The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 161 Fig. 4.35 (Continued) (d) Bell 412 head having elastomeric drag dampers. Flexural members bend to allow flapping and twist to allow pitch change. (e) Aerospatiale spheriflexhead uses spherical elastomeric bearing for flapping, dragging and pitch change. (f) Bell 680 head using flexible elements inside a stiff blade cuff that transmits pitch control and connects to the drag damper. (g) Brantly has conventional flapping and feathering bearings, but dragging bearing is outboard making dragging supercritical. to the swashplate. When the blade drags back, the inboard end of the cuff will move forwards. Thus the drag damper is placed between the inboard end of the cuff and an arm attached to the mast. The hollow cuff approach gives a very clean head. Figure 4.35(g) shows the rotor of the Brantly (now Hynes) helicopter. This has conventional offset flapping hinges, but the dragging hinges are placed part way down the blade. This large drag hinge offset makes the rotor supercritical and no dampers are required.
162 The Art of the Helicopter 4.21 Blade construction The rotor blade has an arduous life being subject to considerable and time-variant forces which in the early days made that life all too short. The construction of rotor blades has been the subject of numerous advances directed towards increasing blade life. Early blades were built up rather like the wings of aircraft. Blades must have a certain weight both to store energy in the case of engine failure and to prevent an excessive coning angle. High torsional stiffness is also required. These requirements were initially met by building the blade around a steel tube. Collars were attached to the tube and these supported wooden ribs. A wooden leading edge was attached to the ribs and this would contain a metal insert to bring the mass centroid of the blade forward. The leading edge of the blade as far back as the spar might be covered with thin plywood. The whole was then covered with canvas and doped. Such construction lent itself readily to the incorporation of blade taper and twist. Machines having such blades had to be hangared because the blades were porous and moisture absorption could cause them to go out of balance. Such blades were of no use in tropical countries where the glues used could not withstand heat, humidity and bacterial/insect attack. The solution was to make all-metal blades. Typically the leading third of the chord was a heavy D-shaped extrusion with the remainder of the chord made up of thin sheet metal, perhaps supported by aluminium honeycomb. Initially riveting was used; later advances in adhesives allowed bonded structures. Metal structures are subject to fatigue, particularly given the alternating loads experienced by the helicopter blade. In some blades a warning of cracks was given by pressurization of the blade and fitting a pressure indicator. In other cases the blade or spar was evacuated and a vacuum indicator was used instead. If the indicator reading was incorrect this might indicate an incipient crack or bonding failure. The use of extrusions made it difficult to incorporate blade taper. As the military helicopter developed, consideration was given to resisting battle damage by designing redundant structures. In civilian applications a redundant structure can have a longer service life because a failure is not catastrophic. Redundant structures work by providing parallel paths for forces. In a rotor blade the single spar can be replaced by a series of smaller parallel box spars. The box sections may be of glass fibre which give support against buckling to steel spars between them. In some cases the spars can be made entirely of glass fibre, but it will usually be necessary to incorporate a metal strip bonded into the leading edge to give the correct mass centroid. The trailing edge of the blade is relatively lightly loaded and it is advantageous if it is not too stiff in bending in comparison with the leading edge because this combination results in the blade washing out as it bends up; a stable condition. Generally a thin outer skin is used which is supported against buckling by a low density filling material such as alloy or composite honeycomb, foam plastic or end grain balsawood. In some machines the trailing edge skin is deliberately weakened by chord-wise slits that allow the blade stiffness to be defined by the spar. The trailing edge of the blade is a relatively non-critical structure and many machines have returned safely following substantial trailing edge damage. As the conditions in which machines can operate are extended, it becomes necessary to protect against lightning strikes. The result of a strike is massive energy dissipation in any material presenting electrical resistance to the current path. The means of protection is to make the outside surface of the blades electrically conductive and to bond the blades together electrically. In this way potential differences across the rotor are minimized along with the damage.
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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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Page 126 and 127: Introduction to helicopter dynamics
Page 132 and 133: 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 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 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
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Engines and transmissions 211 a hot
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Engines and transmissions 213 Fig.
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Engines and transmissions 215 In wa
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Engines and transmissions 217 is a
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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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