The Art of the Helicopter John Watkinson - Karatunov.net

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drag. However, the slots are emitting downward jets and this causes some upthrust that nearly cancels the induced drag leaving a side thrust to counteract the torque. About 60% of the anti-torque thrust comes from the boom. The remainder comes from a nozzle at the extreme end of the boom. This has a stationary inner part and an outer rotating sleeve. The inner part has a port on both sides, whereas the outer part has a single port. The pilot’s foot pedals rotate the sleeve so that it will uncover one or other port to allow yaw control. The degree to which the sleeve turns determines the amount of port opening and so controls the thrust. The boom is pressurized by a variable pitch fan. The pitch required is not a simple function of the pedal position and never becomes negative. Instead it is a function of how much thrust is needed. In other words the application of right or left pedal will cause the fan pitch to increase to provide extra airflow to the yaw control ports. In the case of autorotation the boom will be in upwash and the boundary layer control is no longer effective, but nor is it needed. The fan is driven from the main rotor and yaw control is by rotating the tail sleeve as usual. The NOTAR system is remarkably quiet and safe and demonstration pilots have made their point by backing the tail boom into bushes and using back cyclic so that the machine rests on the rear of the skids and the end of the boom. In the absence of a tail rotor to weathercock, the NOTAR machines, not surprisingly, need a substantial amount of fin area. In the production machine, minor yaw instability is handled with a gyroscopically controlled rudder tab on one of the fins. It is possible that this is not a fundamental problem and further development may eliminate it. 5.12 Tail rotor failure It is often overheard that loss of tail rotor drive will cause the machine to go out of control, but this is not necessarily the case. If the tail rotor drive fails without causing other damage and the machine is put promptly into autorotation, the torque reaction of the main rotor will cease, and the tail rotor will be unnecessary. In some machines the tail boom and fin area are large enough to counteract engine torque by yawing into the slipstream at reasonable translational speed and then autorotation will only be necessary for landing. Nevertheless most pilots would rather lose the engine than the tail rotor drive. Unfortunately tail rotor failures can result in more than loss of drive. A failed component in the rotor can cause serious imbalance and this can tear the whole tail gearbox out of the boom. A broken drive shaft may flail and destroy hydraulics and wiring or even sever the boom. In these cases the machine will suffer a serious forward CM shift and a loss of rear side area. A machine with a zero-offset rotor head probably wouldn’t retain control. Figure 5.19 shows a zero-offset helicopter that has developed a serious nose down trim having lost its tail rotor in a collision with a ship. A machine with a substantial tail boom and a rotor head having a wide CM tolerance would have a much better chance. Despite all the theory, retaining control requires the pilot to take the correct actions and in order to take good decisions the pilot has to know the situation. In the case of tail rotor problems the pilot cannot see what is wrong and has to deduce the problem from whatever symptoms of vibration, noise, yaw and pitch he experiences. When this happens suddenly it is asking a lot of the pilot to do exactly the right thing. Nevertheless many pilots have walked away from tail rotor failure, in some cases with no further damage to the machine. The tail 189
190 The Art of the Helicopter Fig. 5.19 Following a collision with a ship, this helicopter has lost its tail rotor and gearbox. Having a zerooffset rotor head, there is insufficient cyclic authority to oppose the serious trim shift and the nose down attitude that results is apparent. Others were not so fortunate and consequently regular inspection of all components associated with the tail rotor is to be recommended. One point sometimes overlooked is that as the tail rotor is at the back, anything that comes off or out of the helicopter in forward flight can strike it. The designer may try to think of every eventuality, which is why the fuel filler cap is often retained by a length of wire, but there have been cases of engine access covers coming loose in flight and inflicting grievous harm to the tail rotor. Helicopters are often flown with one or more doors open or removed, for film and television shooting, for example. In this case it is essential that everything within the machine is fixed, stowed or tethered. The helicopter with a high set tail rotor is at an advantage here, as debris is more likely to pass below it.
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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
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Rotors in practice 125 Fig. 4.6 (a)
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Rotors in practice 127 assembly tha
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Rotors in practice 129 Pitch change
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× T=c× D Rotors in practice 131 F
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Rotors in practice 133 In a real he
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Rotors in practice 135 Fig. 4.15 Va
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(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 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 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 236 and 237: Engines and transmissions 221 own t
Page 238 and 239: Fig. 6.16 The complex fuel system o
Page 248 and 249: Engines and transmissions 233 and c
Page 252 and 253: Engines and transmissions 237 Sensi
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Engines and transmissions 239 Fig.
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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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