Brief History The history of aircraft structures underlies the history of aviation in general

Brief History
The history of aircraft structures underlies the history of aviation in general. Advances in materials and processes used to construct aircraft have led to their evolution from simple wood truss structures to the sleek modern aircraft used today. Combined with continuous powerplant development, the structures of aircraft have changed significantly.

The key discovery that “lift” could be created by passing an airmass over the top of a curved surface set the development of fixed and rotary-wing aircraft in motion. George Cayley developed an efficient cambered aerofoil in the early 1800s, as well as successful manned gliders later in that century. He established the principles of flight, proving the forces of lift, weight, thrust, and drag. It was Cayley who first stacked wings and created a tri-wing glider that flew a human being in 1853.

In the late 1800s, Otto Lilienthal expanded upon Cayley’s discoveries. He manufactured and flew his own gliders on over 2,000 flights. His willow and cloth aircraft had wings designed from extensive study of the wings of birds. Lilienthal also made standard use of vertical and horizontal fins behind the wings and pilot station. Above all, Lilienthal proved that man could fly.
The work of all of these men was known to the Wright Brothers when they built their successful, powered airplane in 1903. The first powered flying machine to carry a man aloft, the Wright Flyer had thin, cloth-covered wings attached to what was primarily truss structures made of wood. The wings contained forward and rear spars and were supported with both struts and wires. Stacked wings (two sets) were also part of the Wright Flyer. This paved the way for all powered aircraft of which helicopters descended. On September 14, 1939, the VS-300, the world’s first practical helicopter, took flight at Stratford, Connecticut. Designed by Igor Sikorsky and built by the Vought-Sikorsky Aircraft Division of the United Aircraft Corporation, the helicopter was this first to incorporate a single main rotor and tail rotor design.

Airframe General
Helicopter airframes consist of the fuselage, main rotor and related gearbox, tail rotor (on helicopters with a single main rotor), and the landing gear (Figure 1.1). Airframe structural components are constructed from a wide variety of materials.

Figure 1.1: Basic Helicopter Components
The earliest aircraft were constructed primarily of wood. Steel tubing and the most common material, aluminium, followed. Many newly certified aircraft are built from moulded composite materials, such as carbon fibre. Structural members of an aircraft’s fuselage include stringers, longerons, ribs, bulkheads, and more. The main structural member in a wing is called the wing spar.
Major Structural Stresses
Aircraft structural members are designed to carry a load or to resist stress. In designing an aircraft, every square inch of wing and fuselage, every rib, spar, and even each metal fitting must be considered in relation to the physical characteristics of the material of which it is made. Every part of the aircraft must be planned to carry the load to be imposed upon it.

The determination of such loads is called stress analysis. Although planning the design is not the function of the pilot, it is, nevertheless, important that the pilot understand and appreciate the stresses involved. The term “stress” is often used interchangeably with the word “strain.” While related, they are not the same thing. External loads or forces cause stress. Stress is a material’s internal resistance, or counterforce, that opposes deformation. The degree of deformation of a material is strain. When a material is subjected to a load or force, that material is deformed, regardless of how strong the material is or how light the load is.

There are five major stresses (Figure 1.2) to which all aircraft are subjected:
• Tension
• Compression
• Torsion
• Shear
• Bending

Figure 1.2: Stresses imposed during flight on aircraft structural components
Figure 1.2A: Tension is the stress that resists a force that tends to pull something apart. The engine pulls the aircraft forward, but air resistance tries to hold it back. The result is tension, which stretches the aircraft. The tensile strength of a material is measured in pounds per square inch (psi) and is calculated by dividing the load (in pounds) required to pull the material apart by its cross-sectional area (in square inches).

Figure 1.2B: Compression is the stress that resists a crushing force. The compressive strength of a material is also measured in pounds per square inch (psi). Compression is the stress that tends to shorten or squeeze aircraft parts.

Figure 1.2C: Torsion is the stress that produces twisting. While moving the aircraft forward, the engine also tends to twist it to one side, but other aircraft components hold it on course, like the tail rotor. Thus, torsion is created. The torsion strength of a material is its resistance to twisting or torque.

Figure 1.2D: Shear is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two riveted plates in tension subject the rivets to a shearing force. Usually, the shearing strength of a material is either equal to or less than its tensile or compressive strength. Aircraft parts, especially screws, bolts, and rivets, are often subject to a shearing force.

Figure 1.2E: Bending stress is a combination of compression and tension. The rod has been shortened (compressed) on the inside of the bend and stretched on the outside of the bend.

