The Art of the Helicopter John Watkinson - Karatunov.net

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is set by the use of a control knob. Turning the knob will simultaneously change the reference pressure displayed on the subscale and the aneroid capsule spring tension. If, for example, the reference pressure is to be raised, the act of setting the higher pressure on the subscale slightly extends the spring and the capsule will reach equilibrium showing a higher altitude. Conversely if a lower reference pressure is to be used, the spring is allowed to contract slightly and the capsule reaches equilibrium with a lower altitude reading. Throughout this process the absolute pressure at the altimeter did not change, but the pressure difference and the altitude did because the subscale setting was changed. Clearly without an appropriate subscale setting an altimeter reading is meaningless. It should also be clear that, in general, height and altitude are different. There are three main subscale settings used with an altimeter and these are shown in Figure 7.10(a). If the subscale is set to the ground pressure at the elevation of the airfield, the altimeter will read zero prior to take-off and in flight will measure height above the airfield. The barometric pressure at an airfield is called QFE. The ‘Q’ dates from the days of carrier wave radio and Morse code and indicates a question. An aircraft arriving at an airfield may ask for QFE in order to set the altimeter subscale so that the altimeter reads height above the field. QFE is easily remembered as ‘field elevation’. In some parts of the world this technique is not used. For example, the QFE of a mountain airfield may be so low that it is beyond the end of the altimeter subscale. During a flight at moderate altitude the overriding concern is clearance of terrain and structures. These are quoted on maps as height above mean sea level (AMSL). In order to establish the clear height above such obstacles the altimeter reference is set to QNH; the barometric pressure in the area ‘reduced to mean sea level’. This means that the actual barometric pressure is measured at a known elevation and the value the pressure would have had at sea level is computed from the height and temperature. With QNH on the subscale, the altimeter reads feet AMSL and subtracting the height of obstacles in feet AMSL gives the clearance. QNH is easily remembered as ‘nautical height’. Figure 7.10(b) shows a typical low-level flight. The machine takes off with QFE in the subscale to show height above the airfield. Once away from the field the altimeter is set to regional QNH and the machine is trimmed to cruise at 2000 feet AMSL. On the chart is a hill rising to 1200 feet AMSL and the machine will clear it by 800 feet. On approaching the destination airfield, the pilot asks for QFE and sets this on the altimeter. This should read zero as he touches down. Each country is divided up into altimeter setting regions which each have their own QNH value as a function of the weather system across the country. The QNH quoted takes into account the rate at which barometric pressure is changing. If pressure is falling, a machine flying at constant pressure altitude based on QNH would actually descend and not have the expected clearance. QNH is adjusted upwards by the authorities to ensure that clearance is still achieved at the end of the period for which a setting is valid even if barometric pressure falls. On a long flight an altimeter setting region boundary may be crossed. The pilot must obtain the new QNH for the area to ensure his altimeter still reads feet AMSL. The subscale is generally calibrated in millibars (mb) or, more recently, hectoPascals (hP) that are numerically identical. In America, scales calibrated in inches of mercury (inHg) will be found. For flight in airways, the overriding concern is not terrain clearance, but vertical separation between aircraft. For this reason the same reference pressure is always used in airways. This is the pressure of the International Standard Atmosphere (ISA) at Mean Sea Level (MSL): 1013.25 mb. Older texts may use the term QNE to describe this pressure. Aircraft flying with ISA on the subscale are flying at pressure altitude. The flight level is the pressure altitude with the last two digits removed. For example, Control 273
274 The Art of the Helicopter (b) Fig. 7.10 (a) QFE is the local barometric pressure at ground level at an airfield. When the subscale is set to QFE, the altimeter reads height AGL (above ground level). (b) Once away from an airfield, the altimeter may be set to QNH. This is local pressure reduced to MSL (mean sea level) and the altimeter reads height above MSL. As obstructions on maps are described by their height AMSL, the QNH setting allows the pilot to establish that terrain clearance exists. QFE may be used for landing. This may be different from the QFE that was used at take-off. an aircraft at FL 350 is 35 000 feet above ISA MSL. Another aircraft at FL 340 would always be 1000 feet lower whatever the atmospheric conditions. Very few helicopter operations take place in airways, although as tilt-rotor and tilt-wing machines enter service this will change. It should be noted that helicopter altimeters only sense local atmospheric pressure when the rotors are not turning. In the hover, especially in ground effect, the pressure under the rotor disc will be appreciably above ambient. Going from flat pitch on the ground to a low hover will result in a noticeable reduction in the reading on the altimeter, of the order of 30 feet. The effect is negligible in translational flight.
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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
Page 238 and 239: Fig. 6.16 The complex fuel system o
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Page 248 and 249: Engines and transmissions 233 and c
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Page 262 and 263: Fig. 6.37 Displays which may be see
Page 266 and 267: Engines and transmissions 251 slipr
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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
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Page 282 and 283: 7.4 Compass errors A magnetic compa
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Page 286 and 287: Fig. 7.7 The remote indicator for a
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
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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
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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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