The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 135 Fig. 4.15 Various techniques used to allow feathering despite the root tension in the blade. (a) A torsion bar resists the axial pull. (b) Thin steel shims may be laminated to produce a flexible structure. (c) Strap made from strands of wire. (d) Elastomer under compression allows rotation in shear, but bulges under load. (e) Metal shims between layers of elastomer prevent bulging. (f) A practical elastomeric thrust bearing which is failsafe because the blade cannot come off if the bearing fails. so the interior of thick components is unstressed. This is why tubes are used instead of solid bars. A given cross-section of material will be more rugged if it is divided up into many slim elements in parallel and in addition will be easier to twist. This division into many elements also gives a degree of protection against failure because some elements can fail without the whole assembly failing. The blade may be attached by laminated steel shims shown at (b) which can twist as the pitch is changed. PTFE tape may be placed between the shims to allow one to slide over the next. A variation in the idea is used by Bell on the JetRanger. Here the tensile element is a strap made from hundreds of strands of steel wire (c) like the cable in a suspension bridge. The blade is guided by a sleeve bearing that only takes bending loads.
136 The Art of the Helicopter Enstrom use an elastomer in compression to retain the blade. Elastomers are virtually incompressible but flexible synthetic compounds. Figure 4.15(d) shows that if a block of elastomer is placed under compressive stress, it will bulge at the sides. If the bulging is serious, the strength of the material will be exceeded and it will tear. Bulging would also allow the blades to move away from the mast, causing balance problems. Figure 4.15(e) shows that this problem is overcome by interleaving thin shims of metal with layers of elastomer. The metal shims prevent bulging under compression, but have little effect on the ability of the unit to twist so that pitch changes can be made. As the diagram shows at (f), the blade is guided by sleeve bearings, but the elastomer takes the thrust. Note that the system is failsafe because the bearing is in compression. Even if the bearing begins to disintegrate, the blade cannot come off. In the Enstrom only the feathering axis is elastomeric. Aerospatiale pioneered a spherical elastomeric laminated bearing which allows full articulation in addition to feathering. 4.13 Pitch control There are a number of mechanisms suitable for pitch control. No two designs have exactly the same control linkages, but the diagrams here are representative of actual practice. One of the best ways of appreciating how these systems work is to have someone move the controls of a helicopter one at a time whilst observing how the linkages move. The most common control system is the sliding swashplate shown in Figure 4.16(a). The swashplate is made in two halves. The lower part is fitted with a spherical bearing so that the swashplate can tilt to any angle in any direction. The sphere can also slide up and down, often on the outside of the gearbox. A large diameter ball race is fitted between the two halves of the swashplate. A pair of jointed scissors links is fitted. One of these ensures that the lower half of the swashplate cannot revolve and the other ensures that the upper part turns with the rotor shaft. Each rotor blade grip is fitted with a pitch-operating arm. The end of each arm is connected to the periphery of the swashplate by a pushrod fitted with spherical bearings. Movement of the collective lever raises or lowers the swashplate, and this causes all of the pitch operating arms to rotate all of the blades about their feathering axes by the same amount. In addition, movement of the cyclic stick in two axes causes the swashplate to tilt in two axes. Simultaneous lifting and tilting of the swashplate by two controls requires a device called a mixer, an example of which is shown in Figure 4.16(b). One end of the mixing lever is pivoted on the helicopter frame or on the gearbox and is connected to the collective lever by a rod. The opposite end is fitted with a bell crank operated by one axis of the cyclic control. The cyclic pitch changes take place differentially around the average pitch determined by the collective setting. Figure 4.16(b) shows only one dimension of the cyclic mixing for clarity. In practice the cyclic control works in two axes and two bell cranks are fitted to the mixing lever. These bell cranks are connected to the swashplate by vertical rods and to the cyclic stick by horizontal rods. Moving the collective lever moves both bell cranks bodily up and down together, and so the swashplate rises and falls. Moving the cyclic stick rocks the bell cranks and tilts the swashplate about the collective setting in any desired direction. Figure 4.17 shows an alternative arrangement known as a spider. The spider has radial spokes, one for each blade. Pushrods connect the ends of the spokes to the pitch operating arms. The hub of the spider is spherical so it can tilt and slide within the shaft and it is fitted with a vertical post passing down through the centre of the shaft
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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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Page 100 and 101: (a) (c) (b) Introduction to helicop
Page 102 and 103: Introduction to helicopter dynamics
Page 104 and 105: (a) (b) (c) (d) Introduction to hel
Page 106 and 107: Fig. 3.23 Conditions for hover and
Page 110 and 111: (a) (b) Introduction to helicopter
Page 116 and 117: (a) (b) Introduction to helicopter
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 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 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
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Fig. 5.14 (a) Main rotor lateral ro
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long. The cross-section of the duct
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drag. However, the slots are emitti
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6 Engines and transmissions The pow
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Engines and transmissions 193 the D
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(a) (b) Engines and transmissions 1
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(a) (b) (c) Engines and transmissio
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Fig. 6.5 A typical horizontally opp
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Engines and transmissions 201 the v
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Engines and transmissions 203 The c
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6.9 The oil system Engines and tran
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Engines and transmissions 207 (non-
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(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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