The Art of the Helicopter John Watkinson - Karatunov.net

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Introduction to helicopter dynamics 71 Fig. 3.8 (a) When the blade pitch is increased, parts of the blade are brought closer to the axis of rotation. The blades will tend to be thrown to flat pitch when the rotor is turning. (b) So-called Chinese weights are mounted so that they oppose the effect of (a) by moving away from the axis of rotation as pitch is increased. down the rotor. Another possibility is for the rotor head to incorporate centrifugal weights that only apply an upload to the collective when the rotor is turning. 3.7Rotor coning Anyone who has examined a parked helicopter will immediately notice how droopy the blades are. A considerable deflection can be obtained by lifting the tip with one finger. How, then, can the blades lift the weight of the helicopter? The answer is that the rapid rotation in conjunction with the mass of the blades causes them to be pulled out straight; a phenomenon called centrifugal stiffening. Rotating blades are not in static equilibrium and they must be made to accelerate towards the shaft if they are to follow a circular path, and this requires a considerable inward or centripetal force, which will always be an order of magnitude more than the weight of the helicopter. Figure 3.9(a) shows that if the blade bends upwards, the downward component of the centripetal force will balance the lift at some coning angle, and the resultant will be a horizontal force only so no further bending takes place. The reaction of the blades at the rotor head is shown in Figure 3.9(b). The force from the coning blade has an upward component, which is the lift, and an outward component due to the rotation. In the hover, if the blades are properly balanced and all have the same coning angle, the horizontal forces cancel in the rotor head and only lift results. Adjusting all of the blades to the same angle is achieved using the process of tracking which ensures that the collective control applies exactly the same pitch to each blade. Clearly, if the blades
72 The Art of the Helicopter Fig. 3.9 The blades must be accelerated inwards to make them rotate. At (a) the blades cone upwards until the resultant of the lift and the blade tension is perfectly horizontal. At (b) the force balance at the rotor head is shown. The tension in the upwardly coned blades cancels in the horizontal direction leaving only a vertical component to balance the weight of the machine. are not balanced or not tracking, vibration will result. Accurate blade balancing and tracking is important and the necessary techniques will be considered in section 4.19. 3.8 Torque and thrust in rotors Whilst resolving the airfoil reaction into lift and drag is useful for fixed wings, it is less useful for helicopters because of the feathering action of the blades. Figure 3.10 shows how the reaction on a rotor blade can be resolved in a more useful fashion. As the only real force on the airfoil is the reaction, resolving the force into components is only taking place in our imagination, so we can resolve into whatever directions we find useful. In helicopters, whatever the mechanical pitch angle of the blade, and whatever angle of attack results, it is more convenient if the reaction is resolved into the rotor thrust acting substantially at right angles to the tip path plane or rotor disc, and the rotor drag acting in the tip path plane. The angle of attack is the angle by which the pitch angle exceeds the angle of the RAF. The reaction of the airfoil is tilted further back in the presence of inflow, so that a larger component of the reaction is opposing the thrust delivered by the engine, and power is consumed simply driving against it. The rotor thrust is also slightly reduced by the tilt of the reaction. The hover out of ground effect (HOGE) is a particularly power consuming exercise. Inflow tilts back the blade reaction to oppose the engine, so that more torque is required to drive the rotor and more tail rotor power will be needed to balance the torque. HOGE is also one of the worst situations in which to lose power as the high rotor drag will reduce RRPM more before the pilot reacts and lowers the collective lever. Clearly increasing the collective pitch will increase the induced drag, and so more power will be required to maintain rotor speed. In simple piston engine helicopters, the end of the collective lever is fitted with a twist-grip connected to the throttle. Maintaining RRPM is then the responsibility of the pilot who will need to monitor the rev. counter. In more sophisticated piston engine machines and in all turbine machines automatic rotor speed governing systems or throttle correlators are used. The pilot
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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
Page 36 and 37: Introduction to rotorcraft 21 a rot
Page 38 and 39: Technical background 23 Fig. 2.1 At
Page 40 and 41: Fig. 2.3 Effect of force on velocit
Page 42 and 43: C Force A E Resultant (a) Force B (
Page 44 and 45: (a) (b) Technical background 29 Fig
Page 46 and 47: Technical background 31 Fig. 2.11 A
Page 48 and 49: Technical background 33 Fig. 2.12 (
Page 50 and 51: Technical background 35 If the volu
Page 52 and 53: Fig. 2.16 The definition of a radia
Page 54 and 55: Technical background 39 force would
Page 56 and 57: Technical background 41 Figure 2.20
Page 58 and 59: Technical background 43 force where
Page 60 and 61: Technical background 45 the cosine
Page 62 and 63: Technical background 47 Fig. 2.28 F
Page 64 and 65: Technical background 49 Fig. 2.30 T
Page 66 and 67: Technical background 51 centripetal
Page 68 and 69: 2.17 The gyroscope Technical backgr
Page 70 and 71: Technical background 55 of oscillat
Page 72 and 73: Technical background 57 the inertia
Page 74 and 75: Technical background 59 shut down a
Page 76 and 77: 3 Introduction to helicopter dynami
Page 78 and 79: Introduction to helicopter dynamics
Page 100 and 101: (a) (c) (b) Introduction to helicop
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.
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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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Rotors in practice 153 and lag damp
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Rotors in practice 155 disappears a
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(d) (e) (f) Rotors in practice 157
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Rotors in practice 159 hull and a v
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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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