The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.1 (a) A minimal control loop in which the conditions are sensed from simple instruments and the view from the canopy. (b) Power assistance is provided for the controls, making possible the use of a stability augmentation device. Put this way, the pilot’s task sounds onerous, but it need not be so. In a simple light helicopter in good conditions this process is extremely enjoyable. However, many helicopters have to operate in poor conditions, and in addition to flying the machine the pilot may be required to perform other tasks associated with the mission. The goal of the designer may then be to make flight possible in all weathers, and/or to keep the pilot’s stress or workload down to an acceptable level for the expected mission. The pilot’s assessment of the machine’s attitude is assisted by the provision of instruments such as an artificial horizon and a direction indicator. These will allow the pilot to maintain the machine’s attitude despite poor visibility and are essential if the machine is to be flown under instrument flight rules (IFR). The addition of powerassisted controls to Figure 7.1(a) will beneficially reduce the pilot’s physical workload on long flights. IFR instruments improve the quality of information presented, but the stability of the machine is still down to the continuous concentration of the pilot. Flying a simple helicopter on instruments is possible, but difficult. Two pilots might be required, one to fly, the other to handle navigation and communications. Control 259
260 The Art of the Helicopter Once powered controls are available, it is then a small step to make them accept signals from an automatic stabilizing system that can reduce the pilot’s mental workload. The stabilizing action of the pilot is augmented or even replaced by a signal processor programmed to respond to disturbances in the way a skilled pilot would. However, the signal processor has no eyes and needs information about the flight conditions in the form of input signals. Figure 7.1(b) shows that the aircraft instruments no longer just have a visual indication, but also output signals representing the current condition. It will also be seen in Figure 7.1(b) that the outputs of the signal processor can act upon the powered controls. In such a machine there are two parallel control paths. Figure 7.2(a) shows that the control of the machine can be entirely by the pilot, entirely by the signal processor, or by a combination of both, with the possibility that the amount of contribution from each control path may also change throughout the flight. This must be the source of further complexity. It is fundamental to negative feedback that there can be only one overall feedback loop in a system. Two negative feedback loops around the same process will fight each other. For example, if the outputs of the pilot’s stick and the processor are simply added, by fitting what is known as a series actuator in the pushrod, the pilot’s controls cease to function. Figure 7.2(b) shows why. It is the goal of the processor to maintain the attitude of the machine. If the pilot applies, say, left cyclic, the machine will begin to bank left, but the processor will sense this as an error and apply right bank. The machine will continue to fly straight, with the processor precisely opposing everything the pilot does. If the pilot releases the stick, the series actuator may succeed in moving the stick instead of the controls. In early systems using series actuators, engaging the autopilot must lock the stick in the neutral position to give the series actuator a mechanical reference against which to react. The pilot has to disengage the processor if he wants to resume control. In the parallel actuator system shown in Figure 7.2(c) the actuator moves the mechanical reference of the stick centring springs. With the signal processor off, the actuator may perform the trim function. With the signal processor on, the pilot releases the stick and the actuator moves both stick and controls. This gives visual confirmation that the system is working, but more importantly the pilot can override just by moving the stick against the centring springs. In modern systems there is a more convenient way. Figure 7.2(d) shows that the processor is permanently on, but the pilot’s stick is connected differently. If the stick is released, it has no effect and the processor controls the course. However, in order to override the processor, the pilot simply moves the stick as normal. This generates a false error that is fed into the processor. For example, if the pilot wishes to bank left, he moves the cyclic stick left, and this generates a signal that is added to the processor’s bank angle error to indicate falsely that there is a right bank condition. The processor acts to cancels that condition by performing a left bank. Thus it will be seen that the way for the pilot to override a feedback system is not to attempt to oppose the output, which cannot work, but to modify the reference value or the parameter the system is trying to hold constant. In this way the system still stabilizes the override manoeuvre. For example, in the example of the left bank above, the system now acts to hold the bank angle constant. In the event of gusts disturbing the bank angle, the system would correct for them. In this way the combination of control system and real pilot is close to the ideal. Figure 7.1(b) showed a simple stability augmentation system in which the goal is to reduce the naturally divergent behaviour of the helicopter as well as external disturbances from gusts using a parallel actuator. The reduced pilot workload may make single pilot instrument flying possible. Clearly the stability augmentation system must
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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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Page 228 and 229: Engines and transmissions 213 Fig.
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Page 238 and 239: Fig. 6.16 The complex fuel system o
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Page 270 and 271: Engines and transmissions 255 resul
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Page 286 and 287: Fig. 7.7 The remote indicator for a
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Page 290 and 291: Fig. 7.11 The VSI (vertical speed i
Page 292 and 293: Fig. 7.13 A chaser system which all
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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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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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