Introduction to Helicopters Aerodynamics

INTRODUCTION The wings of the airplane create a lift force

when they move through the air. As we known, during flight, there are four forces acting on the helicopter and those are LIFT , DRAG ,

THRUST ,and WEIGHT. In order to make the wings to move through the air , of course, the

helicopter itself has to move. A helicopter works by having its wings move through the air while the body stays still. The helicopter's wings are called Main Rotor Blades. The shape and the

angle of the blades move through the air will

determine how much Lift force is created. After

the helicopter lifted off the ground, the pilot can

tilt the blades, causing the helicopter to tip forward or backward or sideward.

HISTORICAL BACKGROUND The helicopter is arguably one of the earliest ideas for achieving flight. Over two thousand years ago, the Chinese constructed what are

known as Chinese Tops. These simple toys consisted of a propeller attached to a stick that would be spun rapidly through ones hands to spin the propeller and achieve lift.

Chinese top "Helicopter"

Leonardo da Vinci's

Later, in the 15th Century, famed inventor and artist Leonardo da Vinci designed one of the most pleasing concepts for a helicopter, but such a craft was never actually constructed. In

England

constructed

in

1796,

the

first

Sir

George

powered

Cayley

models

of

helicopters that were driven by elastic devices which attained an altitude of 90ft. In 1842, fellow Englishman W. H. Phillips constructed a model helicopter that weighed 20 pounds (9 kg) and was driven by steam. In 1878, Enrico Forlanini,

an

Italian

civil

engineer,

also

constructed a steam driven model helicopter that only weighed 3.5kg.

Sir George Cayley's helicopter

The first manned helicopter to rise vertically completely unrestrained was constructed by Paul Cornu, a French mechanic, in 1907. Cornu's

helicopter had two propellers that were rotated at 90 rpm by a 18 kW engine. Cornu was most probably the first helicopter experimenter who was concerned with control. While cornu’s helicopter

was

historically

significant,

its

performance and control was rather marginal and it was never a practical machine.

Cornu's helicopter

The next influential development in the field of helicopters was brought about by a man who never actually built a helicopter himself. In 1923, Juan de la Cierva successfully flew his

C.4 autogiro, an aircraft that has two propellers, a powered one to provide thrust, and an un powered rotor to provide lift. Cierva's autogiro was noteworthy because it was the first to use an "articulated" rotor that allowed its blades to flap up and down in response to aerodynamic forces on the blades during forward flight.The first recognized helicopter record was set in October 1930 by Italian Corradino D'Ascanio when he flew his helicopter over a distance of one half mile at an altitude of 59 ft (18 m) for 8

minutes

and

45

seconds.

D'Ascanio's

helicopter had two contra rotating coaxial rotors (two rotors on the same shaft) that were

controlled by flaps on booms trailing each blade near its tip.

D'Ascanio's helicopter

Just before and during World War II, Germany made several large, significant steps in helicopter development. The FA-61 helicopter, designed by Heinrich Focke, first flew in June 1936, and was later used in publicity stunts by the Nazis. The FL-282 helicopter, designed by Anton Flettner, became operational with the German Navy, and

over

1000

of

them

were

produced.

This

helicopter utilized twin-intermeshing rotors, had a forward speed of 145 km/h, and could operate at an altitude of 3,965 m with a payload of 360 kg.

Sikorsky's VS-300

The first American helicopter was the VS-300, designed by Igor Sikorsky of the VoughtSikorsky Company. The VS-300 was the first helicopter to use a tail rotor to counteract the torque produced by the main rotor, and it was this innovation that solved the last major hurdle

in making helicopters practical flying vehicles. This design is now the most common in today's helicopters.

NOMENCLATURE AND TECHNICAL TERM

Bernoulli's principle: This principle states that as the air velocity increases, the pressure decreases; and as the velocity decreases, the pressure increases. Airfoil: is technically defined as any surface, such as an airplane aileron, elevator, rudder, wing, main rotor blades, or tail rotor blades designed to obtain reaction from the air through which it moves.

Angle of Attack: is the acute angle measured between the chord of an airfoil and the relative wind.

Angle of Incidence: is the acute angle between the wing's chord line and the longitudinal axis of the airplane. Blades : The blades of the helicopter are airfoils with a very high aspect ratio ( length to chord ). The angle of incidence is adjusted by means of the control from pilots. The main rotor of the helicopter may have two, three, four, five or six blades, depending upon the design. The main rotor blades are hinged to the rotor head in such a manner that they have limited movement up and down and also they can change the pitch (angle of incidence). The controls for the main rotor are called Collective and Cyclic Controls.

The tail rotor is small blades may have two or four blades and mounted on the tail of the helicopter, it rotates in the vertical plane. The tail rotor is controlled by the rudder pedals. Its pitch can be changed as required to turn the helicopter in the direction desired.

Blade Root: The inner end of the blades where the rotors connect to the blade gripos. Blade Grips: Large attaching points where the rotor blade connects to the hub. Rotor Hub: Sit on top of the mast, and connects the rotor

blades to the control tubes. Main Rotor Mast: Rotating shaft from the transmission which connects the main rotor blades to helicopter fuselage.

Pitch Change Horn: to converts control tube movement to blade pitch. Control tube is a push-pull tubes that change the pitch of the rotor blades through the pitch changing horn. Swash Plate Assembly: The swash plate assembly consists of two primary elements through which the rotor mast passes. One element is a disc, linked to the cyclic pitch control. This disc is capable of tilting in any direction but does not rotate as the rotor rotates. This non-rotating disc, often referred to as the Stationary Star is attached by a bearing surface to a second disc, often referred to as the Rotating Star which turns with rotor and linked to the rotor blade pitch horns.

Transmission: The transmission system transmits engine power to the main rotor, tail rotor, generator and other accessories. The engine is operated at a relative high speed while the main rotor turns at a much lower speed. This speed reduction is accomplished through reduction gears in the Transmission System Lift: is produced by a lower pressure created on the upper surface of an airplane's wings compared to the pressure on the wing's lower surfaces, causing the wing to be LIFTED upward. The special shape of the airplane wing (airfoil) is designed so that air flowing over it will have to travel a greater distance and faster resulting in a lower pressure area thus lifting the wing upward. Lift is that force which opposes the force of gravity (or weight).

Lift depends upon (1) shape of the airfoil (2) the angle of attack (3) the area of the surface exposed to the airstream (4) the square of the air speed (5) the air density.

Relative Wind: is the direction of the airflow with respect to an airfoil or to the rotor blades. Pitch Angle : The rotor blade pitch angle is the acute angle between the blade chord line and the rotor plane of rotation. This pitch angle can be varied by the pilot through the use of cockpit controls (collective and cyclic pitch control). Skids: are used mainly because they weigh less than 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). If your helicopter already has the wheels as a permanent feature, it is more convenient to move around when the engine is shut down

or the pilot has wandered off.

GENERAL Lift is obtained by means of one or more power driven horizontal propellers which called Main Rotor. When the main rotor of helicopter turns it produces lift and reaction torque. Reaction torque tends to make helicopter spin. On most helicopters, a small rotor nears the tail which called tail rotorcompensates for this torque. On twin rotor

helicopter the rotors rotate in opposite directions, their reactions cancel each other.

