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.
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.
The main rotor, of course, is the most important part of a helicopter. It's also one of the most complex in terms of its construction and operation. In the next section, we'll peer at the rotor assembly of a typical helicopter.
The Heart of the Helicopter: The Rotor Assembly
A helicopter's main rotor is the most important part of the vehicle. It provides the lift that allows the helicopter to fly, as well as the control that allows the helicopter to move laterally, make turns and change altitude. To handle all of these tasks, the rotor must first be incredibly strong. It must also be able to adjust the angle of the rotor blades with each revolution they make. The pilot communicates these adjustments through a device known as the swash plate assembly.
The swash plate assembly consists of two parts -- the upper and lower swash plates. The upper swash plate connects to the mast, or rotor shaft, through special linkages. As the engine turns the rotor shaft, it also turns the upper swash plate and the rotor blade system. This system includes blade grips, which connect the blades to a hub. Each hub contains a rubbery bearing sandwiched between metal plates that allow its blade to flap up or down. Control rods from the upper swash plate have a connection point on the hubs, making it possible to transfer movements of the upper swash plate to the blades. And the hubs themselves mount to the mast via the Jesus nut, so named because its failure is said to bring a pilot face-to-face with Jesus.
The lower swash plate is fixed and doesn't rotate. Ball bearings lie between the upper and lower swash plates, allowing the upper plate to spin freely on top of the lower plate. Control rods attached to the lower swash plate connect to the cyclic- and collective-pitch levers. When the pilot operates either of those two levers, his or her inputs are transmitted, via the control rods, to the lower swash plate and then, ultimately, to the upper swash plate.
Using this rotor design, a pilot can manipulate the swash plate assembly and control the helicopter's motion. With the cyclic, the swash plate assembly can change the angle of the blades individually as they revolve. This allows the helicopter to move in any direction around a 360-degree circle, including forward, backward, left and right. The collective allows the swash plate assembly to change the angle of all blades simultaneously. Doing this increases or decreases the lift that the main rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude.
Now it's time to see how all these parts work together to get the helicopter airborne
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.
In order to actually control the machine and, say, guide it into a canyon to complete the ultimate rescue, both the main rotor and the tail rotor need to be adjustable. The next three sections explain how pilots guide the helicopter into taking off, hovering or buzzing off in a particular direction.
Flying a Helicopter: Taking Off
The ability of helicopters to move
laterally in any direction or rotate 360 degrees makes them exciting to fly,
but piloting one of these machines requires great skill and dexterity. To
control a helicopter, the pilot grips the
cyclic in one hand, the collective in the other. At the same time, his feet
must operate the foot pedals that control the tail rotor, which allows the
helicopter to rotate in either direction on its horizontal axis. It takes both
hands and both feet to fly a helicopter
During takeoff, the pilot works the
collective and the foot pedals simultaneously. Before we discuss how to take
off, you should know that the collective typically looks like a handbrake whose
grip functions as the throttle. Twisting the grip controls the power output of
the engine,
increasing or decreasing the speed of the main rotor. With that in mind, we're
ready to begin a typical helicopter takeoff:
- First, the pilot opens the throttle completely to increase the speed of the rotor.
- Next, he or she pulls up slowly on the collective. The collective control raises the entire swash plate assembly as a unit. This has the effect of changing the pitch of all rotor blades by the same amount simultaneously.
- As the pilot increases collective pitch, he or she depresses the left foot pedal to counteract the torque produced by the main rotor.
- The pilot keeps pulling up slowly on the collective while depressing the left foot pedal.
- When the amount of lift being produced by the rotor exceeds the weight of the helicopter, the aircraft will get light on its skids and slowly leave the ground.
Flying a Helicopter: Directional Flight
In addition to moving up and down,
helicopters can fly forward, backward and sideways. This kind of directional
flight is achieved by tilting the swash plate assembly with the cyclic, which
alters the pitch of each blade as it rotates. As a result, every blade produces
maximum lift at a particular point. The rotor still generates lift, but it also
creates thrust in the direction that the swash plate assembly is tilted. This
causes the helicopter to lean -- and fly -- in a certain direction. The pilot
can impart additional directional control by depressing or easing up on the
foot pedals, which increases or decreases the counteracting thrust of the tail
rotor.
Let's assume for a moment that the
helicopter we discussed in the last section needs to fly forward. This is the
pilot's procedure:
- First, he or she nudges the cyclic lever forward.
- That input is transmitted to the lower swash plate and then to the upper swash plate.
