Introduction to rotorcraft

Although helicopters must all follow the same laws of physics, the forms that practical machines take vary tremendously due to the range of tasks to which they can be put. To avoid confusion, this book takes the view that an aircraft is any man-made aerial machine capable of climbing out of ground effect. Thus a helicopter must be an aircraft. Neglecting aerostats (balloons etc.) any aircraft that is not a helicopter will be an aeroplane (USA: airplane).

It will be seen later in this book that the slower an aircraft goes, the more power it needs to maintain height. Hovering is the ultimate case of slow flight, suggesting that helicopters must have a high power to weight ratio. This will require heavy engines and a corresponding fuel capacity. These factors limit load carrying capability and range.

The mechanical complexity of the helicopter and the inevitable vibration demand a lot of maintenance. The airspeed of the true helicopter is forever restricted by funda­mental limits. It will generally be more expensive to move a given load by helicopter than by almost any other means, and so if a suitable airstrip exists, a fixed-wing air­craft can do the job at lower cost, and generally at a higher airspeed. In most cases helicopters cannot compete economically with aeroplanes, and so their use is restricted to applications for which aeroplanes, or other forms of transport, are unsuitable.

In remote areas, there will be no airstrips; in wartime, runways are conspicuous targets, and, with the exception of the aircraft carrier, are fixed. The helicopter’s ability to hover means that it can land almost anywhere a fairly flat firm surface exists. Some are genuinely amphibious, landing with equal aplomb on water or land. If the ground is unsuitable (or if the waves are too high) many helicopters can transfer goods and passengers whilst in the hover.

This ability makes the helicopter the ideal rescue vehicle. Many lives have been saved because the helicopter can get to places that would otherwise be difficult or impossible to reach. War casualties, the victims of shipwrecks, mountain climbing accidents and natural disasters such as earthquake or flood today have significantly higher chances of survival because a helicopter can get them rapid treatment.

The accounts of helicopter rescues make more thrilling reading than novels because they are true. Helicopters have flown far out of range of the shore by taking fuel from oilrigs and ships, sometimes taking fuel in the hover if a landing was impossible. This would be remarkable in good weather, but emergencies occur in all weather conditions and the helicopter has evolved to handle the worst.

Despite their life-saving ability, most of today’s helicopters were originally designed for military use. As military fixed-wing aircraft became faster and faster, they found

it harder to attack ground targets or to support ground troops. During the Vietnam War, it was found that the helicopter had a major role. Transport helicopters excelled inserting and withdrawing troops, delivering ammunition and food, evacuating the wounded, recovering the crews of downed planes and even the planes themselves.

Armed helicopters proved to be ideal for attacking ground targets. At first these were general-purpose machines hurriedly fitted with weapons, but later dedicated attack helicopters evolved, complete with armour plating and redundant systems to allow them to withstand ground fire. As their load carrying capability has increased, these machines have virtually rendered the tank obsolete.

Although ideal against ground targets, the helicopter is slow and vulnerable to attack from the air. Fixed-wing planes are necessary to provide the air superiority in which the helicopters operate. As an alternative, helicopters can operate under stealth conditions, avoiding detection by using terrain.

Large military helicopters are very expensive to operate, and armed forces found it worthwhile to have simpler machines specifically for training purposes. A small number of helicopters have been designed specifically for the civil market and these are popular with large companies for executive transport.

For the average private owner, the sheer cost of running helicopters precludes all but the smallest machines with aeroplane-derived piston engines. Virtually all other helicopters are now turbine powered.

This is not a history book and this section must necessarily be brief. The reader inter­ested in the US history of the helicopter is recommended to the comprehensive yet highly readable works of Jay Spenser.1,2 The recent book by Steve Coates, Helicopters of the Third Reich,3 is essential reading to the historian as it shows how far ahead of the rest of the world German helicopter engineers were at that time. For those who read French, two more fascinating volumes are available. LHistoire de l’Helicoptere by Jean Boulet4 contains the words of helicopter pioneers themselves. Les Helicopteres Florine 1920-1950 by Alphonse DuMoulin5 recounts the pioneering work of Nicolas Florine.

