The History and Functioning of Helicopters
Flying a helicopter is complicated. The pilot has to think in three-dimensions and must use both arms and legs constantly to keep the helicopter in the air. Piloting a helicopter requires a great deal of training and skill, as well as continuous attention to the machine.
To control a helicopter, one hand grasps a control called the cyclic, which controls the lateral direction of the helicopter (including forward, backward, left and right). The other hand grasps a control called the collective, which controls the up and down motion of the helicopter (and also controls engine speed). The pilot's feet rest on pedals that control the tail rotor, which allows the helicopter to rotate in either direction on its axis. It takes both hands and both feet to fly a helicopter.
Special Capabilities of Helicopters
Helicopters have a number of unique abilities - The signature of a helicopter is its ability to hover over one point on the ground. While hovering, a helicopter can also spin on its axis so that the pilot can look in any direction. Another unique feature of a helicopter is its ability to fly backwards. A helicopter can also fly sideways just as easy.
Since a helicopter can fly backwards and sideways, it can do a number of interesting tricks - pirouette, in which it rotates 360 degrees while it travels down in a straight line relative to the ground. A helicopter that is flying forward can also stop in mid air and begin hovering very quickly.
How Helicopters Fly
If you are going to provide the upward force with a wing, then the wing has to be motion in order to create lift. Wings create lift by deflecting air downward and benefiting from the equal and opposite reaction that results. A rotary motion is the easiest way to keep a wing in continuous motion. So you can mount two or more wings on a central shaft and spin the shaft. The rotating wings of a helicopter are shaped just like the airfoils of an airplane wing, but generally the wings on a helicopter's rotor are narrow and thin because they must spin so quickly. The helicopter's rotating wing assembly is normally called the main rotor. If you give the main rotor wings a slight angle of attack on the shaft and spin the shaft, the wings start to develop lift.
In order to spin the shaft with enough force to lift a human being and the vehicle, you need an engine - reciprocating gasoline engines and gas turbine engines are the most common types. The engine's drive shaft can connect through a transmission to the main rotor draft. This arrangement works really well until the moment the vehicle leaves the ground. At that moment, there is nothing to keep the engine (and therefore the body of the vehicle) from spinning just like the main rotor does. So, in the absence of anything to stop it, the body will spin in an opposite direction to the main rotor. To keep the body from spinning, you need to apply a force to it.
The usual way to provide a force to the body of the vehicle is to attach another set of rotating wings to a long boom. These wings are known as the tail rotor. The tail rotor produces thrust just like an airplanes propeller does. By producing thrust in a sideways direction, counteracting the engines desire to spin the body, the tail rotor keeps the body of the helicopter from spinning. Normally, the tail rotor is driven by a long drive shaft that runs from the main rotor's transmission back through the tail boom to a small transmission @ the tail rotor. In order to actually control the machine, both the main rotor and the tail rotor need to be adjustable.
The Tail Rotor
The adjustability of the tail rotor is straightforward - what you want is the ability to change the angle of attack on the tail rotor wings so that you can use the tail rotor to rotate the helicopter on the drive shaft's axis, the pilot has two foot pedals that control the angle of attack.
The Main Rotor
A helicopters main rotor is the most important part of the vehicle. It provides the lift that allows the helicopter to fly, as well as the control that allows the helicopter to move laterally, make turns and change altitude.
To handle all these tasks, the rotor must first be incredibly strong. It must also be able to adjust the angle of the rotor blades with each revolution of the hub. The adjustability is provided by a device called the swash plate assembly.
The swash plate assembly has two primary roles:
Under the direction of the collective control, the swash plate assembly can change the angle of the blades simultaneously. Doing this increases or decreases the lift that the main rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude
Under the direction of the cyclic control, the swash plate assembly can change the angle of the blades individually as they revolve. This allows the helicopter to move in any direction around a 360 degree circle, including forward, backward, left and right.
*The swash plate assembly consists of two plates - the fixed and the rotating swash plates
The rotating swash plate rotates with the drive shaft and the rotor's blades because of the links that connect the rotating plate to drive the shaft.
The pitch controls rods allow the rotating swash plate to change the pitch of the rotor blades.
The angle of the fixed swash plate is changed by the control rods attached to the fixed swash plate.
The fixed plate's control rods are affected by the pilot's input to the cyclic and collective controls
The fixed and rotating swash plates are connected with a set of bearings between the two plates. These bearings allow the rotating swash plate to spin on top of the fixed swash plate
The swash plate assembly changes the angle of attack of the main rotor's wings as the wings revolve. A steep angle of attack provides more lift than a shallow angle of attack.
The collective lets you change the angle of attack of the main rotor simultaneously on both blades.
The cyclic control tilts the swash plate assembly so that the angle of attack on one side of the helicopter is greater than it is on the other
The cyclic changes the angle of attack of the main rotor's wings unevenly by tilting the swash plate assembly. On one side of the helicopter, the angle of attack (and therefore the lift) is greater.
Hovering
Hovering in a helicopter requires experience and skill. The pilot adjusts the cyclic to maintain the helicopter's position over a point on the ground. The pilot adjusts the collective to maintain a fixed altitude (especially important when close to the ground. The pilot adjusts the foot pedals to maintain the direction that the helicopter is pointing. Windy conditions can make hovering a real challenge.
Relating the Controls and the Swash Plate
The collective control raises the entire swash plate assembly as a unit. This has the effect of changing the pitch of both blades simultaneously.
The cyclic control pushes one side of the swash plate assembly upward or downward. This has the effect of changing the pitch of the blades unevenly depending on where they are in the rotation. The result of the cyclic control is that the rotor's wings have a greater angle of attack (and therefore more lift) on one side of helicopter and lesser angle of attack (and less lift) on the opposite side. The unbalanced lift causes the helicopter to tip and move laterally.