- The swash plates tilt forward at an amount equal to the input.
- The rotor blades are pitched lower in the front of the rotor assembly than behind it.
- This increases the angle of attack -- and creates lift -- at the back of the helicopter.
- The unbalanced lift causes the helicopter to tip forward and move in that direction.
When the aircraft reaches about 15
to 20 knots of forward airspeed, it begins to transition from hovering flight
to full forward flight. At this point, known as effective translational lift,
or ETL, the pilot eases up on the left foot pedal and moves closer to a
neutral setting. He or she also feels a shudder in the rotor system as the
helicopter begins to fly out of rotor wash (the turbulence created by a
helicopter's rotor) and into clean air. In response, the rotor will try to lift
up and slow the aircraft automatically. To compensate, the pilot will continue
to push the cyclic forward to keep the helicopter flying in that direction with
increasing airspeed.
A helicopter that is flying forward
can stop in mid-air and begin hovering very quickly.
The defining characteristic of a
helicopter is its ability to hover at any point during a flight. To achieve
hovering, a pilot
must maintain the aircraft in nearly motionless flight over a reference point
at a constant altitude and on a heading (the direction that the front of the
helicopter is pointing). This may sound easy, but it requires tremendous
experience and skill.
Before we tackle the technique of
hovering, let's take a moment to discuss nap-of-the-earth (NOE) flight, another
unique characteristic of helicopters. NOE flight describes a helicopter
located just above the ground or any obstacles on the ground. Military pilots
perfected the technique during Vietnam as a means to become more elusive to
ground-based weapons.
In fact, film footage from the era often shows helicopters rapidly skimming the
Earth's surface, machine-gunners firing from open rear doors or hovering with
their skids just a few feet off the ground as troops disembark at a target
location.
Of course, any helicopter taking off
or landing must undertake NOE flight, if only for a few moments. It's a
particularly critical time for a helicopter because a wild attitude adjustment
could tip the craft too far and bring the rotor blades in contact with an
obstacle. Attitude, for our purposes, refers to the helicopter's
orientation in relation to the helicopter's direction of motion. You'll also hear
flight-minded folks talk about attitude in reference to an axis, such as the
horizon.
With that said, here's the basic
technique to bring a helicopter into a hovering position:
- First, the pilot must cease any directional flying. For example, if flying the helicopter forward, the pilot must ease back on the cyclic until the helicopter's forward motion stops and the aircraft remains motionless over a point on the ground.
- Next, it's important that the pilot can detect small changes in the aircraft's altitude or attitude. He or she accomplishes this by locating a fixed point outside the cockpit and tracking how the helicopter moves relative to that point.
- Finally, the pilot adjusts the collective to maintain a fixed altitude and adjusts the foot pedals to maintain the direction that the helicopter is pointing.
To maintain a stabilized hover, the
pilot must make small, smooth, coordinated corrections on all of the controls.
In fact, one of the most common errors of novice pilots is to overcompensate
while trying to hover. For example, if the helicopter begins to move rearward,
the pilot must be careful not to apply too much forward pressure on the cyclic
because the aircraft will not just come to a stop but will start drifting
forward.
Over the years, innovations in
helicopter design have made the machines safer, more reliable and easier to
control. The next page presents a few of these innovations to provide a glimpse
of how far helicopters have come and where they might go in the future.
Helicopter Innovations
The modern helicopter, like any
complex machine, is an accumulation of innovations from numerous inventors and
engineers. Some of these modifications improve performance significantly
without changing the overall appearance of the aircraft. For example, Arthur
Young's stabilizer bar looks small and insignificant when compared to the gross
anatomy of a chopper, but it revolutionized vertical-lift flight. Other
innovations are less subtle and seem to give the helicopter a complete
makeover. Let's check out a few changes.
One significant advancement in the
last decade has been the no-tail rotor, or NOTAR, helicopter.
As you now know, vertical-lift flight is impossible without a tail rotor to
counteract the torque
produced by the main rotor. Unfortunately, the much-smaller tail rotor makes a
lot of noise and is often easily damaged. The NOTAR helicopter solves both of
these problems. Here's how it works: A large fan at the rear of the fuselage
blows spent air from the main rotor down the tail boom. Slots along the side of
the tail boom and at the end of the boom allow this air to escape. This creates
a sideways force that counteracts the main rotor's torque. Varying the amount
of air expelled from the rear slot provides additional directional control.
