered flight, with equal pitch on both rotors there is no net torque. Increasing the pitch of the upper rotor causes its blade reaction to rotate back whilst that of the other rotor rotates forward and a yaw results. Figure 9.14 shows how the yaw control works. The machine is in a power-on condition so the blade reactions will be tilted back with respect to the shaft axis. The vector sum of these two reactions is vertical so there is no net torque and no yaw effect. Figure 9.14 shows that increasing the pitch of the upper blade will increase the thrust and tilt it back against the direction of rotation. At the same time the pitch of the lower blade is reduced. This will reduce the thrust and tilt it forwards, with the direction of rotation. The vector sum will now be non-vertical so that there is a net torque turning the hull the same way as the rotor whose pitch has been reduced. In autorotation, the yaw control of a coaxial machine using differential collective is problematic as it may have no effect, or a reverse effect. This was initially not appreciated, and caused some hair-raising moments. Figure 3.14 introduced the autorotation diagram and Figure 9.15 shows an autorotation diagram appropriate for coaxial helicopters. Here the horizontal axis is the average angle of attack of the two rotors. In autorotation, the blade pitch will be chosen for the best rotor L/D at the bottom of the drag bucket. In most helicopters, the lower collective stop is set at the intersection of the α–θ line and the drag bucket. However, when differential collective is used for yaw, the pitch of one of the rotors can be even less and so operation can be to the left of the intersection. The problem is that when operating at L/Dmax., an increase in pitch or a decrease in pitch will have the same effect: the rotor experiences more drag and slows down. At some collective settings there is no yaw control at all. To make matters worse, helicopter designers often select blade sections having a relatively flat bottomed drag bucket. If the collective stop is slightly lower than the setting for L/Dmax., increasing the pitch will reduce drag and reducing pitch will increase it. As a result the yaw control reverses. To overcome the problem, coaxial helicopters need a significant amount of vertical tail area to give yaw stability in autorotation. The fins are often slatted to prevent stalling at high angles of attack. In some machines a mechanism is fitted so that the action of the pedals is automatically reversed when the collective is fully lowered. An alternative yaw control mechanism for coaxial helicopters was seen on the Gyrodyne helicopter. This consisted of drag brakes in the blade tips that could be
364 The Art of the Helicopter Fig. 9.15 Yaw control in coaxial helicopters is problematic in autorotation. Changing the blade pitch may not have the desired effect. According to where in the drag bucket the rotor is operating, the yaw control may have no effect or may even reverse. extended in one or other rotor. This approach has the advantage that there is no reversal in autorotation. Another suggestion for yaw control is the use of a friction brake between one or other of the rotor shafts and the hull. A moment’s thought will reveal that with a conventional transmission this would not work. If the coaxial helicopter simply has two shafts driven in opposite directions at the same speed by the same input shaft, applying a brake to one of these has no yaw effect at all, but simply slows down both rotors. In order to use brakes, the two rotors have to be driven with a differential gearbox that allows one rotor to speed up as the other slows down. Subject to this complexity, the friction brake system has the advantage of significant reduction of rotating linkage as there is then no need for differential collective or tip drag brakes. There is also no control reversal. In forward flight the coaxial helicopter has the advantage that the retreating blade of one rotor is on the opposite side to the retreating blade of the other rotor. This means that if a suitably rigid rotor head assembly can be built, the lift trough due to a retreating blade can be overcome by the lift from the advancing blade of the other rotor and higher forward speed becomes possible. 9.8 The synchropter The synchropter is a helicopter having two contra-rotating synchronized rotors mounted side by side that are deeply meshed. Figure 1.17 showed that the rotor shafts are set on pylons and are tilted outwards so that the blades of one rotor can pass over the head of the other. In machines having two-bladed rotors the blades are phased 90 ◦ apart so that one blade is safely oriented fore and aft when the other passes overhead. The synchropter principle was invented by Anton Flettner and used on his successful Kolibri (hummingbird) helicopter. Although there is no tail rotor, there is no consequent safety advantage. The outward tilt causes the rotors to pass close to the ground at the sides. Synchropters are usually placarded with warnings to ground personnel to approach from the front. Aerodynamically the synchropter is similar to the coaxial helicopter in that the efficiency loss of
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The Art of the Helicopter
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The Art of the Helicopter John Watk
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Contents Preface xi Acknowledgement
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4.12 Feathering 134 4.13 Pitch cont
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8.5 Power management 326 8.6 Flying
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xii Preface reader to make sense of
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1 Introduction to rotorcraft 1.1 Ap
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Introduction to rotorcraft 3 Fig. 1
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Introduction to rotorcraft 9 for th
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Introduction to rotorcraft 11 Fig.