Main Rotor: The lifting force is produced by the main rotor . As they spin in the air and produced the lift. Each blade produces an equal share of the lifting force. The weight of a helicopter is divided evenly between the rotor blades on the main rotor system. If a helicopter weight 4000 lbs and it has two blades, then each blade must be able to support 2000 lbs. In addition to the static weight of helicopter ,each blade must be accept dynamic load as well . Tail Rotor: The tail rotor is very important. If you spin a rotor with an engine, the rotor will rotate, but the engine and helicopter body will tend to rotate in opposite direction to the rotor. This is called Torque reaction. Newton's third law of

motion states, “to every action there is an equal and opposite reaction”. The tail rotor is used to compensate for this torque and hold the helicopter straight. On twin-rotors helicopter, the rotors spin in opposite directions, so their reactions cancel each other.

The tail rotor in normally linked to the main rotor via a system of driveshaft’s and gearboxes , that means if you turn the main rotor , the tail rotor is also turn. Most helicopter have a ratio of 3:1 to 6:1. That is, if main rotor turn one rotation, the tail rotor will turn 3 revolutions (for 3:1)or 6 revolutions (for 6:1).

Dissymmetry of Lift: All rotor systems are subject to Dissymmetry of Lift in forward flight. At a hover, the lift is equal across the entire rotor disk. As the helicopter gain air speed, the advancing blade develops greater lift because of the increased airspeed and the retreating blade will produce less lift, this will cause the helicopter to roll.

Blade Flapping: Dissymmetry of lift is compensated by Blade flapping. Because of the increased airspeed and lift on the advancing blade will cause the blade to flap up and decreasing the angle of attack. The decreased lift on the retreating blade will cause the blade to flap down and increasing the angle of attack. The combination of decreased angle of attack on the advancing blade and increased angle of attack on the retreating blade through blade flapping action tends to equalize the lift over the two halves of the rotor disc.

The Collective Control: When pilot raises the collective control or pull collective control up, the collective control will raises the entire swash plate assembly as a unit. This has effect to the blades by changing the pitch of all blades simultaneously .This causes to increase angle of attack and give more lift. The Cyclic Control: The cyclic control will push one side of the swash plate assembly up or down. This has the effect to the rotor head system because the cyclic control or cyclic stick controls the angle of the main rotor by angling the rotor head to which all the blades are attached .This cause the helicopter to move left or right, forward or backward.

Anti torque Pedals:

The Thrust produced by the auxiliary (tail) rotor is governed by the position of anti torque pedals. These are not rudder pedals, although they are in the same place as rudder pedals on an airplane. They are linked to a pitch change mechanism in the tail rotor gear box to permit the pilot to increase the pitch of the tail rotor blades. The primary purpose of the tail rotor and its controls is to counteract the torque effect of the main rotor.

Flight Direction Control Function of Controls There are three major controls in the helicopter that the pilot must use during flight. They are : ( 1 ) Collective pitch control. ( 2 ) Anti Torque Pedals or Tail Rotor Control. ( 3 ) Cyclic Stick Control.

Collective Control: The collective pitch lever or stick is located by the left side of the pilot's seat and is operated with the left hand. The collective is used to increase main rotor pitch at all points of the rotor blade rotation. It increases or decreases total rotor thrust. The collective lever is connected to the swash plate by a series of bush pull tubes. Raising the collective

lever increases the pitch on the main rotor blade, lowering the collective lever decreases the main rotor blade pitch. The amount of movement of the lever determines the amount of blade pitch change. As the angle of attack increase, drag increases and Rotor RPM and Engine RPM tend to decrease. As the angle of attack decreases, drag decreases and the RPM tend to increase. Since it is essential that the RPM remain constant, there must be some means of making a proportionate change in power to compensate for the change in drag. This coordination of power change with blade pitch angle change is controlled through a collective pitch lever- throttle control cam linkage which automatically increases power when the collective pitch lever is raised and decreases power when the lever is lowered.

Collective Lever is connected to the rotor system via push pull tubes. It also has droop com pensation devics which sense change in the collective pitch lever and increases or decreases fuel to the engine automatically somewhat in anticipated of a change in power required. This helps to minimize the RPM fluctuations during collective pitch change. Engine Control (Emergency) is the throttle twist grip. During emergency condition, between flight and flight idle positions. This is useful during any event which would cause engine or rotor RPM to go too high or while landing after a tail rotor failure. Idle Release Button, when the throttle is rolled from " off " to " idle " the idle release button snaps into a detent which

prevents the throttle from being rolled back to " off " Starter Button Pushing this button will cause the starter / generator to act as a starter motor ( Starter / Generator is a component that function in either mode as a starter or generator ) , turning over the engine. Landing Light Switch has a three position which are “off”, “forward” and "both”. In forward, only the forward light is activated. In both, the forward and downward lights are activated. Power Trim Switch ,by holding it in " increase " or " decrease " the pilot can set the RPM that the pilot attempt to maintain. Anti-Torque Pedals or Tail Rotor Control: In accordance with Newton's law of action and reaction, the helicopter fuselage tends to rotate in the direction opposite to the rotor blades . This effect is called torque. Torque must be counteracted and controlled to make flight is possible. Compensation for torque in a single main rotor helicopter is accomplished by means of a variable pitch antitorque rotor (tail rotor) located on the end of the tail boom extension at the rear of fuselage.

Heading Control : In addition to counteracted torque, the tail rotor and its control linkage also permit control of the helicopter heading during flight. Application of more control than is necessary to counteract torque will cause the nose of helicopter to turn in the direction of pedal movement.

In forward flight, the pedals are not used to control the heading of the helicopter (except during portions of crosswind takeoff and approach). They are used to compensate for torque to put the helicopter in longitudinal trim so that coordinated flight can be maintained. The thrust of the tail rotor is depend upon the pitch angle

of the tail rotor blades. The tail rotor may have a positive pitch angle or it may have a negative pitch angle which to push the tail to the right or pull the tail to the left.

With the right pedal pressed or moved forward of the neutral position will cause the tail rotor blades to change the pitch angle and the nose of helicopter will yaw to the right. With the left pedal pressed or moved forward of the neutral position will cause the tail rotor blades to change the pitch angle opposite to the right pedal and the nose of helicopter will yaw to the left.

Cyclic Control:

As mention earlier , the total lift force is always perpendicular to the tip-path plane of the main rotor. When the tip path plane is tilt away from the horizontal, the lift -thrust force is divide into two components of forces that are, the horizontal acting force, thrust and the upward acting force, lift.

The purpose of the cyclic pitch control is to tilt the tip path plane in the direction that horizontal movement is desired. The thrust component of force then pulls the helicopter in the direction of rotor tilt. The cyclic control changes the direction of this force, thus controlling the attitude and air speed of helicopter.

The rotor disc tilts in the same direction of the cyclic stick was moved. If the cyclic stick is moved forward, the rotor disc tilts forward: if the cyclic is moved aft, the rotor disc tilt aft, and so on. The rotor disc will always tilt in the same direction that the cyclic stick is moved.

The radio switch is used for pilot to transmit radio by clicking the switch. The trim switch, pilot use this switch to neutralize stick force. Pilot can use the trim switch to put the stick to the right, left, forward or backward. This runs electric motor which will tension the spring which will tend to hold the stick. The cyclic will stay where it is even the pilot were let it go . This also releases tension from pilot. The cargo release switch is the option switch; some manufacturer may have other function switch.

AEROFOIL THEORY AND PROPELLOR ACTION AEROFOIL THEORY:An aerofoil is a streamlined body, which is designed to produce lift or thrust when passed through air. Airplane wings, propeller blades and helicopter main and tail rotor blades are all aerofoil.

