Helicopters

by Chris Woodford. Last updated: August 30, 2011.

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: The US Navy's largest helicopter: the CH53-E Sikorsky Super Stallion. Each one costs almost $25 million! Picture by Joshua Adam Nuzzo courtesy of US Navy.

Rotors are spinning wings

Helicopters fly upward against the force of gravity by using their rotors to throw air down beneath them. Like the wings of an airplane, each blade in a helicopter's rotor is an airfoil (aerofoil): a wing with a curved top and a straight bottom. As the blade spins around, it forces air over its curved upper surface and then throws it down behind it toward the ground, producing 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.

Photo: Mighty rotors: You can see just how big and heavy a helicopter's rotors are in this picture. It takes four US marines to hold this rotor in place while it's being reattached after maintenance. Notice the curved front edge of the rotor blade that cuts like an airfoil as it spins around. Picture by Jeremy L. Grisham courtesy of US Navy.

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.

Why do helicopters need a tail rotor?

Photo: The tail rotor of a Seahawk helicopter. The tail rotor is driven by a drive shaft running back from the main engines, parallel to the body of the helicopter. If you look closely, you'll see that the blades of the rotor can swivel to generate more or less pushing force, giving the helicopter the ability to rotate on the spot as it hovers. Picture by James R. Evans courtesy of US Navy.

According to the laws of motion, 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 (counter-rotating) 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.

Further Reading

On this website

• Airplanes
• Jet engines

On other websites

• Sikorksy Archives: A detailed archive devoted to the Russian-born helicopter pioneer.
• Igor Sikorksy: A basic Wikipedia biography.

Books

For older readers

• Helicopters: An Illustrated History of their Impact by Stanley S. McGowen. ABC-CLIO, 2005. A detailed history of helicopters, starting with Leonardo and ending with tilt-rotor and NOTAR copters. Quite text-heavy, though with plenty of facts and specifications of important craft.
• Learning to Fly Helicopters by R. Randall Padfield. McGraw-Hill Professional, 1992. A pilot's manual, but also a great technical guide to exactly how helicopters work (and why they stay in the sky).
• The Sikorsky Legacy by Sergei I. Sikorsky. Arcadia Publishing, 2007. A mainly pictorial history of Igor Sikorsky's contribution to helicopter flight by his son, with many photos from the company archives.

For younger readers

• Helicopters by Mark Dartford. Lerner Publications, 2003. A basic 48-page introduction for ages 8–10, covering the history of helicopters, a little of the science, and how helicopters are used in different ways.
• Air and Space Travel by Chris Woodford. Facts on File, 2004. One of my own books, this is a longer (96-page) guide to the history of human flight, from gliders and balloons through planes and helicopters to space rockets.