Types of main rotors

There are several different types of main rotor systems, and they are classified depending on how each rotor blade can move in respect to the main hub.

In fully articulated rotor systems, each rotor blade is attached to the main hub through a series of hinges, and each rotor blade can move independently of the others.The three possible movements are called flapping, leading/lagging, and feathering. These rotors usually have 3 or more blades.

The flapping hinge allows the rotor blade to move up and down, and is necessary to compensate for the asymmetry of lift.
The lead-lag hinge allows a rotor blade to move horizontally. The purpose of this hinge is to compensate for the acceleration and deceleration caused by the Coriolis-effect.
The last hinge is the feathering hinge, which allows a rotor blade to rotate along its length. Feathering is necessary to be able to change the lift generated by a rotor blade. Without a feathering hinge, it wouldn't be possible to control a helicopter.

Semi-rigid rotor systems are usually composed out of 2 rotor blades. The blades are connected to the main rotor shaft by a teetering hinge. The teetering hinge allows the two rotor blades to move up and down as a whole. When one blade goes up, the other goes down. Semi-rigid rotors don't have lead-lag hinges, so the lead-lag forces are absorbed through blade bending. These rotor systems do have feathering hinges though, because without them, the helicopter would be uncontrollable.

Rigid rotor systems only allow rotor blades to feather, all other forces are absorbed through blade bending.

Combination rotor systems are the most modern rotor systems, and may use the principles of all previous rotor systems. Some incorporate a flexible hub, which allows the blades to move without the need for bearings or hinges. They use flextures and elastomeric bearings to accomplish this. The advantages of this system include less maintenance, less vibrations, and a longer lifespan.

Source:
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

Rotor blade materials

The main rotor blades are a vital part of a helicopter, because they are responsible for supporting the entire weight of a helicopter. The forces on the rotor blades can increase even more when performing maneuvers, for example: pulling up at 1.5G means that the rotor needs to support 1.5 times the weight of the helicopter.

The first helicopter rotor blades were constructed out of laminated wood and fabric. One of the major drawbacks of using wood to construct the rotor blades is that wood absorbs moisture, which changes the mass of the rotor blade.

Wooden rotor blades were used up until the 1960s, until they were replaced by steel and aluminium. Advantages of steel and aluminium rotor blades is that they're cheaper and easier to produce, and that they do not suffer from moisture absorption. However, disadvantages include a low strength to density ratio and a poor resistance to fatigue.

Major improvements were made to the rotor blades by using composite materials. Composite materials are made by combining two different materials together. For example, glass fiber and plastic can be combined to form a composite material. The plastic binds the fibers together, and distributes the forces among them. The plastic also helps prevent the propagation of cracks. Composite materials are anisotropic, the material's properties depend on the direction of the fibers. Because of this, multiple layers are put on top of each other at 90° angles. Of course, glass fiber isn't the only material used in composite materials, carbon fibres, and many others are also used, depending on the specific requirements of the rotor blades.

Modern rotor blades start out with a core, made out of Nomex (a brand of aramid), or honeycomb aluminium, which is cut to size. Then, precisely cut pieces of composite materials are placed inside a mold, and are partially cured. The core is then placed within the mold, and is crushed into shape by a hydraulic press. The composite material is then cured using pressurized steam, and excessive material is trimmed off.

Rotor blades constructed out of composite materials can be up to 45% lighter than their metal equivalents, and they can be more easily manufactured in complex shapes.

Sources:
http://www.whystudymaterials.ac.uk/casestudies/helicopter.asp
http://www.madehow.com/Volume-1/Helicopter.html
http://www.advancedtechnologiesinc.com/rotor_blade_development.asp

Countering the torque effect

There are several ways to counter the torque the rotor blades generate on the helicopter fuselage. The most common solution is the tail rotor, but that's not the only option.

NOTAR, "no tail rotor", is a system which utilizes the Coanda effect to redirect the air coming from the main rotor to the side, and thus counters the torque caused by the main rotor. Inside the tail boom is a fan which blows a high volume of air down the boom. The air exits the tail boom through slots on the side of the boom. This small air flow attracts and redirects the main air flow from the rotor, which in turn counters the rotor torque.

Normally, a rotor is driven by the mast, however the tip jet system makes the rotor blades turn using nozzles on the tips of the blades. The tips eject a gas in one direction and the blades move in the opposite direction. These two forces are action and reaction, and because of this, no torque is generated on the fuselage.

Another group of helicopters use several counterrotating main rotors, and because the rotors spin in opposite directions, no net torque is generated on the helicopter. Variations include coaxial rotors, tandem rotors, and intermeshing rotors.

