The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 123 opposite occurs. As a result no additional damping is required about the flapping axis, aerodynamic damping being perfectly adequate. 4.5 Droop stops When a rotor is turning at flight RPM, centrifugal stiffening tends to hold the blades straight and they will cone to whatever angle is necessary for a force balance in the rotor head. With flapping hinge offset, if the tip path plane tilts, a couple will be created which tilts the hull to follow. As a result the freely flapping blades will not strike the tail boom. There have been some spectacular incidents in wind tunnels where the hull of the helicopter was fixed and this mechanism could not operate. As a rotor slows down, the blades will droop and stops are needed to restrict the travel. These can be fixed if a tall mast is used. However, Figure 4.4 shows that at flight RPM the blades are nearly straight whereas at rest they droop. Thus the actual angular motion allowable at the head is greater when the blades are turning. This requirement can be met by using moving droop stops which are thrown out of engagement as the rotor speeds up, and which return under the influence of springs as it slows down. Stops are also required to restrict the upward motion of the blade if it ‘sails’ whilst starting up in a wind. 4.6Introduction to dragging In the absence of coning, the flapping hinges would be sufficient for the shaft axis to tilt with respect to the tip path axis, but greater freedom is needed in the presence of coning. Figure 4.5 shows an example of a machine hovering with the CM not directly below the rotor head so that the mast is not vertical. When viewed along the tip path axis at (a), the blades are evenly spaced, but when viewed along the shaft axis (b), the blades are not evenly spaced, and dragging hinges are necessary to allow this angular change with respect to the rotor head so that the blades can travel at constant velocity to conserve momentum. If the blades were not allowed to drag, they would experience alternating bending moments due to the geometrical conflict. The dragging motion in the hover is due to the rotor head turning on the shaft axis whilst the blades turn on the tip path axis. If a flexible joint were installed in the rotor shaft, the rotor head could turn on the same axis as the blades and then there would be no dragging movement due to coning and no need for dragging hinges. A simple flexible joint such as the Hooke or universal joint used in automotive propeller shafts Fig. 4.4 In flight the blades are held straight by centrifugal stiffening. At rest the blades will droop further. Retractable stops may be used to prevent excessive droop without limiting flapping in flight.
124 The Art of the Helicopter Fig. 4.5 When viewed along the tip path axis, the blades are 90 ◦ apart in the hover, but if the CM is offset, the shaft axis and the tip path axis are not aligned. With respect to the shaft axis the blades are not evenly spaced. Dragging hinges allow this geometric conflict to be resolved. is shown in Figure 4.6(a). This, however, does not give constant angular velocity when it is deflected and this would cause torsional stress in the transmission. What is needed is a true constant velocity joint of the type used in front-wheel-drive cars. In the LZ-5 helicopter designed by Glidden Doman a true constant velocity joint was used (b). A rotor head having only a constant velocity joint would not be able to transmit any moments to the mast and so would be classified as a zero-offset head. Such heads are only suitable for lightweight machines. However, it would be possible to fit springs to the head to give it some ability to transmit moments. Clearly the blades are being driven through the air by torque supplied to the rotor head. In an articulated head the dragging bearings prevent torque being transmitted to the blades. The blades actually drag back until the blade tension applied to the rotor head is no longer truly radial but is displaced with respect to the mast axis. Figure 4.7(a) shows that this results in the torque from the transmission being balanced in the rotor head. The rotor head will often be constructed with a typical offset built in as shown in (b). Note that the blade grip attaches to the blade on the axis of the mass centroid of the blade so that no bending stress is caused at the mounting. As the mass centroid 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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Page 88 and 89: Introduction to helicopter dynamics
Page 100 and 101: (a) (c) (b) Introduction to helicop
Page 104 and 105: (a) (b) (c) (d) Introduction to hel
Page 106 and 107: Fig. 3.23 Conditions for hover and
Page 110 and 111: (a) (b) Introduction to helicopter
Page 116 and 117: (a) (b) Introduction to helicopter
Page 132 and 133: 4.1 Introduction 4 Rotors in practi
Page 134 and 135: (a) (b) Rotors in practice 119 Fig.
Page 136 and 137: Rotors in practice 121 Designers of
Page 140 and 141: Rotors in practice 125 Fig. 4.6 (a)
Page 142 and 143: Rotors in practice 127 assembly tha
Page 144 and 145: Rotors in practice 129 Pitch change
Page 146 and 147: × T=c× D Rotors in practice 131 F
Page 148 and 149: Rotors in practice 133 In a real he
Page 150 and 151: Rotors in practice 135 Fig. 4.15 Va
Page 152 and 153: (a) (b) Rotors in practice 137 Fig.
Page 154 and 155: Rotors in practice 139 Fig. 4.18 Th
Page 156 and 157: Rotors in practice 141 swashplate a
Page 158 and 159: Fig. 4.23 A fixed-pitch tilting hea
Page 160 and 161: Rotors in practice 145 Fig. 4.25 Bl
Page 162 and 163: Rotors in practice 147 Fig. 4.27 Ef
Page 164 and 165: Rotors in practice 149 Fig. 4.29 (a
Page 166 and 167: Rotors in practice 151 concerned ar
Page 168 and 169: Rotors in practice 153 and lag damp
Page 170 and 171: Rotors in practice 155 disappears a
Page 172 and 173: (d) (e) (f) Rotors in practice 157
Page 174 and 175: Rotors in practice 159 hull and a v
Page 176 and 177: Rotors in practice 161 Fig. 4.35 (C
Page 178 and 179: Rotors in practice 163 Abrasion is
Page 180 and 181: Rotors in practice 165 There are so
Page 182 and 183: eing more visible to ground personn
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Page 186 and 187: 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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8 Helicopter performance 8.1 Introd
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Helicopter performance 325 column i
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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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