The history of the helicopter has been very short indeed. In comparison with fixed – wing aircraft, helicopters need more power, have to withstand higher stresses, are harder to understand and control and have more moving parts. It is hardly surprising that the development of the helicopter took place well after that of the fixed-wing aircraft.

Early helicopters lacked enough power to fly. Once helicopters were powerful enough to leave the ground, they were found to be uncontrollable. Once the principles of control were understood, they were found to vibrate and to need a lot of maintenance and so on. Today’s helicopters represent the sum of a tremendous number of achievements in overcoming one obstacle after another.

Before World War II helicopters were in an experimental phase. This was the heyday of the gyroplane, invented by the Spaniard Juan de la Cierva and technically refined by Raoul Hafner, an Austrian working in England who would later contribute much to the development of the helicopter.6

The first practical helicopter was the Focke-Wulf Fw-61 of 1938 (Figure 1.1), followed in the same year by the Weir W-5 (Figure 1.2) that flew two years before Sikorsky’s VS-300 (Figure 1.3). The urgencies of war accelerated all technical development with the emergence of production helicopters, where the work of Anton Flettner (Figure 1.4) and Heinrich Focke was far in advance of anything taking place elsewhere.

The Focke-Achgelis Fa-223 shown in Figure 1.5 became, on 6 September 1945, the first helicopter to cross the English Channel when a captured machine was flown to England by Hans Gerstenhauer. This machine had a payload of 2000 kg.7

Helicopter development in the UK was halted by government order during World War II and, being an Austrian, Hafner was locked up, but fortunately not for long.

In Germany, production was hampered by Allied bombing, whereas US helicopters were unrefined. The result was the same: helicopters made little contribution to the war itself.

After World War II great progress was made in the understanding of helicopter dynamics and stability. This led directly to machines that were less stressful to fly and correspondingly safer. The Bell 47 (Figure 1.6) based on the research of Arthur Young was in 1946 the first helicopter to be certified. The Sycamore of 1952 (Figure 1.7) designed by Hafner, was the first British helicopter to be certified and was noted not just for its performance, but also for its light control forces which needed no power assistance.8

Advances in constructional techniques and materials continued to improve the service life of components, especially blades. Possibly the most significant single step

was the introduction of the turbine engine which was much lighter than the piston engine for the same power, yet had fewer moving parts. This allowed greater payload and a reduction in maintenance.

The first turbine-powered helicopter to fly was a modified Kaman K-225 in 1951 and in 1954 the first twin-turbine machine, also a Kaman, flew. The Bell Huey first flew in 1956 (Figure 1.8). It was in the 1960s that the many disciplines in helicopter design were finally mastered allowing the machine to be considered as a system. Some elegant and definitive designs emerged during this period. Such was the validity of their basic concepts that they could accept a steady succession of upgrades that would allow them in some cases to remain in service to the present day. Sikorsky’s S-61 and Boeing’s Chinook are good examples of longevity.

Since that time there have been few breakthroughs; instead there has been a steady process of refinement. The introduction of composite materials in blades, rotor heads and body parts has reduced weight and extended service life. Refinements in mechani­cal engineering have produced lighter engines and transmissions having longer life. Manufacturers have used production engineering to reduce the amount of labour needed to build machines.

Instead of a revolution, the employment of electronics and computers in helicopters has seen steady and relentless progress. There is no wear mechanism in electronics and complicated transfer functions can be realized in lightweight parts that use little power. Items such as turbine engine controllers and rotor rpm governors are ideal applications for electronics, along with stability augmentation systems. The flexible control systems that electronics make possible have enabled developments such as the tilt-rotor helicopter.

