LIFT MECHANISM OF THE HELICOPTER. INTRODUCTION. A helicopter is an aircraft that is lifted and propelled by one or more horizontal rotors, each rotor consisting of two or more rotor blades.
A helicopter is an aircraft that is lifted and propelled by one or more horizontal rotors, each rotor consisting of two or more rotor blades.
A helicopter works by having its wings move through air while the body stays still. The helicopter blades are called main rotor blades. During flight there are four forces on the helicopter and those forces are lift, drag, thrust and weight.
Angle of Attack :is the acute angle measured between the chord of an airfoil and the relative wind.
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.
In a helicopter, the structure making flight possible is the airfoil - a surfaced body that responds to relative motion between itself and the air with a useful, dynamic reaction known as lift. The term airfoil, refers to the rotary wing, and more specifically means the curvature, or camber, of the blade.
As the diagram indicates, the thick end of the section is known as the leading edge. The small tapering end is the trailing edge. The distance between the leading edge and the trailing edge is known as the chord of the airfoil.
The rotary wing blade in a helicopter is asymmetrical, that is it has a curvature that changes along the entire length of the chord. If the blade were symmetrical, then the chord line would be a straight line from the leading to the trailing edges. Since the curvature changes constantly in rotor blade, the result is that the chord of the blade also changes. When computing the chord line of this type of blade, an average or mean aerodynamic chord (MAC) becomes apparent .
When a blade (airfoil) is moved through the air, a stream of air flows over and under it. The blade is designed so that the flow of air will be smooth and will conform to the shape of the moving blade. If the blade is set at the proper angle and made to move fast enough, the airflow will support the weight of the blade. This is the nature of the action that enables rotary wings to furnish enough lift to sustain the helicopter in flight.
Hence by applying bernoulli’s theorem we can observe that lift is produced by a lower pressure created on the upper surface of the helicopter’s wings compared to the pressure on the wing's lower surfaces, causing the wing to be lifted upward. The special shape of the rotor (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 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.
This image shows the streamlines over a NACA 0012 airfoil of the real flow. The flow approaching an airfoil can be divided into two streamtubes, which are defined based on the area between two streamlines.
The upper stream tube constricts as it flows up and around the airfoil, a part of the so-called upwash. From the conservation of mass, the flow speed must increase as the stream tube area decreases.The area of the lower stream tube increases, causing the flow inside the tube to slow down. It is typically the case that the air parcels traveling over the upper surface will reach the trailing edge before those traveling over the bottom.
From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface. The pressure difference thus creates a net aerodynamic force, pointing upward and downstream to the flow direction.
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.
The tail rotor in normally linked to the main rotor via a system of drive shafts and gearboxes .Most helicopter have a ratio of 3:1 to 6:1 . In most helicopter the engine turns a shaft that connected to an input quill in the transmission gearbox.
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.
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 .
In order to overcome this problem blade flapping is done.
Dissymmetry of lift in helicopter aerodynamics refers to an uneven amount of lift on opposite sides of the rotor disc.
The dissymmetry is caused by differences in relative airspeed between the advancing blade and the retreating blade.
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.
Swash plate assembly :
Its primary component is the swash plate, located below the rotor head. This swash plate consists of one non-revolving disc and one revolving disc mounted directly on top. The swash plate is connected to the cockpit control sticks and can be made to tilt in any direction, according to the cyclic stick movement made by the pilot, or moved up and down according to the collective lever movement.
The cyclic control works by tilting the swash plate and changing the pitch angle of a rotor blade at a given point in the rotation.As the pitch angle changes, so the lift generated by each blade changes and as a result the helicopter becomes 'unbalanced', and so tips towards whichever side is experiencing the lesser amount of lift. Thus with its help, helicopter can move right or left , backward or forward.