A single member of the structure may be subjected to a combination of stresses. In most cases, the structural members are designed to carry end loads rather than side loads. They are designed to be subjected to tension or compression rather than bending. Strength or resistance to the external loads imposed during operation may be the principal requirement in certain structures. However, there are numerous other characteristics in addition to designing to control the five major stresses that engineers must consider. For example, cowling, fairings, and similar parts may not be subject to significant loads requiring a high degree of strength. However, these parts must have streamlined shapes to meet aerodynamic requirements, such as reducing drag or directing airflow.

Types of Airframe Structures
Truss Type A truss is a rigid framework made up of members, such as beams, struts, and bars to resist deformation by applied loads. The truss-framed fuselage is generally covered with fabric. The truss-type fuselage frame is usually constructed of steel tubing welded together in such a manner that all members of the truss can carry both tension and compression loads (Figure 1.3). In some aircraft, principally the light, single engine models, truss fuselage frames may be constructed of aluminium alloy and may be riveted or bolted into one piece, with cross-bracing achieved by using solid rods or tubes.

Figure 1.3: Truss Type
Monocoque Type The monocoque (single shell) fuselage relies largely on the strength of the skin or covering to carry the primary loads. The design may be divided into two classes:
1. Monocoque
2. Semi-monocoque
Different portions of the same fuselage may belong to either of the two classes, but most modern aircraft are considered to be of semi-monocoque type construction. The true monocoque construction uses formers, frame assemblies, and bulkheads to give shape to the fuselage (Figure 1.4). The heaviest of these structural members are located at intervals to carry concentrated loads and at points where fittings are used to attach other units such as wings, power plants, and stabilizers. Since no other bracing members are present, the skin must carry the primary stresses and keep the fuselage rigid. Thus, the biggest problem involved in monocoque construction is maintaining enough strength while keeping the weight within allowable limits.

Figure 1.4: Monocoque
Semi-Monocoque Type To overcome the strength/weight problem of monocoque construction, a modification called semi-monocoque construction was developed. It also consists of frame assemblies, bulkheads, and formers as used in the monocoque design but, additionally, the skin is reinforced by longitudinal members called longerons. Longerons usually extend across several frame members and help the skin support primary bending loads. They are typically made of aluminium alloy either of a single piece or a built-up construction. Stringers are also used in the semi-monocoque fuselage. These longitudinal members are typically more numerous and lighter in weight than the longerons. They come in a variety of shapes and are usually made from single piece aluminium alloy extrusions or formed aluminium. Stringers have some rigidity but are chiefly used for giving shape and for attachment of the skin. Stringers and longerons together prevent tension and compression from bending the fuselage (Figure 1.5).

Figure 1.5: Semi-Monocoque
Fuselage of a Helicopter
The airframe, or fundamental structure, of a helicopter can be made of either metal, wood, or composite materials, or some combination of the aforementioned. Typically, a composite component consists of many layers of fibre-impregnated resins, bonded to form a smooth panel. Tubular and sheet metal substructures are usually made of aluminium, though stainless steel or titanium is sometimes used in areas subject to higher stress or heat. Airframe design encompasses engineering, aerodynamics, materials technology, and manufacturing methods to achieve favourable balances of performance, reliability, and cost.

As with fixed-wing aircraft, helicopter fuselages and tail booms are often truss-type or semi-monocoque structures of stress-skin design. Steel and aluminium tubing, formed aluminium, and aluminium skin are commonly used. Modern helicopter fuselage design includes an increasing utilization of advanced composites as well. Firewalls and engine decks are usually stainless steel. Helicopter fuselages vary widely from those with a truss frame, two seats, no doors, and a monocoque shell flight compartment to those with fully enclosed airplane-style cabins as found on larger twin-engine helicopters. The multidirectional nature of helicopter flight makes wide-range visibility from the cockpit essential. Large, formed polycarbonate, glass, or plexiglass windscreens are common.

Figure 1.6: Major Components of a Helicopter
Helicopter Main Rotor Blades
Since the very earliest concepts of rotor-powered aircraft the design and manufacture of helicopters has been greatly developed and refined. In addition to the increased knowledge of the aerodynamics of flight and the avionics of aircraft, one extremely important factor in the advancement of the design, production and performance of helicopters is the use of composite materials. With their great versatility and desirable properties such materials can be found in numerous layers of a helicopter. From the seats and the engine bay door to the fuselage and the tailplane, composites form an integral part of helicopters and their design. However, the component whose performance and service-life has perhaps benefited most significantly from the use of these materials is the rotor blade. A typical cross-section of this component is illustrated in Figure 1.7 below. 

