Helicopters
Last updated: December 1, 2009.
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: Sikorsky's prototype tilt-rotor helicopter/airplane, 1984. Picture courtesy of NASA Glenn Research Center (NASA-GRC).
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: 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.
How helicopter rotors work
Everyone knows a helicopter's rotors rotate (that's why they're called rotors).
But the really clever thing about them is that the blades can swivel back and forth
as they turn around—and that requires some amazingly intricate machinery.
It's easy to mimic a helicopter with your arms and your body's hidden structure makes the movements seem easy.
Stand up with your arms outstretched horizontally. Rotate your whole body slowly on the spot.
As you're turning around, swivel your arms at the shoulders. That's roughly what a helicopter
does with its blades, except that it does it about 3-4 times each second!
Here are the main bits that make it work:
- The blades are shaped like airfoils (airplane wings with a curved profile) so they generate lift as they spin around.
- Each blade can swivel as it spins around.
- Vertical rods push the blades up and down, making them swivel as they rotate.
- A central axle connected to engine makes the entire blade assembly rotate.
- The cap above the rotors is missile proof to protect against enemy attacks.
- There are two turbo-shaft jet engines, one on either side of the rotors. If one engine fails, there should still be enough power from the other engine to land the helicopter safely.
Photo: Top: A US Navy engineer checks the rotor assembly on a Seahawk helicopter.
Picture by Kathaleen A. Knowles courtesy of US Navy.
Bottom: An engineer repairs the amazingly intricate and complex rotor mechanism of a Seahawk, viewed here from
directly above. The engines are the two open cones on either side. You can also see two of the rotor blades
folded back (and pointing upward in this picture). Photo by Oliver Cole courtesy of US Navy.
Why do helicopters need a tail rotor?
Photo: The tail rotor of a Seahawk helicopter.
Picture courtesy of US Navy.
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.
