A helicopter pilot manipulates the helicopter flight controls in order to achieve controlled
aerodynamic flight.
The changes made to the flight controls are transmitted mechanically to the rotor, producing
aerodynamic effects on the helicopter's rotor blades which allow the helicopter to be controlled.
For tilting forward and back, or tilting sideways, the angle of attack of the main rotor blades
is altered cyclically during rotation, creating differing amounts of lift at different points
in the cycle.
For increasing or decreasing overall lift, the angle of attack for all blades is collectively
altered by equal amounts at the same time resulting in ascents, descents, acceleration
and deceleration.
A typical helicopter has three separate flight control inputs.
These are the cyclic stick, the collective lever, and the anti-torque pedals.
Depending on the complexity of the helicopter, the cyclic and collective may be linked together
by a mixing unit, a mechanical or hydraulic device that combines the inputs from both
and then sends along the "mixed" input to the control surfaces to achieve the desired
result.
The manual throttle may also be considered a flight control because it is needed to maintain
rotor speed on smaller helicopters without governors.
The governors also help the pilot control the collective pitch on the helicopters main
rotors, to keep a stable, more accurate flight.
Controls Cyclic
The cyclic control is usually located between the pilot's legs and is commonly called the
cyclic stick or just cyclic.
On most helicopters, the cyclic is similar in appearance to a joystick in a conventional
aircraft.
The control is called the cyclic because it changes the pitch angle of the rotor blades
cyclically.
That is, the pitch or feathering angle of the rotor blades changes depending upon their
position as they rotate around the hub so that all blades will have the same incidence
at the same point in the cycle.
The change in cyclic pitch has the effect of changing the angle of attack and thus the
lift generated by a single blade as it moves around the rotor disk.
This in turn causes the blades to fly up or down in sequence, depending on the changes
in lift affecting each individual blade.
The result is to tilt the rotor disk in a particular direction, resulting in the helicopter
moving in that direction.
If the pilot pushes the cyclic forward, the rotor disk tilts forward, and the rotor produces
a thrust vector in the forward direction.
If the pilot pushes the cyclic to the right, the rotor disk tilts to the right and produces
thrust in that direction, causing the helicopter to move sideways in a hover or to roll into
a right turn during forward flight, much as in a fixed wing aircraft.
On any rotor system there is a delay between the point in rotation where a change in pitch
is introduced by the flight controls and the point where the desired change is manifest
in the rotor blade's flight.
This difference is caused by phase lag, often confused with gyroscopic precession.
A rotor is an oscillatory system that obeys the laws governing vibration which, depending
on the rotor system, may resemble the behaviour of a gyroscope.
Collective The collective pitch control, or collective
lever, is normally located on the left side of the pilot's seat with an adjustable friction
control to prevent inadvertent movement.
The collective changes the pitch angle of all the main rotor blades collectively and
independent of their position.
Therefore, if a collective input is made, all the blades change equally, and the result
is the helicopter increases or decreases its total lift derived from the rotor.
In level flight this would cause a climb or descent, while with the helicopter pitched
forward an increase in total lift would produce an acceleration together with a given amount
of ascent.
The collective pitch control in a Boeing CH-47 Chinook is called a "Thrust Control", but
it serves the exact same purpose, except that it controls two rotor systems and applies
differential collective pitch into the two rotor systems.
Anti-torque pedals The anti-torque pedals are located in the
same position as the rudder pedals in an airplane, and serve a similar purpose, namely to control
the direction in which the nose of the aircraft is pointed.
Application of the pedal in a given direction changes the pitch of the tail rotor blades,
increasing or reducing the thrust produced by the tail rotor and causing the nose to
yaw in the direction of the applied pedal.
The pedals mechanically change the pitch of the tail rotor altering the amount of thrust
produced.
Throttle Helicopter rotors are designed to operate
at a specific rotational speed.
The throttle controls the power produced by the engine, which is connected to the rotor
by a transmission.
The purpose of the throttle is to maintain enough engine power to keep the rotor speed
within allowable limits in order to keep the rotor producing enough lift for flight.
In many helicopters, the throttle control is a single or dual motorcycle-style twist
grip mounted on the collective control, while some multi-engine helicopters have power levers.
In many piston engine-powered helicopters, the pilot manipulates the throttle to maintain
rotor speed.
Turbine engine helicopters, and some piston helicopters, use governors or other electro-mechanical
control systems to maintain rotor speed and relieve the pilot of routine responsibility
for that task.
Flight conditions There are three basic flight conditions for
a helicopter: hover, forward flight and autorotation.
Hover Hovering is considered by some pilots to be
the most challenging aspect of helicopter flight to learn.
This is because helicopters are generally dynamically unstable, meaning that deviations
from a given attitude are not corrected without pilot input.
Thus, frequent control inputs and corrections must be made by the pilot to keep the helicopter
at a desired location and altitude.
The pilot's use of control inputs in a hover is as follows: the cyclic is used to eliminate
drift in the horizontal plane,; the collective is used to maintain desired altitude; and
the tail rotor pedals are used to control nose direction or heading.
It is the interaction of these controls that can make learning to hover difficult, since
often an adjustment in any one control requires the adjustment of the other two, necessitating
pilot familiarity with the coupling of control inputs needed to produce smooth flight.
Forward flight In forward flight a helicopter's flight controls
behave more like those in a fixed-wing aircraft.
Displacing the cyclic forward will cause the nose to pitch down, with a resultant increase
in airspeed and loss of altitude.
Aft cyclic will cause the nose to pitch up, slowing the helicopter and causing it to climb.
Increasing collective while maintaining a constant airspeed will induce a climb while
decreasing collective will cause a descent.
Coordinating these two inputs, down collective plus aft cyclic or up collective plus forward
cyclic, will result in airspeed changes while maintaining a constant altitude.
The pedals serve the same function in both a helicopter and an airplane, to maintain
balanced flight.
This is done by applying a pedal input in whichever direction is necessary to center
the ball in the turn and bank indicator.
Autorotation
Differential pitch control For helicopters with contra-rotating rotors
- also known as "Coaxial mounted" one over the other on the same rotor shaft, helicopter
control requires interaction between the two rotors.
However, a helicopter with tandem rotors - counter rotating rotors on different rotor masts uses
differential collective pitch to change the pitch attitude of the aircraft.
To pitch nose down and accelerate forward, the collective pitch on the front rotor is
decreased and the collective pitch on the rear rotor is increased proportionally.
Conversely, the synchropter and transverse-mounted rotor - a counter rotating rotorcraft have
two large horizontal rotor assemblies mounted side by side, helicopters use differential
collective pitch to affect the roll of the aircraft.
All of these configurations use differential cyclic pitch to control movement about the
yaw axis, tilting the rotors in opposite directions to cause the aircraft to spin in the direction
of the tilted rotors.
See also Aeronautical engineering
Autogyro Helicopter rotor
References Notes
Sources
External links Dynamic Flight
Student's article Helicopter history
Helicopter development in the early 20th century