HELICOPTER’S FLYING MECHANISM

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 or airplane and those are LIFT , DRAG , THRUST ,and WEIGHT .(please go back and see on What makes an airplane fly ? section).In order to make the wings to move through the air , of course, the plane 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.

NOMENCLATURE AND TECHNICAL TERMS USED IN A HELICOPTER

• Bernoulli '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. (usually manufacturer had built the aircraft with the wing has some degrees to the horizontal plane or airplane longitudinal axis).

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.

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

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. 

 

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 (see illustration) 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.( you may understand as the angle of incidence ) . This pitch angle can be varied by the pilot through the use of cockpit controls ( collective and cyclic pitch control ).

Principle of Helicopter Flight

Helicopter, 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 near the tail which called tail rotor compensates 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 . For example, if a helicopter pull up in a 1.5 g maneuver (1.5 time the gravity force), then the effective weight of helicopter will be 1.5 time of static helicopter weight or 6000 lbs. due to gravitational pull.

Tail RotorThe 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 compensates 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 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). In most helicopter the engine turns a shaft that connected to an input quill in the transmission gearbox. the main rotor mast out to the top and tail rotor drive shafts out to the tail from the transmission gear box. 

 

Dissymmetry of LiftAll 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 advanceing blade develops greater lift because of the increased airspeed and the retreating blade will produce less lift , this will cause the helicopter to roll (for example: if rotor speed = 400 km/hr , helicopter move forward=100 km/hr then advancing blade will have speed=500 km/hr but the retreating blade will has moving speed of only 300 kr/hr ) . This has to be compensated for in some way . 

Blade FlappingDissymmetry of lift is compensated for 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. 

 

Relationship between Swash Plate and Controls

 

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 refered to as the Stationary Star is attached by a bearing surface to a second disc, often refered to as the Rotating Star which turns with rotor and linked to the rotor blade pitch horns.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 PedalsThe 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.