The Art of the Helicopter John Watkinson - Karatunov.net

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Other types of rotorcraft 371 the rotors are mounted so that they remain substantially in the same plane in translation. This allows the rear rotor to operate in relatively clean air. In order to keep the cabin level and to provide blade clearance, the rear rotor is mounted on a pylon. It is a characteristic of this approach that ground clearance of the front rotor disc is relatively small. However, the poor clearance exists directly in the pilot’s field of view. With no tail rotor to weathercock and a short moment arm, a large amount of fin area is usually needed and even then tandem rotor machines are aerodynamically marginal in yaw. This is not all bad because many operations require the machine to hover at an arbitrary orientation to the prevailing wind and the tandem does that very well. The rear pylon is always shaped to act as a substantial fin and the front pylon may have aerodynamic treatment to stop it acting as a fin. This may consist of irregularities designed to cause early flow separation when the relative airflow is off the fore-and-aft axis. Figure 9.23(b) shows that the resulting drag causes a smaller yaw component than if the pylon caused lift. Figure 9.23(c) shows that the rear rotor is affected by the front rotor vortices in translational flight. Effectively the rear rotor is operating with a greater inflow velocity than the front rotor. In a machine with a central CM, both rotors will need to produce the same amount of lift. To develop the same thrust the rear rotor will need to use a higher blade pitch and will absorb more torque because of the higher inflow. The swirl of the forward rotor also acts asymmetrically on the hull. For these reasons the torque is not precisely balanced in translation. One prototype tandem helicopter could not initially fly in a straight line in translational flight because these effects were more powerful than the yaw control. Once the nature of the problem was understood a solution was not far behind. There are ways to mitigate the problem. An obvious suggestion would be to move the CM forward. Whilst this would work, it somewhat negates the advantage of CM position freedom. In practice, the two rotors can be set at a different angle. Figure 9.24 shows that the front rotor shaft may be inclined forward with respect to the rear. This is done on the CH-47 in which there is a 5 ◦ difference between the angles of the shafts. This increases the inflow seen by the front rotor and decreases the lift component of its thrust, whereas the inflow of the rear rotor is reduced and the lift component of its thrust is greater. Another possibility is that the fin can be built with an integral camber so that in translational flight it produces a side thrust. The cambered fin of the CH-46 is obvious in a rear view. The interference between the rotors can be minimized by flying the machine sideways as this increases the diameter of the stream tube and improves the aspect ratio, Fig. 9.24 The front and rear rotor shafts may be inclined at different angles.
372 The Art of the Helicopter giving greater translational lift. This technique works well in a steep climb. The drag of the broadside hull will be a problem as speed builds up. However, some advantage can still be gained by flying with sideslip; 15 ◦ is a typical figure. Some piston-engine tandems used as plane guards during carrier operations were operated in this way as standard procedure. In a twin-engine machine, sideslip operation may be used to handle one-engine-inoperative conditions. The direction of sideslip is important as the downwash from the front rotor is not symmetrical. The tandem rotor market is one many manufacturers have stayed away from. In the UK, the most successful tandem was the Bristol Belvedere (Figure 9.25) designed by Raoul Hafner which started life mechanically as a pair of Sycamore mechanics with a synchronizing shaft and which subsequently evolved into a capable twin turbine. Yakovlev built a large tandem machine in the USSR but this was not successful. In the USA Bell built the HSL but it did not enter production. The name most closely associated with the tandem is that of Frank Piasecki whose early fragile canvas covered creations, dubbed ‘flying bananas’ from the bent hull needed for rotor clearance, matured into the enduring CH-46 and CH-47. The CH-46 is about as attractive as a tandem can get and when it was first built its good lifting capacity and amphibious capability made it a favourite with the US Navy who use it for vertical replenishment (VERTREP) which is military-speak for moving material from one ship to another suspended below a helicopter. The army liked the concept but wanted something with a bigger cabin and the CH-47 was born. The CH-47 Chinook is a superb piece of industrial design by any reference. The basic design dates from 1961 and because it is fundamentally well founded it is simply upgraded from time to time. It is amazingly versatile and genuinely amphibious. In other words it can operate normally from water rather than just floating in an emergency. The absence of a tail rotor means that it can land in scrub. Fig. 9.25 The Bristol Belvedere was a twin turbine military machine that was successful in service despite frequent fires caused by its absurd engine starting system. (AugustaWestland)
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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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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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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
Page 350 and 351: 8.7 Climbingand descending Helicopt
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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.
Page 368 and 369: Other types of rotorcraft 353 incre
Page 370 and 371: Other types of rotorcraft 355 and c
Page 372 and 373: Fig. 9.7 The Bell-Boeing Osprey til
Page 374 and 375: (a) (b) Other types of rotorcraft 3
Page 380 and 381: Other types of rotorcraft 365 a low
Page 382 and 383: Other types of rotorcraft 367 the y
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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