Figure 1.7: Cross-section view of a typical rotor blade
Helicopter rotor blades were originally constructed of laminated wood and fabric; this design was retained until the 1960s, when the first steel and aluminium structures were introduced. These metal blades were a huge improvement on previous designs, amongst whose problems was mass alteration due to moisture absorption. However, despite this and other benefits, such as cheapness and ease of manufacture, steel and aluminium blades suffered from various design and structural problems. The most critical of these were poor fatigue resistance, and low strength-to-density ratios. These problems, together with many other design drawbacks were hugely reduced by the use of composite materials for rotor blade construction. 
Radical advancement in rotor blade design was made possible due to the structure and basic ingredients of composite materials, for example, glass fibre reinforced plastics (GFRP). These consist of glass fibres dispersed within a polymeric (chain like molecules) matrix, both of which determine the properties and characteristics of the resulting material. The matrix has several functions, the first being to bind the fibres together, allowing any external stresses to be conveyed and distributed to them. In addition, being ductile, relatively soft and with quite a high plasticity, the matrix is able to play its second role to prevent crack propagation between fibres. The fibres themselves have their own characteristics. They are produced by means of drawing continuous fibres, and are readily available at low cost. Their strength and chemical inertness also make them highly desirable for use in rotor blades.

Thus, composite materials such as GFRPs, offer many advantages over metals, including lightness, ease of manufacture, relative cheapness and strength. GFRPs do, however, have one major drawback; they lack stiffness, a vital property required of helicopter rotor blades. The solution to this problem lies in another variety of composite material called carbon fibre reinforced plastic (CFRP). The high strength constituent fibres used in these materials are manufactured from polyacrylonitrile (synthetic, semi crystalline organic polymer resin) (PAN).However, as is the case with GFRPs, these properties are dependent on fibre direction, since such sheets are anisotropic (Anisotropy, is the property of being directionally dependent, which implies different properties in different directions). To overcome this, sheets of fibre reinforced material are sandwiched together alternately at right-angles, as shown in figure 1.8 below. 
Figure 1.8: Lay-up of FRP’s
Thus, such composite materials can be tailored in such a way as to display desired properties in specific directions and areas.

Rotor Heads
The rotor system is the rotating part of a helicopter which generates lift. The rotor consists of a mast, hub, and rotor blades. The mast is a hollow cylindrical metal shaft which extends upwards from and is driven and sometimes supported by the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. The rotor blades are then attached to the hub by any number of different methods. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. There are three basic classifications: semi-rigid, rigid, or fully articulated. Some modern rotor systems, such as the bearingless rotor system, use an engineered combination of these types.

Semi-Rigid Rotor
A semi-rigid rotor system is usually composed of two blades that are rigidly mounted to the main rotor hub. The main rotor hub is free to tilt with respect to the main rotor shaft on what is known as a teetering hinge. This allows the blades to flap together as a unit. As one blade flaps up, the other flaps down. Since there is no vertical drag hinge, lead/lag forces are absorbed and mitigated by blade bending. The semi-rigid rotor is also capable of feathering, which means that the pitch angle of the blade changes. This is made possible by the feathering hinge (Figure 1.9).

Figure 1.9: The teetering hinge allows the main rotor hub to tilt, and the feathering hinge enables the pitch angle of the blades to change.

The underslung rotor system mitigates the lead/lag forces by mounting the blades slightly lower than the usual plane of rotation, so the lead and lag forces are minimized. As the blades cone upward, the centre of pressures of the blades are almost in the same plane as the hub. Whatever stresses are remaining bend the blades for compliance. If the semi-rigid rotor system is an underslung rotor, the centre of gravity (CG) is below where it is attached to the mast. This underslung mounting is designed to align the blade’s centre of mass with a common flapping hinge so that both blades’ centres of mass vary equally in distance from the centre of rotation during flapping. The rotational speed of the system tends to change, but this is restrained by the inertia of the engine and flexibility of the drive system. Only a moderate amount of stiffening at the blade root is necessary to handle this restriction.
Simply put, under-slinging effectively eliminates geometric imbalance. Helicopters with semi-rigid rotors are vulnerable to a condition known as mast bumping which can cause the rotor flap stops to shear the mast. The mechanical design of the semi-rigid rotor system dictates downward flapping of the blades must have some physical limit.
Mast bumping is the result of excessive rotor flapping. Each rotor system design has a maximum flapping angle. If flapping exceeds the design value, the static stop will contact the mast. It is the violent contact between the static stop and the mast during flight that causes mast damage or separation. This contact must be avoided at all costs. Mast bumping is directly related to how much the blade system flaps. In straight and level flight, blade flapping is minimal, perhaps 2° under usual flight conditions.
Flapping angles increase slightly with high forward speeds, at low rotor rpm, at high-density altitudes, at high gross weights, and when encountering turbulence. Manoeuvring the aircraft in a sideslip or during low-speed flight at extreme CG positions can induce larger flapping angles.