Helicopters can survive engine failures and the failure of a variety of parts, but there remain some parts such as gearboxes and rotor heads in which failure will be catastrophic. The technique of electronically monitoring critical components has made a great contribution to safety. Spontaneous failures are very rare. Generally there are symptoms such as a change in the characteristic of vibration or noise. These may be too slight to be heard by the crew, but a sensor mounted on the affected part in conjunction with a signal processor which knows what the normal sounds, or signature, from the part are, can give vital warning of a potential problem.

The life of a component may be reduced if it is subjected to higher stress. Modern electronic systems (HUMS: Health and Usage Monitoring Systems) can measure stress and the time for which it has been applied in order to calculate the safe remaining life in major components.

Much fundamental research into helicopters was done using models. De la Cierva’s early work was with free flying model gyroplanes. Arthur Young used models exten­sively (Figure 1.9) and it was his demonstration of a model controlled by a trailing wire that convinced Larry Bell to enter the helicopter business. Irven Culver at Lock­heed built what was probably the world’s first radio-controlled model helicopter in the late 1950s.

Advances in radio control equipment in the 1960s made the necessary precision available at reasonable cost and this led to the availability of flying model helicopters

for the model building enthusiast. As will be seen in Chapter 9, such models have become highly sophisticated even though, like their full-sized counterparts, they remain expensive to build and operate.

Conceptually somewhere between the model and the full-size helicopter is the UAV (unmanned autonomous vehicle). These machines are designed to perform surveillance tasks, typically carrying cameras and other sensors. As electronic and sensing devices have become smaller, useful equipment can be carried aloft at much lower cost if there is no need to carry a pilot. Unlike the model, which needs actively to be controlled by the pilot on the ground at all times, the UAV carries enough navigational equipment, automatic stabilization systems and processing power to be self-contained. Also unlike the hobbyist’s machines, UAVs must be built and operated to professional standards.

The definition of a rotorcraft is quite general, embracing any flying machine that produces lift from rotors turning in a plane that is normally close to the horizontal. This definition does not concern itself with the proportion of the machine’s weight carried by the rotors, or whether that proportion changes at different stages of flight. Figure 1.10 shows the main classes of rotorcraft that will be defined here.

The pure helicopter obtains the great majority of its lift in all modes of flight from a power driven rotor or rotors, and any lift due to airflow around the hull is incidental. The majority of today’s rotary wing machines are pure helicopters. The thrust from a rotor is closely aligned with a line drawn perpendicular to the tip path, and the pure helicopter propels itself by tilting the rotor forward to obtain a component of rotor thrust which balances the drag, as shown in Figure 1.10(a).

The gyroplane (also known by de la Cierva’s trade name of autogyro) obtains lift from an undriven rotor that must be tilted away from the direction of flight to make air flow up through it. The rearward rotor-thrust component, along with the drag, is balanced by the forward thrust of a conventional airscrew as shown in Figure 1.10(b). As the rotor needs to be pulled through the air to maintain height, the autogyro cannot hover in still air, although it can give the illusion of hovering by flying into wind. Simple autogyros must taxi to spin up the rotor, but later machines could spin the rotors with engine power on the ground, and use the stored energy to perform a jump take-off.

Between the pure helicopter and the gyroplane is the gyrodyne, which obtains lift from a power driven rotor. Unlike the pure helicopter, the gyrodyne maintains the rotor disc parallel to the direction of flight, as in Figure 1.10(c) and propels itself with a conventional airscrew. The Fairey Gyrodyne (Figure 1.11) replaced the tail rotor with a side-mounted airscrew to cancel torque reaction when hovering, but also to provide thrust for forward flight. More recently the Lockheed Cheyenne (Figure 1.12) had both anti-torque and pusher rotors at the tail. The gyrodyne offers high speed potential, with the penalties of raised complexity, weight and difficulty of control. Some gyrodynes have wings in addition to the rotor.

The compound helicopter is one that hovers like a helicopter, but which may obtain supplementary lift from fixed wings during flight and may incorporate additional means of providing forward thrust.

