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How does a Helicopter Work



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How does a helicopter fly? It flies a little like an airplane, but mostly not like an airplane. A helicopter is more difficult to fly than a fixed wing aircraft due to many extra factors that act on the helicopters flight, but the general aerodynamics are surprisingly similar.

In a fixed wing aircraft the wings produce lift that allows the aircraft to fly. Air that travels faster has less pressure, and all the wings do is make the air pass over the top of the wing faster than the bottom, mostly through the shape of the wing (which is why when you look at a cutaway view of an aircraft wing it’s got a big curve in the upper surface). What keeps a helicopter in the air is also a wing, only it’s one that spins around. This means that lift can be created even when the helicopter is not moving forwards. The actual profile of a helicopter rotor is different to that of an aircraft, changing the angle of the rotor up and down to create lift. That’s about where the similarities end. A fixed wing aircraft has an engine that drives a propeller or a jet to push the aircraft through the air. In the helicopter the rotors lift and pull the helicopter through the air. Most helicopters have three or more rotor blades, unlike the ‘Huey’ of Vietnam fame and the Bell Jet Ranger models, which have only two. For the purposes of this article it will use the three blades or more examples to avoid having to use multiple definitions.

Speed Limits

The advantage of helicopters is of course that they can land in small places due to the ability to descend vertically. The disadvantage is that helicopters are not as fast as fixed wing aircraft in flight. This is because while some rotor blades on a helicopter are moving forwards the others are moving backwards. Each side of the rotor disc has to produce the same amount of lift, which means that you will eventually reach a speed where the ‘retreating’ rotor blades are not actually moving through the air at all, or even moving backwards through the air, causing the helicopter to flip over. The way that most helicopter balance out the lift on each side is that the retreating blades ‘flap’, that is, move up into a position where they create more lift than normal. On the other side the blades are ‘down’ producing less lift than they would ideally produce. This is mostly operated automatically using links and hinges on the base of the rotor blades.

Moving forwards (and turning)

While a fixed wing aircraft uses the engine to push through the air, the helicopter uses the rotor blades and the helicopter literally hangs from them. To move forward, linkages from the controls are used to change the trim of the rotors so that the mast and helicopter tilt forwards. This produces lift in a forward direction, while pushing air behind the helicopter. To turn left or right while moving the blades are set so that the helicopter banks and turns (the tail rotor may be used, but only a little). While hovering the tail rotor is what controls the turning of the helicopter (and also stops it from spinning out of control...)

Torque

You will of course have noticed that helicopters have long tails with an extra propeller sticking out from the end. This is to stop the helicopter from spinning around and around. Torque may not be too familiar to some people but it goes back to one of the laws of physics – for every action there is an equal and opposite reaction. In this case the rotor blades are turning, which is one action, so the reaction is for the rest of the helicopter to turn in the other direction. As the helicopter in flight is hanging directly from the rotor head there’s nothing to stop it from spinning the other way, apart from a tail rotor. This applies an extra countering force so that – especially when hovering – the helicopter will not begin to spin around. Once the helicopter is moving various other aerodynamic forces help take over from the tail rotor so the power required from the tail rotor is reduced. As the tail rotor is connected to the same engine the main rotor is that means that move power can be used for lift and thrust through the air.

Other designs for overcoming torque

There are a few other ways to overcome torque reaction. The most common is the tandem rotor configuration, as seen on the Boeing CH-47 series (most notably currently being used in Afghanistan with US and UK detachments), where there is one rotor at the front and a second at the back, that rotates in the other direction. Another way has been seen on Russian helicopters, notably those built by Kamov, which have two rotor blade systems on the same mast. One rotor blade system rotates one way and the other rotates the other way. Kaman, a US company, has a novel system (currently used on the K-MAX helicopter) where two rotor systems are closely mounted together on small masts, with the rotors intermeshing close to the rotor head. These layouts have the advantages that they require no power sapping tail rotors and therefore can devote all engine power to lifting all the time.

Autorotation

And yes, it is possible to keep a helicopter in the air if the engine breaks down. It’s called autorotation and it allows the helicopter to glide to an emergency landing. When a rotor blade rotates under power it pushes air downwards as a consequence of its movement through the air. With no power the blades can be set at an angle that they now use air pushing upwards through the rotors to keep them spinning. With the right settings a glide of between three and four feet forward for every one foot downwards is possible. This allows the pilot time to decide where to land, just as it would happen in a fixed wing airplane, although in that case it’s the airflow over the wings that is still generating lift. As the helicopter descends and a landing spot is decided upon the pilot waits until about 100 feet off the ground and begins to set the rotor blades as they were in powered flight, producing lift at the end of the autorotation and allowing the helicopter to slow and land. All helicopter pilots after heavily trained in the art of autorotation as part of their training and licensing.

So, simply, the difference between how a fixed wing aircraft and a helicopter flies in that while the plane uses an engine to push it while the wings lift the plane, the helicopter uses what are essentially smaller rotating wings to not only lift the helicopter off the ground but fly it through the air as well.

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