The Art of the Helicopter John Watkinson - Karatunov.net

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or without the AFCS. Systems of this type are intuitive and much more appropriate for helicopters which are generally flown at lower altitude and in closer proximity to potential hazards. Such autopilots can be left engaged at all times. If the pilot wishes to perform an emergency manoeuvre, it is not necessary to disengage the autopilot; an instinctive movement of the controls is all that is needed. The autopilot acts as an AFCS and makes the machine execute the manoeuvre more accurately. However, if a small dead-band is incorporated in the stick transducer, the autopilot can sense when the pilot has released the stick and will then maintain the last attitude indefinitely. An autopilot that can only maintain attitude is not enough. For prolonged use the ability to maintain airspeed and altitude is also required. For instrument landing the ability to maintain a specified rate of descent is essential, as is the ability to move smoothly from altitude control to descent control as the glide slope is entered. An encoding altimeter as described in section 7.10 produces an actual altitude signal and this is subtracted from the desired altitude set on a dial by the pilot. The resulting altitude error will be cancelled by climbing or descending using the collective control. When cruising at a fixed altitude the necessary rates of climb and descent will be very small, but if the pilot directs the system to a new altitude, a significant rate of climb may result. In the absence of a stabilator, this will increase the downwash on the tail plane and give an aft trim effect. However the vertical axis gyro will sense this as an attitude error and try to cancel it with forward cyclic. This will produce undesirable mast bending and vibration. A solution is to feed a small proportion of the collective climb command across to the cyclic system to modify the pitch axis reference allowing a slightly tail down attitude for the duration of the climb. The climb will also require more torque and although the yaw control of the autopilot would handle this a better result may be obtained if the measured torque is fed into the yaw system. Airspeed is easily obtained from the dynamic pressure at the pitot head. In a sophisticated system, the static pressure may also be used to compensate the indicated airspeed (IAS) to true airspeed (TAS). The pilot enters the desired airspeed on a dial and a subtraction produces the airspeed error. An increase in airspeed requires the application of forward cyclic to tilt the thrust vector, an increase in collective pitch to maintain altitude, a change of lateral cyclic trim because of inflow and a change of tail rotor pitch because of the increased torque. In a simple system the airspeed control might simply apply a forward pitch command to the vertical attitude reference. The resulting loss of height would then be sensed by the altitude hold system that would increase collective to maintain height. The increased torque would cause the yaw stabilizer to operate and the roll attitude would be held against inflow changes by the roll axis stabilization. However, all of these changes could take some time to stabilize. Instead of letting an error occur and then allowing the automatic system to cancel it, it is more accurate to anticipate the error by issuing an appropriate command. This is known as feedforward. For example, if the amount of tail rotor pitch needed to cancel torque is known for all combinations of power and airspeed, then if a power change is commanded, the appropriate pitch change can be added directly to the tail rotor. In this way the tail would not yaw at all during a power change unless the feedforward had not been correctly assessed. Part of the development phase of an autopilot system is to map the feedforward parameters over the entire flight envelope. Feedforward of this kind may be used in manually controlled helicopters. For example, the collective lever position can usefully be linked to the tail rotor pitch. An increase in collective pitch will require more rotor drive torque and a corresponding increase in tail rotor pitch. By passing the tail rotor cables over a system of pulleys operated Control 317
318 The Art of the Helicopter by the collective lever, the amount of pedal input needed, for example when entering autorotation, is reduced. Fast forward flight requires a combination of forward cyclic to oppose the advancing/retreating lift difference and raised collective to handle increased inflow. In some helicopters, as the collective lever is raised a degree of forward cyclic may automatically be applied to reduce the amount of retrimming needed. 7.28Coupled systems Once an autopilot is available which is sensitive to external direction, automatic navigation becomes possible. Using inertial sensing, radio navigation, Doppler or other means the course can be corrected automatically to bring the machine to a desired destination. This is known as coupling and a unit known as a flight director allows the pilot to select which control source is coupled to the autopilot. In the heading (HDG) position of the flight director, the yaw reference for the autopilot becomes the magnetic-compass-locked DI such that the helicopter will maintain a fixed magnetic heading. In the VOR position of the flight director, the yaw reference becomes the bearing of a VOR (VHF omni-range) beacon so that the machine can fly directly towards (or away from) the beacon. Figure 7.52 shows that a VOR beacon radiates in the VHF aircraft band and is modulated with two low frequency sinusoidal signals. One of these is radiated in the same way in all directions, whereas the relative phase of the other signal is a function of the direction of radiation. The in-phase condition is aligned with magnetic north and the phase lag is equal to the magnetic bearing on which the radiation leaves the beacon. The beacon also transmits its identifying signal in Morse code. Fig. 7.52 In the VOR (VHF omni-range) system there are two transmitted signals. One of these is the same in all directions whereas the phase of the other is a function of direction. The receiver can determine the direction along which the transmitter was sending and the bearing of the transmitter must be the reciprocal of this.
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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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(a) (b) Fig. 7.4 (a) The earth beha
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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
Page 330 and 331: Fig. 7.50 With the autopilot diseng
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 380 and 381: 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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