The convertiplane is a more extreme example of the compound helicopter in that it reconfigures its means of providing lift and propulsion in different flight regimes. The Fairey (later Westland) Rotodyne was a convertiplane. Figure 1.13 shows that it consisted of a twin turboprop aircraft-like structure with a pylon-mounted rotor driven by tip jets. As a helicopter, the tip jet drive provided lift, and yaw control was obtained by differentially changing the pitch of the turboprops. Forward thrust from the airscrews would bring the machine to cruising speed, where much of the lift was developed by the wing, and the tip jets were turned off such that the rotor free-wheeled and the machine became a compound gyroplane.

Reconfiguring can also be done by tilting the whole wing-engine-rotor assembly (tilt wing) as shown in Figure 1.14(a) or by tilting the engine-rotor units on fixed wings (tilt rotor) as in Figure 1.14(b). The Bell-Boeing Osprey is a tilt rotor. As can be seen in Figure 1.14(c), the diameter of convertiplane rotors is usually such that the machine cannot land in the forward flight configuration, but must return to the hover.

The advantages of the convertiplane over the pure helicopter are that using the rotors as airscrews reduces vibration, it is much more efficient and allows a higher airspeed. This reduces fuel consumed and allows greater range. The tilt rotor has its wing in the downwash which reduces hover performance, whereas in the tilt wing the wing is almost always working. However, the tilt wing needs a supplementary mechanism to control the pitch axis, such as a jet or rotor at the tail.

The various configurations of the pure helicopter will now be considered. The most common configuration is the single main rotor and the anti-torque tail rotor. The remaining configurations, shown in Figure 1.15, handle torque reaction by contra­rotation. Contra-rotating helicopters have no need for the tail rotor, but generally have a tail fin or fins for directional stability at speed.

In the tandem rotor helicopter, Figure 1.15(a), two rotors turn in opposite directions at the opposite ends of a long hull. The rotors are usually synchronized through a transmission system so that the shafts can be little more than a blade length apart. The Chinook in Figure 1.16 is a good example of the type, the large disc area offering good lifting ability, and the long cabin ample load space.

An alternative to the tandem rotor is the side-by-side twin rotor (Figure 1.15(b)). This has aerodynamic advantages in forward flight because the lifting area has a better aspect ratio, but it is difficult to avoid drag due to the structure needed to locate the rotors each side of the hull and to carry the transmission across the machine. See Figure 1.5. In large machines it is also difficult to make the structure stiff enough to avoid resonance and so the type is rare.

A relative of the side-by-side helicopter which has had more success is the synchropter (Figure 1.15(c)) in which the two rotors mesh so closely that the contra-rotating shafts can be driven by the same gearbox. The close meshing is achieved by tilting the shafts outwards so that the blades of one rotor can pass over the rotor head of the other. The German Kolibri of World War II, designed by Anton Flettner (Figure 1.4), was the first successful machine to use the idea. In the USA, Charles Kaman adopted the syn­chropter principle, and produced the famous H-43 Huskie which became the definitive crash rescue helicopter of its time (Figure 1.17). The synchropter is the easiest of all

helicopters to fly, as the interactions and second-order effects of the conventional configuration are eliminated, but replaced by some interesting yaw characteristics.

The final approach to contra-rotation is the coaxial helicopter. Stanley Hiller and Arthur Young both built such machines experimentally, but Nikolai Kamov in the USSR put the idea into production (Figure 1.18). The coaxial helicopter places both rotors one above the other on a common shaft, and drives them in opposite directions. Like the synchropter, control interactions are reduced, but yaw control remains an issue. The main advantage of synchropters and coaxial helicopters is that in the absence of a tail rotor, the machine can be much more compact, a crucial factor in naval aviation, where everything has to be squeezed into limited hangar space, although the height needed may increase. The alternative is to fit a folding tail on a conventional machine.

