The Art of the Helicopter John Watkinson - Karatunov.net

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Other types of rotorcraft 365 a lower blade working in the downwash of an upper blade is offset by swirl recovery. The lateral separation of the rotors means that the disc area is greater than the area of a single rotor and this reduces the disc loading and the induced power needed, although some of the advantage is lost because the rotor thrusts do not align. The horizontal components of the rotor thrusts are in opposition and some power is wasted. In forward flight the rotor separation gives a better disc aspect ratio. From a performance standpoint, the twin rotor heads cause a drag penalty and the synchropter is not appropriate for high speed. However, as the synchropter is practically limited to two-blade rotors, it naturally suggests low solidity and disc loading which is the ideal for low speed, high altitude work or heavy lifting. The synchropter is controlled in a similar way to the coaxial helicopter in that differential collective pitch is used as a yaw control. This will be subject to reversal in autorotation as for the coaxial helicopter. Some yaw control is also possible by the use of differential fore-and-aft cyclic, although the short distance between the rotor heads makes this ineffective. The usual fore-and-aft cyclic control affects both rotors equally, but the lateral cyclic may be adapted so that the discs tilt outwards more than they tilt inwards in order to preserve clearance between the heads and the blades. Generally synchropters have poor yaw control and require a large amount of fin area. The outward tilt of the rotors means that the torque cancellation is not perfect. It can be seen from Figure 9.16 that although the torque cancels in the vertical axis, there is a component of rotor torque in the horizontal axis which is the same for both rotors and therefore adds. If the synchropter is arranged to have the advancing blades on the inside, the torque reaction will tend to pitch the hull nose up until the CM is far enough ahead of the masts to counter the torque. In the hover this will require the application of forward cyclic so that the rotor thrust remains vertical. In forward flight the forward CM is a useful stability aid. As forward speed builds up, the drag on the hull will act below the rotor heads and produce a couple tending to tilt the hull nose down. The rotor torque will counter this. Consequently there is a correct way for synchropter rotors to turn, i.e. with the advancing blades on the inside. The Flettner 282 Kolibri (Figure 1.4) was not only the world’s first synchropter, it was also the world’s first production helicopter, beating Sikorsky’s R-4 by several months as well as being technically superior. The Kolibri had very closely meshed two-bladed rotors. Figure 9.17 shows that these were fully articulated with friction disc dragging dampers. There was substantial flapping hinge offset so that the rotor blades would not flap into contact with the opposite head. This allowed space for the feathering hinge to be mounted inboard. The swashplates and pitch links were a compact and elegant arrangement. A 160 hp BMW radial aircraft engine mounted in the conventional aircraft attitude drove the transmission through an inclined shaft. The foot pedals Fig. 9.16 In the synchropter there is a component of torque along a horizontal axis.
366 The Art of the Helicopter Fig. 9.17 The transmission and rotor heads of the Flettner Kolibri. (Steve Coates) operated the rudder as well as the rotor yaw mechanism. The pilot could also change the angle of incidence of the tail plane. The Kolibri was an advanced machine; it had adequate power and was highly manoeuvrable. In a mock dogfight with a Focke-Wulf 190 the fighter was unable to get the Kolibri in it sights. Following the end of the war, a significant amount of advanced German technology found its way to various allied countries. In the USA, Charles Kaman recognized the potential of the synchropter principle and combined it with the servo-tab control system to produce a highly successful series of machines. The Kellet Co. made some synchropters with three-bladed rotors but these had blade contact problems and did not reach production. Kaman’s early synchropters were piston engine powered, culminating in the HOK-1 into which was shoehorned a 600 hp radial aircraft engine, which still wasn’t powerful enough for some purposes. The piston engine simply isn’t suitable for helicopters above a certain size. The world’s first turbine engine helicopter was a Kaman synchropter, as was the world’s first twin turbine helicopter. Kaman’s most well-known synchropter is the H-43B, aka Husky. It was also known as the Pedro, after a call sign originally used at a Texas air base. The Husky was basically a redesign of the piston engined H-43A using a turbine and it became a classic piece of industrial design. The compact turbine engine was placed on the roof behind the transmission, resulting in a large, open cabin accessed by rear clamshell doors as well as sliding doors at the side. The servo-tab control system meant that the forces needed from the pilot were quite low even though there was no power assistance. The yaw control was aided by relatively wide rotor head spacing and an automatic yaw reverser to make the pedals work in the correct sense in autorotation. The Husky also had an impressive array of vertical fin area to maintain directional stability when
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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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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 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 382 and 383: Other types of rotorcraft 367 the y
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Page 394 and 395: Acceleration: and force, 22-5 and v
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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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