Rigid Rotor System
The rigid rotor system shown in Figure 4-3 is mechanically simple, but structurally complex because operating loads must be absorbed in bending rather than through hinges. In this system, the blade roots are rigidly attached to the rotor hub. Rigid rotor systems tend to behave like fully articulated systems through aerodynamics, but lack flapping or lead/ lag hinges. Instead, the blades accommodate these motions by bending. They cannot flap or lead/lag, but they can be feathered. As advancements in helicopter aerodynamics and materials continue to improve, rigid rotor systems may become more common because the system is fundamentally easier to design and offers the best properties of both semi-rigid and fully articulated systems.
The rigid rotor system is very responsive and is usually not susceptible to mast bumping like the semi-rigid or articulated systems because the rotor hubs are mounted solid to the main rotor mast. This allows the rotor and fuselage to move together as one entity and eliminates much of the oscillation usually present in the other rotor systems.
Other advantages of the rigid rotor include a reduction in the weight and drag of the rotor hub and a larger flapping arm, which significantly reduces control inputs. Without the complex hinges, the rotor system becomes much more reliable and easier to maintain than the other rotor configurations. A disadvantage of this system is the quality of ride in turbulent or gusty air. Because there are no hinges to help absorb the larger loads, vibrations are felt in the cabin much more than with other rotor head designs.

There are several variations of the basic three rotor head designs. The bearingless rotor system is closely related to the articulated rotor system, but has no bearings or hinges. This design relies on the structure of blades and hub to absorb stresses. The main difference between the rigid rotor system and the bearingless system is that the bearingless system has no feathering bearing—the material inside the cuff is twisted by the action of the pitch change arm. Nearly all bearingless rotor hubs are made of fibre-composite materials. The differences in handling between the types of rotor system are summarised at the end of this subsection in figure 1.12.

Fully Articulated Rotor System
Fully articulated rotor systems are found on helicopters with more than two main rotor blades. As the rotor spins, each blade responds to inputs from the control system to enable aircraft control. The centre of lift on the whole rotor system moves in response to these inputs to effect pitch, roll, and upward motion. The magnitude of this lift force is based on the collective input, which changes pitch on all blades in the same direction at the same time. The location of this lift force is based on the pitch and roll inputs from the pilot.
Therefore, the feathering angle of each blade (proportional to its own lifting force) changes as it rotates with the rotor, hence the name “cyclic control.” As the lift on a given blade increases, it tends to flap upwards. The flapping hinge for the blade permits this motion and is balanced by the centrifugal force of the weight of the blade, which tries to keep it in the horizontal plane (Figure 1.10).

Figure 1.10: Lead/Lag Hinge
Fully articulated rotor systems are found on helicopters with more than two main rotor blades. As the rotor spins, each blade responds to inputs from the control system to enable aircraft control. The centre of lift on the whole rotor system moves in response to these inputs to effect pitch, roll, and upward motion. The magnitude of this lift force is based on the collective input, which changes pitch on all blades in the same direction at the same time. The location of this lift force is based on the pitch and roll inputs from the pilot. Therefore, the feathering angle of each blade (proportional to its own lifting force) changes as it rotates with the rotor, hence the name “cyclic control.” As the lift on a given blade increases, it tends to flap upwards. The flapping hinge for the blade permits this motion and is balanced by the centrifugal force of the weight of the blade, which tries to keep it in the horizontal plane (Figure 1.11).

Figure1.11: Flapping Hinge

Figure 1.12: Advantages and disadvantages of different rotor configurations
Transmission Systems
The transmission system transfers power from the engine to the main rotor, tail rotor, and other accessories during normal flight conditions. The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. The freewheeling unit or autorotative clutch allows the main rotor transmission to drive the tail rotor drive shaft during autorotation. In some helicopter designs, such as the Bell BH-206, the freewheeling unit is located in the accessory gearbox. Because it is part of the transmission system, the transmission lubricates it to ensure free rotation. Helicopter transmissions are normally lubricated and cooled with their own oil supply. A sight gauge is provided to check the oil level. Some transmissions have chip detectors located in the sump. These detectors are wired to warning lights located on the pilot’s instrument panel that illuminate in the event of an internal problem. Some chip detectors on modern helicopters have a “burn off” capability and attempt to correct the situation without pilot action. If the problem cannot be corrected on its own, the pilot must refer to the emergency procedures for that particular helicopter.

Main Rotor Transmission System
The primary purpose of the main rotor transmission is to reduce engine output rpm to optimum rotor rpm. This reduction is different for the various helicopters. As an example, suppose the engine rpm of a specific helicopter is 2,700. A rotor speed of 450 rpm would require a 6:1 reduction. A 9:1 reduction would mean the rotor would turn at 300 rpm.

