by Chris Woodford. Last updated: December 4, 2017.
No runway, no problem—have helicopter, will travel! Igor Sikorsky (1889–1972), father of the modern chopper, had no doubt at all about the brilliance of this amazing, flying machine, which he said was the closest thing to "fulfillment of mankind's ancient dreams of the flying horse and the magic carpet." Jet planes are wonderful for screaming us from one side of the planet to the other. But when it comes to tricky rescue missions—plucking stranded sailors from the sea, hurling tubs of water onto forest fires, plucking engineers off wind turbines, dashing the critically injured to hospital—nothing beats a chopper. According to science historians, inventors had been trying to develop flying machines with spinning rotors for over 2000 years before Sikorsky finally built the world's first practical helicopter in 1939. Why did it take so long? Because helicopters are incredibly complex machines—miracles of intricate engineering that take real skill to fly. How exactly do they work? Let's take a closer look!
Photo: The US Navy's largest helicopter: the CH53-E Sikorsky Super Stallion. A newer version, the CH-53K, is currently under development and expected to cost about $100 million per helicopter! Picture by Joshua Adam Nuzzo courtesy of US Navy.
How does a helicopter stay in the air?
The science of a helicopter is exactly the same as the science of an airplane: it works by generating lift—an upward-pushing force that overcomes its weight and sweeps it into the air. Planes make lift with airfoils (wings that have a curved cross-section). As they shoot forwards, their wings change the pressure and direction of the oncoming air, forcing it down behind them and powering them up into the sky: a plane's engines speed it forward, while its wings fling it up. The big problem with a plane is that lots of air has to race across its wings to generate enough lift; that means it needs large wings, it has to fly fast, and it needs a long runway for takeoff and landing.
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.
Helicopters also make air move over airfoils to generate lift, but instead of having their airfoils in a single fixed wing, they have them built into their rotor blades, which spin around at high speed (roughly 500 RPM, revolutions per minute). The rotors are like thin wings, "running" on the spot, generating a massive downdraft of air that blows the helicopter upward. With skillful piloting, a helicopter can take off or land vertically, hover or spin on the spot, or drift gently in any direction—and you can't do any of that in a conventional plane.
Key parts of a helicopter
A typical helicopter has thousands of intricate components, but we only need to worry about a handful of the bigger bits. The main framework is called the fuselage and it's typically made from strong but relatively lightweight composite materials. It contains one or two engines, a transmission, and gearboxes, which power one or two main rotors and a much smaller tail rotor at the back.
Artwork: A quick summary of the essential, mechanical parts of a helicopter. Each rotor blade (1) is connected to the hub (2) and rotating mast by a feathering hinge (3), which allows it to swivel. A pitch link (a short rod) attached to each blade (4, orange) can tilt it to a steeper or shallower angle according to the position of the rotating upper swash plate (5, blue), which spins on bearings around the static lower swash plate (6, red). That's how a chopper hovers and steers and it's described in more detail later in this article. The two swash plates are moved up and down or tilted to the side by the pilot's cyclic and collective cockpit controls (not shown), which are explained below. The rotor is powered by a driveshaft (7) connected to a transmission and gearbox (8, red). The same transmission powers a second, longer driveshaft (9, yellow) connected to a gearbox that spins the tail rotor (10, orange). The power from both rotors comes from one or two turboshaft jet engines (11).
Photo: Helicopter engine: Look under the rotor of this Seahawk helicopter. The long, gray tube between the two sets of numbers ("69") is a turboshaft jet engine. There's a second engine exactly the same on the other side. Photo by Trevor Kohlrus courtesy of US Navy.
Although some small helicopters still use piston engines (also called reciprocating engines, similar to the ones used in cars and trucks), most now use gas turbines more like the jet engines on conventional airplanes. Turbine engines are smoother in operation (vibrating much less), more powerful, less mechanically complex, and more reliable. Some helicopters have a single engine mounted horizontally, underneath and just behind the rotor; most small Bell helicopters, for example, work like this. Others have one engine mounted either side of the rotor mast; military Seahawk and Apache helicopters are powered this way. Most modern choppers have turboshaft engines, which are similar to normal jet engines on airplanes. However, instead of squirting out a hot jet of exhaust gas that thrusts them forward, they use the energy from the burning gas to spin a central turbine and driveshaft that powers the transmission (the mechanism that allows the engine to power the rotors). Our main article on jet engines tells you more about how turbojet engines work.
The huge spinning rotor is the single most noticeable feature of any helicopter, but no chopper can get by with just one rotor. Why? A basic principle of physics called Newton's third law of motion tells us that when a force (called an action) makes something move, another force, just as big (called a reaction), makes something else move in the opposite direction; action and reaction are equal and opposite is another way of putting it. As a helicopter rotor spins around (the action), the entire body of the craft tends to rotate somewhat more slowly in the opposite direction (the reaction). Left to its own devices, this torque (turning force) would make a helicopter completely uncontrollable, so we have to counteract it in some way with what's called counter-torque (a turning force in the opposite direction). One solution is to have a second large rotor spinning the other way. Sometimes this is mounted on the same mast as the first rotor (a design called a coaxial rotor); sometimes, as in the huge military Chinook helicopters, there's a large rotor at either end of the craft (a design called a tandem rotor).
Photo: Tandem rotor: This military Boeing CH-47 Chinook has one rotor at the front and one at the back and they spin in opposite directions to cancel one another's torque. Photo by Tamara Vaughn courtesy of US Navy.
