The Art of the Helicopter John Watkinson - Karatunov.net

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Other types of rotorcraft 373 For easy loading, the Chinook is designed around a constant cross-section cabin with a rear ramp. The fuel and undercarriage are housed in two long side sponsons that provide stability on water and cause no intrusion into the load space. For similar reasons the engines are placed above the cabin, and the synchronizing shaft and control pushrods run in a hump or spine above the cabin. The immense rear pylon/fin is used to house the engine combining transmission, the oil cooler, the rear gearbox and swashplate actuators, the APU and almost anything else that needs a home with the result that the hull is kept free for payload. The control system of the tandem rotor helicopter is more complex than in a tail rotor-type machine. The rotor heads need to be articulated for reasons that will be seen. Figure 9.26(a) shows that the swashplates tilt simultaneously for roll control and tilt differentially for yaw control and to oppose any residual torque imbalance. They move up and down together for thrust control and differentially for fore-and-aft (pitch) control and to handle CM shifts. All of this is achieved in a Chinook in a diabolical mechanical mixer located in the control closet behind the port pilot’s seat which takes the inputs from the pedals, cyclic stick and thrust lever (tandemspeak for collective) in the cockpit floor and adds and subtracts in various ways to obtain four pushrod outputs which go into the roof, one to control each side of each swashplate. The conceptual mechanism is shown in Figure 9.26(b). The autopilot is incorporated in an unusual fashion. Electrical autopilot signals and mechanical pilot inputs are supplied to small duplicated hydraulic actuators in the control closet located before the control mixer. These are called integrated lower control actuators (ILCA) and they operate the control mixer. The mixer pushrod outputs are then led to duplicated high pressure hydraulic actuators at the swashplates. These actuators have mechanical inputs. This approach allows a conventional autopilot to output yaw, pitch, roll and thrust commands to the airframe. If the autopilot had to control the swashplate actuators directly, an autopilot mixer would have been needed. When the cyclic stick is pushed forward, differential collective pitch increases the thrust of the rear rotor relative to the front rotor (Figure 9.27(a)). The hull is tilted forward about the pitch axis (b) and this makes both rotor shafts tilt relative to the tip path axes. This hub tilt causes a cyclic feathering effect such that both rotors pitch forward to follow the hull (c). This is the control mechanism used in hovering. In order to move into forward flight, forward differential collective is maintained so the machine gathers speed. The lift asymmetry due to translational flight in both rotors will cause the tip path axes to flap back with respect to the two control axes as shown in Figure 9.28(a). At a given airspeed, this results in a larger change in cabin floor angle, reduced blade clearance at the rear of the front rotor, more hull parasite drag and a large amplitude of flapping and dragging at the rotor heads because of the large differences between the tip path and shaft axes. Thus in practice a tandem helicopter requires means to apply forward cyclic control to both swashplates in translational flight as a function of airspeed. Figure 9.28(b) shows that this results in closer alignment between the tip path and shaft axes, reducing the amount of flapping and dragging and improving the clearance of the front rotor. In the Chinook the fore-and-aft cyclic controls are not connected to the cyclic stick. Instead they are driven by an automatic system which is airspeed sensitive and starts to apply forward cyclic at speeds above about 70 knots. The application of forward cyclic to reduce flapping will also tend to increase the airspeed. To compensate for this, a series actuator, known as the DASH actuator, in the fore-and-aft cyclic pushrod is also operated by the automatic system in such a way that the pilot sees a fairly constant positive gradient between cyclic stick position and airspeed which is not impaired by the automatic forward cyclic control of the swashplates. The DASH actuator can be seen in Figure 9.26(b).
374 The Art of the Helicopter (a) (b) Fig. 9.26 Tandem control principles. (a) All control is by the two swashplates which tilt together for roll, differentially for yaw and which rise together for collective and rise differentially for pitch. (b) The mixer mechanism needed in a tandem to combine all of the pilot’s controls into the two swashplates.
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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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7.4 Compass errors A magnetic compa
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In most cases the machine will have
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Fig. 7.7 The remote indicator for a
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is set by the use of a control knob
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Fig. 7.11 The VSI (vertical speed i
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Fig. 7.13 A chaser system which all
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e modified if the gyro is in a movi
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must be in steady flight when the D
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arranged on opposite sides of the p
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Fig. 7.19 The turn indicator is spr
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7.17 Airflow-sensing devices The he
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Fig. 7.24 Using RADAR signals. At (
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Fig. 7.27 Transducers used in signa
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Fig. 7.28 In PCM or digital signall
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(a) Fig. 7.30 Using slicing and rec
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Fig. 7.33 The false codes created i
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Fig. 7.35 In a pure binary system,
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Fig. 7.38 Producing two’s complem
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Fig. 7.40 A power-assisted hydrauli
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Fig. 7.42 An electro-hydraulic valv
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path axis and the flybar axis diver
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and autostabilization, it also prov
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Fig. 7.47 The Lockheed gyrobar syst
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Fig. 7.49 In the Lockheed AMCS, the
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Fig. 7.50 With the autopilot diseng
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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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Page 342 and 343: Helicopter performance 327 Fig. 8.1
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Page 348 and 349: Helicopter performance 333 Fig. 8.7
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Page 352 and 353: Helicopter performance 337 In the c
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Page 356 and 357: 8.10 Stability Helicopter performan
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Page 362 and 363: 9 Other types of rotorcraft Althoug
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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
Page 382 and 383: Other types of rotorcraft 367 the y
Page 386 and 387: Other types of rotorcraft 371 the r
Page 394 and 395: Acceleration: and force, 22-5 and v
Page 396 and 397: Convertiplane, 11-12, 355-8 Bell-Bo
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Page 400 and 401: Lockheed flybar system, 309-13 AMCS
Page 402 and 403: centrifugal stiffening, 71, 100 and
Page 404 and 405: tail/main rotor interaction, 173 te
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