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Up to this point our discussion has centered on theory of lift and the shape of airfoils best suited to the rotating
wings of a helicopter. The second phase of this chapter will deal with the functioning of the flight controls. In a
sense, dividing the subject into these two categories parallels the development of the helicopter. The first phase,
roughly up until the early 1920's, dealt largely with the problem of perfecting rotors that were efficient enough
to permit flight. The next step, through the 1930's and into the 1940's, was the perfection of control systems and
mechanisms that would provide the capability for real flight at a fair rate of speed.

Our examination of the pilot's control system will be limited

to one helicopter configuration, surely the best example, the
type which incorporates one main overhead rotor with a small tail rotor at the rear of the fuselage. This classic
form, perfected by Igor I.Sikorsky in the United States between 1939 and 1941, remains the predominant type
today, although there have been many successful aircraft with various other rotor configurations. It should be
noted that the control systems described here are not limited to the single main rotor type but are employed in
one form or another in most helicopters, no matter what the rotor configuration.

With the Sikorsky configuration, the small tail rotor is intended to offset the torque reaction mentioned in earlier
chapters. Torque has no effect while the aircraft is sitting on the ground, sinc e the ship is prevented from rotating
by the weight on the landing gear. But once the ship is clear of the ground, the torque effect tries to take over.
The practical effect of this  as far as the pilot is concerned  is simply that the tail has a tendency to swing in
one direction. To prevent this, a small anti-torque rotor is mounted at the tail, driven by an extension shaft from
the engine transmission. This tail rotor is really more like an airplane propeller, since it turns much faster than
the main rotor. To keep the helicopter moving straight ahead without turning, the pitch of the tail rotor blades
must be just enough exactly to counteract the force of the torque reaction. On most helicopters with a single
main rotor, the pitch of the tail rotor is adjusted so that at cruise speed the aircraft is trimmed to fly a straight
course.

Besides holding the tail end of the fuselage straight against the twisting force of torque, this rear rotor provides
control for steering to the right or left, as does the r udder of an airplane. This is accomplished by the pilot's @

 which control the pitch angle of the tail rotor blades: by pressing on the right pedal, the pitch is
decreased and the tail swings to the left.

Pressing on the left pedal, of course, has the opposite effect: the pitch of the tail rotor is increased so that its
thrust now overcomes the torque and tail swings the other way.
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The term "control is yaw" is used to describe this effect of the tail rotor  that is, the ability to swing the tail to
the right or to the left. Unlike the rudder of an airplane, it isn't necessary for the helicopter to be in forward
motion in order for the tail rotor to be effective; as long as the main rotor is turning over the tail rotor will keep
spinning, owing to the mechanical drive system that links them together, and the tail control will continue to
function. This feature is important in the event of an engine failure or if the engine is slowed deliberately. In this
situation the helicopter descends with the main rotor free-wheeling in autorotation. The fact that the tail rotor
will keep turning with the power cut off means that directional (yawing) conrol can be maintained, and the pilot
will still be able to steer the ai rcraft as it descends.

Another primary control used to fly a helicopter is the u u



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located to the left of the pilot's seat
and mounted on a pivot so the pilot can ease it up and down with his left hand. By means of this control the pilo t
can increase or decrease the pitch angle of all the blades in the main rotor, equally and simultaneously. For
example, to make the aircraft rise, he pulls up on the collective stick  this is called £



u £  the
effect is to increase the pitch angle of the blades and thus, to increase the lift. However, as the blades meet the
air at a greater angle of attack, * the drag on the rotor will increase as well as the lift. Therefore, to maintain a
constant rotational speed it is necessary to increase the power by speeding up the engine as the stick is raised.
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This brings us to yet another control, the


 for the engine. Since the pilot already has both hands and feet
occupied, the problem is solved by providing a rotating handle (similar to the twist grip found on a motorcycle)
mounted directly on the collective stick. As the pilot pulls in pitch by raisi ng the stick with his left hand he also
twists the throttle slightly to speed up the engine.

We can see how this control is used by following the actual motions that the pilot makes during take -off. First,
he will bring the rotor up to the correct speed by opening the twist-grip throttle. He then eases up on the
collective pitch stick, increasing the bite of the main blades while simultaneously adjusting the power of the
engine to maintain the rotor speed. When the lift force developed by the rotor becomes greater than the weight
of the aircraft, the landing gear will begin to clear the ground. In a condition of hovering flight the lift exactly
equals the weight, and the pilot can then raise or lower the helicopter by increasing or decreasing his collective
pitch.

The technique of using the left hand to operate both the throttle and the collective stick is one of the problems
peculiar to flying a helicopter. Coordination of the throttle and the collective pitch is vital in maintaining the
speed of the rotor at the all-important correct number of revolutions per minute. To make the task easier for the
pilot, most of today's helicopters employ a mechanical linkage in the collective control which automatically adds
throttle as the pitch is increased. However, wit h many helicopters the pilot still has the chore of making constant
small corrections in spite of the action of this linkage.
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The last primary flight control is the cyclic control stick. The cyclic stick is mounted vertically from the cockpit
floor, between the pilot's legs, in the same position as the control stick of a n airplane, which it so greatly
resembles. Like the control stick of an airplane, it can be moved slightly in any direction from the vertical. With
this control the pilot can move the helicopter in any direction horizontally - that is, for flying straight ahead,
backing up, and for moving to either side.