Aerofoil features

Chord is the distance or imaginary line between the leading and trailing edge of an airfoil. The amount of curve or departure of the airfoil surface from the chord line is known as camber. Upper chamber refers to the upper surface; lower camber refers to the lower surface. If the surface is flat, the camber is zero. The camber is positive if the surface is convex. The camber is negative if the surface is concave. The upper surface of an airfoil always has positive camber, but the lower surface may have positive, negative, or zero camber.

BERNOULLIS PRINCIPLE: Bernoulli, an eighteenth century physicist, discovered that air moving over a surface decreases air pressure on the surface. As air speed increases, surface air pressure decreases accordingly. This is directly related to the flight of an aircraft. As an airfoil starts moving through the air, it divides the mass of air molecules at its leading edge. The distance across the curved top surface is greater than that across the relatively flat bottom surface. Air molecules that pass over the top must therefore move faster than those passing under the bottom in order to meet at the same time along the trailing edge. The faster airflow across the top surface creates a low-pressure area above the airfoil. Air pressure below the airfoil is greater than the pressure above it and tends to push the airfoil up into the area of lower pressure. As long as air passes over the airfoil, this condition will exist. It is the difference in pressure that causes lift. When air

movement is fast enough over a wing or rotor blade, the lift produced matches the weight of the airfoil and its attached parts. This lift is able to support the entire aircraft. As airspeed across the wing or rotor increases further, the lift exceeds the weight of the aircraft and the aircraft rises. Not all of the air met by an airfoil is used in lift. Some of it creates resistance, or drag, that hinders forward motion. Lift and drag increase and decrease together. The airfoil’s angle of attack into the air, the speed of airflow, the air density, and the shape of the airfoil or wing therefore affect them.

Bernoulli’s principle The amount of lift that an aerofoil develop depends on 1. Area (size or surface area of the air foil) 2. Shape (shape or design of airfoil sections) 3. Speed (Velocity of the air passing over the aerofoil)

4. Angle of attack (angle at which air strikes the aerofoil) 5. Air density (amount of air in a given space)

PROPELLORS:The production of thrust in helicopters is based on the propeller action. The rotation of propeller causes the air to accelerate from one side to the other side of it, which results in the development of thrust in the opposite direction of the flow. A propeller does the conversion of torque into axial thrust by changing the momentum of the fluid in which it is submerged. When a propeller submerged in an undisturbed fluid rotates, it exerts a force on the fluid and pushes the fluid backwards. The reaction to this force on the fluid provides a forward thrust, which is used for propulsion. Although the complete design of a propeller cannot be done according to the momentum theory, yet the application of this theory leads to some useful results s indicated by simple analysis of problem below.

Propeller

Let U be the upstream velocity and u be the downstream velocity. Let A be the propeller disc area and Q the mass flow rate of air. By Bernoulli’s principle we get the velocity through the propeller equal to average of upstream and far down stream velocities. Therefore the induced velocity u through the propeller equals, u

U u u U U  2 2

Pr opulsive Power  Thrust  Propulsive velocity Uu    A   u  U U 2    u2 U 2  2 

Uu  Power Input   A  2  

Propulsive Efficiency 

1

 

Propulsive Power Power Input

1





u 2U

If P is the power supplied and T the thrust developed then from momentum theory we have P 1  T 2

T A

This formula is applied for hovering condition of the helicopter where torque T equals weight to be supported. The actual flow through the propeller differs considerably from the model depicted above since the propeller works in an “infinite sea of air “; there is no well-defined boundary between the fluid at rest and fluid motion; therefore the actual thrust will differ considerably from the values in the above expressions.

CONFIGURATION OF HELICOPTERS

SINGLE ROTOR HELICOPTER:The most popular helicopter arrangement is that of single rotor using a tail rotor. The single rotor helicopter is relatively lightweight and is fairly simple in design with one rotor one main transmission and one set of controls. The disadvantage of single rotor machine are its limited lifting and speed capabilities and a severe safety hazard during ground operation with the tail rotor position several feet behind the pilot and out of line of his vision.

Single rotor helicopter

TANDEM ROTOR HELICOPTER:This helicopter uses two synchronized rotor rotating in opposite direction. The opposite rotation of the rotors causes one rotor to cancel the torque of the other.

Each rotor is fully articulated and has three blades. It is capable of lifting large loads. A disadvantage of the tandem type is that it is not efficient in forward flight because one rotor is working in the wake of the other.

Tandem rotor helicopter

SIDE-BY-SIDE HELICOPTER:It has two main rotors mounted on pylons or wings positioned out from the sides of the fuselage. The side by side has rotors turning in opposite direction, which eliminates the need for a tail rotor.The advantages are its excellent stability and disadvantage is having high drag and structural weight both resulting from structure necessary to support the main rotor.

Side by side helicopter COAXIAL HELICOPTER: In this fuselage torque is eliminated by two counter rotating rigid main rotors mounted one above the other on common shaft.

Coaxial helicopter

TILT ROTOR AIRCRAFT:The tilt rotor has the ability to combine the vertical take off low speed capabilities of the helicopter with high-speed performance of a turboprop airplane.

GYROSCOPIC PRECESSION The term gyroscopic precession describes an inherent quality of rotating bodies in which an applied force is 0

manifested 90 in the direction of rotation from the point where the force is applied. Since the rotor of a helicopter has a relatively large diameter and turns at several hundred revolutions per minute precession is a prime factor in controlling the rotor operation. The cyclic pitch control causes variation in the pitch of the rotor blades as they rotate about the circle of the tip path plane. The purpose of this pitch change is in part to cause the rotor disc to tilt in the direction in which it is desired to make the helicopter move. When only the aerodynamic effects of blades are considered it would seem that when the pitch of the blades is high the lift would be high and

the blade would rise. Thus if the blades had high pitch as they passed through one side of the rotor disc the side of the disc having low pitch should rise and the side having low pitch should fall. This would be true except for gyroscopic precession. Gyroscopic precession is caused by a combination of a spinning

force

and

an

applied

acceleration

force

perpendicular to the spinning force. Thus if force is applied perpendicular to the plane of rotation the precession will 0

cause the force to take effect 90 from the applied force in the direction of rotation.As a result of the fore going principle, if a pilot wants the main rotor of a helicopter to tilt in a particular direction, the applied force must be at a 0

angular displacement 90 ahead of the desired direction of tilt. The required force is applied aerodynamically by changing the pitch of the rotor blades through the cyclic pitch control. When the cyclic control is pushed forward the blade at left increases its pitch as the blade on right

decreases pitch. This applies an up force to the left hand side of the rotor disc, but the up movement is therefore at rear of the rotor plane and the rotor tilts forward. This applies a forward thrust and causes the helicopter to move forward.

VIBRATION Any type of machine vibrates. However greater than normal vibration usually means that there is a malfunction. Malfunctions can be caused by worn bearings, out-ofbalance conditions, or loose hardware. If allowed to continue unchecked, vibrations can cause material failure or machine destruction. Aircraft -- particularly helicopters -have a high vibration level due to their many moving parts. Designers

have

been

forced

to

use

many

different

dampening and counteracting methods to keep vibrations at acceptable levels. Some examples are

1. Driving secondary parts at different speeds to reduce harmonic vibrations; this method removes much of the vibration build up. 2. Mounting high-level vibration parts such as drive shafting on shock-absorbent mounts. 3. Installing vibration absorbers in high-level vibration areas of the airframe. LATERAL: Lateral vibrations are evident in side-to-side swinging rhythms. An out-of-balance rotor blade causes this type of vibration. Lateral vibrations in helicopter rotor systems are quite common. VERTICAL:Vertical vibrations are evident in up-and-down movement that produces a thumping effect. An out-of-track rotor blade causes this type vibration.

