The Art of the Helicopter John Watkinson - Karatunov.net

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must be in steady flight when the DI is aligned to the compass or the gyro axis will be incorrect and it will be set to repeat the compass turning error. 7.13 The gyromagnetic compass More sophisticated DIs can tie themselves to a magnetic flux gate compass to produce a gyromagnetic compass which combines the best features of both devices. The flux gate compass was described in section 7.5. The tying process is heavily filtered so that compass errors during manoeuvres do not affect the reading. Gyromagnetic compasses have the advantage that they do not need to be reset. Figure 7.16 shows how the DI follows the magnetic compass. The signals from the flux gate compass are applied to a set of three coils encircling the shaft driving the display card in the DI. On the shaft is a single secondary coil. When the DI is accurately aligned with the compass, this coil will be transverse to the resultant flux of the three input coils and so no voltage will be induced in it. However, if any angular difference develops, a signal will be induced in the secondary coil and this can be amplified and used to drive torque motors that apply a rolling torque to the inner gimbal of the gyro. The gyro will precess this torque into yaw so that the display card will turn, and with it the secondary Fig. 7.16 A slave DI that follows a flux gate compass. If the DI reading and the flux gate output are different, a torque motor precesses the DI until the two have the same indication. Control 281
282 The Art of the Helicopter coil. In this way the gyro is precessed until the signal in the secondary coil nulls and the precession stops. At this point the display card will be showing the heading coming from the compass. The gain in the system will be quite low so that the gyro follows very slowly. In some units it is possible to rotate the three coils using a front panel variation control so that the DI card reads true heading rather than magnetic. In most conditions tying the DI to magnetic north is useful, but when flying very close to the magnetic poles it may be better to turn the slaving off and to rely on the earth rate and transport error compensation only. A switch will be provided for this purpose. A gyromagnetic compass may also form the heading reference for an autopilot. In this case the display unit has additional controls allowing the pilot to enter the desired heading. This is displayed with respect to the DI card markings by an indicator or bug on the periphery of the instrument. If the autopilot is engaged and a new heading is selected, the machine will carry out a two-minute rate turn to the new heading at which point the bug will align with the lubber line of the DI. 7.14 The artificial horizon The artificial horizon is a gyroscopic instrument displaying a realistic copy of what the real horizon does for use in poor visibility. When the helicopter banks, the horizon in the display remains parallel to the real one. When the machine dives, the displayed horizon rises up the face of the instrument. A small symbolic aircraft on the face of the instrument then has the same attitude to the horizon as the real one, and if the pilot flies the symbolic aircraft the real one will follow. In a machine equipped with an autopilot, in addition to the visible display, the artificial horizon may provide attitude output signals that are fed into the control system to stabilize the flight attitude. The gyroscope in an artificial horizon must be compensated for drift. This is done by an erection mechanism to maintain the axis of the gyro vertical with respect to gravity. For this reason, this type of instrument is sometimes called a vertical gyro. The addition of gravity sensing creates what is known as an earth gyro. Figure 7.17 shows how gravity sensing works. The rotating wheel is fitted inside a case. Two small pendula are fitted to the case in such a way that they can swing in orthogonal directions. The electrically driven rotor is fitted with vanes at one end so that it acts as a centrifugal blower. Air from the blower is arranged to leave the casing at four equally spaced points. Each pendulum has two vanes that block or reveal the blower orifices. The vanes are Fig. 7.17 Gravity sensing is used in earth-tied instruments such as the artificial horizon. Small pendula (c) and (d) hang in the efflux (b) from a blower (a) built into the gyro rotor. If the rotor axis is not vertical the pendula will tilt and the reaction from the blower will be imbalanced so a precessive torque is created.
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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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Page 246 and 247: Engines and transmissions 231 Fig.
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Page 262 and 263: Fig. 6.37 Displays which may be see
Page 266 and 267: Engines and transmissions 251 slipr
Page 268 and 269: Engines and transmissions 253 a fau
Page 270 and 271: Engines and transmissions 255 resul
Page 272 and 273: Engines and transmissions 257 The p
Page 274 and 275: Fig. 7.1 (a) A minimal control loop
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Page 278 and 279: fixed point in the hover despite ex
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Page 282 and 283: 7.4 Compass errors A magnetic compa
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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
Page 294 and 295: e modified if the gyro is in a movi
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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
Page 320 and 321: Fig. 7.42 An electro-hydraulic valv
Page 322 and 323: path axis and the flybar axis diver
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
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(a) (b) Helicopter performance 331
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Helicopter performance 333 Fig. 8.7
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8.7 Climbingand descending Helicopt
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Helicopter performance 337 In the c
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Helicopter performance 339 of its c
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8.10 Stability Helicopter performan
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Fig. 8.13 The origin of pitchup ins
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Helicopter performance 345 moment f
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9 Other types of rotorcraft Althoug
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Other types of rotorcraft 349 De la
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Other types of rotorcraft 351 Fig.
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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 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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