Coaxial rotors

Tandem rotors

Intermeshing rotors

Sources:
http://en.wikipedia.org/wiki/Helicopter_rotor
http://en.wikipedia.org/wiki/NOTAR
http://en.wikipedia.org/wiki/Tip_jet
http://en.wikipedia.org/wiki/Tandem_rotors
http://en.wikipedia.org/wiki/Coaxial_rotors
http://en.wikipedia.org/wiki/Intermeshing_rotors

Cockpit controls

Due to the innate complexity of flying in general, and helicopters in particular, there are quite a few controls necessary to pilot a helicopter.



First of all, there's the throttle. The throttle controls how much fuel goes to the engine of the helicopter. The rotor blades of a helicopter are designed to rotate at a specific speed. Because of this, it is necessary to be able to influence the power output of the engine by means of a throttle. This is controlled using a twist throttle on the collective control.

Next, there are the anti-torque pedals. These pedals control the pitch of the tail rotor blades. By changing the pitch, the tail rotor will produce more or less force, which causes the helicopter to yaw.

The collective control is a lever which moves the swashplate up and down. This causes the pitch of all the blades to change by the same amount, which means that this lever controls the lift of the helicopter. The collective control is usually at the left side of the pilot. The name is derived from the fact that it changes the angle of attack of all the blades collectively (i.e. at the same time).

Finally, there's the cyclic control. The cyclic control is the stick located between the pilot's legs and controls the tilt direction of the swashplate. The cyclic control changes the pitch of the blades cyclically (i.e. depending on the position of the blade in the rotation cycle). By pushing the cyclic forward, the helicopter will move forward. Moving the cyclic left or right will make the helicopter roll in that direction. Moving it backward will make the helicopter move backward.

Sources:
http://en.wikipedia.org/wiki/Helicopter_flight_controls
http://science.howstuffworks.com/transport/flight/modern/helicopter4.htm

How helicopters fly

The first step necessary to achieve flight is generating a force which counters gravity. By moving an airfoil through the air, lift is generated due to the shape and angle of the airfoil. In the case of a helicopter, multiple airfoils are attached to a shaft, and by spinning that shaft, lift is generated.

This works well up until the moment the helicopter no longer touches the the ground. Newton's laws of motion dictate that to every action there is an equal and opposite reaction. This means that if the helicopter's rotor turns in one direction, the body of the helicopter will have a tendency to turn in the opposite direction. There are multiple solutions to this problem, but the most common solution is the tail rotor. The tail rotor generates a force which counters the helicopter's tendency to spin.

The next problem is movement, a helicopter with only the previous two mechanisms will only be able to take off and land. One way to make a helicopter move is by constantly changing the angle of attack of the blades. This in turn causes the blades to generate more or less lift on different parts of the surface the rotor blades cover, and this causes the helicopter to move.

This is implemented using a swashplate mechanism. The blades are connected to the rotor shaft using a bearing. A rod is also connected to the blade, and by moving this rod up and down, the blade rotates around its length axis. This means that the rod controls the angle of attack of the blade, and thus controls the lift the blade generates.

All these rods are connected to a ring-shaped plate around the main rotor shaft, and this rotating plate is then connected to a stationary plate using a bearing. Tilting the stationary plate causes the blades to different lift depending on their position, and this causes the helicopter to move. Moving the plate up and down changes the angle of attack of all the blades by the same amount, and thus allows the pilot to control how much lift the helicopter generates.

The combination of these mechanisms allow the helicopter to move in virtually any direction.

Sources:
http://en.wikipedia.org/wiki/Helicopter_rotor
http://en.wikipedia.org/wiki/Swashplate_%28helicopter%29
http://science.howstuffworks.com/transport/flight/modern/helicopter1.htm

Basic function of an airfoil

Lift is defined as the force perpendicular to the direction of the air flow, and is responsible for negating gravity and thus making an airplane or helicopter fly. Lift is the result of 2 different factors. The first factor is quite obvious: an airfoil changes the direction of the airflow downwards, this is known as downwash.

The second factor is caused by pressure differences. When air hits the front of the airfoil, the airflow is split in two. Because of the angle of attack, the air below the wing is compressed, and the air flowing above the wing, flows well over the surface of the wing, in an arc shape. The air closest to the upper surface is traveling slower than the air above it, and this creates a low pressure pocket because of the Bernouilli effect.

It is possible to fly with a completely flat wing, but most wings have a typical airfoil shape which helps the air arc over the upper surface. Helicopter blades do not have a pronounced airfoil shape because drag is of less concern, but they use the same mechanics to produce a lifting force.

Sources:
http://warp.povusers.org/grrr/airfoilmyth.html
http://en.wikipedia.org/wiki/Airfoil
http://en.wikipedia.org/wiki/Lift_(force)