Some helicopters started receiving a
second engine, which can operate the main rotor if the main engine fails. For
example, the UH-60 Black Hawk helicopter, the workhorse of the U.S. Army,
features this design improvement. Either engine can keep the aircraft aloft on
its own, enabling the pilot to land safely in the event of an emergency.
Scientists have also fiddled with
the main rotor assembly in an attempt to simplify one of the most complex parts
of a helicopter. In the late 1990s, researchers developed a solid-state
adaptive rotor system incorporating piezoelectric sheets. A piezoelectric
material is one in which its molecules bend and twist in response to an
electric field. In a rotor assembly, piezoelectric sheets -- not mechanical
linkages -- twist sections of the blade root, thereby changing the pitch of the
blades as they rotate. This eliminates parts in the rotor hub and decreases the
chance of a mechanical failure.
Finally, it's worth mentioning those
strange machines, known as tiltrotors, that bring together the best
features of helicopters and airplanes. A tiltrotor aircraft takes off like a
helicopter, with its two main rotors upright. But when it's airborne, the pilot
can tip the rotors forward 90 degrees, enabling the machine to fly like
conventional turboprop airplane. The V-22 Osprey, which completed a successful
test flight in 1989, operates in this fashion.
None of these innovations has made helicopters
less absurd-looking. Some, like the tiltrotor, only increase the aircraft's
awkward visual appearance. All of which brings us back to Harry Reasoner's 1971
commentary about helicopters:
Mark Twain once noted that he lost belief in conventional pictures
of angels of his boyhood when a scientist calculated for a 150-pound men to fly like a bird,
he would have to have a breast bone 15 feet wide supporting wings in
proportion. Well, that's sort of the way a helicopter looks.
Mr. Reasoner may be right, but a
helicopter's peculiar design and configuration haven't diminished its impact.
It's become one of the most versatile and widely used aircraft in the world
today.
Helicopter Parts & Functions
A helicopter flies using the same
basic principles of lift and thrust to overcome drag and gravity as an
airplane. But the application of those principles is different and allow the
helicopter to routinely maneuver in ways that most airplanes cannot. Features
such as the rotor system, tail rotor and the transmission make the helicopter
unique machines.
Turbine Engine
·
The turbine engine in a helicopter operates
nearly identically to its counterpart in an airplane. The engine's compressor
heats and compresses intake air, directing it into a burner where a mixture of
the air and fuel is burned. The heat and energy from combustion is directed
across two sets of turbines. The first drives the compressor and the second
turns a main driveshaft. Unlike the engine in an airplane, most of the energy
produced by a helicopter engine is directed through a power turbine shaft to a
transmission rather along the axis of the engine as thrust.
Transmission
·
A typical helicopter transmission is mounted
forward of the engine and directly below the main rotor. The transmission
reduces the high-speed revolutions from the main drive shaft and drives the
mast that turns the main rotor as well as a second drive shaft to the tail
rotor.
Main Rotor
·
There are multiple designs for a helicopter's
main rotor system. Some helicopters have two blades such as the Army's UH 1
(Huey). Other larger helicopters have as many as eight rotor blades. In flight,
a fuel control governor maintains the rotor speed at constant revolutions per
minute. When rotating at the proper RPM, the individual rotor blades act as a
single disk. The pilot's inputs of pitch using the collective and the cyclic
flight controls control the helicopter's altitude, direction and speed.
Tail Rotor and Pedals
·
The helicopter's tail rotor is driven from the
transmission through a tail rotor driveshaft and is designed to counter the
torque produced by the rotation of the main rotor blades. Torque causes the
aircraft's fuselage to spin in the opposite direction to the main rotor. As the
pitch is increased to the main rotor blades, the pilot counteracts the torque
by increasing the pitch to the tail rotor blades with the pedals in the
cockpit. By changing the tail rotor's pitch, the pilot can also turn the
aircraft to the right and left.
Collective
·
The collective flight control is used for
climbing and descending. Located to the left of the pilot's seat, it is placed
at approximately a 35-degree angle to the floor. By pulling up on the
collective, the pilot can uniformly increase pitch to all of the blades of the
main rotor system. This increased pitch creates additional lift, and also
increases the power demanded from the engine.
The Cyclic
·
The cyclic control system (sometimes called the
stick) is used for direction and speed. It is attached to the floor in front of
the pilot's seat and aft of the pedals. The cyclic increases pitch to the main
rotors unevenly; thus causing the disc formed by the main rotors to tilt in one
direction or another. The aircraft travels in the direction the pilot moves the
cyclic. It is this control that allows the helicopter to fly backward, forward
and sideways.
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