The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 5.5 A right-side wrong-direction tail rotor (a) can be made into a wrong-side right-direction tail rotor by turning the gearbox over (b). The improvement due to the right direction exceeds the loss due to being on the wrong side. contracted slipstream which also has a higher relative airspeed. The thrust loss due to blockage will be significantly higher. There is thus a ‘right’ side and a ‘wrong’ side to mount a tail rotor; the blower installation will always be more efficient in the hover. In an ab-initio design, the right side, right direction tail rotor is an automatic choice. The existence of the right side was appreciated well before that of the right direction, and many machines appeared with this configuration as shown in Figure 5.5(a). When it was realized that the tail rotor should be reversed, cost considerations often dictated the simple expedient of turning the tail rotor gearbox through 180 ◦ about its input shaft. This had the desired effect on the rotation, but then put the tail rotor on the ‘wrong’ side as shown in Figure 5.5(b). The improvement obtained by turning the right way exceeds the loss experienced by the increased blockage. As a result there are quite a few right-direction wrong-side machines. The Westland Lynx and the Mil Mi-24 are both in this category. In the Enstrom F-28A, the tail rotor originally went the ‘wrong’ way, but by turning over the gearbox as described the blockage increase was negligible because the tail rotor is mounted on a slim tube. The result was a significant improvement in tail rotor authority. Figure 3.24(b) showed that in forward flight the main rotor produces a trailing vortex structure not unlike that of a wing. The streamlines converge above the plane of the rotor and diverge below it. The tail rotor operates in this airflow and the yaw-stabilizing action is affected by it. When the tail is mounted high, as in Figure 5.6(a) the amount of inflow change for a given yaw is increased by the converging flow, increasing the yaw-stabilizing action. However, a low mounted tail rotor may be operating in the diverging flow (b) where the yaw-stabilizing action is actually reduced. There are thus many good reasons to mount the tail rotor high on a cranked boom. In addition to power saving in the hover, a high tail rotor is less likely to strike the ground in a low level quick stop, and it enables a safe landing to be made in scrub. The blades are less likely to suffer leading edge erosion from grit blown up by the downwash during hovering. In forward flight a raised tail rotor will produce greater directional stability, and encounters a cleaner airflow. The elimination of tail rotor roll as shown in section 5.2 is an elegant but not an essential bonus. Unfortunately to site the tail rotor on a cranked tail boom requires an extra gearbox in the tail rotor shaft, and a stiffer tail boom. This adds expense that is unwelcome in smaller machines. In this case the requirement for a little extra power is not a problem because the turbine engine is usually heavily derated. In larger machines, the power loss becomes significant and in very large machines a correspondingly large tail rotor will be required and the issue of ground clearance also arises. Thus a cranked boom is The tail 175
176 The Art of the Helicopter Fig. 5.6 The pressure drop across a main rotor in forward flight causes a vortex system as shown. A high mounted tail rotor (a) operates in the converging area of the vortex system and will have good weather-cocking properties. A low-mounted tail rotor (b) may operate in a diverging area of the vortex system and be less effective. Fig. 5.7 (a) A high-mounted tail rotor applies a moment to the tail boom. (b) By canting the tail rotor, the weight of the rotor and gearbox produces an opposing moment. a better option on a large machine, good examples being the Mi-26 and the Sikorsky CH-54 Skycrane. In some helicopters the tail rotor is mounted in an unusual way. The Sikorsky series S-65/Sea Stallion/Sea Dragon is one example. This is a very powerful machine having a high disc loading, three engines and a seven-bladed main rotor needing an equally powerful tail rotor on a cranked boom. Figure 5.7(a) shows that in a conventional installation the side thrust from the high mounted tail rotor puts the tail boom under torsional loading. By tilting over the fin and tail rotor as in (b), the weight of the tail rotor assembly applies a torsional load in the opposite direction and the vertical component of the tail rotor thrust helps to carry the weight of the tail so that stress in the tail boom is reduced. The extra power consumed by the tail rotor is offset by a slight reduction in main rotor power as the latter is no longer carrying the whole weight of the machine. The Sikorsky Blackhawk also has a canted tail rotor, but for a different reason. In solving various contradictory requirements, this machine turned out with a rearward
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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
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 184 and 185: otates the rod in the screw thread.
Page 186 and 187: In the case of a rotor having offse
Page 188 and 189: The tail rotor designer is faced wi
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
Page 234 and 235: Engines and transmissions 219 Fig.
Page 236 and 237: Engines and transmissions 221 own t
Page 238 and 239: Fig. 6.16 The complex fuel system o
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Engines and transmissions 225 Fig.
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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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