The Art of the Helicopter John Watkinson - Karatunov.net

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Technical background 31 Fig. 2.11 At (a) a helicopter flying at 30 kt into a 30 kt wind is doing 30 kt aerodynamically but inertially it is hovering. At (b), flying downwind the aerodynamic conditions are the same but the machine has KE corresponding to 60 kt. Figure 2.11(a) shows a helicopter flying at 30 knots directly into a 30 knot wind. Aerodynamically it is doing 30 knots but inertially it is stationary and has no kinetic energy. Inertially it is hovering. Figure 2.11(b) shows a helicopter flying at 30 knots down wind in a 30 knot wind. Aerodynamically the conditions are the same, but inertially the machine is now doing 60 knots and possesses the kinetic energy due to that groundspeed. The difference in kinetic energy becomes obvious when an attempt is made to change heading. If the helicopter in (b) does a 180 ◦ turn, it will conserve its kinetic energy and will exit the turn with 60 knots of groundspeed and 90 knots on the ASI. The result will be that the machine will tend to climb as the surplus airspeed is converted to potential energy. If the helicopter in (a) tries a 180 ◦ turn it will be in difficulty and will lose height because it lacks 60 knots worth of kinetic energy. If the machine isn’t powerful and doesn’t have height to lose it could crash. And the pilot who thought that the helicopter doesn’t know what the ground is doing wouldn’t know what he did wrong. 2.8 Efficiency Efficiency is generally defined as the ratio of useful output power to input power in any mechanism. In the case of a gearbox the useful power is the output shaft power. This will always be less than the input power and the difference will be converted to heat.
32 The Art of the Helicopter In the case of an engine, the input power is the heat released by burning the fuel. In practical engines, this exceeds the output shaft power considerably. The percentage of the thermal energy in the fuel that emerges as shaft power is the thermal efficiency of the engine. The waste heat has to be removed by a cooling system to prevent the temperature of the engine rising to the point where components are damaged. Cooling systems waste further power in driving fans and pumps and usually increase the drag of the airframe. Given the necessary high power to weight ratio in helicopters, the power plant and fuel form a significant part of the all-up weight. It is beneficial to explore means to improve the thermal efficiency of the engine. Not only will this reduce the weight of the fuel to be carried for a given range, but it may also allow the cooling system to be lighter, to consume less power and to cause less intake drag. Thus a small improvement in thermal efficiency may result in a significant increase in performance. Passenger aircraft may be compared using specific air range (the mass of fuel used per unit of distance), but in a hovering helicopter this figure is meaningless. In helicopters, it may be better to compare the power actually used to hover with the theoretical power needed by an ideal rotor under the same conditions. 2.9 Gases and the atmosphere The atmosphere is the medium in which helicopters fly but it is also one of the fuels for the engine and the occupants breathe it. It is a highly variable medium that is constantly being forced out of equilibrium by heat from the sun and in which the pressure, temperature, and humidity can vary with height and with time and in which winds blow in complex time- and height-variant patterns. The effect of atmospheric conditions on flight is so significant that no pilot can obtain qualifications without demonstrating a working knowledge of these effects. The atmosphere is a mixture of gases. About 78% is nitrogen–arelativelyunreactive element – whereas about 21% is oxygen, which is highly reactive. The remainder is a mixture of water in the gaseous state and various other traces. The reactive nature of oxygen is both good and bad. The good part is that it provides a source of energy for life and helicopters alike because hydrocarbons can react with oxygen to release energy. The bad part is that many materials will react with oxygen when we would rather they didn’t. Chemically, combustion and corrosion are one and the same thing. The difference is based on the human reaction to the chemical reaction. Gases are the highest energy state of matter, for example the application of energy to ice produces water and the application of more energy produces water vapour. The reason that a gas takes up so much more room than a liquid is that the molecules contain so much energy that they break free from their neighbours and rush around at a high speed, which is a function of absolute temperature. As Figure 2.12(a) shows, the innumerable elastic collisions of these high speed molecules produce pressure on the walls of any gas container. In fact the distance a molecule can go without a collision, the mean free path, is quite short at atmospheric pressure. Consequently gas molecules also collide with each other elastically, so that if left undisturbed, in a container at a constant temperature, every molecule would end up with essentially the same energy and the pressure throughout would be constant and uniform. Pressure is measured by physicists and by engineers in units of force per unit of area using imperial units of pounds per square inch or SI units of Newtons per square metre. At sea level, the atmosphere exerts a pressure of about 15 pounds per square inch and has a density of about 0.075 pounds per cubic foot or in metric units about
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 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 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
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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 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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