Rotordisc and Forward Flight. In this scenario, each rotor-blade encounters the same airspeed, which is defined by the speed of rotation of the rotorsystem. As a result, all of the blades in the rotordisc contribute the same amount of lift. Note that we can't speak of an advancing or a retreating blade in these circumstances. It is doing exactly the same thing as a seesaw rotor in forward flight. Javier is correct inasmuch as 2/rev is a result of aerodynamic input, ie, 2 hits/revolution when the rotor is broadside to the airstream and nothing can be done about it except to isolate the vibration via a limber mast or something similar such as a slider.
Location of flight controls in a helicopter
A helicopterpilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight.[1] Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter the angle of attack of the main rotor blades cyclically during rotation, creating differing amounts of lift (force) at different points in the cycle. To increase or decrease overall lift requires that the controls alter the angle of attack for all blades collectively by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration.
A typical helicopter has three flight control inputs—the cyclic stick, the collective lever, and the anti-torque pedals.[2] 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 helicopter's main rotors, to keep a stable, more accurate flight.
- 1Controls
- 2Flight conditions
- 5References
Controls[edit]
Cyclic[edit]
Cyclic control in a H145
The cyclic control, commonly called the cyclic stick or just cyclic, is similar in appearance on most helicopters to a control stick from a conventional aircraft. The cyclic stick commonly rises up from beneath the front of each pilot's seat. The Robinson R22 has a 'teetering' cyclic design connected to a central column located between the two seats. Helicopters with fly-by-wire systems allow a cyclic-style controller to be mounted to the side of the pilot seat.
The cyclic is used to control the main rotor in order to change the helicopter's direction of movement. In a hover, the cyclic controls the movement of the helicopter forward, back, and laterally. During forward flight, the cyclic control inputs cause flight path changes similar to fixed-wing aircraft flight; left or right inputs cause the helicopter to roll into a turn in the desired direction, and forward and back inputs change the pitch attitude of the helicopter resulting in altitude changes (climbing or descending flight).
The control is called the cyclic because it changes the mechanical pitch angle or feathering angle of each main rotor blade independently, depending on its position in the cycle. The pitch is changed so that each blade will have the same angle of incidence as it passes the same point in the cycle, changing the lift generated by the blade at that point and causing each blade to fly up or down in sequence as it passes the same point. If the pilot pushes the cyclic forward, the rotor tilts forward. If the pilot pushes the cyclic to the right, the rotor disk tilts to the right.
Cyclic control in a Robinson R22
Any rotor system has a delay between the point in rotation where the controls introduce a change in pitch and the point where the desired change in the rotor blade's flight occurs. This difference is caused by phase lag, often confused with gyroscopic precession. A rotor is an oscillatory system that obeys the laws that govern vibration—which, depending on the rotor system, may resemble the behaviour of a gyroscope.
Collective[edit]
Collective control in a Cabri G2 (viewed from above)
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 (i.e., all at the same time) and independent of their position. Therefore, if a collective input is made, all the blades change equally, and as a result, 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 serves the same purpose, except that it controls two rotor systems, applying differential collective pitch.[3]
Anti-torque pedals[edit]
The anti-torque pedals are located in the same place as the rudder pedals in an airplane, and serve a similar purpose—they control the direction that the nose of the aircraft points. Applying the pedal in a given direction changes the tail rotor blade pitch, increasing or reducing tail rotor thrust and making the nose yaw in the direction of the applied pedal.
Throttle[edit]
Helicopter rotors are designed to operate at a specific rotational speed. The throttle controls the power of the engine, which is connected to the rotor by a transmission. The throttle setting must maintain enough engine power to keep the rotor speed within the limits where the rotor produces enough lift for flight. In many helicopters, the throttle control is a single or dual motorcycle-style twist grip mounted on the collective control (rotation is opposite of a motorcycle throttle), 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. (There is normally also a manual reversion available in the event of a governor failure.)
