Helicopters : Flying a Helicopter
Add comment November 20th, 2005
The helicopter is type of aircraft in which lift is obtained by means of one or more power-driven horizontal propellers called rotors. When the rotor of a helicopter turns it produces reaction torque which tends to make the craft spin also. On most helicopters a small rotor near the tail compensates for this torque. On twin-rotor craft the rotors spin in opposite directions, so their reactions cancel each other. The helicopter is propelled in a given direction by inclining the axis of the main rotor in that direction. The helicopter’s speed is limited by the fact that if the blades rotate too fast they will produce compressibility effects on the blade moving forward and stall effects on the rearward moving blade, at the same time.
Although the helicopter was only recently fully developed, its concept can date back to the late 1400’s. Since then, helicopters have been put into use by society in many ways. One can find helicopters in both civil and military areas. The early helicopters were mainly developed for military use, but later became certified for civilian use. Since then helicopters have evolved greatly, specifically with the design. Because a helicopter can perform more actions than a fixed-wing aircraft can, it is more complicated to fly. The helicopter must compensate for a variety of forces, like the spinning force induced by the main rotors. The engineering behind designing a helicopter is complex with a variety of issues to be understood .
The Main Rotor : The lifting force is produced by the rotors. As they spin they cut into the air and produce lift. Each blade produces an equal share of the lifting force. Spinning the rotor against the air causes lift, allowing the helicopter to rise vertically or hover. Tilting the spinning rotor will cause flight in the direction of the tilt.
The Tail Rotor : The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction. This is called TORQUE REACTION. By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder.
The tail rotor is connected to the main rotor through a gearbox.
When using the tail rotor trying to compensate the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate. The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first.
On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon. The helicopter is steered in any direction by inclining the axis of the main rotor in that direction. Flying a helicopter requires great concentration. You must use one hand on the control lever that is at your side (the collective control stick) to raise or lower the helicopter, while at the same time controlling the throttle (not an easy task). This is a control which is only found in helicopters and is linked to the engine power. Moving this up and down changes the pitch of the main rotors. As the pitch is increased more power is required from the engines so that the rotor speed is kept at the same level.
You must use your other hand on the control lever that is just in front of you (the cyclic control stick) to move the helicopter forward, backward and to either side, as if you were in a conventional aircraft.
Moving it forward or back will point the nose of the helicopter up or down. It does this by varying the angle of the rotor blades as they go round, tilting the rotor back and forth. When moved left or right the rotor tilts in that direction and the helicopter banks and rolls.
And finally you must use the tail rotor pedals, on the floor, to control the pitch of the tail-rotor. For straight flight, the pitch of the tail rotor is set to prevent the helicopter from turning to the right as the main rotor turns to the left. The pilot pushes the left pedal to increase the pitch of the tail rotor and turn to the left. Pushing the right pedal decreases the pitch of the tail rotor and turns the helicopter to the right.
Flying a helicopter requires entirely different skills than flying conventional aircraft. This is why it is difficult to fly a vertical take-off or landing. On tandem rotors helicopters, like Boeing ’s Chinook, that had not tail rotor, the pedals are connected to the swashplates and cyclicly change pitch on both rotors in equal, but opposite directions. For example, if the left pedal is pressed at a hover, the front rotor disk tips left and the rear rotor tips right so that the helicopter yaws to the left.
How the Helicopter Flies
Flight of a helicopter is governed by the pitch or angle of its rotor blades as they sweep through the air. For climbing and descending, the pitch of all the blades is changed at the same time and in the same degree.
To Climb, the angle ot pitch of the blades is increased. To descend, the pitch of the blades is decreased. Because all blades are acting simultaneously, or collectively, this is known as collective pitch .
For forward, backward and sideways flight an additional change of pitch is provided. By this means the pitch of each blade increases at the same selected point in its circular pathway. This is the cyclic pitch.
With these two controls in mind let us make an imaginary flight. With the engine warmed up and the rotor blades whirling above us in flat pitch, that is, with no angle or bite in the air, we are ready to start. We increase the collective pitch. The rotor blades bite into the air, each to the same degree, and lift the helicopter vertically. Now we decide to fly forward. We still have collective pitch to hold us in the air and we adjust the cyclic pitch so that as each blade passes over the tail of the helicopter, it has more bite on the air than when it passes over the nose. Naturally the helicopter travels forward.
Now we decide to stop and hover motionless so we put the cyclic pitch in neutral, the rotor blades now have the same pitch throughout their cycle, and the collective pitch holds the helicopter suspended in space without moving in any direction. In short, it is the cyclic and collective pitch which gives the helicopter its unique ability to fly forward, backwards, sideways, rise and descend vertically and hover motionless in the air , making it one of the most versatile vehicles known by man.