Most helicopters use a dual-needle tachometer or a vertical scale instrument to show both engine and rotor rpm or a percentage of engine and rotor rpm. The rotor rpm indicator is used during clutch engagement to monitor rotor acceleration, and in autorotation to maintain rpm within prescribed limits. It is vital to understand that rotor rpm is paramount and that engine rpm is secondary. If the rotor tachometer fails, rotor rpm can still be determined indirectly by the engine rpm since the engine supplies power to the rotor during powered flight. There have been many accidents where the pilot responded to the rotor rpm tachometer failure and entered into autorotation while the engine was still operating.

Look closer at the markings on the gauges in Figure 1.13. All gauges shown are dual tachometer gauges. The two on the left have two needles each, one marked with the letter ‘T’ (turbine) the other marked with the letter ‘R’ (rotor). The lower left gauge shows two arced areas within the same needle location. In this case, both needles should be nearly together or superimposed during normal operation. Note the top left gauge shows two numerical arcs. The outer arc, with larger numbers, applies one set of values to engine rpm. The inner arc, or smaller numbers, represents a separate set of values for rotor rpm. Normal operating limits are shown when the needles are married or appear superimposed. The top right gauge shows independent needles, focused toward the middle of the gauge, with coloured limitation areas respective to the needle head. The left side represents engine operational parameters; the right, rotor operational parameters.

Figure 1.13: Various types of tachometers
Structural Design
In helicopters with horizontally mounted engines, another purpose of the main rotor transmission is to change the axis of rotation from the horizontal axis of the engine to the vertical axis of the rotor shaft (Figure 1.14). This is a major difference in the design of the airplane power plant and power train whereas the airplane propeller is mounted directly to the crankshaft or to shaft that is geared to the crankshaft.

Figure 1.14: Helicopter Transmission systems
The importance of main rotor rpm translates directly to lift. RPM within normal limits produces adequate lift for normal manoeuvring. Therefore, it is imperative not only to know the location of the tachometers, but also to understand the information they provide. If rotor rpm is allowed to go below normal limits, the outcome could be catastrophic.

Tail Rotor Transmission System
The tail rotor drive system consists of a tail rotor drive shaft powered from the main transmission and a tail rotor transmission mounted at the end of the tail boom. The drive shaft may consist of one long shaft or a series of shorter shafts connected at both ends with flexible couplings. This allows the drive shaft to flex with the tail boom. The tail rotor transmission provides a right angle drive for the tail rotor and may also include gearing to adjust the output to optimum tail rotor RPM. See figure 1.15 below.

Figure 1.15: Tail Rotor Transmission
Clutch Systems
In a conventional airplane, the engine and propeller are permanently connected. However, in a helicopter there is a different relationship between the engine and the rotor. Because of the greater weight of a rotor in relation to the power of the engine, as compared to the weight of a propeller and the power in an airplane, the rotor must be disconnected from the engine when the starter is engaged. A clutch allows the engine to be started and then gradually pick up the load of the rotor.

Freewheeling turbine engines do not require a separate clutch since the air coupling between the gas producer turbine and the power (take-off) turbine functions as an air clutch for starting purposes. When the engine is started, there is little resistance from the power turbine. This enables the gas producing turbine to accelerate to normal idle speed without the load of the transmission and rotor system dragging it down. As the gas pressure increases through the power turbine, the rotor blades begin to turn, slowly at first and then gradually accelerate to normal operating rpm.

On reciprocating and single-shaft turbine engines, a clutch is required to enable engine start. Air, or wind milling starts are not possible. The two main types of clutches are the centrifugal clutch and the idler or manual clutch.

How the clutch engages the main rotor system during engine start differs between helicopter designs. Piston powered helicopters have a means of engaging the clutch manually just as a manual clutch in a car. This may be by means of an electric motor that positions a pulley when the engine is at the proper operating condition (oil temperature and pressure in the appropriate range), but it is controlled by a cockpit mounted switch.

Belt Drive Clutch
Some helicopters utilize a belt drive to transmit power from the engine to the transmission. A belt drive consists of a lower pulley attached to the engine, an upper pulley attached to the transmission input shaft, a belt or a set of V-belts, and some means of applying tension to the belts. The belts fit loosely over the upper and lower pulley when there is no tension on the belts (Figure 1.16).

Some aircraft utilize a clutch for starting. This allows the engine to be started without requiring power to turn the transmission. One advantage this concept has is that without a load on the engine starting may be accomplished with minimal throttle application. However, caution should also be used during starting, since rapid or large throttle inputs may cause overspeed. Once the engine is running, tension on the belts is gradually increased.
When the rotor and engine tachometer needles are superimposed, the rotor and the engine are synchronized, and the clutch is then fully engaged. Advantages of this system include vibration isolation, simple maintenance, and the ability to start and warm up the engine without engaging the rotor. When the clutch is not engaged, engines are very easy to overspeed, resulting in costly inspections and maintenance. Power, or throttle control, is very important in this phase of engine operation.

