Helicopter Main Blade

Helicopters

Helicopters

The main difference between fixed-wing and rotary-wing aircraft is the way lift is achieved.

Lift

The fixed-wing aircraft gets its lift from a fixed airfoil surface. The helicopter gets lift from rotating airfoils called rotor blades. The word helicopter comes from the Greek words meaning helical wing or rotating wing. A helicopter uses two or more engine-driven rotors from which it gets lift and propulsion. The helicopter's airfoils are the rotor blades. The airfoils of a helicopter are perfectly symmetrical. This means that the upper and lower surfaces are shaped the same. This fact is one of the major differences between the fixed-wing aircraft's airfoil and the helicopter's airfoil. A fixed-wing aircraft's airfoil has a greater camber on the upper surface than on the lower surface. The helicopter's airfoil camber is the same on both surfaces. The symmetrical airfoil is used on the helicopter because the center of pressure across its surface is fixed. On the fixed-wing airfoil, the centre of pressure moves fore and aft, along the chord line, with changes in the angle of attack. If this type of airfoil were used on a rotary-wing aircraft, it would cause the rotor blades to jump around (dive and climb) uncontrollably. With the symmetrical airfoil, this undesirable effect is removed. The airfoil, when rotated, travels smoothly through the air. The main rotor of a helicopter consists of two or more rotor blades. Lift is accomplished by rotating the blades through the air at a high rate of speed. Lift may be changed by increasing the angle of attack or pitch of the rotor blades. When the rotor is turning and the blades are at zero angle (flat pitch), no lift is developed. This feature provides the pilot with complete control of the lift developed by the rotor blades. 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.

Directional Control

A pilot controls the direction of flight of the helicopter by tilting the main rotor. If the rotor is tilted forward, the force developed by the rotor is directed downward and aft. Now, apply Newton's third law of motion (action and reaction). Lift will be developed in an upward and forward direction, and the helicopter will tend to rise and move forward.

Centre of pressure.

You should realize that a pilot can move a helicopter forward or rearward, or to the right or left, simply by tilting the main rotor in the desired direction. This points out another major difference between fixed-wing and rotary-wing aircraft. The fixed-wing aircraft can't move up or down or right or left without forward movement. Remember, a fixed-wing aircraft's primary direction is forward. However, a helicopter can move in any direction, with or without forward movement.

Hovering

Hovering is defined as maintaining a position above a fixed spot on the ground. A helicopter has the ability to remain in one spot in the air with little or no movement in any direction. This is done by equalizing all the forces acting on the helicopters (lift, drag, weight, and thrust). This action also allows a helicopter to take off or land without a runway. This is another advantage the rotary-wing aircraft has over the fixed-wing aircraft.

Torque Reaction

As the helicopter's main rotor turns in one direction, the body (fuselage) of the helicopter tends to rotate in the opposite direction (Newton's third law). This is known as torque reaction. In a single main rotor helicopter, the usual way of getting rid of torque reaction is by using a tail rotor (anti-torque rotor). This rotor is mounted vertically on the outer portion of the helicopter's tail section. The tail rotor produces thrust in the opposite direction of the torque reaction developed by the main rotor.

   

 

   

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 manouver (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 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 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 driveshafts 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 revelutions (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 tranmission gear box.

   

 

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 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 Flapping

Dissymmetry 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 swashplate 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.

 

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The author is aviation specialist.