The Art of the Helicopter John Watkinson - Karatunov.net

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The tail rotor designer is faced with permanent compromise. Chapter 3 showed that the least power is needed if a rotor has a large diameter and a low, uniform, induced velocity. Profile power is reduced if the tip speed is low. Unfortunately the solidity will have to go up and the resulting tail rotor will be very heavy. To make matters worse, the large tail rotor will have to be mounted further aft to maintain clearance with the main rotor and this will require a longer and heavier tail boom. Ground clearance may also be an issue. There are further problems with large, slow, tail rotors. With a very low tip speed the translational speed will be limited by the growth of the reverse flow area. In the hover if the induced velocity is too low the tail rotor can enter a vortex ring condition during a yaw. In fact minimizing tail rotor power is not the goal. A more useful goal is to minimize the ratio of the total power absorbed by both rotors to the overall machinery weight. A highly efficient but heavy tail rotor may need more total power as the main rotor has to work harder to lift it. As a result the designer will settle for a tail rotor diameter that contains the weight and clearance problems, and a tip speed similar to that of the main rotor so that the same advance ratio is obtained. As a result the only room for variation is in the solidity and taper. As a design evolves, the solidity can be adapted by changing the number of blades. For example, the Super Puma has four tail blades, whereas the original Puma had five. In some helicopters the high tip speed tail rotor was replaced in a later model with a slower version to reduce noise. This may need more blades to increase the solidity. 5.4 Tail rotor location The tail rotor and the main rotor are both actuator discs and both produce thrust by virtue of the induced velocity they impart to their respective inflows. The main and tail rotors cannot be considered independently because of their proximity. They can and do affect each other by interaction of inflow to a degree that varies considerably from one regime of flight to another. In the hover a low mounted tail rotor will actually draw air in from the edge of the main rotor. This will result in an increase in the induced velocity experienced by the main rotor and it will need more power. This might amount to 10–20 kW in a mediumsized helicopter. If the tail rotor is mounted higher the effect is largely eliminated and some power can be saved. Figure 5.4(a) shows that the increased pressure below the main rotor disc and the reduced pressure above causes air to flow in a toroidal path around the edge of the disc. This is the mechanism of tip loss. The tail rotor operates in this region and there is a strong interaction. When the tail rotor turns in the same direction as the main rotor vortices (b) the relative airspeed of the tail blades is reduced and the available thrust is limited. When the tail rotor turns against the main rotor vortex (c) the performance is considerably enhanced because of the square-law connection between thrust and speed. There appears to be no detrimental aerodynamic effect of this direction, and so it is now considered to be the only direction to employ. This phenomenon was understood relatively late in the history of the helicopter with the result that many machines were designed with the tail rotor going the ‘wrong’ way. In many cases in later models the tail rotor direction was reversed to universal acclaim. Oddly enough many model helicopters are still designed (if that is the word) with the wrong tail rotor direction. The author has modified a number of models to have the ‘right’ rotation and can vouch for the improvement in these cases also. The tail 173
174 The Art of the Helicopter (a) (b) (c) (d) Fig. 5.4 (a) In the hover there is a toroidal flow around the edge of the main rotor in which the tail rotor operates. (b) A wrong-way tail rotor has its effective speed of rotation reduced by the main rotor flow. (c) A right-way tail rotor has its effective speed of rotation increased by the main rotor flow and will thus have greater authority. (d) Owing to wake contraction, it is more efficient to use a ‘blower’ tail rotor that takes in air over the mounting structure. If the tail rotor is on the wrong side the structure blocks a larger proportion of the wake. As was seen in Chapter 3, with axial flow in the hover the wake of the tail rotor contracts and speeds up just as it does in the main rotor. This means that there is a right way and a wrong way to mount the tail rotor with respect to its supporting structure, which inevitably causes some blockage. The supporting structure may be a slim tube or a substantial tail fin. Figure 5.4(d) shows that if the tail rotor slipstream is directed away from the fin, a so-called ‘blower’, the proportion of the inflow which is blocked is reduced. The relative airspeed at the blockage is also lower. If instead the thrust is directed away from the fin, this blocks a larger proportion of the now
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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
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 184 and 185: otates the rod in the screw thread.
Page 186 and 187: In the case of a rotor having offse
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
Page 234 and 235: Engines and transmissions 219 Fig.
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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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