The helicopter contains a large number of systems and components, but these can generally be broken down into a smaller number of major areas. Figure 1.19 shows a cutaway drawing of a conventional tail rotor type helicopter. The main systems to consider in a helicopter are the hull or airframe, the engine and transmission, the fuel system, the landing gear, the rotors, the controls, electrical and hydraulic power,

instrumentation and avionics. These subjects will briefly be introduced in the remain­ing sections of this chapter, and references will be made to more detailed treatments elsewhere in the book.

The fuselage or hull has a number of jobs to perform. One obvious task is to hold all of the components in the correct position and to transfer forces from the rotors, the tail surfaces, the landing gear and any internal or underslung payload. It also protects the occupants and the mechanisms from the elements whilst still allowing good visibility for the pilot. Space has to be found for fuel tanks close to the centre of mass (CM) as single rotor machines are sensitive to trim shifts as fuel burns off. It will also have a more or less aerodynamic shape, although the other requirements often combine to prevent this. In some cases the hull will also be designed to float in case the machine is forced down over water, whereas in others, amphibious operation is planned.

Fuselage construction varies considerably, but the materials and techniques are not much different from those used in any aircraft. Early machines such as the Bell 47 were no more than a steel tube lattice frame with a blown acrylic canopy for the crew. Aerodynamic improvements came when the hull was faired in. The tail cone is often a stressed skin structure, but the centre section has too many doors, windows and access hatches for the skin to carry all the loads, and alloy frames or steel tubes are often used beneath the skin. The main landing gear often shares the same framework as the engine and transmission so that the skin is not unduly stressed on landing.

Figure 1.20 shows the structure of an Enstrom F-28. The landing skids, engine, fuel tanks and transmission are all attached to a welded tube frame known as a pylon. A stressed skin tail cone is attached to the rear of the pylon structure, and the aluminium seat and cabin floor is attached to the front. The cabin is glass fibre with plastics glazing. The cabin lines are faired into the tail cone by unstressed panelling attached to the pylon, so the machine has a sleek outline. Much of the unstressed centre panelling can be removed for servicing.

Composite materials are ideal for airframe construction and are becoming increas­ingly important since they are less dense than metals and are inherently well damped, which helps to control the inevitable vibration that characterizes helicopters. They can also have an indefinite fatigue life.

The engine or engines and transmission are generally close together. Piston engines are heavy, and they are almost always placed below the rotor head to balance the machine. Turbines are much smaller and lighter and are often built into the roof of the hull to maximize internal space. Both piston engines and turbines turn much faster than the rotor shaft, and the transmission must incorporate reduction gearing.

The gearbox will generate a good deal of heat on a large machine, and require an oil cooler. This will have its own air intake near the rotor shaft; often in the front of the pylon. The gearbox also drives the tail rotor shaft, which runs the length of the tail boom to the tail rotor gearbox. The tail rotor shaft is often mounted outside the tail cone for ease of inspection and maintenance.

The rotors may take some time to come to rest after the engines have been shut down, and this may be inconvenient. The civil user wants to disembark passengers with the rotors stopped. The naval user wants to put the machine on the elevator down to the hangar and the army user wants the rotors stopped quickly on a clandestine mission so that the machine can be camouflaged. This can be achieved by fitting a rotor brake on the transmission.

Helicopters have also been built with jets at the blade tips, and this has the advantages that no gearbox is required and there is no torque reaction. Unfortunately tip jets have short fatigue life, very high fuel consumption and noise level to match and are little used. They will not be considered further here.

As it is a major subject, the whole of Chapter 6 is devoted to helicopter engines and transmissions.

The fuel system can be simple or complex depending on the type of machine. In early piston engine machines using carburettors, the fuel system was little more than a pair of tanks that fed fuel by gravity through the pilot’s cut-off valve and a filter to the engine. The tanks in a gravity fed system are mounted one each side of the mast so that lateral and longitudinal trim is unaffected as fuel is consumed. The tanks are clearly visible on the Bell 47 and the Hughes 300. Larger machines will use the space below

the floor to put tanks close to the CM, in which case pumps will be required to feed the engines.

