The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.11 The VSI (vertical speed indicator) is a leaky barometer that responds to a rate of pressure change. At constant altitude the pressure in the instrument will be the same as ambient owing to the leak. In a climb, air will try to flow out of the leak and the pressure across the orifice will be sensed to operate the display. 7.8The vertical speed indicator As its name suggests, the vertical speed indicator (VSI) displays the rate at which altitude is changing. Whilst this can be established by looking at the speed with which the hand of the altimeter turns, this takes some time and it is quicker to look at a separate display. Figure 7.11 shows how the VSI is constructed. The case of the instrument is connected to the static port of the aircraft. The mechanism includes a thin metal capsule sealed except for a very small orifice of carefully determined size. Should the aircraft climb, ambient pressure will fall and the capsule will expand, moving a pointer across a scale. The reduced ambient pressure will cause the air inside the capsule to flow out, but the flow is slowed down by the restriction. At constant altitude, the capsule will eventually have the same pressure inside and out, and the pointer will read zero. The faster the ambient pressure changes the more the restriction opposes the balancing airflow and the greater the deflection of the pointer. As it is self-balancing the VSI needs no pilot adjustment and has no controls. The VSI is particularly useful in cruise where it should read zero. Any discrepancy indicates that the helicopter needs retrimming to fly at constant height. 7.9 The airspeed indicator The airspeed indicator (ASI) is connected between the forward facing pitot head and the static vents. This arrangement cancels ambient air pressure so that it measures only the dynamic pressure due to forward flight. Dynamic pressure is proportional to air density and to the square of the velocity. As a result the expansion of the pressure-sensing capsule is proportional to the square of the speed, and it is necessary to convert the square law response to drive a practical scale. Figure 7.12 shows that this can be done using a straight spring (a) whose effective stiffness increases with deflection. At low speeds the whole length of the spring is active (b) and the restoring force is relatively low. When the spring contacts a stop (c) the active length is reduced and the spring becomes stiffer. The system does not achieve complete linearity. Practical ASIs have decidedly nonlinear scales and the designer positions the stretched part of the scale at the most useful Control 275
276 The Art of the Helicopter (a) Fig. 7.12 Dynamic pressure rises as the square of the airspeed and instrument manufacturers use various techniques to linearize the scale to some extent. This may include the provision of stops to make the return spring progressively stiffer as speed increases. range of flying speeds. In a light helicopter the stiffer spring may come into operation at about 100 mph. Pointer movement from 70 to 80 mph may be twice that from 100 to 110 mph. A further consequence of the square law is that the dynamic pressure at low speed is very small indeed and comparable to the friction in the linkages. The ASI is unreliable at low speeds because of the low dynamic pressures involved and because of the effect of downwash. Thus an ASI generally has no calibrations below 20 mph. According to the country of origin ASIs will be found calibrated in mph, knots and kph, some have dual scales like car speedometers. The reading of the ASI is also proportional to air density, so it will underread at high altitude. The scale on the ASI displays indicated air speed (IAS) proportional to density and the square of the speed. Since the rotors behave proportionally to these parameters, IAS is the best way of displaying speed from the point of view of the airframe. Clearly IAS is not the actual speed through the air, and for navigational purposes it needs compensating for ambient density to give true air speed or TAS. Density is affected by altitude and temperature. At 10 000 feet the IAS must be increased by around 17% to compensate for density change. An increase of 10 ◦ C raises TAS about 2%. If TAS, windspeed and direction are known, the groundspeed (GS) can be calculated. In real ASIs the scale markings are not perfect, and frequently the position of the pitot head with respect to the hull causes errors at certain airspeeds. A calibration table is supplied so that IAS can be corrected by looking it up on the table. The result is called rectified or calibrated airspeed (RAS or CAS) and is the value the IAS would have read if it were free of error. In a helicopter airspeed is relatively slow and uncertainty about windspeed causes more error than the ASI. In this case it is probably not worth correcting the ASI for navigational purposes. 7.10 Airspeed and altitude sensing Airspeed and altitude are both sensed by measuring pressure, one dynamic and one static. Both of these processes are often combined in a single unit. As was seen above, pressure is measured using the deflection of a corrugated capsule. The amount of (b) (c)
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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 219 Fig.
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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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Page 272 and 273: Engines and transmissions 257 The p
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Page 286 and 287: Fig. 7.7 The remote indicator for a
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Page 292 and 293: Fig. 7.13 A chaser system which all
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Page 300 and 301: Fig. 7.19 The turn indicator is spr
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Page 304 and 305: Fig. 7.24 Using RADAR signals. At (
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
Page 320 and 321: Fig. 7.42 An electro-hydraulic valv
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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
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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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