The Art of the Helicopter John Watkinson - Karatunov.net

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In most cases the machine will have a gyroscopic direction indicator (see section 7.12) set to the same heading as the compass during steady flight. The direction indicator is used for turns because it does not suffer from acceleration errors so it is only necessary to invoke the theory if the DI fails. In everyday use there is very little that can go wrong with a compass. It should be inspected for leaks of the liquid, for bubbles or for some obvious broken or cracked component, and if the transparent housing is plastic, this may darken as the material degrades with age and indicates that it is becoming brittle as well as difficult to see through. The deviation card must be present and the machine must have been swung on the appropriate occasions. 7.5 The flux gate compass Figure 7.6(a) shows how a flux gate compass works. There are three radial pole assemblies and the side view shows that these are split so that a single coil with a vertical axis can be wound at the centre. There are a further three coils, one on each limb. The pole pieces are made of a highly permeable material such as permalloy. The entire assembly is suspended below a Hooke joint so that in steady flight it will hang level. An alternating excitation signal, typically 400 Hz, is applied to the central coil. In the absence of any external magnetic field, there would be no signal induced in the three sense coils because they are wound around both poles and the flux from the excitation coil would cancel out. However, in the presence of the earth’s magnetic field, the highly permeable material is more prone to saturation in one direction than the other. Figure 7.6(b) shows that the earth’s field adds to the excitation and shifts the operating point along the characteristic curve of the magnetic material. It will be seen in Figure 7.6(b) that the curvature of the magnetization characteristic has the effect of asymmetrically compressing the flux waveform so that there is a resultant flux in the sensor coil. The relative amplitudes of the three alternating signals induced in the sensor coils are a function of the direction of the earth’s field. The three signals are sent to any remote instrument needing magnetic heading information. This could be a simple display or a tied gyroscopic DI. Figure 7.7 shows how a remote display works. The three signals from the flux gate are supplied to three coils arranged at 120 ◦ . There is also a second single coil that can rotate with respect to the three input coils. The resultant of the flux produced by these three coils will have a direction determined by the direction of the earth’s field at the sensor. As this is an alternating flux it will induce a voltage in the secondary coil. However, if this coil is exactly at right angles to the resultant flux, no voltage will be induced. Figure 7.7 shows that the secondary coil signal is connected to an amplifier that drives a motor to turn the secondary coil and the display card. The amplifier will drive the motor until the secondary coil is transverse to the resultant field of the three input coils. At this point the induced signal will disappear and the motor will stop. Thus as the magnetic heading of the machine changes, the signals from the flux gate change the resultant flux direction in the display. The secondary coil is driven until it once more is in a null. In this way the motor and secondary coil follow the angle of the earth’s field with respect to the flux gate. The flux gate compass has no controls and can be mounted anywhere convenient. In a fixed-wing aircraft the wing tip is a favoured location as it is well away from heavy ferrous objects. In a helicopter the tail boom could be used. Some flux gate compasses contain additional coils fed with carefully calibrated currents from a control box in Control 269
270 The Art of the Helicopter Fig. 7.6 (a) In the flux gate compass, the earth’s magnetic field disturbs the conditions (b) in each of the three limbs of the device differently and its direction can be established from the relationship between the three output signals.
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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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Page 238 and 239: Fig. 6.16 The complex fuel system o
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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
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 286 and 287: Fig. 7.7 The remote indicator for a
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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 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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Thus if a VOR receiver detected a 9
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7.29 Fault tolerance In feedback sy
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8 Helicopter performance 8.1 Introd
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Helicopter performance 325 column i
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Helicopter performance 327 Fig. 8.1
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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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