The Art of the Helicopter John Watkinson - Karatunov.net

More documents

Recommendations

Info

Fig. 4.23 A fixed-pitch tilting head needs no more than a gimbal and control rods leading to the cockpit. Rotors in practice 143 translational flight a steady application of cyclic feathering is required to overcome lift asymmetry. Early de la Cierva autogyros had aircraft-style control surfaces and no rotor control at all. The rotor was equipped only with flapping hinges. In forward flight, lift asymmetry would result in extra lift on the advancing side, but this would precess to an aft tilt of the disc. Figure 1.10 showed that the thrust of the airscrew overcomes the rearward component of rotor thrust due to the aft tilt. As the attitude of the hull does not change when the rotor tilts aft, the mast is now tilted forwards with respect to the tip path plane. The result is that the rotor has cyclic pitch applied. In an autogyro of this kind the rotor will flap back until the correct amount of cyclic pitch is obtained for the airspeed. Whilst the autogyro flies in this way, it should be emphasized that the helicopter cannot and does not. In later de la Cierva machines, the aircraft control surfaces were dispensed with and the pilot could tilt the rotor head for pitch and roll. This approach is used to this day for control of lightweight fixed-pitch autogyros. Where there is no shaft drive or collective control, the rotor head can be very simple, as shown in Figure 4.23. The rotor requires just a teetering hinge and the head is mounted on a gimbal so the pilot can tilt it. The tilting head can only apply cyclic pitch. Where collective control is required, a feathering bearing will be needed and so the tilting head has no advantage. Hafner was the first to build an autogyro with cyclic and collective control and the mechanical solutions he devised would be used in his subsequent and successful Sycamore helicopter. 4.16Dragging dynamics In-plane blade motion is variously called ‘lead lag’, ‘lag’ or ‘dragging’. This author prefers the term dragging because lead lag is cumbersome and lag can be confused with phase lag; another thing altogether. Section 4.6 illustrated the mechanisms responsible for dragging. In many rotor designs, dragging hinges or flexible members are provided
144 The Art of the Helicopter Fig. 4.24 When a blade drags, the CM moves closer to the rotor axis. In rotating co-ordinates, centrifugal force creates a restoring moment proportional to deflection such that the blade has a resonant response to excitation in dragging. Unlike flapping, the aerodynamic damping is low. to reduce stress. In other designs, the rotor is designed to be extremely stiff, but as stiffness is always finite, there will still be some dragging motion. In all cases the dynamics of the system must be considered in order to avoid resonances. In a traditional articulated rotor head, the dragging hinge cannot apply a restoring moment to the blade. The only restoring force for dragging motion in this case is due to rotation. Figure 4.24 shows that when a blade drags on a hinge having an offset from the rotor axis, the CM of the blade moves closer to that axis. When the rotor is turning, the CM will tend to be at the greatest possible radius and so if the blade drags forward or back, there will be a restoring force. Figure 4.24 is drawn in a rotating frame of reference in order to arrest the rotation. The acceleration of the blade is now zero and has been replaced by an equivalent centrifugal force. It will be seen that the centrifugal force, which must pass through the rotor axis, creates a small moment due to the drag hinge offset. The restoring moment is proportional to the deflection and the result is that a blade which suffers an in-plane disturbance will tend to execute simple harmonic motion at its resonant frequency. In an articulated rotor, resonant frequency is proportional to the square root of the stiffness. The stiffness is due to centrifugal