Helicopter flight controls

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Helicopter flight controls
Location of flight controls in a helicopter

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 with aerodynamic effects on the helicopter's rotor blades, allowing the helicopter to be controlled. For tilting forward and back (pitch), or tilting sideways (roll), the angle of attack of the main rotor blades is altered—cycled—during the rotation creating different amount of lifts at different points in the cycle. For increasing or decreasing lift, the angle of attack for all blades is collectively changed the same amount at the same time resulting in climbs, descents, and changes in airspeed.

A typical helicopter has three separate flight control inputs. These are the cyclic, the collective, 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 throttle is also considered a flight control because it is required to maintain rotor RPM on smaller helicopters without governors.

[edit] Controls

[edit] Cyclic

Enstrom F28A Cyclic control the buttons do:
Blue: Trimming/Centering cyclic
Red: 1st click intercom, 2nd radio
Green: Cargo release\No Function

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 to a joystick. By contrast, the Robinson R22 and R44 have a unique teetering bar cyclic control system. The original flight control design of the Gyrobee had a cyclic control that descends into the cockpit from overhead.^[1] However, this design (only on the original prototype) was changed to a conventional flight control configuration.^[2]

The control is called the cyclic because it changes the pitch of the rotor blades cyclically. That is, the pitch or feathering angle of the rotor blades change depending upon their position as they rotate around the hub so that all blades will change their angle the same amount 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 change in lift.

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 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 hover sideways or to roll into a right turn during forward flight.

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. Sometimes termed gyroscopic precession but more appropriately referred to as phase lag, this delay is generally close to ninety degrees. The delay is caused by the time it takes for the blade to change its flapping angle after the change in lift.

[edit] Collective

The collective pitch control or collective is located on the left side of the pilot's seat with a settable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e. all at the same time) and independently of their position. Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increasing or decreasing in altitude.

[edit] 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.

[edit] Throttle

Helicopter rotors are designed to operate at a specific RPM. 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 RPM within allowable limits in order to keep the rotor producing enough lift for flight. In single-engine helicopters, the throttle control is a motorcycle-style twist grip mounted on the collective control, while dual-engine helicopters have power levers.

In many piston engine-powered helicopters, the pilot manipulates the throttle to maintain rotor RPM. Turbine engine helicopters, and some piston helicopters, use governors or other electro-mechanical control systems to maintain rotor RPM and relieve the pilot of routine responsibility for that task.

[edit] Flight conditions

There are two basic flight conditions for a helicopter; hover and forward flight.

[edit] Hover

Hovering is the most challenging part of flying a helicopter. This is because a helicopter generates its own gusty air while in a hover, which acts against the fuselage and flight control surfaces. The end result is constant control inputs and corrections by the pilot to keep the helicopter where it is required to be. Despite the complexity of the task, the control inputs in a hover are simple. The cyclic is used to eliminate drift in the horizontal plane, that is to control forward and back, right and left. The collective is used to maintain altitude. The pedals are used to control nose direction or heading. It is the interaction of these controls that makes hovering difficult, since an adjustment in any one control requires an adjustment of the other two, creating a cycle of constant correction.

[edit] 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 (power) 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.

[edit] Differential pitch control

For helicopters with contra-rotating rotors, helicopter control requires interaction between the two rotors. A helicopter with tandem rotors uses differential collective pitch to change the attitude of the nose 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 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.

[edit] See also

• Helicopter
• Helicopter rotor
• Aeronautical engineering
• Autogyro

[edit] Notes

[edit] References

• Flight Standards Service (2001). Rotorcraft Flying Handbook: FAA Manual H-8083-21. Federal Aviation Administration, United States Department of Transportation. ISBN 1560274042. 

[edit] External links

• Dynamic Flight
• Student's article
• Helicopter history
• Helicopter history
• Helicopter development in the early 20th century