The Art of the Helicopter John Watkinson - Karatunov.net

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Introduction to helicopter dynamics 113 Fig. 3.42 In an active anti-vibration system, the transmission is mounted on actuators that are driven by a suitable forcing function to cancel the vibration. require increased static pressure in the actuators to prevent them extending. In practice a feedback mechanism measures the average extension of the actuator and centralizes each one with a relatively long time constant. No practical hull can be rigid, and so there will be various modes of vibration excited in it. One of the sources of vibration will be due to the non-uniform downwash impinging on the hull surface. Conventional isolators between hull and transmission can do nothing about this because they only oppose structure-borne vibration. However, an active system can use the transmission as a reaction mass to create forces opposing vibration from any source. Unfortunately the amplitude of vibration in the transmission will be increased. Better results may be obtained by applying active control to a raft carrying the engines, etc. as described above. 3.28 Harmonic pitch control It was seen in Figure 3.30 that the use of sinusoidal feathering in a cyclic pitch control is incorrect. The airspeed seen by a blade element changes sinusoidally in forward flight, but the lift is proportional to the square of the airspeed. This non-linear function generates flapping harmonics that cannot be cancelled by sinusoidal feathering. It follows immediately that if there are to be no flapping harmonics, the feathering function must contain harmonics. A harmonic cyclic pitch waveform is approximately a square root function of a raised sine wave although some modification will be necessary because of inflow effects. When multiplied by the sine-squared function due to the speed, the product, namely the lift, is constant. The pitch function above will need further refinement to account for upwash at the leading edge of the disc. Figure 3.43 shows how an actuator between the swashplate and the blade control arm could add the harmonic control. In practice perfect constant lift can only be achieved at one radius, but it should be possible to find a pitch function that makes the average blade lift moment essentially constant. A more advanced system would have means to control the angle of attack of different parts of the blade independently. This may be possible using, for example, piezo-electric actuators. It is impossible to obtain constant lift over the whole blade because of the reverse flow area, and so some harmonic flapping will always remain. Whilst theoretically obvious, harmonic blade control, also known as higher harmonic control, is rarely found. One of the difficulties is that the cyclic control becomes a
114 The Art of the Helicopter Fig. 3.43 In harmonic pitch control, the blade pitch is not sinusoidal, but has a waveform calculated to make the lift more uniform around the disc. This may be controlled by an actuator between the swashplate and the blade pitch control arm. function of airspeed, returning to a near sinusoidal function in the hover. This is probably beyond what might be achieved mechanically but some kind of electronic system would easily generate the required functions. High speed actuators are necessary to drive the blade feathering, and the blades must be torsionally stiff in order to respond to the rapid pitch variations. Another consideration is that as the number of blades increases the effects of lift troughs are averaged out more and there is less of a problem. However, the elegant result of constant lift moment around the disc is one that is most desirable. The reduction in vibration makes the flight more comfortable for passengers and prolongs the life of the machine. A further consequence of constant lift is a reduction in power needed in forward flight. In a conventional rotor the increased power needed where the lift is greatest is not balanced by a reduction in power where the lift troughs are. Consequently constant lift needs the least power. It would seem clear that the greatest cost benefit of harmonic blade control would be in the two-bladed helicopter where the 2P hop could be eliminated. The improvement in ride would be greatest but the number of actuators needed is least. In some light helicopters a two-bladed harmonically controlled rotor could replace a conventional three-bladed rotor and the overall cost might be the same. At some point designers will find ways of economically achieving the goal of constant lift. Some experiments have been done with systems which rock a conventional swashplate, but this cannot give the required result as each blade needs to be individually controlled. 3.29 Blade design The helicopter blade is a complex subject because of the number of interacting variables involved. These will be considered here. It might be thought that the aerodynamic characteristics of the blade would be determined only by shape. In general this is untrue. Figure 3.44 shows some of the issues. The long thin structure of a practical blade can never be rigid and flight loads will cause the blade to flex. However, it is not enough to know the stiffness of the blade to calculate the result. In forward flight the loads have a powerful alternating component and this generates harmonics. At the frequencies of the higher harmonics the motion may be mass controlled. A further complexity is that the flexing of the blade will affect the aerodynamics; a phenomenon
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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
Page 78 and 79: Introduction to helicopter dynamics
Page 100 and 101: (a) (c) (b) Introduction to helicop
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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 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
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Rotors in practice 163 Abrasion is
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Rotors in practice 165 There are so
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eing more visible to ground personn
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otates the rod in the screw thread.
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In the case of a rotor having offse
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The tail rotor designer is faced wi
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Fig. 5.5 A right-side wrong-directi
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centre of mass. The solution was to
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safe to start the rotors. However,
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Fig. 5.10 Tail plane locations: (a)
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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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