The Art of the Helicopter John Watkinson - Karatunov.net

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7.4 Compass errors A magnetic compass is simple and reliable and needs no power, but it has a number of characteristics that need to be understood if it is to be used correctly. The compass can only align itself to the field it experiences. If something external to the compass disturbs the direction of that field, the compass cannot know and will point to a false north. The result is known as deviation and should not be confused with variation. This is shown in Figure 7.4(c). A helicopter contains a wealth of components that are necessarily magnetized such as the magnetos, the alternator, the electric fuel pump, the motors in the trim system and gyroscopic instruments and the moving coil drive units in the headsets. Additionally many of the parts of a helicopter whilst not magnetic themselves are ferrous and can distort the earth’s field by their presence. Any steel tubing in the hull, the engine and gearbox and the crankshaft, con rods and gears are all ferrous. Aluminium, brass, glass fibre and plastics are non-ferrous and have no effect. The sum of all of the effects of the helicopter’s structure on the compass determines the deviation. Before a helicopter can be released to service the deviation must be measured and displayed on a deviation card (Figure 7.4(d)) mounted next to the compass. The deviation card is completed at the end of a procedure called swinging the compass. The helicopter is taken away from buildings and aligned with each cardinal point in turn. The deviation due to the machine’s own magnetism can be detected by this process as at some points it will increase the compass reading and at other points it will reduce it. Sometimes the deviation can be reduced by the installation of ferrous compensators adjacent to the compass, but in any case the remaining deviation must be measured every 30 ◦ and recorded on the deviation card. This should be done with the engine running as a rotating permanent magnet has less effect than when it is stationary. Since radios can also generate magnetic fields, these must be tested for deviation effects. It was stated above that the dip of the compass is overcome provided the disc stays horizontal. Unfortunately there are occasions in the normal flight of a helicopter when this is not the case and the compass dip is not overcome but acts to give an erroneous reading. The problem is caused whenever the machine accelerates in a horizontal plane. The acceleration can be due to a speed change or due to flying at steady speed in a turn. Figure 7.5(a) shows a helicopter which has just taken off and is accelerating forwards. The rotor thrust is inclined well forward and is accelerating the machine in a horizontal plane. The helicopter hangs from the rotor head like a pendulum, and the compass disc CM hangs from the pivot in the same way. As a result the compass disc in a pure helicopter usually stays very nearly parallel to the cockpit floor. The effect of the tilted disc during acceleration depends on the direction of flight and the hemisphere in which the machine is flying. If the machine in the example were to be heading 000 ◦ M the disc tilt will be in the same direction as the dip and there is no effect. However, in the example here the machine accelerates along 270M and the disc is inclined west down/east up. The vertical component of the earth’s field will rotate the north pole of the disc downwards and increase the heading shown against the lubber line. In the southern hemisphere, or if accelerating east, the opposite effect is obtained and the heading shown will reduce. On the equator the effect is absent. Acceleration in the horizontal plane will also occur in a banked turn, which is the only kind of turn a helicopter can make at speed. Figure 7.5(b) shows a helicopter in a sharp bank to port. As the nose of the machine passes through magnetic north the acceleration is to the west and the disc tilts west down/east up. The vertical component of the earth’s field once more rotates the disc down at the north pole and increases the heading displayed. Control 267
268 The Art of the Helicopter (a) (b) Fig. 7.5 (a) When a helicopter accelerates at 270M in the northern hemisphere, dip will pull the north pole of the compass down and increases the apparent heading. (b) In a sharp bank there will also be acceleration towards the centre of the turn and a further acceleration error will occur. Erroneous explanations will be encountered attributing some of the rotation of the compass disc to the acceleration alone, but a moment’s thought will confirm that a couple cannot be conveyed through a single pivot. It is almost certain that there will be a question on compass acceleration errors in one of the pilot’s examinations, and the correct answer will be obtained following the process below: (a) Visualize the manoeuvre and the attitude of the rotor disc. The compass disc will be very nearly parallel to the rotor disc. (b) If the disc is tilted north down/south up or vice versa there is no effect. If the tilt is east down/west up or vice versa there will be an error. (c) Visualize the tilted compass disc in the earth’s dipping magnetic field. In the northern hemisphere the north-seeking pole dips; in the southern hemisphere the south-seeking pole dips. (d) The dip will turn the tilted disc. Visualize whether this increases or reduces the heading read off against the lubber line. Here is an example: a machine turns to port through north in the southern hemisphere. The rotor disc and compass disc will be tilted down on the west side, so the dip in the southern hemisphere will pull the south-seeking pole down. The compass disc is turned clockwise when seen from above. This causes the heading in degrees to be less than it should be. The pilot should turn to a greater reading or ‘overshoot the turn’ to compensate. In practice the error is countered by turning to a greater or a lesser indicated heading that will become the correct heading when the turn ceases and the compass settles.
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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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Page 234 and 235: Engines and transmissions 219 Fig.
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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 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 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
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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
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or without the AFCS. Systems of thi
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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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