HIGH-FREQUENCY High-frequency vibrations are evident in buzzing and a numbing effect on the feet and fingers of crewmembers. High-frequency vibrations are caused by an out-of-balance condition or a high-speed, moving part that has been torque incorrectly. The balancing of high-speed parts is very important. Any build-up of dirt, grease, or fluid on or inside such a part (drive shafting for example) causes a high-frequency vibration. This type vibration is more dangerous than a lateral or vertical one because it causes crystallization of metal, which weakens it. This vibration must be corrected before the equipment can be operated.

GROUND RESONANCE

Ground resonance is the most dangerous and destructive of the vibrations discussed here. Ground resonance can destroy a helicopter in a matter of seconds. It is present in helicopters with articulated rotor heads. Ground resonance occurs while the helicopter is on the ground with rotors turning it will not happen in flight. Ground resonance results when unbalanced forces in the rotor system cause the helicopter to rock on the landing gear at or near its natural frequency. Correcting this problem is difficult because the natural frequency of the helicopter changes as lift is applied to the rotors. With all parts working properly, the design of the helicopter landing gear, shock struts, and rotor blade lag dampeners will prevent the resonance building up to dangerous levels. Improper adjustment of the landing gear shock struts, incorrect tire pressure, and defective rotor blade lag dampeners may cause ground resonance. The quickest way to remove ground resonance is to hover the helicopter clear of the ground.

CYCLIC CONTROL The tip path plane, or TPP, is the plane connecting the rotor blade tips as they rotate. While hovering, the thrust vector of a helicopter is oriented upward, perpendicular to the tip path plane. In order for the helicopter to travel forward, this thrust vector needs to be rotated slightly in the forward direction. To rotate the thrust vector, it is in turn necessary to rotate the TPP by the same amount.

Tip path planes and thrust vectors for hovering and forward flight

Since tilting the rotor hub or rotor shaft is impractical, an alternative means of rotating the TPP is needed. Most modern helicopters use a system of swash plates. Seen in the following diagram, the swash plate system is composed of upper and lower swash plates.

Cyclic control and swash plates

The lower swash plate remains stationary relative to the helicopter. The upper swash plate rotates with the rotor, while remaining parallel to the lower swash plate. By utilizing what is called cyclic control, the swash plates can be angled so as to vary the Pitch of the blades depending on their azimuth angle. As the swash plates are tilted in the

proper direction, there is an increased lift on the aft portion of the rotor, causing the blades to flap up, which in turn causes the TPP to rotate forwards. As the TPP rotates forwards, the thrust vector does as well, imparting a forward acceleration to the helicopter.

MOMENTUM THEORY The first analytical theory to consider for a helicopter in forward (no axial) flight is the momentum theory. The analysis for vertical (axial) flight is very similar to that of a simple propeller, and will not be discussed here. One notable result of that analysis, however, is the induced velocity of the rotor in hover.

Where w is the disc loading, given by

In the terms of basic momentum theory, the thrust of a rotor in no axial flight is very difficult to derive. In the context of this discussion, a relationship for the thrust that was proposed by Glauert in 1928 will be used. A simple diagram of an actuator disk in no axial flow is depicted below.

Actuator disk in no axial flow

The thrust of the actuator disk can be given by

Far downstream from the disk, the downwash vf is doubled. Also, the term

becomes the mass flow through the

stream tube that is defined by the actuator disk. Some validity

for

these

relationships

can

be

inferred

by

comparing them to the formula for the lift of a wing having 2R span with a uniform downwash. The lift of such a wing is expressed by an equation similar to that shown above.

After

assuming

that

this

equation

is

valid,

determining the thrust requires that the induced velocity in forward flight be determined.

These two equations allow the determination of thrust and induced velocity of a helicopter in forward flight.

STRENGTH DESIGN REQUIREMENTS

AND

The helicopter structure must be strong enough to with stand all the loads expected to be experienced in service life. This comprises large loads, which are experienced rarely, and repetitive small to medium loads which are experienced in a normal flight. Where as large loads are important in designing the non-rotating parts of helicopter like the fuselage, the tail boom, the landing gear etc. The repetitive loads are important in designing the rotating parts such as the main rotor, the tail rotor, the shafts, the main rotor gearbox, the tail rotor gearbox etc. ROTOR STRUCTURE: The rotor blade structure must possess sufficient strength to with stand not only the aerodynamic loads generated on the blade surface but also the inertial loads arising from the centrifugal, the coriolis, the gyroscopic and the vibratory effects produced by the blade movement .the blade must also possess sufficient stiffness and rigidity to prevent excessive deformation and

to assure that the blades will maintain the desired aerodynamic characteristics. VIBRATION: The vibration, its causes and reduction are as discussed previously. SERVICE LIFE:While considering the expected service life of the helicopter or its components all types of expected loads must be considered. Three basic factors, which govern the service life, are 1. Corrosion 2. Creep and 3. Fatigue STRUCTURAL MATERIALS:Some of the important factors, which govern the selection of material for airframe and the primary load selection of material for airframe and the primary load bearing members of the helicopter, are 1. A high strength to weight ratio 2. Stiffness 3. Specific gravity

4. Resistance to impact loads 5. Temperature effects 6. Corrosion resistance 7. Fatigue strength 8. Rate of crack propagation

The Rotor Mechanism

The Autogiro's blades had evolved into long slender units with a good airfoil shape - true rotating wings, as opposed to the primitive, fan-shaped "airscrew" rotors found on many early helicopters. The lengthy blades of the Autogiro turned through a greater circle than the stubby short-span rotors then being tried for helicopters, thus providing that much more disc area to support the weight of the aircraft. An aeronautical engineer would describe this advantage as a "lower disc loading" (less weight for each square foot of disc area), and eventually helicopter experimenters followed this lead. Another basic improvement, stemming from Cierva's work and perhaps even more important than the shape of the blades, was the system for hinging each blade to the hub. This arrangement permitted each blade to flap and to adjust to the unequal lift forces created on opposite sides of the rotor disc as the aircraft sped into forward flight.

The airflow patterns created by the forward flight of an airplane an Autogiro, and a helicopter are compared here. With the Autogiro, the rotor mast is inclined to the rear and the airflow into the rotor is from the front and below. In the helicopter, the rotor is inclined forward and the flow enters from above

There were other important benefits gained from experience with the Autogiro. The use of autorotation itself to turn an unpowered rotor pointed the way to a means of making a safe power-off landing in a helicopter. The late-model direct-control and jump-take-off giros of the 1930's went even further: improved systems of control to the rotor head, methods for making pitch changes to the blades, mechanical drive systems-all were developed, and frequently by trial and error. Not that current methods are so very different. It is probably more true of helicopter

design than any other phase of aerospace engineering that the personal element still enters into the equation. The creative process of designing a successful helicopter particularly the rotor system — to this day has something in common with the free-for-all experimentation of the aeronautical pioneers. Although designers have created an impressive number of rotor systems, it is possible to narrow the field down to three basic types: articulated, semi-rigid, and rigid rotors. There are rotor systems that seem to fall outside this threefold classification, but for the most part these are only variations or combinations of the three types. In this connection, it should be appreciated that the term "rotor" or "rotor system" refers to a single unit only, composed of just one hub and the blades attached to it. A helicopter may have more than one main rotor; multiple arrangements of two, three, four or even more rotors have been found on various aircraft at different times in history, but each rotor is considered a separate system. For the purposes of the explanations that follow (which deal primarily with the various types of rotor hubs and the workings of cyclic

pitch control) we will be concerned primarily with the most widely used type, the Sikorsky configuration, which has just one main rotor, in combination with a small tail rotor. The most important part of the system is the hub at the center. Here are concentrated all the forces generated by the movement of the blades through the air; aerodynamic, centrifugal, and inertia factors are involved that create very great loads which simultaneously pull the blades upward and outward. The hub is designed, for the most part, to accommodate and control these forces automatically, and the working of its mechanism is the very essence of the helicopter's mechanical nature. In dealing with the three basic types of rotor systems we will describe how some of these forces affect the rotor hub. The articulated rotor system is the oldest; it appeared on the Autogiros of the 1920's and was incorporated in the first workable helicopters of the 1930's. (The Autogiro ancestry of the articulated rotor prompted an earlier name, the "Cierva rotor.") Today it is perhaps still the most widely used type, in one form or another.