An electric clutch uses the belts to transfer the power from the engine to the rotor system (one shaft connected to both main and TR). These belts run round a third pulley which can be moved by an electric motor. When the aircraft is stopped and at rest (engine not running), the belts are slack. Once the aircraft has been started then a switch on the panel is used to run the electric motor, pull the pulley and tighten the belts. The belts slowly tighten, the rotor starts to move and increases rotational velocity. Engine and rotor rpm are monitored on the gauge, and when the needles line up (‘join’), the switch is left on, covered and caged to avoid accidentally switching off in flight

Figure 1.16: Belt Drive Clutch
Centrifugal Clutch
The centrifugal clutch is made up of an inner assembly and an outer drum. The inner assembly, which is connected to the engine driveshaft, consists of shoes lined with material similar to automotive brake linings. At low engine speeds, springs hold the shoes in, so there is no contact with the outer drum, which is attached to the transmission input shaft. As engine speed increases, centrifugal force causes the clutch shoes to move outward and begin sliding against the outer drum. The transmission input shaft begins to rotate, causing the rotor to turn slowly at first, but increasing as the friction increases between the clutch shoes and transmission drum. As rotor speed increases, the rotor tachometer needle shows an increase by moving toward the engine tachometer needle. When the two needles are superimposed, the engine and the rotor are synchronized, indicating the clutch is fully engaged and there is no further slippage of the clutch shoes.

The turbine engine engages the clutch through centrifugal force, as stated above. Unless a rotor brake is used to separate the automatic engagement of the main driveshaft and subsequently the main rotor, the drive shaft turns at the same time as the engine and the inner drum of the freewheeling unit engages gradually to turn the main rotor system.

Figure 1.17: Centrifugal Clutch

Figure 1.18: Centrifugal Clutch
Swashplate
A swashplate is a device that is used to transmit the pilot’s commands from the non-rotating fuselage to the rotating rotor hub and blades. The fact that the rotor blades are rotating at a very high speed makes the swashplate mechanism’s task more challenging. The mechanism consists of two main parts: a stationary and a rotating swashplate. The stationary swashplate is able to tilt in all directions and move vertically. The rotating swashplate is mounted on the stationary swashplate by means of a bearing, and is allowed to rotate with the main rotor mast.

A swashplate mechanism controls the cyclic and collective pitch of the rotor blades. The cyclic pitch of the rotor blades is used to change a helicopter’s roll and pitch. To tilt the helicopter forward, the difference of lift around the blades should be at a maximum along the left-right plane, creating a torque that, due to the gyroscopic effect, will tilt the helicopter forward instead of sideways. This is accomplished by tilting the swashplate assembly through pushrods. Collective pitch of the rotor blades, responsible for the average lift force, can be changed by moving the swashplate assembly vertically without tilting it. See figure 1.19 below which illustrates where the rotating and non-rotating parts are in relation to the transmission and rotors.

Figure 1.19: Swashplate
Helicopter Flying Controls
There are three major controls in a helicopter that the pilot must use during flight. They are the collective pitch control, the cyclic pitch control, and the anti-torque pedals or tail rotor control. In addition to these major controls, the pilot must also use the throttle control, which is usually mounted directly to the collective pitch control in order to fly the helicopter. In this chapter, the control systems described are not limited to the single main rotor type helicopter but are employed in one form or another in most helicopter configurations. All examples in this chapter refer to an anti-clockwise main rotor blade rotation as viewed from above. If flying a helicopter with a clockwise rotation, left and right references must be reversed, particularly in the areas of rotor blade pitch change, anti-torque pedal movement, and tail rotor thrust.

Collective Pitch
The collective pitch control (or simply “collective” or “thrust lever”) is located on the left side of the pilot’s seat and is operated with the left hand. The collective is used to make changes to the pitch angle of the main rotor blades and does this simultaneously, or collectively, as the name implies. As the collective pitch control is raised, there is a simultaneous and equal increase in pitch angle of all main rotor blades; as it is lowered, there is a simultaneous and equal decrease in pitch angle. This is done through a series of mechanical linkages and the amount of movement in the collective lever determines the amount of blade pitch change (Figure 1.20). An adjustable friction control helps prevent inadvertent collective pitch movement.

Changing the pitch angle on the blades changes the angle of incidence on each blade. With a change in angle of incidence comes a change in drag, which affects the speed or revolutions per minute (rpm) of the main rotor. As the pitch angle increases, angle of incidence increases, drag increases, and rotor rpm decreases. Decreasing pitch angle decreases both angle of incidence and drag, while rotor rpm increases. In order to maintain a constant rotor rpm, which is essential in helicopter operations, a proportionate change in power is required to compensate for the change in drag. This is accomplished with the throttle control or governor, which automatically adjusts engine power.