force and is proportional to the square of the RRPM. Thus the dragging resonant frequency will be proportional to RRPM as was the case for articulated flapping. However, unlike flapping, the only aerodynamic damping available to the dragging motion is due to changes of profile drag. Damping will often need to be augmented by mechanical means. In hingeless rotors there will be some degree of structural stiffness that will provide a restoring force for the blade even when the rotor is not turning. The dragging resonant behaviour of a rotor can be characterized by the Southwell coefficients. Figure 4.25 shows how the ratio of dragging resonant frequency to rotor frequency is determined by K1and K2. K1reflects the resonant frequency when the rotor is not turning, as determined by the blade inertia and the stiffness of the rotor/head system in the dragging plane and is called the structural stiffening component. For an articulated head, K1 is zero because the dragging hinge has no stiffness. K2 is determined by the effective or actual dragging hinge offset and represents what is usually called the centrifugal stiffening component. Figure 4.25(a) shows that in the articulated rotor, the hinge offset and consequent restoring forces are generally small and so the dragging resonant frequency is a small proportion of the rotor frequency, typically between 20 and 30%. In a hingeless rotor, shown in Figure 4.25(b), at low RRPM the structural stiffness will dominate, whereas at high RRPM rotational forces dominate and the dragging
Page 2 and 3:
The Art of the Helicopter
Page 4 and 5:
The Art of the Helicopter John Watk
Page 6 and 7:
Contents Preface xi Acknowledgement
Page 8 and 9:
4.12 Feathering 134 4.13 Pitch cont
Page 10:
8.5 Power management 326 8.6 Flying
Page 13 and 14:
xii Preface reader to make sense of
Page 16 and 17:
1 Introduction to rotorcraft 1.1 Ap
Page 18 and 19:
Introduction to rotorcraft 3 Fig. 1
Page 20 and 21:
Page 22 and 23:
Page 24 and 25:
Introduction to rotorcraft 9 for th
Page 26 and 27:
Introduction to rotorcraft 11 Fig.
Page 28 and 29:
(a) (b) (c) (d) Introduction to rot
Page 30 and 31:
Fig. 1.18 The contra-rotating coaxi
Page 32 and 33:
Fig. 1.20 The structure of a light
Page 34 and 35:
Introduction to rotorcraft 19 wings
Page 36 and 37:
Introduction to rotorcraft 21 a rot
Page 38 and 39:
Technical background 23 Fig. 2.1 At
Page 40 and 41:
Fig. 2.3 Effect of force on velocit
Page 42 and 43:
C Force A E Resultant (a) Force B (
Page 44 and 45:
(a) (b) Technical background 29 Fig
Page 46 and 47:
Technical background 31 Fig. 2.11 A
Page 48 and 49:
Technical background 33 Fig. 2.12 (
Page 50 and 51:
Technical background 35 If the volu
Page 52 and 53:
Fig. 2.16 The definition of a radia
Page 54 and 55:
Technical background 39 force would
Page 56 and 57:
Technical background 41 Figure 2.20
Page 58 and 59:
Technical background 43 force where
Page 60 and 61:
Technical background 45 the cosine
Page 62 and 63:
Technical background 47 Fig. 2.28 F
Page 64 and 65:
Technical background 49 Fig. 2.30 T
Page 66 and 67:
Technical background 51 centripetal
Page 68 and 69:
2.17 The gyroscope Technical backgr
Page 70 and 71:
Technical background 55 of oscillat
Page 72 and 73:
Technical background 57 the inertia
Page 74 and 75:
Technical background 59 shut down a
Page 76 and 77:
3 Introduction to helicopter dynami
Page 78 and 79:
Introduction to helicopter dynamics
Page 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
(a) (c) (b) Introduction to helicop
Page 102 and 103:
Page 104 and 105:
(a) (b) (c) (d) Introduction to hel
Page 106 and 107:
Fig. 3.23 Conditions for hover and
Page 108 and 109: Introduction to helicopter dynamics
Page 110 and 111: (a) (b) Introduction to helicopter
Page 116 and 117: (a) (b) Introduction to helicopter
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 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 168 and 169: Rotors in practice 153 and lag damp
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
