The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.13 A chaser system which allows the position of an altimeter mechanism to be measured without adding friction. See text. movement and the power available is small and it is difficult to drive a conventional encoder directly. Instead a form of electromechanical amplification is used. Figure 7.13 shows one example. The sensing capsule is mounted on a platform that can be moved by an electric motor. The moving diaphragm of the capsule carries only a low mass vane or plate placed in a light beam. The light falls on differential photocells so that very small off-centre movements of the diaphragm produce an electrical signal because more light falls on one cell and less on the other. This signal is amplified and supplied to the electric motor such that the motor is driven in a direction that recentres the optical sensor. Alternate types of sensing may be used, for example a differential transformer as will presently be described. A motor used in this way is known as a chaser. Effectively the chasing action means that the diaphragm of the capsule remains in the same place. Pressure changes must then be reflected in the position of the motor drive, which has enough power to drive the pressure encoder. In addition to the encoder output, the speed of the motor may be measured to produce a rate output. When used for the altitude hold function in an autopilot, the chaser motor may be disconnected and the signal from the optical sensor will be used to make the aircraft climb or descend as needed to maintain altitude. Altitude is sensed using an evacuated capsule, and means are required to allow the appropriate reference pressure to be set. Airspeed is sensed using a capsule exposed on one side to dynamic pressure at the pitot head and to static pressure on the other. For automatic approach or landing systems a RADAR altimeter is a necessity. In fact it is not an altimeter as it does not consider air pressure; it is a height meter because it works by timing the reflection of a radio signal to give height AGL which is the requirement for landing. 7.11 Gyroscopic instruments Gyroscopic instruments are those depending on a rapidly rotating flywheel for their operation and include the direction indicator (DI), the turn and slip indicator (T + S), also known as the turn and bank indicator, and the artificial horizon. These display, in various ways, the attitude and heading of the helicopter. It is possible to fly a helicopter in good conditions without them, relying only on the magnetic compass for heading Control 277
278 The Art of the Helicopter information and assessing the attitude of the machine from the view of the world outside. In poor conditions or at night the horizon may be indistinct and without attitude clues even the finest pilot could lose control owing to disorientation. The gyroscopic instruments provide sufficient clues that in conjunction with the other flight instruments the machine can be flown without looking outside at all. Whilst the instruments are optional in a VFR light helicopter, most have them fitted and it is worth knowing how they operate as they could make a significant difference if an unanticipated deterioration in the weather takes place or if cloud is inadvertently entered. The basic principle of the gyroscope was considered in Chapter 2. In practical instruments the gyro rotor is spun by an air jet or by an electric motor. Many light planes have a venturi in the slipstream that generates suction. This is applied to the case of the gyro instrument. Filtered cabin air is drawn into the instrument through a nozzle by the suction and blows the rotor round. Clearly this arrangement is not suitable for a helicopter as there is not necessarily a slipstream. A further alternative is to have a vacuum pump driven by the engine. The solution adopted in most helicopters is to drive the gyroscopes electrically. Power is fed through the gimbals by miniature sliprings. As the operation of the gyro is dependent on the rotational speed, many gyros, but not all, have a flag coloured red or orange which swings into view if the rotor is not running fast enough. No flight action should be based on the reading of an instrument showing a flag. If a gyroscope is mounted in gimbals and set running with the rotor axis parallel to the earth’s surface, it will not remain in this condition due to drift. Drift has two unrelated components. First, the gyroscope is built to finite accuracy and minute imbalances in the gimbals and bearing friction will result in the spin axis changing slowly in an unpredictable manner; a genuine drift. Second, as the earth is turning at 15 ◦ per hour the gyro axis fixed in space will display apparent drift. Figure 7.14 shows that the effect of the earth’s rotation on apparent drift is a function of latitude. At the equator the gyro axis and the earth’s axis are parallel and no drift is observed. However, at the geographic pole, the full 15 ◦ /hr will be observed. The apparent drift due to earth rotation is a sinusoidal function of latitude. The drift may Fig. 7.14 Apparent drift in a gyroscope is not a random effect but is due to the earth’s rotation. At the poles the full rotational rate of the earth (15 ◦ /hr) will be apparent. This falls to zero at the equator as a sinusoidal function of latitude.
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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 242 and 243: Engines and transmissions 227 Fig.
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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
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Page 300 and 301: Fig. 7.19 The turn indicator is spr
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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 336 and 337: 7.29 Fault tolerance In feedback sy
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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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