The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 153 and lag dampers are in any case present to prevent ground resonance. Furthermore the motion of the hull is decoupled to some extent by the articulation. In a hingeless rotor the blades lag by flexing and the stiffness is higher, leading to a higher natural frequency of lagging motion. The relatively stiff connection between the rotor and the hull means that large rotor moments can excite hull flexing. If such hull flexing is resonant then an unstable system could exist. A bending mode of the hull could result in a lateral motion at the rotor head that is similar to the rocking experienced in ground resonance. It should be appreciated that resonance may also be excited by the tail rotor. This will be discussed in Chapter 5. 4.19 Dynamic rollover Dynamic rollover is a hazard that can only occur when a helicopter is in contact with the ground in some way. Normally the cyclic control has complete authority over the attitude of the helicopter, but interference from the ground or from external loads can reduce or even overcome the cyclic authority. Section 4.15 introduced the tilting hub mechanism for cyclic control. If the hull of a helicopter is forcibly tilted, the rotor will follow. Dynamic rollover occurs when external tilting forces overpower the cyclic control. Teetering or zero-offset rotors are more prone than offset or hingeless rotors because their cyclic authority is small to begin with. Dynamic rollover becomes a possibility when the conditions on one side of the hull are different from those on the other side. This may be due to an attempt to land or take-off from a slope, an underslung load snagging a skid or a skid being stuck in mud or the surface tension of wet sand. Most winches for personnel recovery are fitted at one side of the machine and if a winch rope snags on the ground it can cause difficulty. A side wind and the thrust of the tail rotor complicate the matter. Figure 4.33(a) shows what can happen with a stuck skid. The pilot raises the collective lever to lift off, but the stuck skid combines with the rotor thrust to produce a roll couple that could exceed the power of the cyclic control. In many cases the machine will be on its side before the pilot can react and the cyclic authority becomes academic. The roll accelerates because as the hull tips it increases the cyclic input to the rotor and causes further roll. A variation of this hazard is where the stuck skid suddenly comes free and the machine ends up on its other side because the pilot does not remove the lateral cyclic quickly enough. Figure 4.33(b) shows a helicopter about to lift a slung load. Unknown to the pilot the load strap has snagged a skid. As the machine rises, once more the rotor thrust and the load strap tension create a couple on the hull and the dynamic rollover takes place. Most helicopter winches are installed with a view to bringing loads alongside one of the hull doors. The load is applied with a considerable moment arm from the machine CM. Moments due to normal loads can be resisted by the rotor, but in the event that the winch line became snagged an attempt to climb would result in dynamic rollover. This is one of the reasons why the pilot is provided with means to jettison loads in an emergency. Underslung loads can be jettisoned by releasing the load hook that may be electrically operated. Winches are fitted with cable cutters that may be explosively actuated. Figure 4.33(c) shows an attempted landing on a slope. After the first skid has touched, the machine rotates about that skid. As it comes down, the control axis must be kept vertical by the application of cyclic control. The question is, what happens when the cyclic reaches the end of its travel and the other skid still is not in contact? The rotor disc cannot be kept horizontal and there is a danger of dynamic rollover. If the landing
154 The Art of the Helicopter Fig. 4.33 The origins of dynamic rollover. (a) A skid sticks in mud and applies a roll moment as the helicopter attempts to take-off. (b) A load strap snags around a skid and applies a roll moment as the tension is taken up. (c) An attempted slope landing in which the cyclic control has reached the end of its travel with the lower skid still not in ground contact. (d) A landing made with an upslope wind is easier, but if the wind changes to downslope, there may not be enough cyclic travel to level the disc in the presence of flapback. is continued, the hull will roll and carry the rotor with it due to the tilting head effect. When the downhill skid contacts, there may be enough roll momentum that the roll continues, with the uphill skid lifting. Once the CM of the machine is outside the skid base there is no recovery. This explanation is the origin of the slope landing limit laid down for each type of machine. These figures should be ignored at one’s peril. Slope landings can be made harder or easier by wind. Figure 4.33(d) shows the wind blowing upslope. This is an advantage because more cyclic travel is available. However, if the wind subsequently reverses, it may become impossible to take-off because insufficient cyclic travel exists to tilt the disc into wind when the mast is tilted downwind. An experienced pilot might perform a ‘hop’ take-off which consists of bringing the rotor to flight RPM with neutral cyclic and then pulling collective rapidly to exit at a right angle to the slope. Once the gear is clear of the ground full cyclic authority becomes available. The preferred technique in all external load handling or slope landing situations is to proceed very slowly, and if the cyclic travel approaches the limits the manoeuvre will have to be halted. This means in the event of attempting a take-off, rotor thrust should be applied very gradually so that any roll does not proceed too far. If anything goes wrong, the collective should be lowered. With the rotor thrust gone, the couple
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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
Page 118 and 119: Introduction to helicopter dynamics
Page 132 and 133: 4.1 Introduction 4 Rotors in practi
Page 134 and 135: (a) (b) Rotors in practice 119 Fig.
Page 136 and 137: Rotors in practice 121 Designers of
Page 138 and 139: Rotors in practice 123 opposite occ
Page 140 and 141: Rotors in practice 125 Fig. 4.6 (a)
Page 142 and 143: Rotors in practice 127 assembly tha
Page 144 and 145: Rotors in practice 129 Pitch change
Page 146 and 147: × T=c× D Rotors in practice 131 F
Page 148 and 149: Rotors in practice 133 In a real he
Page 150 and 151: Rotors in practice 135 Fig. 4.15 Va
Page 152 and 153: (a) (b) Rotors in practice 137 Fig.
Page 154 and 155: Rotors in practice 139 Fig. 4.18 Th
Page 156 and 157: Rotors in practice 141 swashplate a
Page 158 and 159: Fig. 4.23 A fixed-pitch tilting hea
Page 160 and 161: Rotors in practice 145 Fig. 4.25 Bl
Page 162 and 163: Rotors in practice 147 Fig. 4.27 Ef
Page 164 and 165: Rotors in practice 149 Fig. 4.29 (a
Page 166 and 167: Rotors in practice 151 concerned ar
Page 170 and 171: Rotors in practice 155 disappears a
Page 172 and 173: (d) (e) (f) Rotors in practice 157
Page 174 and 175: Rotors in practice 159 hull and a v
Page 176 and 177: Rotors in practice 161 Fig. 4.35 (C
Page 178 and 179: Rotors in practice 163 Abrasion is
Page 180 and 181: Rotors in practice 165 There are so
Page 182 and 183: eing more visible to ground personn
Page 184 and 185: otates the rod in the screw thread.
Page 186 and 187: In the case of a rotor having offse
Page 188 and 189: The tail rotor designer is faced wi
Page 190 and 191: Fig. 5.5 A right-side wrong-directi
Page 192 and 193: centre of mass. The solution was to
Page 194 and 195: safe to start the rotors. However,
Page 196 and 197: Fig. 5.10 Tail plane locations: (a)
Page 198 and 199: Fig. 5.12 (a) A top-mounted fin is
Page 200 and 201: Fig. 5.14 (a) Main rotor lateral ro
Page 202 and 203: long. The cross-section of the duct
Page 204 and 205: drag. However, the slots are emitti
Page 206 and 207: 6 Engines and transmissions The pow
Page 208 and 209: Engines and transmissions 193 the D
Page 210 and 211: (a) (b) Engines and transmissions 1
Page 212 and 213: (a) (b) (c) Engines and transmissio
Page 214 and 215: Fig. 6.5 A typical horizontally opp
Page 216 and 217: 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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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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