Helicopters

Last updated: April 26, 2006.

Helicopters are highly maneuverable aircraft that fly not by forcing air over a pair of fixed wings, like an airplane, but by spinning a rotor blade at high speed. Leonardo da Vinci (1452–1519) is generally credited with inventing the helicopter, but the first practical helicopter was developed only in 1939 by Russian-born Igor Sikorsky (1889–1972). Today, typical uses for helicopters include military transportation and air-sea rescue.

Photo of Sikorsky's prototype helicopter/airplane, 1984. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

Rotors are spinning wings

Each blade in a helicopter rotor is an airfoil, a wing with a curved top and a straight bottom. As the blade spins around, air travels faster over the top surface than under the bottom. This reduces air pressure above the blade and produces an upward force called lift. The pitch of the blades (the angle they make to the incoming airflow) controls the amount of lift. During takeoff, the pilot increases the pitch with a control called the collective pitch stick. The lift produced is greater than the helicopter's weight and this makes the helicopter rise upward. If the lift exactly equals the weight, the helicopter hovers. If the weight is greater than the lift, the helicopter descends to Earth. Turning the throttle increases the speed of the blades and also increases lift.

Normally the lift produced by the rotor aims straight upward, but the pilot can tilt the rotor blades with a device called the cyclic pitch control to make the helicopter fly in a particular direction. Although most of the lift force still points upward, some of it now also points to the front, back, left, or right, tilting the entire helicopter and pushing it in that direction.

The pilot's movements are transmitted from the cockpit to the rotor blades by two disks called the upper and lower swash plates. The lower swash plate does not rotate, but can tilt or move up and down. The upper swash plate spins with the rotors on ball bearings on top of the lower swash plate. When the pilot pushes the controls, the lower swash plate nudges the upper swash plate, and the blades are tilted in turn by a system of control rods.

Rotors and torque

According to the laws of physics, any force (or action) produces an equal force (or reaction) in the opposite direction. This means the torque (rotating force) produced by a helicopter's blades tends to turn the fuselage (the main helicopter body) in the opposite direction. All helicopters have either a second propeller or another device to counteract the torque of the main blade. In most helicopters, a tail rotor balances the torque by pushing in the opposite direction to the main rotor. Some helicopters have two rotors mounted on the same shaft, which turn in opposite directions to cancel the torque. Others (notably the large military Chinook helicopters) have a rotor at the front and a rotor at the back and cancel the torque by turning in opposite directions. Tail rotors solve one problem but can cause others. Noisy and dangerous to passengers, the tail rotor of a helicopter is also highly susceptible to damage from passing birds or debris. This is a big problem, because a helicopter with a damaged tail rotor is dangerously uncontrollable. NOTAR helicopters have a giant fan inside the fuselage that sucks in air just behind the cockpit and blows it out again through a side hole near the tail. This produces the same sideways force as a tail rotor, but is quieter and safer.