Piston engines burn AVGAS or aviation gasoline, which is basically similar to auto­mobile fuel but made to tighter quality standards. Turbines burn AVTUR that has a similar relationship to kerosene. As a piston engine will stop if AVTUR reaches it, pilots like to know they have taken on the right fuel. AVGAS fillers are marked red and AVTUR fillers are marked black to help prevent a dangerous mistake.

Recently there have been significant advances in the development of Diesel engines, allowing a similar power to weight ratio to a gasoline engine to be obtained. The advantage of the aeroDiesel is that it can burn AVTUR and its improved fuel economy allows better payload or range. A more detailed treatment of fuel systems can be found in Chapter 6.

The landing gear is subject to considerable variation. Utility and training helicopters are invariably fitted with skids to allow a landing on unprepared ground even with forward speed. Small wheels, known as ground handling wheels, can be fitted so the machine can be moved around. Some skids are broader than usual so they can act as skis for landing on snow or soft ground. Inflatable or rigid floats can be attached to the skids to permit operation over water, but there will be a drag and payload penalty. Larger machines invariably have wheels, as skids would make them too difficult to move. Naval helicopters need wheels to allow them to be moved below decks. In many cases the wheels can be locked so as to be tangential to a circle. The machine can be turned into the wind, but will not roll as the ship heels.

Landing gears often incorporate a telescopic section containing oil and a compressed gas acting as a spring. These are formally known as oleo-pneumatic struts, invariably abbreviated to oleos. When the length of the oleo changes, the oil is forced through a small orifice to damp the movement. The struts that hold up automobile tailgates work on the same principle, but these are sealed units whereas the type of oil and the gas pressure in an oleo may be adjusted to give the correct spring rate and damping.

One obvious purpose of the oleo is to absorb the impact of landing, but a more important role is to control ground resonance. Ideally the rotor blades rotate with perfectly even spacing when run up on the ground, but it is possible for them to be disturbed from that condition. This results in the CM of the rotor moving away from the shaft axis and the rotor tries to whirl the top of the hull in a circular orbit. Under certain conditions this motion becomes uncontrollable unless there is damping to dis­sipate the energy. The origin of ground resonance will be discussed in Chapter 4 where it will be shown that the rotor head may also need dampers to prevent the problem.

The main rotor takes the place of the wings of a conventional aircraft, and it is not unrealistic to think of a helicopter as being supported by the lift from wings that rotate instead of flying in a straight line. The main difference between rotor blades and

wings is that the former are in comparison very thin and flexible, and the forces acting upon them are greater and more rapidly varying. The rotors have more to do than an aeroplane wing, because they are also the control system. Chapter 3 explains how the rotors produce lift and introduces the control of the machine.

The control system cannot be treated in isolation, but must be integrated into the design of a machine from the outset. In the pure helicopter, control of the machine is achieved entirely by changing the pitch of the main and tail rotor blades in various ways. This will then determine the amount of engine power needed. The rotors are generally designed to turn at constant speed and the throttle setting will have to be modified whenever the rotor power demand changes so that the speed does not change. Chapter 6 considers engines and power control.

There are two main forces acting on a helicopter, the force due to gravity, which is always downwards, and the rotor thrust vector, which is always at right angles to the tip path plane, otherwise called the rotor disc. Chapter 2 explains how the result of forces acting in various ways can be predicted and Chapter 3 shows how rotors develop thrust. The pilot can control the magnitude of the rotor thrust with the collective pitch lever held in his left hand, and the direction of the rotor thrust with the cyclic stick held in his right hand. The cyclic stick works in two dimensions: if the stick is pushed in any direction, the rotor thrust tilts the same way. These two fundamental controls are illustrated in Figure 1.21.