In a helicopter with an articulated rotor system, there are three kinds of movement for the rotor blade as it turns around the mast: up and down (flapping), back and forth in the horizontal plane (lead and lug), and changes in the pitch angle

In this system, exclusive of the rotation of the blades about the mast, each individual blade is attached so that it can move in three different ways about the hub. One movement is common to almost all helicopters and types of rotor systems: the turning of the blades along their span-wise axis, owing to the action of the pilot's controls, in order to change the pitch angle. The other two kinds of motion, however, are not under the pilot's immediate control. These are movements the blades make in response to the powerful natural forces acting on the rotor, for which the articulated hub provides the necessary mechanisms — specifically, hinges — which permit freedom of movement so the blades can "articulate," or flap up and down and move back

and forth slightly in the horizontal plane. The pivot which permits the up-and-down movement is usually called the "flapping" hinge, while the fore-and-aft pivot, mounted vertically, which allows the blade to move back and forth slightly in the horizontal plane, is called the "drag" hinge.

Flapping hinges in the rotor hub help to adjust the unequal lift forces in

the right and left halves of the rotors circle as the helicopter moves through the air in forward flight. The hinges permit the blades to rise and

fall as they turn, thus varying the angle of attack so as to equalize the lift forces

The flapping hinge provides the blades with flapping freedom, which permits each blade to rise and fall, as it turns, so the tip rides higher or lower in its circular path. While the hinge may be located very close to the center of the rotor drive shaft, it is more frequently designed to be a short distance from this center line. This is termed an "offset" flapping hinge, and it offers the designer a number of important advantages. The flapping motion is the result of the constantly changing balance between lift, centrifugal, and inertial forces; this rising and falling of the blades is characteristic of most helicopters and has often been compared to the beating of a bird's wing. One other point should be mentioned; the flapping hinge, in company with the natural flexibility found in most blades, permits the blade to droop considerably when the helicopter is at rest and the rotor is not turning over. During flight the necessary rigidity is provided by the powerful centrifugal force which results from the rotation of the blades; this

force pulls outward from the tip, stiffening the blade, and is actually the only factor which keeps it from folding up.

Drawing shows root attachment of rotor blade to an articulated hub. The flapping hinge permits each blade to rise and fall as it turns, and the vertically mounted drag hinge allows lead-lag motion

The vertically mounted drag hinge as we have already noted, permits each blade to move back and forth slightly in the horizontal plane independently of the movement of the other blades. The terms "dragging," "hunting," and "lead-lag" are also used to describe this movement, which is necessary to relieve the powerful forces that might otherwise bend and even break the blades. To prevent this back-and-forth hunting from developing into serious vibration, it is restricted by hydraulic dampers which slow

down and "damp" the movement; this action is very similar to the damping effect of an ordinary hydraulic door-closer. The early Autogiros, incidentally, used friction discs to accomplish the same thing. The position that the blades actually assume while the helicopter is in flight obviously is the result of the various forces acting upon them. Normally, the blades will be lagged back slightly on the drag hinge and tilted up a few degrees on the flapping hinge; this upward tilt is termed the "coning angle" and is the result of the lifting force pulling upward on each blade while, simultaneously, centrifugal force is pulling outward. Since the centrifugal loading is so much greater, the blades only tilt upward a few degrees, and their path through the air takes the form of a shallow cone. The articulated type of rotor is designed to leave the blades as free as possible, to avoid trying to restrict their natural tendencies to flap up and down or move in the horizontal plane. One effect is that the blades can be very slender and light, since great strength is required only to resist the

tension of the powerful centrifugal force pulling along the span of the blade. An articulated blade is designed to have the inertial, centrifugal, and aerodynamic forces developed in flight all balancing about the same point on the blade chord—this is ordinarily one-fourth of the way back from the leading edge, or, as it is called, the "quarter chord point." Balancing the forces in this manner makes it possible for the pilot to control the blades with a minimum of effort and tends to hold down vibration as well. These highly desirable characteristics are reasons why the traditional articulated rotor is still so widely used. The other two types of rotors are the semi-rigid and the rigid (or "hingeless"). Both are primary types currently in use, and both duplicate the function of the articulated rotor. Though different mechanisms are involved, the aerodynamic effects are essentially the same. In the semi-rigid rotor (sometimes called a "rocking hub" or "teetering" rotor), the blades are attached rigidly to the hub but the hub itself is free to tilt in any direction about the top of the mast. Although there is no lead-lag

movement, the blades can still flap or, in the true sense, rock up and down in order to compensate for dissymmetry of lift when moving forward. Semi-rigid rotors have appeared on helicopters with two, three, and four blades and provide some simplification, although they cause other problems. One important advantage is the fact that there are no drag hinges, and therefore no drag dampers are required. But there are complications including the necessity for providing a type of universal joint between the drive shaft and the rotor hub. The rigid rotor, which until fairly recently was still in the experimental stage, is used in relatively few helicopters. In theory the rigid rotor is similar to an ordinary propeller; the blades are fixed to the hub without hinges and the hub in turn is fixed to the shaft. Of the various systems, it is closest to the elemental concept of the airscrew which tantalized experimenters in centuries past. (Obviously, there can be no such thing as a completely rigid rotor, since all blades inherently exhibit some degree of flexibility—from a structural viewpoint it would be almost impossible to build a truly rigid blade.) Since there are no flapping

hinges, or any other provisions for movement at the hub, other systems have been developed to overcome the unequal forces on the rotor, including pre-coning and feathering of the blades. Pre-coning, as the word suggests, is an arrangement for presetting the blade at a slight upward angle from the hub to the tip. This is the same angle that the blade would ordinarily take, due to its coning upward in normal flight. If the upward tilt for average operating conditions is determined, and the blades mounted on the hub at this angle, the bending loads can be reduced materially. Preconing is thus a fairly simple design approach for dealing with the stresses on a semi-rigid or rigid rotor. Feathering, on the other hand, involves the incorporation of an entirely new mechanism in the rotor head. This system compensates for the lift differential between the advancing and retreating blades by reducing the angle of attack as the blade starts to rise and decreasing it as the blade starts to fall; this, of course, means that the blade has to be mounted on the hub, so that it can be rotated along its