The blades of a helicopter's main rotor come in three basic kinds that allow increasing amounts of movement as they spin around: they're called rigid, semi-rigid, and fully articulated. As the name suggests, rigid blades are firmly attached to the rotor hub (the "wheel" to which the blades are fixed at the top of the spinning rotor mast) by a swiveling connection called a feathering hinge (or pitch hinge). This allows them to "feather" (swivel as they rotate, which, as we'll discover in a moment, is how a helicopter steers). Semi-rigid blades have the same feathering hinge, but they also have a teetering hinge (or flapping hinge) that lets them flap up and down. Fully articulated blades can feather and flap, and they also have a third hinge (a drag hinge) that allows them to move slightly ahead of ("lead") or behind ("lag") their normal position. Each of these blade types has advantages and drawbacks.
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.
Apart from adding a second large rotor, another way to counteract the torque from the main rotor is by using a small, sideways-pointing propeller called a tail rotor, powered by a driveshaft from the engine that runs through the tail end of the craft. Sometimes, for safety reasons, the tail rotor is built right inside the tail (a design called a fenestron or fan-tail). Another alternative is called a NOTAR® ("no tail rotor"), which uses a jet of air, fired through a vent on the tail, to counteract the main rotor torque instead. If a helicopter has a single main rotor blade, it has to have a tail rotor, fenestron, or NOTAR or it can't fly safely; similarly, any damage to the tail rotor—such as a bird strike or missile hit—makes a copter dangerously uncontrollable and usually results in it crashing quite quickly afterward. Most helicopters have a vertical tail fin (pylon) that also helps to counteract some of the torque from the main rotor.
How does a helicopter hover and steer?
A helicopter's rotors are ingenious things that allow it to hover in mid-air or steer in any direction. The pilot has five basic movement and steering controls: two hand levers called the collective and cyclic pitch, a throttle, and two foot pedals. Most maneuvers that a pilot executes involve a complex interplay between these different controls, which is why flying a helicopter requires such skill and concentration.
As they start to spin around, the airfoils on the rotor blades generate lift that overcomes the weight of the craft, pushing it up into the air. If the lift is greater than the weight, the helicopter climbs; if it's less than the weight, the helicopter falls. When the lift and the weight are exactly equal, the helicopter hovers in mid-air. The pilot can make the rotor blades generate more or less lift using a control called the collective pitch (or "collective"), which increases or decreases the angle ("pitch") that all the blades make to the oncoming air as they spin around. For takeoff, the blades need to make a steep angle to generate maximum lift.
Artwork: How a helicopter hovers and 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 does that happen? As we've already seen, the main rotor is connected to the hub at the top of the mast by a feathering hinge that allows each blade to swivel as it spins, so it makes a steeper or shallower angle to the oncoming air. The blades have short vertical rods (pitch links) attached to them that are connected to a rotating metal disc called a swash plate, a bit lower down the mast. This swash plate slides on bearings around a second, similar plate directly underneath that doesn't rotate. When the pilot moves the collective one way, both swash plates move upward, pushing up on the pitch links that tilt the rotor blades to a steeper angle. Moving the collective the other way moves the swash plates back down, pulling on the pitch links and tilting the blades to a shallower angle.
At the end of the collective, there's a throttle connected by a cable to the engine. This is like the accelerator of a car or the throttle of a motorbike, increasing or decreasing the engine speed so the rotor makes more or less lift.
The rotors also provide the steering for a helicopter by making more lift on one side than the other. They do this by swiveling back and forth (feathering) as they rotate, so, for example, they make a steeper angle when they're on the left side of the craft than when they're on the right. That means they generate more lift on the left, tilting the craft over to the right and steering it in that direction. The pilot steers like this using a second lever called the cyclic pitch (also known as the "cyclic stick" or just "cyclic"), similar to a joystick, which makes the blades swivel as they cycle around. The ingenious swash plate mechanism translates the pilot's movements into appropriate movements of the rotor blades. Suppose the pilot wants to fly to the right. First, she moves the cyclic to the right, and a system of connected levers makes the two swash plates tilt to the right as well. This makes the rotor blades tilt to a steep angle when they're on the left and a shallow angle when they're on the right, so the rotor produces more lift on the left hand side, steering the craft to the right.
Artwork: How the swash plate steers a helicopter. In the center, you can see a simplified view of the swash plate mechanism. There are two discs at the top of the rotor mast, an upper one (red) that rotates on ball bearings (orange) around a lower one (blue) that doesn't rotate at all. Four pitch links (green) connect the upper swash plate to the rotor blades. Now suppose you want to fly to the right. You tilt the cyclic in that direction. That tilts both swash plates over to the right. As the rotor blades rotate, the tilted swash plates force the pitch links up when they're on the left and down when they're on the right. That makes each rotor blade tilt to a steeper angle when it's on the left and a shallower angle when it's on the right. This produces more lift on the left, steering the chopper to the right.
The pilot can also steer the nose of a helicopter in a certain direction using a pair of foot controls, known as antitorque pedals, which change the pitch of the tail rotor blades so they make more or less sideways thrust than in normal straight flight. That makes the entire craft rotate slowly clockwise or counterclockwise so it heads in a different direction. On tandem rotor helicopters like the Chinook, which have no tail rotor, the foot pedals tilt the swashplates for the front and back rotors in opposite ways, steering the craft accordingly.