The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.50 With the autopilot disengaged, the pushrod from the pilot’s controls operates the spool valve upper right. With the autopilot engaged, the solenoid valve admits pressure to the engage piston which moves up, locking the pilot’s pushrod to the actuator and connecting the EHV outputs to the hydraulic ram. possible. The second opens the autopilot select valve connecting the EHV spool to the main ram. Now the autopilot can move the control by supplying a signal to the EHV and can measure the control position using the feedback signal from the LVDT in the ram. Note that as the pilot’s controls are locked to the actuator body by the engage cylinder, they will move as the autopilot operates, giving the pilot confirmation that the machine is literally flying itself. Two nested feedback loops are involved here. The outer loop consists of the attitude gyro to produce an error if the machine turns around the stabilized axis for any reason. The attitude error will compensate for factors such as the moment of inertia of the helicopter about the controlled axis and will then be fed to the actuator loop. The actuator loop converts the attitude error into a control movement in such a sense that the error is cancelled. The actuator feedback loop stabilizes the controls and gives the servo stiffness against flight loads. The machine will maintain the attitude dictated by the gyro indefinitely. When the autopilot is engaged it has full authority. If the pilot tries to move the controls of a parallel system, the stiffness of the hydraulic actuator will oppose him. The LVDT will sense an unwanted movement and the autopilot will operate the EHV to oppose it. Instead the autopilot control panel carries knobs the pilot can turn to give the autopilot a new reference. Effectively the pilot’s control knob adds an offset to the output of the reference sensor. For example, if the pilot wished to make a constant rate turn, he would set the turn control knob to an appropriate angle. This would add an offset to the roll axis gyro output that the autopilot would sense as an attitude error. The autopilot would roll the helicopter until the attitude error was apparently cancelled. Thus only a 5 ◦ bank angle signal from the gyro would cancel a −5 ◦ bank input from the control knob. The helicopter would indefinitely remain at a bank angle of 5 ◦ and Control 315
316 The Art of the Helicopter Fig. 7.51 Autopilot interlock. The autopilot cannot be engaged unless all of the necessary conditions are present. make a constant rate turn. Note that a hazard could exist if the autopilot were to be engaged with the control knobs inadvertently set away from neutral. Accordingly the control knobs have centre detents that operate interlock switches so that engagement is impossible unless the control knobs are in a safe position. Figure 7.51 shows an illustrative autopilot interlock. A number of conditions must be met before power can reach the engage relay. All necessary power must be present, flight directors must be off, the gyros must be erected and the control knobs must be neutral before the engage switch receives power. The engage switch is spring biased to return to the off position, but if all the interlock conditions are met it will be held in the engage position by a solenoid. When the engage switch is operated this feeds power to a relay which applies power to the solenoid, holding the switch in the engaged position. If the interlock conditions are not met the engage switch will return to the off position as soon as it is released. The relay has further contacts which bypass the control knob centre switches so that these can subsequently be moved. Power is also fed to the autopilot solenoid in the actuator to enable the EHV. If while the autopilot is engaged any autopilot interlock condition fails, the holding current to the engage switch will be interrupted and the switch will spring back to the off position. The pilot may also disengage by pressing a button on the control column to break the holding current. In the modern AFCS, the control knobs have been dispensed with. Instead the autopilot reference can be changed intuitively using the normal controls. When the autopilot is engaged, in a traditional system the manual controls are locked and any attempt to move them would be met by the rigidity of the actuator. However, if a force sensor is fitted in the control stick or in a pushrod it is possible to sense that the pilot is trying to move the stick. Figure 7.2 showed that the stick force signal can be used to modify the AFCS reference attitude so that the AFCS acts to implement the new attitude. In doing so, the AFCS moves the pilot’s stick. When all of the parameters are correctly set, the pilot’s stick moves by virtually the same amount for a given manoeuvre with
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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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Engines and transmissions 255 resul
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Engines and transmissions 257 The p
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Fig. 7.1 (a) A minimal control loop
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Fig. 7.2 (a) When attitude conditio
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fixed point in the hover despite ex
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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
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
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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
Page 350 and 351: 8.7 Climbingand descending Helicopt
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
Page 370 and 371: Other types of rotorcraft 355 and c
Page 372 and 373: Fig. 9.7 The Bell-Boeing Osprey til
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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 369 Fig.
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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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The Art of the Helicopter John Watkinson - Karatunov.net

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