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When the cyclic stick is moved fore or aft, to either side, or in any combination of these, the whole whirling
rotor system-termed the "disc of rotation"  will be tilted in the same direction and the movement of the
helicopter will be in this direction. This is accomplished by changing the pitch angle  and thus the angle of
attackof each blade in cycles, from a maximum to a minimum position, as the blade sweeps through a
complete revolution. The way this works is illustrated by what happens when the rotor disc is tilted forward, to
move the helicopter into forward flight. The pilot pushes his control stick forward slightly; the result is that the
blade which happens to be at the rear of the disc has its pitch increased by the control linkage, while the blade
which happens to be moving around at the front has its pitch decreased. Since each blade is attached to the hub
by a hinge which allows it to flap up and down slightly as it turns (here we are describing a standard articulated
rotor), the blade at the rear will rise upward and then, as it moves around to the front, it will ride at a lower than
 normal position. The over-all effect is that the rotor disc  actually the path the blades take describes the shape
of a shallow cone rather than a disc  will be inclined forward so that its lift is no longer straight up but is now
pulling slightly forward as well. In the same way, the cyclic stick can be used to incline the rotor to move the
helicopter in other directions.

An important function of the cyclic control system is that it allows the pilot to
correct for 

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 This phenomenon refers to the unequal lifting
forces created as the helicopter speeds up and moves forward through the air that, unless corrected, can cause the
aircraft to roll to one side. To understand this, it is helpful to visualize the position of e ach blade in relation to
the airflow coming from the front as it sweeps through the full 360 -degree circle of rotation. Each blade can be
thought of as advancing when it sweeps forward against the airflow from the front, and retreating when it swings
past the straight-ahead position and begins to move to the rear on the other side of the circle. Since the blades
obtain lift from both the airflow induced by their rotation and from the flow from the front, it is not surprising
that the advancing blade, which is meeting the flow from the front head on, will create more lift than the
retreating blade. Because of this, the advancing blade will tend to flap up while the retreating blade, which is
"running away" from the airflow from the front, will flap down. The effect is that the rising blade is actually
decreasing its angle of attack, since the movement of the air against the blade is no longer horizontal but is now
slightly downwardowing to the blade moving On the other side of the rotor the situation is reversed;
there the blade is actually increasing its angle of attack since it is "diving down" against the airflow. This
flapping effect is quite beneficial because it results in an equalization of the angle of attack  and therefore the
lift force  created on the right and left sides of the rotor disc, and eliminates the tendency for the helicopter to
roll to one side.

However, there is an undesirable result from this flapping action which can be overcome through use of the
cyclic pitch control. As the advancing blade flaps upward it reaches its highest flapped position directly in front,
one-quarter of a revolution from the point where the increased lift began to take effect. Conversely, the
retreating blade reaches its lowest flapped position directly to the rear, over the tail of the helicopter. Therefore
the tendency of the aircraft to roll, eliminated by the flapping of the blades, is now changed to a tendency for the
rotor disc to pitch to the rear and thus for the helicopter to climb as it goes in to forward flight. This might seem
to call for still more complications in the control system, but fortunately this is not the case. To start the ship
moving forward initially, the pilot had pushed his cyclic stick forward. To overcome the climbing tendenc y he
simply pushes the stick still farther forward, responding naturally to the nose -up position, and the rotor disc
remains in the desired attitude.

One of the important characteristics of lift dissymmetry is that it becomes much worse as the helicopter speeds
up and flies faster. Since the lift on the blades on the advancing side cannot be permitted to exceed the lift on the
retreating side, this factor tends to limit the top speed at which the fastest helicopters (with conventional rotor
systems) can fly; for even the largest and most powerfully-engined of helicopters, at speeds just above 200 miles
per hour the lifting and propelling characteristics of the rotor are affected, and a phenomenon termed "blade
stall" is encountered. *

With these primary controls  pedals for steering, the collective pitch stick (with the twist throttle), and the
cyclic stick  the pilot can move his ship in any direction, turning it around on the axis of the main rotor,
moving it up, down, forward, to the rear, and sideways. Obviously, a helicopter pilot can be very busy indeed.
Each hand and both feet may be at work at the same time, and the pilot's left hand will have two jobs to do
simultaneously, pulling up on the pitch stick while feeding in power with the throttle. Added to this is the fact
that most copters do not possess the inherent stability found in airplanes, being in this respect more comparable
to an automobile than an airplane; you can't let go of the controls for more than a few seconds at a time. But
these seemingly complicated motions are easily absorbed, and a natural pattern of action and reaction is quickly
established by most student pilots.