The Art of the Helicopter John Watkinson - Karatunov.net

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Engines and transmissions 203 The capacitor is a vital part of the system, because without it the current would have nowhere to go when the points open, and most of the magnetic energy would be dissipated in a spark at the points, which would then be rapidly eroded away. The spark in the cylinder would then be very weak. This can happen if the capacitor becomes disconnected or fails open circuit. If the capacitor fails short circuit the points are bypassed, and no spark will be produced at all. This type of failure is rare. The HT wiring to the spark plugs needs to be well insulated so that the HT does not jump to the nearest piece of metal instead of going to the plug. At high altitude the reduced air pressure reduces the insulating ability of the air, and as a result the ignition leads must have somewhat better insulation than those found in cars. Part of the pre-flight check is an inspection of the condition of the HT leads. Needless to say they should not be touched when the engine is running. Whilst the magneto has the advantage that it is completely independent of the helicopter’s electrical system, it does suffer from one disadvantage. When starting, the engine speed is low, and the rate of change of flux in the magneto will be correspondingly low and a poor spark results. One solution to this problem is a device known as an impulse coupling which is basically a torsion spring in the magneto drive shaft. The presence of the permanent magnet makes the torque needed to rotate the magneto vary with the angle of rotation. This can easily be demonstrated by attempting to turn a bicycle dynamo by hand. At starting speed, when the torque is large, the spring in the impulse coupling is wound up as the magneto lags behind the shaft. As the torque falls, the spring tension is released, and the magneto is temporarily turned faster than the shaft. This allows a larger spark to be generated. A further use of the impulse coupling is that the lag delays the time at which the spark is generated. Under normal conditions, the spark is supplied a little before top dead centre (TDC) in order to allow the charge time to start burning before the power stroke. At low starting RPM, the advanced spark might cause the engine to kick back, or fire during the compression stroke. The delay of the impulse coupling prevents this happening. At normal operating speed the torsion spring does not have time to twist and the magneto runs at constant speed with the correct timing. Dual ignition systems are provided to increase the reliability of the engine. This reliability increase is only present if both ignition systems are working properly. Part of the pre-flight check is to test the ignition system. This is done using the ignition switch. As the magneto is self-contained, the only way it can be switched off is by shorting out the points. The ignition switch is connected across the points and the magneto is switched off by closing the ignition switch contacts. This is a safer approach since a broken wire from the ignition switch will not stop the engine. The ignition switch is constructed so that either or both magnetos can be switched on. In aircraft the ignition switch may have a further position to operate the starter, but this is not used in helicopters. Instead the starter button is fitted on the end of the collective lever where it may be operated by the pilot without letting go of the controls. When the engine is warm, the dual ignition may be tested. At a specified RPM, the ignition switch is turned from ‘both’ to ‘left’. This disables the right magneto. The result should be a slight reduction in RPM, known as a magneto drop, because only one spark is being generated per cylinder. The drop is normally that which is measured after 5 seconds running on one magneto. The engine is run on both systems again for a short time to clear any fouling which may have built up on the plugs whilst they were out of use. The switch is then set to ‘right’, when an identical drop should be obtained. If the drops are grossly different or do not occur, there is a problem which must be rectified before flight.
204 The Art of the Helicopter To take an example, if on switching from ‘both’ to ‘left’ there is no drop, either the right magneto was not working or it has not been switched off due to a broken wire. Switching to ‘right’ will show which. If the engine cuts, the right magneto is inoperative. If a drop is obtained, the right magneto is permanently on due to a wiring fault. This can be proved by switching off the ignition. If the engine continues to run, the wire to the ignition switch is broken. The engine can be stopped by the idle cut-off as normal. If the drops are not identical, one ignition system is working better than the other for some reason. If only an RPM loss is noted, the different drop could be due to a timing difference between the magnetos. If one magneto gives a larger drop and a misfire, consideration should be given to the possibility of a failed spark plug or capacitor or a fractured HT lead. The cylinders are numbered from what would be the front in an aircraft, which in a helicopter corresponds to being numbered away from the drive pulley. In a Lycoming engine mounted back to front in a helicopter, the odd numbered cylinders are on the port side. The sequence of power strokes is designed to reduce torsional stress in the crankshaft by firing cylinder pairs alternately. This gives the firing order of 1-3-2-4. 6.8 The starter The starter is basically a powerful electric motor used to turn the engine. A reduction gearing system is needed so that the starter can develop enough torque to overcome the engine compression. A small pinion on the starter engages with a large ring gear on the engine flywheel. The gears are only engaged during starting. This can be done by a helical thread mechanism that screws the pinion along the motor shaft or by a solenoid that slides it. In the latter type a one-way clutch prevents the engine driving the starter. Recently geared starter motors have become available. These have a high speed electric motor and a reduction gearbox and are lighter than conventional starters and consume less current. The weight saving may be compounded because they allow a smaller battery and thinner wiring to be used. The motor must be quite powerful, and it will require a current of several hundred amperes to operate it. In order to minimize power loss in the cables, these run direct from the battery to the motor. This current is beyond the capability of a switch, and a relay is used. The starter button on the collective lever switches a small current through a solenoid coil, and this attracts the solenoid armature, sliding the starter gear into engagement with the flywheel and pulling the main motor contacts closed. Normally the starter motor gear is driven through a one-way clutch so that it can run at high speed when the engine fires without damaging the starter motor. A warning light is fitted which should go out if the starter button is released. If it does not the starter may still be engaged. If the starter fails to operate, this could indicate a faulty solenoid. It is generally possible to hear the solenoid operating. If the solenoid chatters or vibrates this indicates a loose battery connection or a discharged battery. The starter motor dissipates a lot of heat when starting, and if the engine does not start promptly because it is flooded or vapour locked, prolonged cranking will flatten the battery and overheat the starter motor. It is better to allow a cooling period between starting attempts than to burn out the starter motor. The starter motor should never be operated when the engine is running as engaging the starter pinion with the fast turning gear on the flywheel could remove teeth. The engine must be allowed to come to a complete halt before making a further starting attempt.
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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.
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Rotors in practice 139 Fig. 4.18 Th
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Rotors in practice 141 swashplate a
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Fig. 4.23 A fixed-pitch tilting hea
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Rotors in practice 145 Fig. 4.25 Bl
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Rotors in practice 147 Fig. 4.27 Ef
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Rotors in practice 149 Fig. 4.29 (a
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Rotors in practice 151 concerned ar
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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
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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 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
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Page 214 and 215: Fig. 6.5 A typical horizontally opp
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Page 220 and 221: 6.9 The oil system Engines and tran
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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
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Page 238 and 239: Fig. 6.16 The complex fuel system o
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Page 252 and 253: Engines and transmissions 237 Sensi
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Page 262 and 263: Fig. 6.37 Displays which may be see
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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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