Page 218 and 219:
Engines and transmissions 203 The c
Page 220 and 221:
6.9 The oil system Engines and tran
Page 222 and 223:
Engines and transmissions 207 (non-
Page 224 and 225:
(a) (b) Engines and transmissions 2
Page 226 and 227:
Engines and transmissions 211 a hot
Page 228 and 229:
Engines and transmissions 213 Fig.
Page 230 and 231:
Engines and transmissions 215 In wa
Page 232 and 233:
Engines and transmissions 217 is a
Page 234 and 235:
Page 236 and 237:
Engines and transmissions 221 own t
Page 238 and 239:
Fig. 6.16 The complex fuel system o
Page 240 and 241:
Page 242 and 243:
Page 244 and 245:
Page 246 and 247:
Page 248 and 249:
Engines and transmissions 233 and c
Page 250 and 251:
Page 252 and 253:
Engines and transmissions 237 Sensi
Page 254 and 255:
Page 256 and 257:
Page 258 and 259:
Engines and transmissions 243 the A
Page 260 and 261:
Page 262 and 263:
Fig. 6.37 Displays which may be see
Page 264 and 265:
Page 266 and 267:
Engines and transmissions 251 slipr
Page 268 and 269:
Engines and transmissions 253 a fau
Page 270 and 271:
Engines and transmissions 255 resul
Page 272 and 273:
Engines and transmissions 257 The p
Page 274 and 275:
Fig. 7.1 (a) A minimal control loop
Page 276 and 277:
Fig. 7.2 (a) When attitude conditio
Page 278 and 279:
fixed point in the hover despite ex
Page 280 and 281:
(a) (b) Fig. 7.4 (a) The earth beha
Page 282 and 283:
7.4 Compass errors A magnetic compa
Page 284 and 285:
In most cases the machine will have
Page 286 and 287:
Fig. 7.7 The remote indicator for a
Page 288 and 289:
is set by the use of a control knob
Page 290 and 291:
Fig. 7.11 The VSI (vertical speed i
Page 292 and 293:
Fig. 7.13 A chaser system which all
Page 294 and 295:
e modified if the gyro is in a movi
Page 296 and 297:
must be in steady flight when the D
Page 298 and 299:
arranged on opposite sides of the p
Page 300 and 301:
Fig. 7.19 The turn indicator is spr
Page 302 and 303:
7.17 Airflow-sensing devices The he
Page 304 and 305:
Fig. 7.24 Using RADAR signals. At (
Page 306 and 307:
Fig. 7.27 Transducers used in signa
Page 308 and 309:
Fig. 7.28 In PCM or digital signall
Page 310 and 311:
(a) Fig. 7.30 Using slicing and rec
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
Page 320 and 321:
Fig. 7.42 An electro-hydraulic valv
Page 322 and 323:
path axis and the flybar axis diver
Page 324 and 325:
and autostabilization, it also prov
Page 326 and 327:
Fig. 7.47 The Lockheed gyrobar syst
Page 328 and 329:
Fig. 7.49 In the Lockheed AMCS, the
Page 330 and 331:
Fig. 7.50 With the autopilot diseng
Page 332 and 333:
or without the AFCS. Systems of thi
Page 334 and 335:
Thus if a VOR receiver detected a 9
Page 336 and 337:
7.29 Fault tolerance In feedback sy
Page 338 and 339:
8 Helicopter performance 8.1 Introd
Page 340 and 341:
Helicopter performance 325 column i
Page 342 and 343:
Helicopter performance 327 Fig. 8.1
Page 344 and 345:
Helicopter performance 329 to skid
Page 346 and 347:
(a) (b) Helicopter performance 331
Page 348 and 349:
Helicopter performance 333 Fig. 8.7
Page 350 and 351:
8.7 Climbingand descending Helicopt
Page 352 and 353:
Helicopter performance 337 In the c
Page 354 and 355:
Helicopter performance 339 of its c
Page 356 and 357:
8.10 Stability Helicopter performan
Page 358 and 359:
Fig. 8.13 The origin of pitchup ins
Page 360 and 361:
Helicopter performance 345 moment f
Page 362 and 363:
9 Other types of rotorcraft Althoug
Page 364 and 365:
Other types of rotorcraft 349 De la
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 376 and 377:
Page 378 and 379:
Page 380 and 381:
Other types of rotorcraft 365 a low
Page 382 and 383:
Other types of rotorcraft 367 the y
Page 384 and 385:
Page 386 and 387:
Other types of rotorcraft 371 the r
Page 388 and 389:
Other types of rotorcraft 373 For e
Page 390 and 391:
Page 392 and 393:
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
show all

The Art of the Helicopter John Watkinson - Karatunov.net

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?