The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.47 The Lockheed gyrobar system in which each blade has its own flybar. See text for details. disc should tilt for any reason, the inter-axis coupling causes the feathering bearing to turn slightly, and a couple is applied to the gyrobar. If a given blade is imagined to rise, an upward couple will be applied to its gyrobar. The gyrobar will then precess by 90 ◦ causing the blade pitch to increase 90 ◦ after the stimulus. As the blade is also gyroscopic, the blade would respond by rising a further 90 ◦ after the stimulus. As a result the blade responds at 180 ◦ to the stimulus and is thus stable. Cyclic control is obtained by applying couples to the gyrobar using the swashplate. The inertia of the gyrobar means that it will control the following rate of the main rotor. In 1961 a ring gyro having direct control of blade cyclic pitch was fitted below the rotor. A collective pitch mechanism was also fitted so that constant RPM could be used. In this form the CL-475 was widely demonstrated and achieved a reputation for stability that represented the state of the art at the time. The machine still exists with the ring gyro and is at Fort Rucker at the time of writing. Subsequently the Model 286 (XH-51) was developed. This hingeless machine initially had three blades, but changed to four blades again for vibration reasons. The ring gyro is replaced by a rigid four-legged flybar, again directly coupled to the cyclic pitch arms of the rotor blades such that the flybar axis forms the control axis of the main rotor. Gyroscopic rigidity of the flybar means that any unwanted disturbance of the main rotor attitude due to gusts is automatically opposed. The Lockheed system achieves cyclic control by applying control couples to the flybar via the swashplate to make it precess in the required direction. The cyclic control system is unlike that of any other helicopter because it provides forces, not displacements. This is no more than a scaled-up version of the use of torque motors to erect gyroscopic instruments as was seen earlier in this chapter. Figure 7.48 shows that couples are applied to the flybar by a system of springs. Each of the control runs, pitch and roll, contains a pair of springs, one positive and one negative. With the cyclic stick at neutral, the geometry is such that if the swashplate moves because the flybar has tilted, one spring is compressed and the other extends Control 311
312 The Art of the Helicopter Fig. 7.48 In the Lockheed system the flybar is caused to precess by the application of couples. A double-spring arrangement allows the precession without interference from the other cyclic axis. See text. by a similar amount. The springs will balance each other over a significant range of movement of the flybar. Thus if the flybar tilts it will not experience a restoring force from the controls. However, if the controls are displaced from neutral, the spring imbalance is upset and the result is a couple applied to the flybar. Control is achieved as follows. If the pilot wishes to roll the machine, this can only be done if the flybar is made to roll. As the flybar is gyroscopic, it can only be made to roll by applying a couple in the pitch axis. Consequently the controls are rigged such that the roll cyclic pushrod applies a pitch couple to the flybar through the spring system. The flybar will sense this couple and correctly precess with a 90 ◦ lag to perform the commanded roll. The flybar is allowed to precess in this way by the balanced springs in the pitch pushrod. The hingeless model 286 was aerobatic and performed loops and rolls with ease, whilst remaining easy to fly. Essentially the same control approach was adopted in the first models of the Lockheed Cheyenne compound helicopter (see Chapter 9). The high speed of this machine increased the loads that the rotors fed into the gyro through the pitch links, undermining gyro authority. The solution was the system shown in Figure 7.49. Lockheed called this the AMCS (advanced mechanical control system). Here the gyro has been moved inboard, below the transmission. The absence of an external flybar reduces drag. The gyro is fitted with a swashplate through which it controls the mechanical inputs to hydraulic actuators. These provide an irreversible control over the cyclic pitch so that pitch link forces from the blades cannot reach the gyro. The hydraulic actuators operate a spider that passes through the centre of the gyro and the transmission and emerges above the rotor head where it operates pitch links. The gyro is supplied with couples by the pilot’s cyclic control using the positive and negative balanced spring system as before. These are not shown in the diagram. Having eliminated pitch link loads with the hydraulic actuators, the gyro is now made to follow the hull attitude using springs attached to the mast. Lockheed’s AMCS represented the ultimate mechanical helicopter gyrostabilizer, but even as it was being tested, systems were being developed using miniature gyros and electronic amplification and the future of helicopter stabilization would lie in that direction. Flybars are still found in flying model helicopters. Models are harder to fly than the full size and the flybar allows a welcome degree of stability for the pilot who won’t have
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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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Page 306 and 307: Fig. 7.27 Transducers used in signa
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Page 312 and 313: Fig. 7.33 The false codes created i
Page 314 and 315: Fig. 7.35 In a pure binary system,
Page 316 and 317: Fig. 7.38 Producing two’s complem
Page 318 and 319: Fig. 7.40 A power-assisted hydrauli
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Page 342 and 343: Helicopter performance 327 Fig. 8.1
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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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Other types of rotorcraft 361 Fig.
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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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