The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.42 An electro-hydraulic valve. Small electrical signals can control very powerful actuators with this mechanism. 7.23 Stability augmentation Early helicopters were beset by weight and stability problems where solving one often made the other worse. A further issue is that by definition there was not initially a pool of experienced pilots to fly the first helicopters and any extra stability would be very welcome. The flybar was an appropriate and elegant solution at the time and played a pivotal role in the development of the helicopter, but subsequent developments have made it obsolete in the full size, although it is still found in most model helicopters. Early helicopter blades were relatively lightly built because power was limited. This resulted in a rotor with low inertia having a high following rate in response to a cyclic control input or an external disturbance. Making the blades heavier would reduce the following rate, but the weight could not be accepted. The flybar essentially stabilizes the machine by reducing the rate at which the rotor can change its attitude. Although the flybar works, it raises the drag and complexity of the rotor head and causes a performance penalty. Later systems were developed which sensed the machine’s attitude using miniature gyroscopes. Correcting signals were then fed into the powered control system. In a sense the flybar was miniaturized and put in a black box. 7.24 The Bell bar Figure 7.43(a) shows how the Bell flybar system works. This was developed by Arthur Young, using initially electrically driven models. It was subsequently used on a number of full-sized machines including the Bell 47 and the Huey. Use is restricted to two-bladed rotors. The flybar is mounted across the rotor on a bearing parallel to the feathering Control 305
306 The Art of the Helicopter (a) (b) Fig. 7.43 (a) The Bell/Young flybar system. The flybar is essentially a gyroscope. (b) In this example the main rotor will stay parallel to the plane of the flybar because there is a unity relationship between the flybar attitude and the amount of cyclic pitch applied. axis of the blades. The control rods from the swashplate are attached to a pair of mixer arms pivoting in the flybar. Further rods from the mixer arms then operate the pitch control arms of the blades. It will be seen from Figure 7.43(b) that if the swashplate remains fixed, with a suitable linkage the rotational axis of the flybar effectively can become the control axis of the rotor, because the blades tend to stay parallel to the flybar. This is called a unity mixing system. A smaller stabilizing effect is still obtained if the feathering of the blades is some proportion of the flybar motion. The flybar is influenced by dampers on a bar fixed to the rotor shaft. If the flybar rotational axis and the shaft axis are different the dampers will oscillate because the flybar is effectively flapping with respect to the shaft. This action tries to bring the flybar axis parallel to the shaft axis at a rate proportional to the divergence between the axes. If the flybar is tilted in any axis, the maximum flapping velocity is reached at 90 ◦ to that axis. As the dampers produce a couple proportional to velocity, this couple will be about an axis at 90 ◦ to the flapping axis. This couple will cause the flybar to precess into alignment with the rotor shaft. Thus the flybar is a gyro loosely tied to the rotor shaft. The flybar axis will align with the shaft axis when the machine is in equilibrium, but in any manoeuvre the divergence will be proportional to the rate of change of the attitude of the shaft axis, i.e. the roll rate or the pitch rate of the hull. Thus the angle of the flybar relative to the mast is a measure of the roll rate of the helicopter. The flybar is gyroscopic so it will tend to hold a constant rotational axis. If, in steady flight or in the hover, the tip path plane of the rotor is disturbed by a gust, the tip
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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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Page 274 and 275: Fig. 7.1 (a) A minimal control loop
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Page 280 and 281: (a) (b) Fig. 7.4 (a) The earth beha
Page 282 and 283: 7.4 Compass errors A magnetic compa
Page 284 and 285: In most cases the machine will have
Page 286 and 287: Fig. 7.7 The remote indicator for a
Page 288 and 289: is set by the use of a control knob
Page 290 and 291: Fig. 7.11 The VSI (vertical speed i
Page 292 and 293: Fig. 7.13 A chaser system which all
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Page 298 and 299: arranged on opposite sides of the p
Page 300 and 301: Fig. 7.19 The turn indicator is spr
Page 302 and 303: 7.17 Airflow-sensing devices The he
Page 304 and 305: Fig. 7.24 Using RADAR signals. At (
Page 306 and 307: Fig. 7.27 Transducers used in signa
Page 308 and 309: Fig. 7.28 In PCM or digital signall
Page 310 and 311: (a) Fig. 7.30 Using slicing and rec
Page 312 and 313: Fig. 7.33 The false codes created i
Page 314 and 315: Fig. 7.35 In a pure binary system,
Page 316 and 317: Fig. 7.38 Producing two’s complem
Page 318 and 319: Fig. 7.40 A power-assisted hydrauli
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Page 324 and 325: and autostabilization, it also prov
Page 326 and 327: Fig. 7.47 The Lockheed gyrobar syst
Page 328 and 329: Fig. 7.49 In the Lockheed AMCS, the
Page 330 and 331: Fig. 7.50 With the autopilot diseng
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Page 334 and 335: Thus if a VOR receiver detected a 9
Page 336 and 337: 7.29 Fault tolerance In feedback sy
Page 338 and 339: 8 Helicopter performance 8.1 Introd
Page 340 and 341: Helicopter performance 325 column i
Page 342 and 343: Helicopter performance 327 Fig. 8.1
Page 344 and 345: Helicopter performance 329 to skid
Page 346 and 347: (a) (b) Helicopter performance 331
Page 348 and 349: Helicopter performance 333 Fig. 8.7
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Page 352 and 353: Helicopter performance 337 In the c
Page 354 and 355: Helicopter performance 339 of its c
Page 356 and 357: 8.10 Stability Helicopter performan
Page 358 and 359: Fig. 8.13 The origin of pitchup ins
Page 360 and 361: Helicopter performance 345 moment f
Page 362 and 363: 9 Other types of rotorcraft Althoug
Page 364 and 365: Other types of rotorcraft 349 De la
Page 366 and 367: Other types of rotorcraft 351 Fig.
Page 368 and 369: Other types of rotorcraft 353 incre
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Other types of rotorcraft 355 and c
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Fig. 9.7 The Bell-Boeing Osprey til
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(a) (b) Other types of rotorcraft 3
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Other types of rotorcraft 361 Fig.
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Other types of rotorcraft 365 a low
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Other types of rotorcraft 367 the y
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Other types of rotorcraft 371 the r
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Other types of rotorcraft 373 For e
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Acceleration: and force, 22-5 and v
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Convertiplane, 11-12, 355-8 Bell-Bo
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tanks, 219-20 turbine engines, 237-
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Lockheed flybar system, 309-13 AMCS
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centrifugal stiffening, 71, 100 and
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tail/main rotor interaction, 173 te
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