Helicopter

Ever since Daedalus fashioned wings of feathers and wax for himself and his son Icarus, humans have yearned to master powered, heavier-than-air flight. In the early 20th century, a few daring inventors turned the dream into reality by designing and building flying machines that actually lived up to their names. Everyone knows the story of the Wright brothers and their famed flight across the dunes of Kitty Hawk, N.C., so we won't dwell here on their accomplishments or how airplanes work. Instead, we want to focus on a lesser-known personality -- Igor Sikorsky -- and his vision of the modern helicopter: an aircraft without wings that achieves vertical flight from the rotation of overhead blades.

One thing that has characterized the helicopter since its invention in the 1930s has been the absurdity of the machine. The contraption simply looks unable to deliver on its promise, which is to fly up and down, backward and forward, right and left. The famous U.S. broadcast journalist Harry Reasoner discussed this apparent paradox in a 1971 commentary he delivered about the use of helicopters in the Vietnam conflict:

An airplane by its nature wants to fly. … A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other, and if there is any disturbance in this delicate balance, the helicopter stops flying, immediately and disastrously. There is no such thing as a gliding helicopter.

Reasoner laid bare the fundamental reality of helicopters -- that the machines have complex designs and that flying them is extraordinarily complicated. The pilot has to think in three dimensions and must use both arms and both legs constantly to keep a helicopter in the air. Piloting a helicopter requires a great deal of training and skill, as well as continuous attention to the machine.

History of the Helicopter: From Feathers to Hoppers

The modern mechanical marvel we know as the helicopter began as a Chinese top consisting of a shaft -- a stick -- adorned with feathers on one end. Really. When a person placed the stick between his hands and spun it rapidly, the top would rise vertically into the air. Try it for yourself if you're feeling experimental.

Eventually, a few inventors decided to give the Chinese top a power boost. In 1754, a Russian by the name of Mikhail Lomonosov modeled a small rotor on the design of a Chinese top, then used a windup spring to power the device. (A helicopter rotor, by the way, just refers to a rotating part with airfoils, or blades.) Approximately 30 years later, the French naturalist Christian de Launoy built a similar rotor using turkey feathers mounted to both ends of an axle. A string, wound round the axle and tensioned by a crossbow, generated the power. When the tension was released, the counterrotating blades generated lift and carried the device vertically.

These early designs were more toy than transport, but some of the greatest minds in the history of science and engineering were working hard to make vertical-lift flight something humans could enjoy as passengers. Leonardo da Vinci created elaborate sketches for several flying machines, including one he dubbed the aerial screw. The contraption consisted of a linen wing wrapped around an axis, or screw. Four pilots aboard the machine would turn the axis using a pumping action. As the screw turned, so da Vinci theorized, the machine would lift from the ground. And perhaps if the design were lighter, it would have. Sir George Cayley came up with another fanciful machine -- the aerial carriage -- that had two counterrotating rotors mounted on each side of the craft. He attempted to power the device using a gunpowder-based engine, but the results were far from satisfactory.

Eventually, engines evolved enough to move helicopters from the theoretical to the practical. Thomas Edison, who experimented with several helicopter designs in the early 1900s, demonstrated that both high aerodynamic efficiency of the rotor and serious power from an engine were required for successful vertical flight. Other innovations and design refinements quickly followed. The first generation of engine-powered helicopters -- known as hoppers -- emerged between about 1904 and the 1920s. The engineers who built these machines hailed from France, Great Britain, Russia and the Netherlands, and their inventions could make short, tethered flights of just a few seconds. Some of the machines carried pilots, while some were unmanned. Almost all of them were unreliable and difficult to control.

Anatomy of a Helicopter: The Blade Are Spinning and the Engine Is Running

Sikorsky and a few of his contemporaries brought a technical rigor to the field that finally made vertical flight safe, practical and reliable. As the flight-crazy Russian continued to refine his helicopter designs, he worked out the fundamental requirements that any such machine needed to have to be successful, including:

• a suitable engine with a high power-to-weight ratio
• a mechanism to counteract rotor torque action
• proper controls so the aircraft could be steered confidently and without catastrophic failures
• a lightweight structural frame
• a means to reduce vibrations

Many of the basic parts seen on a modern helicopter grew out of the need to address one or more of these basic requirements. Let's look at these components in greater detail:

Main rotor blade -- The main rotor blade performs the same function as an airplane's wings, providing lift as the blades rotate -- lift being one of the critical aerodynamic forces that keeps aircraft aloft. A pilot can affect lift by changing the rotor's revolutions per minute (rpm) or its angle of attack, which refers to the angle of the rotary wing in relation to the oncoming wind.

