The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 127 assembly that is particularly important on multi-blade heads, where the head can become a serious source of drag if it is larger than necessary. Figure 4.2 showed an example of a head in which the flapping and dragging hinges are combined in a single Hooke joint having a cruciform central member. The logical place for the feathering bearing is outboard. Figure 4.9(a) shows that the three hinge axes are not necessarily orthogonal. This results in coupling between the axes, sometimes deliberate, sometimes an unavoidable geometric compromise. A hinge has an axis and this can only uniquely be defined in three dimensions. In Figure 4.9(a) O is the rotor head, OZ is the rotor shaft axis and OX is the blade span axis. OXY is the plane of the rotor. In an orthogonal head, OY would be the flapping axis, OZ would be the dragging axis and OX the feathering axis. (a) (b) (c) (d) Fig. 4.9 (a) The axes of hinges in the head are not always orthogonal. Two of the axes may be turned as shown by amounts delta-1 and delta-3. (b) To avoid coupling between flapping and feathering, the pitch change rod must terminate on the flapping hinge axis. (c) The pitch change rod may terminate outboard of the flapping axis. This results in a delta-3 effect (d).
128 The Art of the Helicopter OP is an arbitrary hinge axis and could be a flapping or a lagging hinge. In the literature, the flapping hinge is called the δ hinge and the dragging hinge is called the α hinge. These hinges can be turned away from the mutually orthogonal in three axes, 1, 2 and 3. Only two of these axes are shown in Figure 4.9(a); axis 2 is redundant. If OP is a flapping hinge and is projected into the plane of the rotor, OXY, the angle between the projection and OY is called δ3 (delta-three). If OP is projected onto the plane OZY, the angle between the projection and OY is δ1 (delta-one). In an orthogonal head these angles are both zero. Figure 4.9(b) shows that to avoid coupling between flapping and feathering, the spherical bearing between the pitch control rod and the pitch control arm must be on the axis of the flapping hinge which must also be at 90 ◦ to the feathering axis. The pitch control arm is generally on the leading edge of the blade. If the pitch control arm is made slightly shorter as in Figure 4.9(c), there will be an interaction between flapping and feathering. Figure 4.9(d) reveals that upward flapping will reduce the pitch of the blade, tending to lower the blade, whereas downward flapping would have the reverse effect. Effectively a positive δ3 hinge has been created. This is a stable condition, whereas the reverse relationship between the control rod joint and the flapping axis to give a negative δ3 hinge would be unstable. In some autogyros the dragging hinge was inclined to produce a δ1 hinge. This had the effect of coupling the dragging and the blade pitch to allow jump take-off. The blades would stay in fine pitch whilst being driven, but upon the drive being disconnected the blades would swing forward, increasing the pitch. In some toy free-flying helicopters a reverse δ1 hinge is used which sets the machine automatically into autorotation when the fuel runs out. 4.8 Types of rotor head For many years the designer was faced with a choice between a teetering two-bladed head and an articulated multi-bladed head. These techniques were developed because they greatly reduce bending loads on the blades. The teetering head imposes the least stress on the mast, but has some drawbacks as will be seen in section 4.10. In the early years of helicopter design, available blade materials and designs precluded the use of other types of rotor head and the drawbacks had to be accepted. Although it is less demanding on the blades and the feathering bearings, the articulated rotor head contains a mass of bearings subject to oscillating motion that causes wear. These bearings require frequent replacement. Large machines may require continuous oil feed to the bearings, whereas smaller machines require the periodic application of a grease gun. The hingeless rotor head is a desirable goal if only because it reduces the amount of maintenance required. The effect of blade flapping in articulated and teetering heads is also used to prevent excessively rapid response to cyclic control inputs. This will be made clear in section 4.11. The stress due to coning can be relieved by fitting the blades at a preset coning angle. In practice, the disc and shaft axes will be slightly different since the hull blowback will never be perfectly balanced by the tail plane, particularly if the CM is displaced. As the hinges are only resolving a small geometric problem, it is clear that a rotor strong enough or flexible enough to accommodate the geometric conflict can dispense with actual flapping and dragging hinges. The result is a hingeless rotor. The term ‘rigid rotor’ is sometimes used to describe such a system, but clearly it is a misnomer and the term hingeless is to be preferred.
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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 92 and 93: Introduction to helicopter dynamics
Page 100 and 101: (a) (c) (b) Introduction to helicop
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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 138 and 139: Rotors in practice 123 opposite occ
Page 140 and 141: Rotors in practice 125 Fig. 4.6 (a)
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
Page 188 and 189: The tail rotor designer is faced wi
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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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