span-wise axis. As part of the system, the hub mechanism can be designed so the pitch changes are made automatically by the flapping (in this case the term "coning" is sometimes used, as well) of the blades as they turn. As the blade starts to flap, it activates linkage which changes the blade's angle of attack. This technique has been incorporated in many modern helicopters; the arrangement has been called "pitch-cone coupling." The semirigid and rigid rotor systems represent attempts to simplify helicopter design, but the end result more often than not has usually been the need for added complications such as pre-coning or pitch-cone coupling, which tend to defeat the designer's original aim. This pattern has been repeated over and over again in the development of new rotor systems and of other parts of the helicopter, as well; the designer succeeds in simplifying one mechanism and finds that he has to add another device somewhere else in the system. The problem of trying to reduce complexities that refuse to be banished has plagued designers since the days of the

first helicopters. One experimenter, D. H. Kaplan, in writing of the intricacies of the rotor cyclic control system, summed up one part of the puzzle thus: "In a cycliccontrolled rotor, every time the designer tries to deny the blade a freedom, it demands compensation somewhere else in the rotor mechanism. The history of the helicopter is filled with attempts to reduce complication... invariably this turns into a game of Chinese checkers as the designer feverishly moves the complicated problem from one part to another, never getting rid of it." As with the other mechanisms found on a modern helicopter, the rudiments of the cyclic system can be traced back to the Autogiro, on which the first effective rotatingwing controls were developed. The designers of the first Autogiros of the early 1920's did not attempt to control the rotor blades directly. Instead, conventional airplane-type controls were furnished-rudder, elevators, ailerons mounted on stub wings—and the rotor was controlled by the aerodynamic forces on these surfaces.

Since the ailerons were outside the propeller slipstream (the direct blast of air from the propeller), at low airspeeds they were the weakest link in the system; eventually a method was devised for obtaining lateral control by "rocking" the rotor hub from side to side. This meant that the ailerons and the stub wings that supported them could be dispensed with, and the wingless Autogiro appeared. The rocking head played a part in the development of cyclic pitch control systems. When the system was applied to some of the early helicopters, it was used for rocking the head not only from side to side but in all directions; in effect the hub was now mounted on a kind of universal joint. The idea was that by tilting the movable head (when the pilot moved his control stick) the axis of rotation would be inclined slightly from the vertical, and thus its lift would pull slightly in that direction. However, there were problems when this system was used with the power-driven rotor of a helicopter, caused by the drive shaft as it rotated the tilted hub. When the hub was tilted the mass of the rotor was no longer "on center" over the shaft, and this

caused serious vibration. Also, very great control forces were needed to move the rotor head. The answer to this was the cyclic pitch control used on the majority of helicopters today. It had been discovered that you could get the same effect as rocking the hub by increasing the pitch of the blades in cycles as they rotated. As each blade swept through its full 360-degree circle, it changed pitch cyclically—that is, it assumed a high pitch at one point in the disc and then assumed a low pitch as it moved around to the opposite position. As the pitch was increased or decreased, the blades rose or fell on their flapping hinges, thus inclining the disc slightly from its vertical axis. The effect was that the tip path of the blades, as the rotor whirled around, was very much the same as it would have been with a rocking head rotor inclined in that direction. What made this approach particularly attractive was that at the time of this experimentation, many rotating-wing aircraft, helicopters and rotorplanes alike, were already furnished with collective pitch control. This was a system

for changing the pitch on all blades to the same degree, simultaneously, in order to take off vertically, and the blades were therefore mounted on bearings so they could be moved for pitch control along the span-wise axis. All that was needed was the mechanical system for controlling the pitch of the blades cyclically as well as collectively. The device in a helicopter control system which accomplishes this, feeding the cyclic control movements to the rotor hub, is known by the rather interesting name of "swash plate." It is a doughnut-shaped unit that fits around the mast, actually consisting of two plates—an upper one and a lower one—with a bearing between the two. The upper plate is connected to the rotor hub by rods and consequently it spins around on the bearing as the rotor turns. The lower plate is linked to the pilot's cyclic control stick system and does not rotate. However this lower plate is mounted on pivots—either a spherical bearing or a gimbal ring—so that it can be tilted in any direction. As it tilts, the upper plate (which is moving in company with the hub) will be tilted as well. This results in a constant up and down movement in the link rods which connect the

upper plate to the hub, accomplishing the cyclic pitch changes in the blades as they sweep around, and thus transmitting the pilot's control movements to the rotor. While it is not intended here to delve too deeply into the design of the cyclic control system, there is another factor which should be considered, since it helps explain the workings of the linkage from the swash plate to the rotor hub. This is called the "90-degree phase lag" or time lag. When the pitch of a rotor blade is increased, the blade does not immediately rise but has to rotate for approximately another 90 degrees (a quarter of a revolution) before it reaches the highest flapped position; thus, there is a lag of approximately 90 degrees, attributed to blade inertia and gyroscopic factors, between the point at which the pitch is increased or decreased and the point where the full effect registers on the blade. Most helicopters have the control linkage from the swash plate to the hub offset by approximately 90 degrees to compensate for this; the pitch change is fed into the rotor at a point one-quarter of a revolution early in the plane of rotation. For example, when the pilot pushes the cyclic control stick forward so as to

incline the rotor forward, as each blade comes around it will receive the decrease in pitch at the 90-degree point on the right (advancing) side and the increase in pitch at the opposite point on the left (retreating) side. Because of the time lag, each blade is in its highest flapped position directly over the tail of the helicopter and its lowest flapped position directly over the nose. This, of course, inclines the rotor disc forward as desired to propel the helicopter into forward flight. In connection with the design of cyclic systems, one vital consideration is that the forces and loads acting on the tip of the blade are hundreds of times greater than the control forces which can be transmitted from the hub to the blade. The tip is going to go where it pleases, and the hub must be designed either to provide it with mechanical freedom through the use of hinges or, through structural flexibility, to move as it must under its dynamic loads. Even the socalled rigid rotor tends to behave like an articulated rotor because of the bending of the blades. (A blade rigid enough to resist these forces would be too heavy to fly.)

There is one important structural rigidity, however, that is essential to the correct functioning of a cyclic control system. The blade must be constructed so that it will not twist when pitch changes are made at the hub; it must have what a designer calls "torsional rigidity." If the blade failed to have this stiffness it would not be possible to transmit the pitch changes from the hub along the span of the blade out to the tip. Nevertheless, as with many of the basic concepts in rotor design, there are exceptions to this rule. One important example is the torsionally flexible blade used on some helicopters. With this system a type of pitch control is used that requires a blade that is deliberately flexible in torsion. On these rotors the pitch control is accomplished by actually warping the blades through the leverage obtained from a small, controllable, aerodynamic surface mounted on the trailing edge of the blade, similar to the trim tab used on airplane control surfaces.

