by Chris Woodford. Last updated: April 3, 2015.
H elicopters 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 design was developed only in the 1930s 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.
How does a helicopter stay in the air?
Rotor blades work like 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.
Photo: Right: 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.
Artwork: How a helicopter steers: Top drawing: The collective pitch control changes the angle (or pitch) of each of the rotor blades by the same amount at the same time (green arrows)—in other words, collectively. If the blades make a steeper angle, they generate more lift so the entire craft moves straight upward (orange arrow). Bottom drawing: The cyclic pitch control changes the angle of selective rotor blades as they spin, so (in this case) whichever blade is on the left always produces slightly more lift, while the opposite blade (shown here on the right) always produces slightly less lift. That means more lift is produced on the left side of the helicopter, so the overall lift (orange arrow) is tilted to the right, steering the entire helicopter in that direction.
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 as the blades are spinning round! 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.
- Each blade can swivel as it spins.
- Vertical rods push the blades up and down, making them swivel as they rotate.
- A central axle connected to the 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 with annotations by Explain that Stuff. 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 along the fuselage (and pointing upward in this picture), which means the Seahawk can be parked on aircraft carriers in much less space. Photo by Oliver Cole courtesy of US Navy.
How Sikorsky designed the modern helicopter rotor
All this sounds ingenious—and it is! The person who made it possible was brilliant Russian-born inventor Igor Sikorsky. Here are two of his original helicopter design drawings, taken from the patent for a Direct Lift Aircraft (helicopter) he filed in June 1931:
Notice how similar the mechanism is to what we find on a modern helicopter? The patent is extremely detailed and quite complex (you can check it out for yourself), so I've removed most of the labels and numbers and highlighted just
a few key features:
- There's an aileron at the end of each rotor blade, shown in orange.
- The ailerons can be tilted (as they rotate) by the blue rods.
- There are two main rotor blades (which Sikorsky referred to collectively as the "lift propeller."
- The entire rotor blades can swivel on the green rods and can also be tilted as they rotate.
- The main rotor blade rotates around a central hub (yellow) with an engine beneath it.
- A single engine powers both the main rotor blade and the tail rotor. One of Sikorsky's key innovations was to produce a helicopter that needed only one main rotor blade, with a tail rotor to balance it, for reasons discussed below. As Sikorsky noted in his patent, having only one rotor means a helicopter is "light in weight, simple to construct, and cheap to produce"—three powerful advantages over earlier designs!
Artwork: Sikorsky's original patent drawings courtesy of US Patent and Trademark Office. Colors and annotations have been added (for clarity) by explainthatstuff.com.
Why do helicopters need a tail rotor?
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.
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 be tilted by the pilot as they spin around, which generates more or less pushing force and gives the helicopter the ability to rotate on the spot as it hovers. Picture by James R. Evans courtesy of US Navy.
Vertical/Short Takeoff and Landing (V/STOL) aircraft
Airplanes fly fast but need super-long runways for taking off and landing. Helicopters can take off and land almost anywhere, but their complex and relatively clumsy rotor systems mean they can travel at only a fraction of a plane's speed. If you want the best of both worlds—high speed and land-anywhere versatility—you need a V/STOL aircraft. one that's capable of "vertical/short takeoff and landing," such as the famous Harrier jump jet or the tilt-rotor Osprey.
Photo: The Harrier—the most famous V/STOL aircraft of them all. This one is an AV-8B II, landing vertically on the deck of a relatively small US Marine Corps amphibious assault ship. Photo by Angel Roman-Otero courtesy of US Navy.
How can planes take off vertically?
Airplanes have to travel at high speeds to produce enough lift for takeoff, but because they are immensely heavy and often carry substantial cargoes, they can accelerate only very slowly. A typical runway for a large airliner such as a Boeing 747 is around 2 miles (3 km) long, simply because the plane has to travel this far before it has picked up enough speed to get off the ground.
Long runways may be fine for passenger aircraft, but military fighters need to take off in much more confined spaces (for example, from the deck of an aircraft carrier). Vertical/Short Takeoff and Landing (V/STOL) aircraft solve this problem by having jet engines whose nozzles can be swiveled in different directions. During takeoff and landing, the jets point straight downward so the plane can rise or fall on the spot or hover like a helicopter. (Some V/STOL aircraft can even point their nozzles forward so they can fly backward!) Once the plane is airborne, the nozzles swivel so they're pointing backward and the plane shoots forward like a conventional airplane.
Photo: A Harrier can hover because, unlike a traditional jet engine, it has four extra nozzles on the side that can swivel around to direct the engine's exhaust gases straight downward. Picture by Staci Bitzer courtesy of US Navy.
The best known plane of this sort is the Harrier "jump-jet" extensively used by the UK Royal Navy and the US Marine Corps. The Joint Strike Fighter (JSF) currently being developed by Boeing and Lockheed for the US military will also be a VTOL aircraft. The US Airforce Osprey plane works in a similar way, but has tilting propellers instead of jet engines. To land vertically, like a helicopter, it tilts the propellers upward. To fly horizontally, it points them forward.
Tilt-rotor aircraft combine the maneuverability of a helicopter with the speed, range, and economy of a small airplane. Like an airplane, they have wings and propellers. But the propellers can be rotated to point upward, enabling the airplane to take off and land vertically in a confined space. Once the craft is airborne, the propellers can be turned back so it can fly along like a conventional airplane. Bell Boeing's V-22 Osprey is an example of a tilt-rotor craft like this. Ospreys can have their rotors angled forward to fly like planes, pointed upward to hover like helicopters, or folded up for easy storage on aircraft carriers: