An airplane by its nature wants to fly. … A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other, and if there is any disturbance in this delicate balance, the helicopter stops flying, immediately and disastrously. There is no such thing as a gliding helicopter.The helicopter is type of aircraft in which lift is obtained by means of one or more power-driven horizontal propellers called rotors. When the rotor of a helicopter turns it produces reaction torque which tends to make the craft spin also. On most helicopters a small rotor near the tail compensates for this torque. On twin-rotor craft the rotors spin in opposite directions, so their reactions cancel each other. The helicopter is propelled in a given direction by inclining the axis of the main rotor in that direction. The helicopter's speed is limited by the fact that if the blades rotate too fast they will produce compressibility effects on the blade moving forward and stall effects on the rearward moving blade, at the same time.
Although the helicopter was only recently fully developed, its concept can date back to the late 1400's. Since then, helicopters have been put into use by society in many ways. One can find helicopters in both civil and military areas. The early helicopters were mainly developed for military use, but later became certified for civilian use. Since then helicopters have evolved greatly, specifically with the design. Because a helicopter can perform more actions than a fixed-wing aircraft can, it is more complicated to fly. The helicopter must compensate for a variety of forces, like the spinning force induced by the main rotors. The engineering behind designing a helicopter is complex with a variety of issues to be understood . But lets make it simple.
The Helicopter Blade
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.
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 Principle that makes plane fly section)The helicopter blades serves all the flight function in the helicopter. 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 sideways, that is, giving it thrust.
The Main Rotor :
The lifting force is produced by this rotors. As they spin they cut into the air and produce lift. Each blade produces an equal share of the lifting force. Spinning the rotor against the air causes lift, allowing the helicopter to rise vertically or hover. This is based on the principle of thrust as discussed in Principle that makes plane fly .
Lift is produced by a lower pressure created on the upper surface of an airplane's wings compared to the pressure on the blade's lower surfaces,causing the LIFTING upward. The special shape of the airplane blade (airfoil) is designed so that air flowing over it will have to travel faster resulting in a lower pressure area at the top of the wing(see illustration). At the base of the blade, air travel at a slower speed thus resulting in a higher pressure area at the bottom.The aerofoil nature of the blade causes a pressure difference thus lifting the helicopter upward due to the higher pressure air at the bottom of the blade pushing the blade upward due to the deficiency in pressure to balance it up at the top. Lift is that force which opposes the force of gravity (or weight).
Tilting the spinning rotor will cause flight
in the direction of the tilt. This is because the thrust would be somehow in the direction of motion wanted by the pilot as shown by the diagram.
The Tail Rotor :
The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction.
This is called
TORQUE REACTION.
The tail rotor is used like a small propeller, to pull against torque reaction and hold the helicopter straight.
By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor through a gearbox.
When using the tail rotor trying to compensate the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate. The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon. The engine(s) simply drive the rotors and did not assist directly with forward flight (like they would with a plane). There is a very small amount of thrust that comes from engine exhaust, but it is so small that it does not effect flight performance.
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