Figure 1.20: Raising and lowering the collective lever
Cyclic Pitch Control
The cyclic pitch control is usually projected upward from the cockpit floor, between the pilot’s legs or between the two pilot seats in some models (Figure 1.21). This primary flight control allows the pilot to fly the helicopter in any direction of travel: forward, rearward, left, and right. The total lift force is always perpendicular to the tip-path plane of the main rotor. The purpose of the cyclic pitch control is to tilt the tip-path plane in the direction of the desired horizontal direction. The cyclic controls the rotor disk tilt versus the horizon, which directs the rotor disk thrust to enable the pilot to control the direction of travel of the helicopter.

The rotor disk tilts in the same direction the cyclic pitch control is moved. If the cyclic is moved forward, the rotor disk tilts forward; if the cyclic is moved aft, the disk tilts aft, and so on. Because the rotor disk acts like a gyro, the mechanical linkages for the cyclic control rods are rigged in such a way that they decrease the pitch angle of the rotor blade approximately 90° before it reaches the direction of cyclic displacement, and increase the pitch angle of the rotor blade approximately 90° after it passes the direction of displacement.
An increase in pitch angle increases AOA; a decrease in pitch angle decreases AOA. For example, if the cyclic is moved forward, the AOA decreases as the rotor blade passes the right side of the helicopter and increases on the left side. This results in maximum downward deflection of the rotor blade in front of the helicopter and maximum upward deflection behind it, causing the rotor disk to tilt forward.

Figure 1.21: Cyclic pitch control
Anti-torque Control
The anti-torque pedals, located on the cabin floor by the pilot’s feet, control the pitch and therefore the thrust of the tail rotor blades or other anti-torque system. Newton’s third law states that for every action there is an equal but opposite reaction. This holds true for the rotor system which rotates at high rpm. As such there must be an equal and opposite force on the fuselage which is realised as a twisting force or torque. To make flight possible and to compensate for this torque, most helicopter designs incorporate an anti-torque rotor or tail rotor. The anti-torque pedals allow the pilot to control the pitch angle of the tail rotor blades, which in forward flight puts the helicopter in longitudinal trim and, while at a hover, enables the pilot to turn the helicopter 360°. The anti-torque pedals are connected to the pitch change mechanism on the tail rotor gearbox and allow the pitch angle on the tail rotor blades to be increased or decreased thus moving the nose either left or right (Figure 1.22).

Figure 1.22: Anti-torque pedals
Landing Gear
The undercarriage or landing gear in aviation is the structure that supports an aircraft on the ground and allows it to taxi, take-off and land. Typically wheels are used, but skids, skis, floats or a combination of these and other elements can be deployed, depending on the surface. Landing gear usually includes wheels equipped with shock absorbers for solid ground, but some aircraft are equipped with skis for snow or floats for water, and/or skids or pontoons (helicopters). Skids are used mainly because they weigh less than wheels. The weight limitation for skid type landing gear is accepted to be approximately 4000Kg, anything above this and a helicopter will incorporate a wheel design undercarriage.

Skid Undercarriage
The standard landing gear assembly (landing gear) supports the helicopter when it is in contact with the ground. The landing gear can withstand loads made during landing, ground handling, and provides a stable platform to prevent ground resonance (Diverging oscillations). The landing gear primarily absorbs normal landing forces, with the capabilities to absorb severe landing forces during overload conditions. The landing gear dimensions are based on the required minimum roll-over and minimum pitch-over angles. A minimum angle of 27 degrees is maintained from the centre of gravity (CG) location to the skid-to ground contact point.

Skid Landing gear has the following parts:
1. Forward and Aft Cross Tubes
2. Forward and Aft Saddle Assemblies
3. Side Stop Clamp Assemblies
4. Abrasion Strip
5. Landing Gear Damper Assemblies
6. Skid Tubes
In light weight Helicopters skids are mostly used because they possess less weight , it’s very easy for maintenance and they weigh less than wheels but the main disadvantages of skids helicopters are
They can’t taxi on the runway, so they can’t reposition to the required place like hangers, dispersal and they can’t move from one place other place on ground.

When the helicopters lands at high speed the skids get damaged because at that time weight acts fully on the skids.