Stabilizer -- The stabilizer bar sits above and across the main rotor blade. Its weight and rotation dampen unwanted vibrations in the main rotor, helping to stabilize the craft in all flight conditions. Arthur Young, the gent who designed the Bell 47 helicopter, is credited with inventing the stabilizer bar.

Rotor mast -- Also known as the rotor shaft, the mast connects the transmission to the rotor assembly. The mast rotates the upper swash plate and the blades.

Transmission -- Just as it does in a motor vehicle, a helicopter's transmission transmits power from the engine to the main and tail rotors. The transmission's main gearbox steps down the speed of the main rotor so it doesn't rotate as rapidly as the engine shaft. A second gearbox does the same for the tail rotor, although the tail rotor, being much smaller, can rotate faster than the main rotor.

Engine -- The engine generates power for the aircraft. Early helicopters relied on reciprocating gasoline engines, but modern helicopters use gas turbine engines like those found in commercial airliners.

Anatomy of a Helicopter: Working the Controls

Fuselage -- The main body of the helicopter is known as the fuselage. In many models, a frameless plastic canopy surrounds the pilot and connects in the rear to a flush-riveted aluminum frame. Aluminum wasn't widely used in aeronautical applications until the early 1920s, but its appearance helped engineers make their helicopters lighter and, as a result, easier to fly.

Cyclic-pitch lever -- A helicopter pilot controls the pitch, or angle, of the rotor blades with two inputs: the cyclic- and collective-pitch levers, often just shortened to the cyclic and the collective. The cyclic, or "stick," comes out of the floor of the cockpit and sits between the pilot's legs, enabling a person to tilt the craft to either side or forward and backward.

Collective-pitch lever -- The collective-pitch lever is responsible for up-and-down movements. For example, during takeoff, the pilot uses the collective-pitch lever to increase the pitch of all the rotor blades by the same amount.

Foot pedals -- A pair of foot pedals controls the tail rotor. Working the pedals affects which way the helicopter points, so pushing the right pedal deflects the tail of the helicopter to the left and the nose to the right; the left pedal turns the nose to the right.

Tail boom -- The tail boom extends out from the rear of the fuselage and holds the tail rotor assemblies. In some models, the tail boom is nothing more than an aluminum frame. In others, it's a hollow carbon-fiber or aluminum tube.

Anti-torque tail rotor -- Without a tail rotor, the main rotor of a helicopter simply spins the fuselage in the opposite direction. It's enough to make your stomach heave just thinking about all that endless circling. Thankfully, Igor Sikorsky had the idea to install a tail rotor to counter this torque reaction and provide directional control. In twin-rotor helicopters, the torque produced by the rotation of the front rotor is offset by the torque produced by a counterrotating rear rotor.

Landing skids -- Some helicopters have wheels, but most have skids, which are hollow tubes with no wheels or brakes. A few models have skids with two ground-handling wheels.

How Helicopters Fly

Imagine that we would like to create a machine that can simply fly straight upward. Let's not even worry about getting back down for the moment -- up is all that matters. If you are going to provide the upward force with a wing, then the wing has to be in motion in order to create lift. Wings create lift by deflecting air downward and benefiting from the equal and opposite reaction that results (see How Airplanes Work for details -- the article contains a complete explanation of how wings produce lift).

A rotary motion is the easiest way to keep a wing continuously moving. You can mount two or more wings on a central shaft and spin the shaft, much like the blades on a ceiling fan. The rotating wings of a helicopter are shaped just like the airfoils of an airplane wing, but generally the wings on a helicopter's rotor are narrow and thin because they must spin so quickly. The helicopter's rotating wing assembly is normally called the main rotor. If you give the main rotor wings a slight angle of attack on the shaft and spin the shaft, the wings start to develop lift.

In order to spin the shaft with enough force to lift a human being and the vehicle, you need an engine, typically a gas turbine engine these days. The engine's driveshaft can connect through a transmission to the main rotor shaft. This arrangement works really well until the moment the vehicle leaves the ground. At that moment, there is nothing to keep the engine (and therefore the body of the vehicle) from spinning just as the main rotor does. In the absence of anything to stop it, the body of the helicopter will spin in an opposite direction to the main rotor. To keep the body from spinning, you need to apply a force to it.

Enter the tail rotor. The tail rotor produces thrust like an airplane's propeller does. By producing thrust in a sideways direction, this critical part counteracts the engine's desire to spin the body. Normally, the tail rotor is driven by a long driveshaft that runs from the main rotor's transmission back through the tail boom to a small transmission at the tail rotor.