The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 5.10 Tail plane locations: (a) forward location almost always in downwash. (b) fin-top location is never in the downwash. (c) Asymmetrical tail plane opposite the tail rotor. (d) Low aft-mounted tail plane causes strong trim shifts but protects tail rotor. efficiently produces downthrust in forward flight but which stalls in autorotation. The Bell 206 JetRanger uses this approach. Another concern is avoiding tail plane stall in the climb where the (negative) angle of attack becomes large. Sometimes leading edge slats will be used. Another solution is to use a device called a Gurney flap. This non-intuitive device consists of a thin tab set across the trailing edge mounted at 90 ◦ to the chord line. The effect is to delay stalling up to extremely large positive and negative angles of attack, with a small amount of increased drag at low angles of attack. The Sikorsky S-76B uses such a device. 5.7 The stabilator The stabilator is basically a variable-incidence tail plane that is moved to different angles, generally automatically, according to the flight regime. The stabilator and its actuator will be more costly than a fixed tail plane, so there must be a good reason to employ it. The advantage of the stabilator is that it can be aligned with the downwash so that it can give the stabilizing effect of a tail plane without the unwanted down- or uploads in climb or autorotation. A beneficial reduction in pilot workload will result, along with improved hover performance. Figure 5.11(a) shows that in forward translational flight the stabilator may assume a slightly negative angle of attack to counter the hull drag moment. In the hover and in slow forward flight the stabilator will move to a large positive angle (b) in order to minimize the download. In a steep climb, the stabilator may also adopt a small positive angle (c). This not only reduces download and aids the climb, but it also levels the hull. In a tandem seat attack helicopter this may give the pilot in the rear seat a better view forward in a climb. Figure 5.11(d) shows that in autorotation the stabilator should move to a negative pitch to avoid an upload which would pitch the machine nose down. In addition to these deterministic movements, the stabilator may also be linked to an artificial stability system based on gyroscopic pitch sensors in the hull. The tail 181
182 The Art of the Helicopter Fig. 5.11 The stabilator is a variable incidence tail plane that aligns itself with the direction of airflow in different flight regimes to improve performance. In forward flight, (a) a slight negative angle of incidence produces a downthrust to counter hull drag which acts below the rotor head. At (b) in the hover the stabilator adopts a large positive angle of incidence to prevent the creation of a download. At (c) in the climb a smaller positive angle of attack is beneficial. In autorotation (d) a negative angle of incidence prevents an unwanted upthrust. Although stabilators are generally considered a good thing, some thought has to be given during the design process to the results of an actuator or control system failure leaving the stabilator in an inappropriate attitude. In a machine with a zero-offset head this would probably not be acceptable. In a machine with higher control power the incorrect tail loads following an actuator failure could be overcome by the cyclic control at least at low airspeed. In light helicopters the actuator could be a pneumatic device driven by a combination of dynamic pressure from a pitot so that it responds to airspeed and manifold pressure so it responds to power level. It may be advantageous to mount the tail plane on a viscous damper with a suitable time constant that would stabilize brief perturbations whilst aligning to long-term trends. 5.8 Fins Fins may be applied to helicopters for the same reason as on fixed-wing aircraft: to enhance yaw stability in forward flight. Secondary purposes of fins may be to allow the machine to fly on following a tail rotor failure or to enhance control in autorotation, especially in helicopters not having a tail rotor. Fins are seen in a variety of locations as designers seek various compromises. In the case of the high-mounted tail rotor, the tail rotor pylon is naturally extended and streamlined to form the fin. Low-mounted tail rotors with a single fin tend to suffer blockage as the tail rotor and the fin have to be in the same place to clear the main rotor. Blockage can be avoided by using end plates on the tail plane as fins. These also make the tail plane more efficient as tip loss is reduced.
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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
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 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 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 236 and 237: Engines and transmissions 221 own t
Page 238 and 239: Fig. 6.16 The complex fuel system o
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Engines and transmissions 231 Fig.
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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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