Ground effect The high power requirement needed to hover out of ground effect is reduced when operating in ground effect. Ground effect is a condition of improved performance encountered when operating near (within 1/2 rotor diameter) of the ground. It is due to the interference of the surface with the airflow pattern of the rotor system, and it is more pronounced the nearer the ground is approached. Increased blade efficiency while operating in ground effect is due to two separate and distinct phenomena. First and most important is the reduction of the velocity of the induced airflow. Since the ground interrupts the airflow under the helicopter, the entire flow is altered. This reduces downward velocity of the induced flow. The result is less induced drag and a more vertical lift vector. The lift needed to sustain a hover can be produced with a reduced

angle of attack and less power because of the more vertical lift vector:

The second phenomenon is a reduction of the rotor tip

vortex:

When operating in ground effect, the downward and outward airflow pattern tends to restrict vortex generation. This makes the outboard portion of the rotor blade more

efficient and reduces overall system turbulence caused by ingestion and recirculation of the vortex swirls. Rotor efficiency is increased by ground effect up to a height of about one rotor diameter for most helicopters. This figure illustrates the percent increase in rotor thrust experienced at various rotor heights:

At a rotor height of one-half rotor diameter, the thrust is increased about 7 percent. At rotor heights above one rotor diameter, the thrust increase is small and decreases to zero at a height of about 1 1/4 rotor diameters. Maximum ground effect is accomplished when hovering over smooth paved surfaces. While hovering over tall grass, rough terrain, revetments, or water, ground effect may be seriously reduced. This phenomenon is due to the partial

breakdown and cancellation of ground effect and the return of large vortex patterns with increased downwash angles. Two identical airfoils with equal blade pitch angles are compared in the following figure:

The top airfoil is out-of-ground-effect while the bottom airfoil is in-ground-effect. The airfoil that is in-groundeffect is more efficient because it operates at a larger angle of attack and produces a more vertical lift vector. Its increased efficiency results from a smaller downward

induced wind velocity which increases angle of attack. The airfoil operating out-of-ground-effect is less efficient because of increased induced wind velocity which reduces angle of attack. If a helicopter hovering out-of-ground-effect descends into a ground-effect hover, blade efficiency increases because of the more favourable induced flow. As efficiency of the rotor system increases, the pilot reduces blade pitch angle to remain in the ground-effect hover. Less power is required to maintain however in-ground-effect than for the out-ofground-effect hover.

LIMITATIONS There are a number of factors that govern the maximum speed of a helicopter : Drag In aerodynamics, drag is the force opposing thrust.

Drag is present in helicopters in two main types: a. Parasite drag Parasite drag is the drag forces created by the components that protrude into the airflow around the helicopter. Because this drag is opposing thrust it is reducing the amount of thrust available to make the helicopter fly faster. Parasite drag includes the landing gear, antennas, cowlings, doors, etc. The shape of the fuselage will also produce parasite drag. On later helicopters where the manufacturer has attempted to raise the speed of the helicopter, the landing gear is retractable to reduce the amount of parasite drag produced. Generally, for a given structure, the amount of parasite drag is proportional to the speed that the structure is passing through the air and therefore parasite drag is a limiting factor to airspeed. b. Profile drag Profile drag is the drag produced by the action of the rotor blades being forced into the oncoming airflow. If a rotor blade was cut in half from the front of the blade (leading edge) to the rear of the blade (trailing edge), the resulting shape when looking at the cross-section is considered to be the blade "profile". For a rotor blade to produce lift, it must have an amount of thickness from the

upper skin to the lower skin, which is called the "camber" of the blade. In general terms the greater the camber, the greater the profile drag. This is because the oncoming airflow has to separate further to pass over the surfaces of the rotor blade. The blade profile for a given helicopter has been designed as a compromise between producing sufficient lift for the helicopter to fulfil all of its roles, and minimising profile drag. To alter the amount of lift produced by the rotor system, the angle of attack must be altered. As the angle of attack is increased then the profile drag also increases. This is generally referred to as "induced drag", as the drag is induced by increasing the angle of attack. Retreating Blade Stall To understand retreating blade stall it is first necessary to understand a condition known as"Dissymetry of Lift". Consider a helicopter hovering in still air and at zero ground speed. The pilot is maintaining a constant blade pitch angle with the collective pitch control lever and the aircraft is at a constant height from the ground. The airflow velocity over the advancing blade and the retreating blade is equal.

If the tip of the advancing blade is travelling at 300mph then the tip of the retreating blade must also be travelling at 300mph. The velocity of the airflow over the blade is progressively reduced as we look closer toward the root end of the blade (toward the rotor hub) as the distance that the observed point has to travel around the circle is reduced. In this condition the amount of lift being generated by each blade is the same because the amount of lift produced is a function of velocity and angle of attack. However, if the helicopter started to move forward then the airflow velocity over the advancing blade would be increased by the amount of the forward speed as the blade is moving in the opposite direction to the flight. If the helicopter was then travelling forward at 100mph, then the airflow at the advancing blade tip would be: Velocity induced by the

300m

blades turning:

ph

Plus the velocity from

100m

forward flight:

ph

Total effective velocity at

400m

the tip:

ph

At the retreating blade the velocity is reduced by the amount of forward speed as the blade is travelling in the same direction as the airflow created by forward flight. So the tip is now effectively travelling at 200mph, or half the speed of the advancing blade. From the Formula for Lift, it is known that the amount of lift produced varies as the square of velocity. From the example above this means that the advancing blade will produce four times more lift than the retreating blade. If this situation was not corrected, the helicopter could not fly forward in a straight line when forward flight was attempted. (It would actually pitch noseup, but that's another story!) To correct for this the rotor system is allowed to "flap" whereby one blade tip can rise above the other with reference to the rotor plane of rotation. The effect this has is to reduce lift on the advancing blade and increase lift on the retreating blade. The lift across both blades is then equalised. Now that we understand "Dissymetry of Lift", we can look at retreating blade stall. You will recall that the retreating

blade has a lower airflow velocity than the advancing blade in forward flight. If we were to accelerate our helicopter from the above example to 300mph, then the advancing blade would have an airflow velocity of 600mph, and the retreating blade would be zero. For the blade to produce lift it must have some airflow over it, so in this case the blade would "stall". Stall is a condition where there is a breakdown of smooth laminar airflow over the surfaces of an aerofoil (rotor blade). With each blade entering a stall condition as it passed down the left side of the helicopter, forward flight could not be maintained at this speed. Before the blade actually stalled it would produce a series of harsh vibrations known as "buffeting". When a manufacturer produces a new helicopter, the speed at which this buffeting will occur is established during flight test trials and a lower figure is subsequently published which is commonly known as VNE or Velocity - Never Exceed . This establishes a safety margin below the speed where retreating blade stall may occur.

Airflow Reversal Airflow Reversal will normally occur before retreating blade stall. You will recall that the airflow velocity is progressively reduced along a blade from being highest at the tip, to lowest at the root end. If the velocity is 300mph at the tip, it is feasible for the velocity to be as low as 100mph at the root. Therefore when forward speeds as low as 100mph (approx. 60 Kts) are encountered, the root end of the blade is effectively stalled. When higher speeds are attempted, the airflow across the root end of the blade can actually reverse and travel from the trailing edge to the leading edge. This is because the airflow velocity produced by the forward speed is greater than that being produced by the rotor blades turning. Airflow reversal is counter-productive to producing lift and rotor thrust. To reduce the effects of lift variations from the root to the tip of a blade the manufacturer will either twist the blade along its length, or apply a taper to the blade. Twist is the reduction of angle of attack from the root to the tip. Remember that lift increases with velocity and angle of attack? Because the tip is travelling faster than the root, the

angle of attack must be reduced toward the tip to maintain the same amount of lift at the tip and the root ends. Taper is the gradual reduction of the width of a blade from the leading edge to the trailing edge. A straight line drawn from the centre of the leading edge to the centre of the trailing edge is called the "Chord Line". By reducing the chord line from the root to the tip, less surface area is available for the airflow to act on to produce lift. On higher speed helicopters (Westland Lynx), the root end of the blade is a blade spar and attachment area only. The aerofoil shape does not start until several feet out from the centre of the rotor system. This is done to reduce the effects of airflow reversal by placing the lift-producing surface further out where the rotational velocity is higher. Air Compressibility Air is a gas and therefore conforms to the properties of a gas, namely the ability to be compressed. When studying aerodynamics however, air must also be considered to have some of the properties of a fluid. A fluid has far less compressibility than a gas. When the airflow over a rotor blade strikes the leading edge, it is split into two streams, which then pass above

