The Art of the Helicopter John Watkinson - Karatunov.net

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(a) (b) (c) (d) Introduction to helicopter dynamics 89 Fig. 3.22 The reverse flow region can be shown to be circular. See text for details. Its diameter is determined by the ratio of the forward speed to the tip speed, which is the advance ratio µ. is given by the distance D from the shaft multiplied by the angular speed ω. When the blade is at 270 ◦ , the edge of the reverse flow region will be where ωD equals the airspeed V . In (b) the method for finding what happens at an arbitrary blade angle is shown. At any distance d from the shaft, the vector resultant R of ωd and V can be found. Figure 3.22(c) shows the derivation of the expression for d, the distance from the
90 The Art of the Helicopter shaft to the boundary of the reverse flow region. Figure 3.22(d) shows the expression for d graphically and it will be seen that the reverse flow area is circular and that its diameter increases with speed. Instantaneously the rotor is turning with respect to the outer edge of the reverse flow region. The ratio of forward speed to tip speed is known as the advance ratio µ which is the same as the ratio of the diameter of the reverse flow region to the blade radius. The retreating blade suffers reverse flow near the root; a small downward force may be experienced because the retreating blade effectively has a negative angle of attack due to flow reversal. Here is another reason for root cut-out, to reduce the amount of useless reverse-flowed blade. However, as the area is proportional to the square of the diameter, the amount of disc area lost to the reverse flow region is small. For example, at µ = 0.4, the diameter of the reverse flow region is 0.2 of the rotor diameter but the area lost is only 4%. Owing to the huge difference between the airspeeds, if nothing were done, the advancing blade would produce more lift and the retreating blade less. This would apply to the rotor a roll couple toward the retreating side. The gyroscopic action of the rotor would result in a response delayed by 90 ◦ of rotation corresponding to a rearward pitch effect. This rearward couple is colloquially known as ‘flapback’ and it tends to reduce the forward tilt of the machine and consequently reduces the forward component of the rotor thrust, slowing the machine down. Thus sustained forward flight requires a continuous application of forward cyclic control that reduces the angle of attack of the advancing blade and increases that of the retreating blade in order to balance the lift moments on the two sides of the rotor. In cruise the amount of cyclic pitch applied may be of the order of 5 ◦ . Flapback results in speed stability because it automatically opposes the cyclic control. The more forward cyclic is applied, the higher will be the forward speed. Thus in translational flight the fore-and-aft position of the cyclic stick primarily controls the airspeed. If the airspeed is changed, the collective lever will need to be adjusted to maintain height because tilting the rotor thrust vector alters its vertical component. In forward flight the rotor has access to a greater mass of air. Thus the same momentum increase and the same thrust can be obtained with a smaller induced velocity and this reduces the power needed. Figure 3.23 shows that there is a minimum induced power speed where the inflow velocity is the least. 3.21 Inflow and coning roll In flight, there is a region of low pressure above the rotor and a region of high pressure below. This pressure difference results in a tendency for air in the plane of the rotor to move upwards. Figure 3.24(a) shows that this happens for some distance from the edge of the disc. Simple inertia keeps the air moving upwards so that in translational flight, the leading edge of the disc encounters upwash. The upwash ceases when the downward impulse of the rotor cancels the upward momentum of the air mass. In slow forward flight, Figure 3.24(b), the low pressure above the rotor has time to bend the inflow increasingly downwards as it reaches the trailing edge of the disc. The disc can be thought of as a low aspect ratio wing producing tip vortices which roll inward at the top and down through the rear of the disc. The result is a reduction of the angle of attack of the blade at the rear of the disc compared to that at the front. The loss of lift at the rear of the disc is subject to the phase lag, or precession, of the rotor and manifests itself as a roll to the advancing side, which is known as inflow roll.
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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
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 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.
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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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