The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.49 In the Lockheed AMCS, the gyro is caused to precess by springs as before, but the gyro motion controls actuators that drive the swashplate. This prevents control forces disturbing the gyro. the same level of training as his full-size counterpart. Beginners often fit extra weights to the flybar to reduce the following rate even further. The flybar also allows the use of inexpensive and lightweight wooden blades whose following rate alone would be too fast. The Bell system was difficult to replicate in miniature because of the hydraulic dampers, and the pure Hiller system was found to produce a very low following rate in models. As a result a hybrid system combining both and aptly called the Bell-Hiller system evolved. This uses the cyclic mixing of the Bell bar, as well as the Hiller paddles and control rotor feathering. There are thus three rods coming from the swashplate: one to each of the flybar mixer arms, and one to feather the control rotor. This arrangement gives a good balance between stability and response, although the number of links involved makes it look like a dog’s dinner. The falling cost of the piezo-electric gyroscope and its minute size mean that the flybar can now be eliminated in the model helicopter. 7.27 Autopilots and AFCS Autopilots vary in complexity, but the general goal is that the pilot is relieved of the need to maintain some parameter of the flight. In a helicopter it is possible to begin with stabilization of the roll axis only, and then add extra features. Automatic control of the tail rotor allows balanced turns and flight at minimum drag. Control of the pitch axis allows the airspeed to be held, and altitude can be held with collective control. Some systems can achieve a stable hover in a fixed location without pilot intervention. Control 313
314 The Art of the Helicopter Achieving any or all of this requires an intelligent combination of flight parameter sensing, error measurement, power-operated controls stabilization and ergonomics. In addition to these basics there are some additional criteria. In the same way that two pilots cannot simultaneously have control, it is not possible for a human pilot and an automatic system to have full authority over the helicopter at the same time. In stability augmentation systems, also called AFCS (automatic flight control systems), pilot and AFCS share control so that the pilot actually flies the helicopter but the added inputs from the AFCS make the helicopter follow the pilot’s wishes more readily. In simple autopilots there may be no control sharing at all and some mechanism needs to be provided so that the pilot can engage or disengage the autopilot so that one or other has full authority. These control reconfigurations must take place smoothly so that there is no sudden disturbance of the machine when the autopilot engages or disengages. Clearly safety is paramount and autopilots must use extensive interlocking so that they will not engage if correct conditions are not present and so that they will disengage and hand back control to the pilot in the case of a failure. In a simple system the servo output is physically disconnected from the controls when the pilot has control, but is connected by a clutch when the servo has control. Clearly the clutch is either engaged or disengaged and so this is not a control sharing arrangement and would not be appropriate for an AFCS. An autopilot that maintains the machine on an absolutely stable course is in one sense ideal, but in practice the course or altitude needs to be changed from time to time. If the autopilot had to be disengaged and re-engaged every time a course change was required its value in reducing pilot workload would be diminished. Thus it is necessary to provide a means whereby the pilot can direct the flight through the autopilot. In fixed-wing practice this happens relatively infrequently and it is adequate to give the pilot an auxiliary set of control wheels or knobs which are used to direct the autopilot to new flight conditions without touching the main controls. Figure 7.2 showed ways of dividing control between a servo and a pilot. Either a series or parallel arrangement can be used to share control between a small electric servo and the pilot on the mechanical input to a conventional hydraulic actuator. This approach is appropriate if the AFCS or autopilot are optional equipment because the same hydraulic actuator can be used with or without the servo. Where the autopilot is standard equipment, the electric servo can be dispensed with. Figure 7.50 shows a parallel-connected hydraulic actuator designed to allow full control authority by the pilot with the autopilot disengaged and full autopilot authority when engaged. Fluid flow to the main ram is controlled by one of two spool valves. One of these is mechanically operated by pushrod from, for example, the cyclic stick to give pilot authority. The other is operated by electro-hydraulic valve (EHV) to give autopilot authority. Note that the ram is grounded to the airframe and the body of the actuator moves. Figure 7.50 shows that with the autopilot disengaged the EHV and its spool valve are isolated from the main ram by the autopilot select valve which is closed by spring pressure. If the pilot moves the input pushrod this will displace the manual spool valve and admit fluid to the ram so that the actuator moves bodily (‘follows up’) until the spool valve is in the neutral position again. In this mode the system is working as a conventional power-assisted actuator as was described in section 7.21. Provided all of the interlock conditions are met, when the pilot engages the autopilot, the engage interlock output signal operates a solenoid valve admitting hydraulic pressure to a cylinder. This drives a tapering lock pin that moves the manual spool valve to the neutral position, locking it to the actuator body so that no relative movement is
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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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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
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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
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 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
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
Page 374 and 375: (a) (b) Other types of rotorcraft 3
Page 376 and 377: Other types of rotorcraft 361 Fig.
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Other types of rotorcraft 363 Fig.
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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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