The Art of the Helicopter John Watkinson - Karatunov.net

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otates the rod in the screw thread. Alternatively the drum may be replaced with a sprocket engaging a length of roller chain inserted in the wire. An elegant improvement is to couple the collective pitch setting into the tail rotor pitch controls so that to some extent torque variations are automatically compensated. The Westland Lynx has such a system. In larger machines a power assistance system may be necessary and this will be considered in Chapter 7. 5.2 Balancing the torque The torque reaction of the main rotor is a pure couple, which is to say it has only a turning effect, and no resultant force in any direction. The tail rotor is mounted pointing sideways at the tail, and so the thrust it produces results in a moment about the centre of gravity of the machine. Unfortunately a moment cannot cancel a couple. The thrust of the tail rotor can be adjusted completely to cancel the rotation of the machine, but if the main rotor thrust is vertical, the tail rotor thrust will cause the machine to move sideways, a phenomenon called tail rotor drift. Tail rotor drift is prevented in the hover by tilting the main rotor disc in the opposite direction. Figure 5.2(a) shows that the main rotor thrust can be resolved into a vertical component opposing the weight of the machine, and a horizontal component. When the latter is equal to the tail rotor thrust, a perfect couple has been produced that opposes the main rotor torque reaction completely. The pitch of the tail rotor blades is controlled using the foot pedals. By disturbing the balance of the couples, yaw control is obtained. A very slight increase in rotor thrust is necessary so that the vertical component still balances the weight. The tilted attitude of the disc during hover can readily be seen. The hull may or may not tilt due to the disc tilt, depending on the design of the rotor head, the way the transmission is installed and the location of the tail rotor. If the hull tilts, the CM of the helicopter, being some way below the rotor hub, will develop a rolling couple that can only be zero when the CM is directly below the rotor head. The final attitude of the hull will be at whatever angle this restoring couple balances the tilting couples from the rotor head and the tail rotor. Figure 5.2(b) shows that if the tail rotor thrust is not in the plane of the main rotor, there will be a couple about an axis through the rotor head running in a fore-and-aft direction. The figure shows the situation viewed from the rear of a helicopter having a clockwise-from-the-top rotor and the tail rotor below the plane of the disc. In this case, the side force from the main rotor is above the side force from the tail rotor and this causes a clockwise roll couple called tail rotor roll. If the tail rotor is above the plane of the rotor (seldom seen in practice), the side force results in an anticlockwise couple. Clearly it is only when the tail rotor shaft is in the plane of the main rotor hub that this couple will be zero, and helicopters which mount the tail rotor high on the top of the fin can achieve this elegant result, although there are more important reasons for such a location, which will be explored in section 5.5. The reader is cautioned that some texts assert that the tail rotor must be mounted at the same height as the centre of mass to eliminate tail rotor roll, but this is incorrect. In fact if the tail rotor is mounted at the vertical centre of mass of the hull, the couple due to the different heights of the main and tail rotor side thrusts makes the hull tilt until it is balanced by the couple due to the laterally displaced CM. This is shown in Figure 5.2(c). The result is that the shaft axis and the tip path axis align and so there is no lateral flapping and the type of rotor head is irrelevant because, with no flapping, The tail 169
170 The Art of the Helicopter (a) (b) (c) Fig. 5.2 (a) The force from the tail rotor cannot cancel a couple. In practice the main rotor is tilted so that a horizontal component of the main rotor thrust exists. When this is exactly equal to the tail rotor thrust a pure couple has been created. If this is equal to the torque reaction due to driving the main rotor the helicopter will not yaw. (b) If the tail rotor is not at the same height as the rotor head there will be a rolling couple. (c) If the tail rotor is aligned with the vertical location of the helicopter’s CM, the tail rotor roll couple is exactly balanced by the horizontally offset CM so that the main rotor does not tilt with respect to the hull. With all other configurations the hull and the main rotor will have different attitudes. there can be no rolling couple from the head. In all other cases the shaft and tip path axes are different and so the type of rotor head becomes significant. As was seen in Chapter 4, the zero-offset rotor head is effectively a universal joint that allows the hull to hang straight down. Other rotor heads are stiffer and the hull will hang at whatever angle causes all of the couples to cancel one another. If the helicopter has a zero-offset rotor, and the tail rotor is in the plane of the rotor hub, the fuselage will hang so that the CM is vertically below the rotor head. This will result in a laterally level cabin, assuming there is no payload asymmetry.
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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
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 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
Page 186 and 187: In the case of a rotor having offse
Page 188 and 189: The tail rotor designer is faced wi
Page 190 and 191: Fig. 5.5 A right-side wrong-directi
Page 192 and 193: centre of mass. The solution was to
Page 194 and 195: safe to start the rotors. However,
Page 196 and 197: Fig. 5.10 Tail plane locations: (a)
Page 198 and 199: Fig. 5.12 (a) A top-mounted fin is
Page 200 and 201: Fig. 5.14 (a) Main rotor lateral ro
Page 202 and 203: long. The cross-section of the duct
Page 204 and 205: drag. However, the slots are emitti
Page 206 and 207: 6 Engines and transmissions The pow
Page 208 and 209: Engines and transmissions 193 the D
Page 210 and 211: (a) (b) Engines and transmissions 1
Page 212 and 213: (a) (b) (c) Engines and transmissio
Page 214 and 215: Fig. 6.5 A typical horizontally opp
Page 216 and 217: Engines and transmissions 201 the v
Page 218 and 219: Engines and transmissions 203 The c
Page 220 and 221: 6.9 The oil system Engines and tran
Page 222 and 223: Engines and transmissions 207 (non-
Page 224 and 225: (a) (b) Engines and transmissions 2
Page 226 and 227: Engines and transmissions 211 a hot
Page 228 and 229: Engines and transmissions 213 Fig.
Page 230 and 231: Engines and transmissions 215 In wa
Page 232 and 233: Engines and transmissions 217 is a
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Engines and transmissions 219 Fig.
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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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