The blade movements necessary to produce lift and to achieve control will be outlined in Chapter 3, whereas Chapter 4 treats the construction and dynamics of the blades

and rotor head. Chapter 7 integrates the control system and considers power assistance and auto-stabilization.

The pilot controls the yaw axis by altering the pitch of the tail rotor blades using foot pedals. Chapter 5 considers the tail system of the helicopter.

It should be mentioned in passing that the No. 1 pilot in a helicopter conventionally occupies the right hand seat; the opposite of aeroplane practice. In some early machines, only one collective lever was provided, between the seats. The test pilots would sit in the left seat, as per aeroplane practice. Because of the difficulty of reversing the function of left and right hands, a pilot under training would be placed in the right seat and so the right seat became the conventional location for a helicopter pilot. A further factor is that in early helicopters, the forces fed back to the cyclic stick from the rotor were such that it was not safe to let go of the stick for an instant. However, the collective lever could be released temporarily. Given that most secondary controls such as the radio and instruments are centrally disposed so that pilot and co-pilot can both see them, it made sense to put the pilot in command on the right where his free left hand would be of most use.

The power systems of a typical helicopter are not that much different from those of any aircraft. Electrical power is needed for instruments, radios, autopilots, lighting, engine starting, navigation and intercom systems, as well as a host of further avionics that might be needed for special purposes.

A light helicopter may have no power assistance, but as machines get larger the control forces may cause fatigue and in very large machines they will be beyond the strength of the pilot. Power-assisted controls then become essential. Power controls are also needed if some kind of automatic stabilization or autopilot is fitted so that the low powered electronic signals can operate the controls.

When powered controls operated by electrical signals have faultless reliability, the mechanical controls from the pilot can also be replaced by electrical controls, leading to the concept of fly-by-wire. The pilot operates controls having no mechanical connection to the rotors, but which instead produce electrical signals. This concept is explored in Chapter 7.

Electric motors are useful for low powered control purposes such as the trim mecha­nism, but hydraulics allows greater forces to be developed within small actuators, and so they will be used for powered flying controls.

Electrical and hydraulic power is vital to the safety of the machine, and the hydraulic pump and the generator may be driven from the rotor shaft so that power is still available even in the case of engine failure. In small machines the generator is driven from the engine, as battery capacity is enough to keep the electrical system working in the case of an engine failure. Larger machines may have two or more engines and each will have a generator so that electric power is still available in case of engine failure.

The electrical system is discussed in Chapter 6, and an explanation of hydraulic controls will be found in Chapter 7.

Many of the instruments fitted to helicopters are the same as those used in other aircraft, but in addition to the usual engine-related gauges the helicopter will also need

a rotor tachometer. The response of a helicopter to control inputs depends on rotor speed, and this needs to be controlled to close limits. The rotor tachometer and engine- related instruments are detailed in Chapter 6, whereas flight instruments are covered in Chapter 7.

References

1 Spenser, J. P., Whirlybirds, University of Washington Press, Seattle, ISBN 0-295-97699-3 (1998)

2 Spenser, IP., Vertical Challenge, University of Washington Press, Seattle, ISBN 0-295-97203-3

(1998)

3 Coates, S., Helicopters of the ThirdReich, Classic Publications, Hersham, ISBN 1-903223-24-5 (2002)

4 Boulet, J., LHistoire de l’Helicoptere, Editions France-Empire, Paris, ISBN 2-7048-0040-5 (1982)

5 DuMoulin, A., Les Helicopteres Florine 1920-1950, Fonds National Alfred Renard, Brussels

(1999)

6 Everett-Heath, J., British Military Helicopters, Arms and Armour Press, London, ISBN 0-85368-805-2(1986)

7 Nowarra, H. J., German Helicopters, Schiffer, West Chester, ISBN 0-88740-289-5 (1990)

8 Dowling, J., RAF Helicopters: the first twenty years, HmSo, London, ISBN 0-11-772725-3 (1992)