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The Incredible Science Behind How a Helicopter Flies

How Does a Helicopter Fly?
The helicopter is a fascinating engineering achievement, and arguably ranks among the best of the 20th century.
Tanmay Tikekar
Last Updated: Dec 17, 2018
Did You Know?
Despite the apparent ease, hovering is among the most difficult maneuvers to perform in helicopters.
Helicopters are driven by rotating blades known as rotors. This differentiates them from fixed-wing aircraft, such as jet engines or microlights.
All aircraft become airborne by the same principle. When the front tip of a wing is raised, it forces the air flying over it to flow faster than the air flying under it. This means that there is less air at any moment in time above the wing than under it. This imbalance causes lift, and literally sucks the aircraft up.
A fixed-wing aircraft flies due to the lift generated by air flying over its wings. This requires moving air (as in the case of gliders), or some form of propulsion.
In contrast, rotary-wing aircraft generate their own thrust by moving their 'wings' against the air. A helicopter's rotors actually carry out the same function as a plane's wings, except that a plane's wings need thrust to generate forward motion.
Gliders, which don't have an engine, need to drop their height in order to move forward (which they achieve by lowering their 'nose'). A helicopter's engine, on the other hand, powers the rotors and doesn't provide thrust directly; the tiny amount of thrust gained from the engine's exhaust is negligible in the larger scheme.
The ability to generate its own lift gives a helicopter obvious advantages over fixed-wing aircraft. Helicopters can take off and land vertically (though an exclusive class of planes can also do that), are able to hover in midair, and are much more maneuverable if space is at a premium.
But this also makes controlling helicopters much trickier to control than airplanes. The basic shape of airplanes inherently allows them to passively glide forward for a considerable distance even if the piloting and propulsion stops. However, the flexible, active nature of the helicopter means that it is not designed to glide.
It has to be constantly kept airborne without any help from the laws of physics. Pilots need to constantly feed input to the helicopter's propulsion, even while hovering. The rotary mechanism also limits a helicopter's top speed, and makes them inherently slower than fixed-wing aircraft.
So, how exactly do helicopters fly, and how do helicopters steer?

Helicopters are piloted by a combination of three controls.
Working of a Helicopter
Collective
Angle of Attack
This controls the angle of attack (AOA) of all the rotor blades collectively - thus the name. The AOA is the angle formed by the raised or lowered blade against the imaginary plane of its base.
When the angle of attack is increased, the air flowing over the top of the blades has to flow even faster, inducing lift. Likewise, the angle of attack must be decreased in order to land a helicopter.
The collective is located to the left of the pilot, at the base of his seat. It is often accompanied by a motorcycle-like twist grip throttle, which is twisted sideways to increase or decrease the angle.
As the angle of attack is increased, air resistance to the rotors also increases. To counter this, the engine rpm has to be increased to maintain a steady rotor rpm. Thus, the throttle is either paired with the collective lever, or a component known as the governor is built into the chopper.
The governor automatically changes the engine rpm when the angle of attack is changed, and keeps the rotor rpm constant.
Cyclic
Helicopter Rotor Arrangement with Naming
The cyclic controls the direction of the helicopter. It is called the cyclic control, because it alters the angle of each blade cyclically.
Instead of raising a single blade, the cyclic system changes the elevations of certain points in the rotating section of the swash plate. This, in effect, causes the rotor disc (the rotating section of the swash plate) to tilt towards a certain direction.
This causes the same aerodynamic effect as lift, only with the added element of a lateral direction. This propels the copter towards that direction.
The cyclic control is often found in the form of a joystick, either in front of the driver's seat or between the pilot and copilot. Like any other joystick, the direction of the joystick corresponds with its effect, i.e., if the cyclic control is shifted to the left, the rotor disc tilts towards the left, and the chopper goes left.
Anti-torque Pedals
According to Newton's third law of motion―every action has an equal and opposite reaction―the main rotor's rotation causes the rest of the helicopter to rotate in the opposite direction. This is known as torque reaction. The rear rotor on helicopters counters this effect and keeps the chopper in a straight line. The rear rotor can be used as a rudder.
Two anti-torque pedals, situated at the pilot's feet, control the pitch of the rear rotor disc. When the right pedal is pressed, the rear rotor disc is turned towards the right, and the nose of the helicopter turns towards the right, and vice versa.
This is just like how the main rotor generates lift, only sideways. These pedals are used to induce yaw and turn the helicopter. If used while hovering, the anti-torque pedals allow a helicopter to turn a full 360°.
Hovering
This is what defines helicopters better than anything else. Keeping a helicopter stationary in the air may look easy, but in reality, it is one of the most challenging things a pilot has to do.
In order to hover, a pilot must first stop any directional motion by bringing the cyclic control in the middle. Then he must stabilize the collective lever at the required height, and make the required adjustments to the yaw pedals to maintain the exact position with respect to all three axes.
Helicopters are used to put out forest fires, in rescue work, and, of course, in the armed forces. They are crucial civil and military equipment all over the world, and with their extremely useful attributes, that is hardly a surprise.