and below the blade. At lower speeds, this splitting action occurs relatively easily requiring little energy. As speeds increase, the air striking the leading edge tends to be compressed before separating into two streams. Think of this as slapping your hand onto a water surface. If you chop your hand into the water, like a karate chop, you can separate the water fairly easily. If you slap your open hand onto the water however, it takes considerably more force to submerge your hand. The airflow at the leading edge is very similar. As the air at the leading edge is progressively compressed, it requires considerably more rotor thrust for the blade to separate the airflow into two streams. Cyclic Control Stick design Helicopter designers are forever trying to fit more equipment into the cockpit of a helicopter to satisfy market demands. At the same time, they are trying to minimise the weight of the aircraft so that it can carry and lift more. When designing the pilot and copilots workstations the designers attempt to place the controls in a position where the crew can easily and comfortably operate all controls without excessive reaching

or stretching. This places limitations on the amount of movement available at the cyclic control stick. The designers could feasibly arrange the controls such that very small amounts of stick movement were required for normal flight, but this would make control in the hover very difficult as the controls would be super sensitive to small inputs. For this reason, the controls are arranged so that a reasonable control movement is available, generally 6-8 inches of stick movement depending on the particular aircraft model. Available Engine Power The engine system in a helicopter is required to provide power for a range of demands, not only the rotor system. In the rotor system, thrust is required to overcome drag. As speed is increased, so does drag. If more power is available to overcome drag, then potentially the helicopter can fly faster. It can be seen that from these factors that it is very difficult for helicopter designers to increase the maximum speed of a helicopter as many factors are beyond their control. Much research and development has occurred in

areas such as reducing drag, better rotor blade designs and increasing available engine power. The current World Helicopter Speed Record is held by the Westland Lynx at 217.5 Kts (402 km/h) using specially designed high-speed rotor blades.

USES OF HELICOPTER 

Sikorsky S-64 Skycrane lifting a prefab house

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Kern County (California) Fire Department Bell 205 dropping water on fire

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A British Westland WAH-64 Apache attack helicopter

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HH-65 Dolphin demonstrating hoist rescue capability

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A Sikorsky S-76C+ air ambulance being loaded by fire-fighters

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RAF Westland Sea King for rescue of people

A helicopter used to carry loads connected to long cables or slings is called an aerial crane. Aerial cranes are used to place heavy equipment, like radio transmission towers and large air conditioning units, on the tops of tall buildings, or when an item must be raised up in a remote area, such as a radio tower raised on the top of a hill or mountain. Helicopters are used as aerial cranes in the logging industry to lift trees out of terrain where vehicles cannot travel and where environmental concerns prohibit the building of roads. These operations are referred to as logline because of the long, single sling line used to carry the load. Helitack is the use of helicopters to combat wildland fires.The helicopters are used for aerial firefighting (or water bombing) and may be fitted with tanks or carry helibuckets. Helibuckets, such as the Bambi bucket, are usually filled by submerging the bucket into lakes, rivers, reservoirs, or portable tanks. Tanks fitted onto helicopters are filled from a hose while the helicopter is on the ground or water is siphoned from lakes or reservoirs through a hanging snorkel as the helicopter hovers over the water source. Helitack helicopters are also used to deliver

firefighters, who rappel down to inaccessible areas, and to resupply firefighters. Common firefighting helicopters include variants of the Bell 205 and the Erickson S64Aircrane helitanker. Helicopters are used as air ambulances for emergency medical assistance in situations when an ambulance cannot easily or quickly reach the scene. Helicopters are also used when a patient needs to be transported between medical facilities and air transportation is the most practical method for the safety of the patient. Air ambulance helicopters are equipped to provide medical treatment to a patient while in flight. The use of helicopters as an air ambulance is often referred to as MEDEVAC, and patients are referred to as being "airlifted", or "medevaced". Police departments and other law enforcement agencies use helicopters to pursue suspects. Since helicopters can achieve a unique aerial view, they are often used in conjunction with police on the ground to report on suspects' locations and movements. They are often mounted with lighting and heat-sensing equipment for night pursuits.

Military forces use attack helicopters to conduct aerial attacks on ground targets. Such helicopters are mounted with missile launchers and miniguns. Transport helicopters are used to ferry troops and supplies where the lack of an airstrip would make transport via fixed-wing aircraft impossible. The use of transport helicopters to deliver troops as an attack force on an objective is referred to as Air Assault. Unmanned Aerial Systems (UAS) helicopter systems of varying sizes are being developed by companies for military reconnaissance andsurveillance duties. Naval forces also use helicopters equipped with dipping sonar for antisubmarine warfare, since they can operate from small ships. Oil companies charter helicopters to move workers and parts quickly to remote drilling sites located out to sea or in remote locations. The speed over boats makes the high operating cost of helicopters cost effective to ensure that oil platforms continue to flow. Various companies specialize in this type of operation. Other uses of helicopters include, but are not limited to:

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Aerial photography

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Motion picture photography

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Electronic news gathering

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Reflection seismology

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Search and Rescue

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Tourism or recreation

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Transport

PAWAN HANS HELICOPTERS Pawan Hans Helicopters Limited is a helicopter service company based in India. The operations are based at Vile Parle(West). Other than providing helicopter services to ONGC to

its off-shore locations, this public sector company is often engaged for providing services to various state governments in India, particularly in North-east India, Vaishno Devi Helicopter service for devotees. History

Pawan Hans was incorporated on 15 October 1985 as the Helicopter Corporation of India

(HCL), the country's national helicopter company with the objective of providing helicopter

support services to the oil sector for its off-shore exploration operations, services in remote areas

and charter services for promotion of tourism. It

is a government owned enterprise with 78.5% in government hands & 21.5% with ONGC.

ONGC has upped its stake to 49% recently, a

move that will see the equity base of PHHL

being enhanced to 245 crore from the existing 113 crore.

The corporate office is located at Noida with regional offices at Delhi and Mumbai. The

company has a net worth of 37,015 million and equity capital is 1,137 million. Pawan Hans is "Approved Maintenance Centre of Eurocopter" and also the first ISO 9001 : 2000 certified aviation company in India.

It offers helicopter services for 

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Off Shore operations Inter island transportation Connecting inaccessible areas Customs and pipeline surveillance Casualty and rescue work

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Charter services VIP transportation Film shooting and aerial photography Flower dropping and other Customized services.

CONCLUSION Even though the concept of the helicopter is arguably older than that of the airplane, there is still a great amount of research and advancement yet to occur. As the

political climate of our world continues to change and military conflicts approach the small-scale urban warfare of recent years, the importance of the helicopter will continue to grow. It is rather ironic that an idea first conceived long before the Common Era will be key to winning military conflicts in the 21st century.

Rotary wing research and development is a complex interrelated challenge. The advanced tools used are Computational fluid dynamics (CFD), Finite element method (FEM), and Computational structural dynamics (CSD)

for

physical

understanding

of

complex

aerodynamics and structural phenomena. Integration of these will enable us to design rotorcraft, which will have superior productivity, enlarged mission capabilities and improved environmental acceptance.

The future of helicopter is bright with its ability to land in any small clear area; the helicopter finds use in air taxi service, police work, Inter city mail, and rescue work, power line patrolling and other areas. The development is still to continue.