The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 151 concerned are positive. It was seen above that the dragging resonant frequency is proportional to rotor frequency. Thus as rotor frequency increases, the upper and lower whirling frequencies will be found symmetrically above and below rotor frequency. The horizontal line in the figure is the rocking resonant frequency of the hull. As such a rotor is started from rest, there will first be random padding due to the initial blade orientation. At a rotor frequency corresponding to point A, the upper whirling frequency coincides with the hull frequency. As the upper whirling coincidences are always stable, the hull acts as a vibration absorber and this will be noted as a lull in the padding. If the padding motion is noted, it will be seen to reverse phase after point A because the system has passed through a resonance. Further increase in speed makes the rotor frequency coincide with the hull frequency at B. Only if there is any mechanical imbalance in the rotor (a defect) will there be any response. Further increase in rotor speed results in coincidence between the hull frequency and the lower (backwards) whirling frequency. This is the coincidence responsible for ground resonance and the dampers will be working to prevent it. The padding may be worse at this rotor speed. Flight RRPM will be above C where rotation should be smooth. A ‘soft-in-plane’ rotor has no dragging hinges, but a relatively compliant blade root or hub so that there is a small restoring force even when the rotor is stationary. Consequently the dragging resonant frequency starts at a minimum value and increases with RRPM. Figure 4.31shows that when this characteristic is added to the rotor frequency, the forward whirling is always progressive and the frequency increases from the static value to become asymptotic to the value determined by centrifugal stiffening. Points A and B are stable as for the articulated rotor. However, the backwards whirling frequency is initially higher than the rotor frequency. The whirling is actually in the opposite direction to the rotor and so is regressive. If this regressive whirling coincides with a hull frequency, the result is stable because the criterion for ground resonance is not met. In regressive backwards whirling the whirling is backwards in both rotating and stationary co-ordinates and the phase reversal needed for instability is absent. However, when the rotor frequency in Figure 4.31becomes equal to the backwards whirling frequency at C, the whirling frequency becomes zero. At any higher rotor frequency the rotor frequency exceeds the whirling frequency and the backwards whirling Fig. 4.31 A soft-in-plane rotor has a regressive dragging region as far as point C at low RRPM, but this is not unstable.
152 The Art of the Helicopter Fig. 4.32 A supercritical rotor has such a high dragging frequency that the rotor can never reverse it. No damping is then needed. becomes progressive again. The frequency then rises to become asymptotic to the value determined by centrifugal stiffening. Point D in Figure 4.31corresponds to a coincidence between a hull frequency and a progressive backwards whirling frequency, at which ground resonance can occur. Consequently soft-in-plane rotors still require damping. A ‘stiff-in-plane’ or ‘supercritical’ rotor has a powerful dragging restoring force due to its construction and so has a significant dragging resonant frequency, even at rest. When the rotor is turning, the dragging frequency increases even further due to centrifugal stiffening. Figure 4.32 shows that, by definition, in the case of a ‘stiff-in-plane’ rotor, the whirling frequency is always higher than the rotor frequency. The behaviour of the forward whirling is benign as before. However, as the backwards whirling frequency is so high, the rotor frequency can never reverse it and so backwards whirling is always regressive in the case of a stiff-in-plane rotor. In that case the unstable combination of progressive backwards whirling can never occur and such a rotor can function with no damping at all. Two-bladed teetering rotors take advantage of this concept. They are built without dragging hinges to obtain supercritical dragging behaviour. There is a small possibility of a damper failure and so it is as well to know how to handle it. There are two ways to recover from incipient ground resonance, depending on the rotor speed at the time it happens. If sufficient revs are available to permit flight, the answer is to take-off immediately. With no ground reaction against the undercarriage, the hull rocking resonance cannot occur and the shaking stops immediately. An attempt at a gentle landing may succeed, but if not then a landing should be attempted on a soft or high friction surface which is more likely to absorb energy than tarmac. It is important that there is no groundspeed whatsoever at the moment of touchdown. If insufficient rotor revs exist for flight, the solution is to change the frequency of the excitation away from the hull/undercarriage resonance. This may be done by reducing rotor speed by cutting the throttle and using the rotor brake if fitted. 4.18 Air resonance Air resonance is a condition primarily relevant to helicopters having hingeless rotors. In articulated rotors the natural frequency of the blade about the lagging hinge is low
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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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Page 116 and 117: (a) (b) Introduction to helicopter
Page 118 and 119: 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 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 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
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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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