Friday, 30 September 2011

Controlling a helicopter

Controlling a Helicopter

Helicopters require a completely different method of control than airplanes and are much harder to master. Flying a helicopter requires constant concentration by the pilot, and a near-continuous flow of control corrections.
A conventional helicopter has its main rotor above the fuselage which consists of 2 or more rotor blades extending out from a central rotor head, or hub, assembly.
The primary component is the swash plate, located at the base of the rotor head. This swash plate consists of one non-revolving disc and one revolving disc mounted directly on top. The swash plate is connected to the cockpit control sticks and can be made to tilt in any direction, according to the cyclic stick movement made by the pilot, or moved up and down according to the collective lever movement.
But first, to explain how the main rotor blades are moved by the pilot to control the movement of the helicopter, we need to understand pitch.

The basics of pitch

Each rotor blade has an airfoil profile similar to that of an airplane wing, and as the blades rotate through the air they generate lift in exactly the same way as an airplane wing does [read about that here]. The amount of lift generated is determined by the pitch angle (and speed) of each rotor blade as it moves through the air. Pitch angle is known as the Angle of Attack when the rotors are in motion, as shown below:

This pitch angle of the blades is controlled in two ways - collective and cyclic.

Collective control

The collective control is made by moving a lever that rises up from the cockpit floor to the left of the pilot's seat, which in turn raises or lowers the swash plate on the main rotor shaft, without tilting it.

This lever only moves up and down and corresponds directly to the desired movement of the helicopter; lifting the lever will result in the helicopter rising while lowering it will cause the helicopter to sink. At the end of the collective lever is the throttle control, explained further down the page.

As the swash plate rises or falls, so it changes the pitch of all rotor blades at the same time and to the same degree. Because all blades are changing pitch together, or 'collectively', the change in lift remains constant throughout every full rotation of the blades. Therefore, there is no tendency for the helicopter to move in any direction other than straight up or down.
The illustrations below show the effect of raising the collective control on the swash plate and rotor blades. The connecting rods run from the swash plate to the leading edge of the rotor blades; as the plate rises or falls, so all blades are tilted exactly the same way and amount.
Of course, real rotor head systems are far more complicated than this picture shows, but the basics are the same.

Cyclic control

The cyclic control is made by moving the control stick that rises up from the cockpit floor between the pilot's knees, and can be moved in all directions other than up and down.
Like the collective control, these cyclic stick movements correspond to the directional movement of the helicopter; moving the cyclic stick forward makes the helicopter fly forwards while bringing the stick back slows the helicopter and even makes it fly backwards. Moving the stick to the left or right makes the helicopter roll and turn in these directions.
The cyclic control works by tilting the swash plate and increasing the pitch angle of a rotor blade at a given point in the rotation, while decreasing the angle when the blade has spun through 180 degrees.

As the pitch angle changes, so the lift generated by each blade changes and as a result the helicopter becomes 'unbalanced' and so tips towards whichever side is experiencing the lesser amount of lift.

The illustrations below show the effect of cyclic control on the swash plate and rotor blades. As the swash plate is tilted, the opposing rods move in opposite directions. The position of the rods - and hence the pitch of the individual blades - is different at any given point of rotation, thus generating different amounts of lift around the rotor disc.

To understand cyclic control another way is to picture the rotor disc, which is the imaginary circle above the helicopter created by the spinning blades, and to imagine a plate sat flat on top of the cyclic stick. As the stick is leaned over in any direction, so the angle of the plate changes very slightly. This change of angle corresponds directly to what is happening to the rotor disc at the same time ie the side of the plate that is higher represents the side of the rotor disc generating more lift

Rotational (yaw) control

At the very rear of the helicopter's tail boom is the tail rotor - a vertically mounted blade very similar to a conventional airplane propeller. This tail rotor is used to control the yaw, or rotation, of the helicopter (ie which way the nose is pointing) and to explain this we first need to understand torque.

Torque is a natural force that causes rotational movement, and in a helicopter it is caused by the spinning main rotor blades; when the blades are spinning then the natural reaction to that is for the fuselage of the helicopter to start spinning in the opposite direction to the rotors. If this torque isn't controlled, the helicopter would just spin round hopelessly!
So to beat the reaction of the torque, the tail rotor is used and is connected by rods and gears to the main rotor so that it turns whenever the main rotor is spinning.
As the tail rotor spins it generates thrust in exactly the same way as an airplane propeller does. This sideways thrust prevents the helicopter fuselage from trying to spin against the main rotor, and the pitch angle of the tail rotor blades can be changed by the pilot to control the amount of thrust produced.

Increasing the pitch angle of the tail rotor blades will increase the thrust, which in turn will push the helicopter round in the same direction as the main rotor blades. Decreasing the pitch angle decreases the amount of thrust and so the natural torque takes over, letting the helicopter rotate in the opposite direction to the main rotors.
The pilot controls the pitch angle of the tail rotor blades by two pedals at his feet, in exactly the same way as the rudder movement is controlled in an airplane.
NOTAR is an alternative method of yaw control on some helicopters - instead of a tail rotor to generate thrust, compressed air is blown out of the tail boom through moveable slots. These slots are controlled by the pilot's pedals in the same way as a tail rotor is. To generate more thrust, the slots are opened to let out more air, and vice versa.
NOTAR helicopters respond to yaw control in exactly the same way as tail rotor models and have a big safety advantage - tail rotors can be very hazardous while operating on or close to the ground and in flight a failing tail rotor will almost always result in a crash.

Throttle control

The throttle control is a 'twist-grip' on the end of the collective lever and is linked directly to the movement of the lever so that engine RPM is always correct at any given collective setting. Because the cyclic and collective pitch control determines the movement of the helicopter, the engine RPM does not need to be adjusted like an airplane engine does. So during normal flying, constant engine speed (RPM) is maintained and the pilot only needs to 'fine tune' the throttle settings when necessary.

There is, however, a direct correlation between engine power and yaw control in a helicopter - faster spinning main rotor blades generate more torque, so greater pitch is needed in the tail rotor blades to generate more thrust.

It's worth noting that each separate control of a helicopter is easy to understand and operate; the difficulty comes in using all controls together, where the co-ordination has to be perfect! Moving one control drastically effects the other controls, and so they too have to be moved to compensate. His continuous correction of all controls together is what makes flying a helicopter so intense. Indeed, as a helicopter pilot once said... 
"You don't fly a helicopter, you just stop it from crashing"!

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