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(a) (b) (c) (d) Introduction to rot
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Fig. 1.18 The contra-rotating coaxi
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Fig. 1.20 The structure of a light
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Introduction to rotorcraft 19 wings
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Introduction to rotorcraft 21 a rot
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Technical background 23 Fig. 2.1 At
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Fig. 2.3 Effect of force on velocit
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C Force A E Resultant (a) Force B (
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(a) (b) Technical background 29 Fig
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Technical background 31 Fig. 2.11 A
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Technical background 33 Fig. 2.12 (
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Technical background 35 If the volu
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Fig. 2.16 The definition of a radia
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Technical background 39 force would
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Technical background 41 Figure 2.20
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Technical background 43 force where
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Technical background 45 the cosine
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Technical background 47 Fig. 2.28 F
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Technical background 49 Fig. 2.30 T
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Technical background 51 centripetal
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2.17 The gyroscope Technical backgr
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Technical background 55 of oscillat
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Technical background 57 the inertia
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Technical background 59 shut down a
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3 Introduction to helicopter dynami
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Introduction to helicopter dynamics
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(a) (c) (b) Introduction to helicop
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(a) (b) (c) (d) Introduction to hel
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Fig. 3.23 Conditions for hover and
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(a) (b) Introduction to helicopter
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(a) (b) Introduction to helicopter
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4.1 Introduction 4 Rotors in practi
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(a) (b) Rotors in practice 119 Fig.
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Rotors in practice 121 Designers of
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Rotors in practice 123 opposite occ
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Rotors in practice 125 Fig. 4.6 (a)
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Rotors in practice 127 assembly tha
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Rotors in practice 129 Pitch change
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× T=c× D Rotors in practice 131 F
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Rotors in practice 133 In a real he
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Rotors in practice 135 Fig. 4.15 Va
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(a) (b) Rotors in practice 137 Fig.
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Rotors in practice 139 Fig. 4.18 Th
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Rotors in practice 141 swashplate a
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Fig. 4.23 A fixed-pitch tilting hea
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Rotors in practice 145 Fig. 4.25 Bl
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Rotors in practice 147 Fig. 4.27 Ef
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Rotors in practice 149 Fig. 4.29 (a
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Rotors in practice 151 concerned ar
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Rotors in practice 153 and lag damp
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Rotors in practice 155 disappears a
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(d) (e) (f) Rotors in practice 157
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Rotors in practice 159 hull and a v
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Rotors in practice 161 Fig. 4.35 (C
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Rotors in practice 163 Abrasion is
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Rotors in practice 165 There are so
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eing more visible to ground personn
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otates the rod in the screw thread.
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In the case of a rotor having offse
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The tail rotor designer is faced wi
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Fig. 5.5 A right-side wrong-directi
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centre of mass. The solution was to
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safe to start the rotors. However,
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Fig. 5.10 Tail plane locations: (a)
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Fig. 5.12 (a) A top-mounted fin is
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Fig. 5.14 (a) Main rotor lateral ro
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long. The cross-section of the duct
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drag. However, the slots are emitti
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6 Engines and transmissions The pow
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Engines and transmissions 193 the D
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(a) (b) Engines and transmissions 1
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(a) (b) (c) Engines and transmissio
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Fig. 6.5 A typical horizontally opp
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Engines and transmissions 201 the v
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Engines and transmissions 203 The c
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6.9 The oil system Engines and tran
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Engines and transmissions 207 (non-
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(a) (b) Engines and transmissions 2
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Engines and transmissions 211 a hot
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Engines and transmissions 213 Fig.
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Engines and transmissions 215 In wa
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Engines and transmissions 217 is a
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Engines and transmissions 221 own t
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Fig. 6.16 The complex fuel system o
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Engines and transmissions 233 and c
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Engines and transmissions 237 Sensi
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Engines and transmissions 243 the A
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Fig. 6.37 Displays which may be see
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Engines and transmissions 251 slipr
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Engines and transmissions 253 a fau
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Engines and transmissions 255 resul
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Engines and transmissions 257 The p
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Fig. 7.1 (a) A minimal control loop
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Fig. 7.2 (a) When attitude conditio
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fixed point in the hover despite ex
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(a) (b) Fig. 7.4 (a) The earth beha
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7.4 Compass errors A magnetic compa
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In most cases the machine will have
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Fig. 7.7 The remote indicator for a
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is set by the use of a control knob
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Fig. 7.11 The VSI (vertical speed i
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Fig. 7.13 A chaser system which all
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e modified if the gyro is in a movi
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must be in steady flight when the D
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arranged on opposite sides of the p
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Fig. 7.19 The turn indicator is spr
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7.17 Airflow-sensing devices The he
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Fig. 7.24 Using RADAR signals. At (
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Fig. 7.27 Transducers used in signa
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Fig. 7.28 In PCM or digital signall
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(a) Fig. 7.30 Using slicing and rec
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Fig. 7.33 The false codes created i
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Fig. 7.35 In a pure binary system,
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Fig. 7.38 Producing two’s complem
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Fig. 7.40 A power-assisted hydrauli
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Fig. 7.42 An electro-hydraulic valv
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path axis and the flybar axis diver
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and autostabilization, it also prov
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Fig. 7.47 The Lockheed gyrobar syst
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Page 342 and 343: Helicopter performance 327 Fig. 8.1
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Page 348 and 349: Helicopter performance 333 Fig. 8.7
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Page 362 and 363: 9 Other types of rotorcraft Althoug
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Page 366 and 367: Other types of rotorcraft 351 Fig.
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Page 380 and 381: Other types of rotorcraft 365 a low
Page 382 and 383: Other types of rotorcraft 367 the y
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Page 394 and 395: Acceleration: and force, 22-5 and v
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Page 404 and 405: tail/main rotor interaction, 173 te
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