Tricycle Undercarriage (Wheels)
On larger, more powerful helicopters, wheels are used because the utility and convenience can be more important than the savings in weight. In order to move a skid-equipped helicopter on the ground, one has to attach a set of ground-handling wheels, jack up the helicopter, and roll it (into the hangar for maintenance, for example). If your helicopter already has the wheels as a permanent feature, is more convenient to move around when the engine is shut down or the pilot has wandered off. The design decision between retractable or fixed wheels becomes a trade-off between the complexity/weight but increased aerodynamic efficiency of retractable gear and the simplicity of fixed gear (and increased drag/reduced efficiency). It really depends on the primary use of the helicopter; if you are logging, skids make sense because you can lift larger loads and are more concerned with hovering performance. If the primary mission is medevac or air transport, retractable wheels allow greater speed and increased fuel economy over long distances. 
Wheels are generally used in heavy and more powerful helicopters because in order to move such helicopters on the ground one has to attach a set of ground-handling wheels, roll it into the hangar for maintenance or for other purpose. As this is a heavy and well equipped helicopter it needs more maintenance. Some advantages of wheeled Helicopters are
They can taxi on the runway.

They can land on the ground at great speed with causing any damage to the helicopter because the load will be equally distributed among the wheels. Finally the special feature of this type of helicopter is they can Take Off by moving at high speed on the ground as the way Aircraft’s Take Off. In addition, at heavier weights, hotter climates, and higher altitudes it may be impossible to make a hovering take-off or landing, and a rolling one may be required.

Wheels and Tyres
Aircraft wheels are an important component of the landing gear system. With tyres mounted upon them, they support the entire weight of the helicopter during taxi, repositioning and landing. The typical wheel is lightweight, strong and made from aluminium alloy. Some magnesium alloy wheels also exist. Early wheels were of a single piece construction, much the same as the modern automobiles of today. As aircraft tyres were improved for the purpose they serve (Figure 1.24), so were the wheels. They were made stiffer to better absorb the forces of landing without blowing out or separating from the rim. Stretching such a tyre over a single piece wheel rim was not an option and so a two piece wheel was developed. As the design progressed, it developed into a design with nearly two identical halves. This is mostly used in aviation today (Figure 1.23).

Figure 1.23: Split Wheel

Figure 1.24: Helicopter Tyres
Helicopter Braking Systems
Braking is provided with a power brake system used to operate the main wheel brakes. The pressure is manually generated in slave control units (Brake Master Cylinders), which in turn operates a main control unit (Brake Control Valve) which meters fluid from a pressure generating system. The power brake system operates at a system pressure of 1000 psi (68 bars), 2000 psi (137bar) and 3000 psi (206bar), obtained by reducing aircraft hydraulic system pressure using a Pressure Reducing Valve. Brake Master Cylinders (BMC) are mounted below each yaw pedals for both crew members. They are toe operated to generate hydraulic pressure outputs, meaning this is an on-demand system. The hydraulic pressure outputs from BMCs are connected to Brake Control valves (BCV). The BCV meters the brake system pressure in proportion to the outputs from the BMCs and sends to each brake. For each brake higher of two demands from both seats controls the metering.

Advantages of braking other than stopping the aircraft are steering of the aircraft on ground, restriction of forward speed when taxying, residual thrust from the engines often being applicable, holding the aircraft stationary when applying collective keeping in mind that to taxi the entire rotor disk is tilted forward, and for parking (Figure 1.25).

Figure 1.25: Braking system layout.

Helicopter Shocks
An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations.

It is undesirable for an airplane to bounce on landing—it could lead to a loss of control. The landing gear should not add to this tendency. A steel coil spring stores impact energy from landing and then releases it (One of the disadvantages of skids). An oleo strut absorbs this energy, reducing bounce.
As the strut compresses, the spring rate increases dramatically, because the air is being compressed, while the viscosity of the oil dampens the rebound movement (Figure 1.27)
It is important that the oleo struts are charged to the correct pressure to avoid undesirable effects like ground resonance which will be discussed in a separate chapter.

Figure 1.27: Extension and Compression of an Oleo Strut
Helicopter Gust Locks
A gust lock on a helicopter is a mechanism that locks control surfaces and keeps open aircraft doors in place while the aircraft is parked on the ground and non-operational. Gust locks prevent wind from causing unexpected movements of the control surfaces and their linked controls inside the aircraft, as well as aircraft doors on some aircraft. Otherwise wind gusts could cause possible damage to the control surfaces and systems, or nearby people, cargo, or machinery. Some gust locks are external devices attached directly to the helicopter control surfaces, while others are attached to the flight controls inside the cockpit.

Generally helicopters have hydraulic control locks for the main rotor system which is a damping system within the hydraulic system. The tail rotor however has a mechanical lock to avoid the tail rotor blades flapping excessively in the wind and causing damage to the stops.

If the helicopter does not have control locks then tie downs can be used for the main and tail rotor. This prevents excessive flapping in high wind and inadvertent rotation of the main and tail rotor (Figure 1.28).

Figure 1.28: Helicopter with rotor tie downs.