The Art of the Helicopter John Watkinson - Karatunov.net

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Other types of rotorcraft 355 and control systems then this is it. A tail-mounted pusher propeller for forward flight would be naturally complemented by a NOTAR system for hovering, leading to a very clean external appearance with low drag. 9.4 The convertiplane A convertiplane changes its operating principle according to the flight regime, most obviously between hover and cruise. The Fairey Rotodyne (Figure 1.11) is a convertiplane. This machine has two turboprop engines that in addition to driving variable pitch propellers can also produce significant amounts of compressed air. In the hover the compressed air is fed along the rotor blades and fuel is burned at the tips to drive the rotor. There is no torque reaction and yaw control is obtained using differential propeller pitch. The tip jet drive is inefficient, but is only used for take-off and landing. Once forward speed has been obtained, the rotor drive is switched off and the rotor is tilted back so the machine becomes a gyroplane. The rotor is partially unloaded by the wing. Landing is achieved by reigniting the tip jets and reverting to helicopter mode, although landing on a runway in gyroplane mode is also possible. The winged McDonnell XV-1 has a single piston engine driving either a propeller or an air compressor. In order to hover the compressor output is fed along the blades to tip jets as in the Rotodyne. No yaw control could be obtained with a single prop, and instead a pair of small fixed pitch tail rotors is installed, powered by small reversible hydraulic motors. The XV-1 can then take off as a helicopter. In forward flight the engine power can be diverted to the prop so that the machine becomes a compound gyroplane. The XV-1 has a substantial wing area and full conventional aircraft controls and in cruise these will become functional. The fore-and-aft cyclic control is then automatically adjusted to control rotor speed. This is reduced to 50% of the hover RPM, so that the wing is carrying about 80% of the machine weight. Thus the XV-1 can fly as a helicopter, a gyroplane or an airplane. Following the progression of unloading the rotor in forward flight, the next step in the view of some designers is to dispense with the rotor altogether. There have been numerous proposals. A two-blade rotor is stopped transversely in flight to become a wing, a four-blade rotor becomes an X-wing, a rotor folds and retracts into the hull. All of these proposals suffer from the same practical difficulty – as the rotor slows down it loses the centrifugal stiffening effect. As has been seen earlier in this book, as a rotor slows down it ceases to be gyroscopic and the phase lag starts to reduce from 90 ◦ . This causes a control problem. A more serious difficulty shown in Figure 9.6 is that at some stage a near-stationary rotor blade will have significant sweep forward and this is an unstable condition. In conventional wings and blades flutter is avoided by bringing the mass centroid towards the leading edge. However, at low rotor speed the reverse flow region would envelop the retreating blade such that leading and trailing edges interchange so this technique cannot be used. The blade and hub would need to be immensely rigid in torsion and bending to survive the loads. As a result no practical machine has emerged and it is unlikely that one will. The only in-flight rotor-folding technique that appears viable is to tilt the rotor backwards and to feather the blades. As the rotor slows down it will cone up 90 ◦ and ultimately stop with the blades trailing in the slipstream. However, if the rotor is going to be tilted, why not use it as a propeller?
356 The Art of the Helicopter Fig. 9.6 The flaw in stopped rotor proposals. As rotor speed falls, the reverse flow region becomes enormous and leading and trailing edges interchange. Tip jet convertiplanes are interesting and educational, but are unacceptably noisy as production items. More significantly, developments in fixed-wing STOL technology have delivered simpler solutions in many applications. Stopping the rotor in flight is simply too difficult. Consequently most of the recent progress made in convertiplanes has been in machines that tilt the rotors in some way to become propellers creating forward thrust. This is an elegant approach because there are no redundant propellers in the hover and no redundant rotors in cruise. A prop rotor or tilt rotor machine can take off like a helicopter, but by turning the rotors about a horizontal axis forward flight will be achieved. When the rotor axes are horizontal there will be no rotor lift at all and the wing must produce 100% of the lift. In this mode the tilt rotor is an aeroplane with large propellers. Compared to a helicopter, drag is reduced considerably and the top speed and range are significantly improved. The load factor also becomes that of an airplane and so the tilt rotor should be highly manoeuvrable. A rotor that provides enough hover thrust will be somewhat overspecified in forward flight and will be inefficient because it will work at a low L/D ratio with a high profile drag penalty. The solution currently adopted is to make the cruise more efficient by reducing the rotor size. This increases the disc loading in hover, and drives up the downwash velocity and the power required, but this power is not used for very long. It may be an advantage to use different RPM in the hover and in forward flight. With the rotor always in axial flow, the alternating forces experienced by the helicopter blade do not exist and the rotor does not need to be detuned against vibration. It is thus quite easy to change the rotor RPM for cruise. The ideal would be a variable diameter rotor and although the forces involved are considerable this is not considered insoluble. With high disc loading and a wing, the tilt rotor is not expected to have good autorotation performance and it would appear inadvisable to rely on a single engine. The Bell-Boeing Osprey (Figure 9.7) tilt rotor is a true helicopter in the hover. It has contra-rotating rotors that have cyclic and collective pitch. There are two engines, one in each wing tip nacelle. The entire engine and transmission tilts with the rotor. To give protection against engine failure the two rotors are connected by a cross shaft so that
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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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(a) (b) Introduction to helicopter
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4.1 Introduction 4 Rotors in practi
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(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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Page 342 and 343: Helicopter performance 327 Fig. 8.1
Page 344 and 345: Helicopter performance 329 to skid
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Page 348 and 349: Helicopter performance 333 Fig. 8.7
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Page 352 and 353: Helicopter performance 337 In the c
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Page 356 and 357: 8.10 Stability Helicopter performan
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Page 362 and 363: 9 Other types of rotorcraft Althoug
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Page 366 and 367: Other types of rotorcraft 351 Fig.
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Page 372 and 373: Fig. 9.7 The Bell-Boeing Osprey til
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Page 380 and 381: Other types of rotorcraft 365 a low
Page 382 and 383: Other types of rotorcraft 367 the y
Page 386 and 387: Other types of rotorcraft 371 the r
Page 388 and 389: Other types of rotorcraft 373 For e
Page 394 and 395: Acceleration: and force, 22-5 and v
Page 396 and 397: Convertiplane, 11-12, 355-8 Bell-Bo
Page 398 and 399: tanks, 219-20 turbine engines, 237-
Page 400 and 401: Lockheed flybar system, 309-13 AMCS
Page 402 and 403: centrifugal stiffening, 71, 100 and
Page 404 and 405: tail/main rotor interaction, 173 te
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