Name | Directly controls | Primary effect | Secondary effect | Used in forward flight | Used in hover flight |
---|---|---|---|---|---|
Cyclic (longitudinal) | Varies main rotor blade pitch with fore and aft movement | Tilts main rotor disk forward and back via the swashplate | Induces pitch nose down or up | To adjust forward speed and control rolled-turn | To move forwards/backwards |
Cyclic (lateral) | Varies main rotor blade pitch with left and right movement | Tilts main rotor disk left and right through the swashplate | Induces roll in direction moved | To create movement to sides | To move sideways |
Collective | Collective angle of attack for the rotor main blades via the swashplate | Increase/decrease pitch angle of all main rotor blades equally, causing the aircraft to ascend/descend | Increase/decrease torque. Note: in some helicopters the throttle control(s) is a part of the collective stick. Rotor speed is kept basically constant throughout the flight. | To adjust power through rotor blade pitch setting | To adjust skid height/vertical speed |
Anti-torque pedals | Collective pitch supplied to tail rotor blades | Yaw rate | Increase/decrease torque and engine speed (less than collective) | To adjust sideslip angle | To control yaw rate/heading |
Flight conditions[edit]
There are three basic flight conditions for a helicopter: hover, forward flight and autorotation.
Hover[edit]
Some pilots consider hovering the most challenging aspect of helicopter flight.[4] 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, (e.g., forward, aft, and side to side motion); the collective is used to maintain desired altitude; and the tail rotor (or anti-torque system) 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[edit]
In forward flight, a helicopter's flight controls behave more like those in a fixed-wing aircraft. Moving the cyclic forward makes the nose pitch down, thus losing altitude and increasing airspeed. Moving the cyclic back makes the nose pitch up, slowing the helicopter and making it climb. Increasing collective (power) while maintaining a constant airspeed induces a climb, while decreasing collective (power) makes the helicopter descend. Coordinating these two inputs, down collective plus aft (back) cyclic or up collective plus forward cyclic causes 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 the direction necessary to center the ball in the turn and bank indicator.
Autorotation[edit]
Differential pitch control[edit]
For helicopters with two horizontally-mounted rotors, changes in attitude often require having each rotor behave inversely in response to the standard control inputs from the pilot. Those with coaxial rotors (like a Kamov Ka-50) have both rotors mounted on the same mast, one above the other on concentric drive shafts contra-rotating—spinning in opposite directions on a shared axis—and make yaw changes by increasing the collective pitch of the rotor spinning in the direction of the desired turn while simultaneously reducing the collective pitch of the other, creating dissymmetry of torque.
Tandem-rotor craft (like a Boeing CH-47 Chinook) also employ two rotors spinning in opposite directions—termed counter-rotation when it occurs from two separate points on the same airframe—but have the rotors on separate drive shafts through masts at the nose and tail. This configuration uses differential collective pitch to change the overall pitch attitude of the aircraft. When the pilot moves the cyclic forward to pitch the nose down and accelerate forward, the helicopter responds by decreasing collective pitch on the front rotor and increases collective pitch on the rear rotor proportionally, pivoting the two ends around their common center of mass. Changes in yaw are made with differential cyclic pitch, the front rotor altering cyclic pitch in the direction desired and the opposite pitch applied to the rear, once again pivoting the craft around its center.
Conversely, the synchropter and transverse-mounted rotor counter rotating rotorcraft have two large horizontal rotor assemblies mounted side by side, (like a Bell/Boeing V-22 tilt rotor) helicopters use differential collective pitch to affect the roll of the aircraft. Like tandem rotors, differential cyclic pitch is used to control movement about the yaw axis.
See also[edit]
References[edit]
Notes[edit]
- ^Gablehouse, Charles (1969) Helicopters and Autogiros: a History of Rotating-Wing and V/STOL Aviation. Lippincott. p.206
- ^Flying a Helicopter at helis.com
- ^Tandem Rotors at www.helicopterpage.com
- ^Learning to Fly Helicopters, see section titled: First Lesson: Air
Sources[edit]
- Flight Standards Service. Rotorcraft Flying Handbook: FAA Manual H-8083-21. Washington, DC: Flight Standards Service, Federal Aviation Administration, U.S. Dept. of Transportation, 2001. ISBN978-1-56027-404-9.
- AOPA: Aircraft Owners and Pilots Association http://www.aopa.org/News-and-Video/All-News/2013/November/27/rotocraft-rookie-helicopter-controls
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Helicopter_flight_controls&oldid=918992893'
Dissymmetry of lift in rotorcraftaerodynamics refers to an uneven amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters and autogyros in forward flight.
retreating blade side | advancing blade side |
A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade.
Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportional to the square of its airspeed. In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc.[1]
![Rotor Blades In Forward Flight Rotor Blades In Forward Flight](http://avstop.com/ac/Aviation_Maintenance_Technician_Handbook_General/images/fig3_84.jpg)
![Rotor Blades In Forward Flight Rotor Blades In Forward Flight](http://www.copters.com/aero/pictures/Fig_2-50.gif)
Effects[edit]
When dissymmetry causes the retreating blade to experience less airflow than required to maintain lift, a condition called retreating blade stall can occur. This causes the helicopter to roll to the retreating side and pitch up. This situation, when not immediately recognized can cause a severe loss of aircraft controllability.
Analysis[edit]
Envisage a viewpoint above a single-rotor helicopter in still air. For a stationary (hovering) helicopter, whose blades of length of r metres are rotating at ω radians per second, the blade tip is moving at a speed rω meters per second. At all points around the disc mapped out by the blade-tips, the speed of the blade-tip relative to the air is the same: everything is balanced.
Now imagine the helicopter in forward flight at, say, v meters per second. The speed of the blade-tip at point A in the diagram relative to the air is the sum of the blade-tip speed and the helicopter forward-flight speed: rω+v. But the blade-tip speed at point B, relative to the air, is the difference of its rotational speed and the forward-flight speed: rω-v.
Since the lift generated by an aerofoil increases as its relative airspeed increases, on a forward-moving helicopter the blade-tip at position A produces more lift than that at point B. So the rotor disc produces more lift on the right hand side than on the left hand side. This imbalance is the 'dissymetry of lift'.
Counter-measures[edit]
Dissymmetry is countered by 'blade flapping': rotor blades are designed to flap: the advancing blade flaps up and develops a smaller angle of attack due to a change in relative wind vectors, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack due to a change in relative wind vectors, and generates more lift.
Dissymmetry of lift is also countered by cyclic feathering, i.e. the variation in blade pitch arising from delta-three coupling of blade motions, or the forward input of cyclic during blowback.
To reduce dissymetry of lift, modern helicopter rotor blades are mounted in such a manner that the angle of attack varies with the position in the rotor cycle, the angle of attack being reduced on the side corresponding to position A in the diagram, and the angle of attack being increased on the side corresponding to position B in the diagram. However, there exists a limit to the degree by which dissymetry of lift can be diminished by this means, and therefore, since the forward speed v is important in the phenomenon, this imposes an upper speed limit upon the helicopter. This upper speed limit is known as VNE, the never-exceed speed. This speed is the speed beyond which the aerodynamic conditions at the rotor tips would enter unstable régimes - if v was sufficiently fast, the rotor tip at position A would be travelling fast enough through the air for the airflow to change radically as the rotor tip became supersonic, while the rotor tip at position B might have insufficient net linear speed through the air to generate meaningful lift (the stall condition - known as retreating blade stall). Entry of the rotor tip into either of these aerodynamic régimes is catastrophic from the point of view of the pilot, and the maintenance of stable forward flight.
The situation becomes more complex when helicopters with two sets of rotor blades are considered, since in theory at least, the dissymetry of lift of one rotor disc is cancelled by the increased lift of the other rotor disc: the two rotor discs of twin-rotor helicopters rotate in opposite senses, thus reversing the relevant directions of vector addition. However, as entry of the rotor tip into the supersonic aerodynamic realm is one of the unstable conditions that affects forward flight, even helicopters with two rotor discs rotating in opposite senses will be subject to a never-exceed speed. In the case of tandem-rotor helicopters such as the CH-47 Chinook, additional factors such as the aerodynamic drag of the entire design, and the available engine power, may conspire to ensure that the helicopter is incapable of achieving the VNE imposed upon it by dissymetry of lift. In the case of the Kamov Ka-50 'Black Shark', which is a coaxial design, it is possible for the helicopter to enter this aerodynamic régime as it has sufficient engine power, and pilots of this machine need to take this into consideration during the operation of the helicopter.
See also[edit]
References[edit]
- ^Helicopter Flying Handbook (FAA-H-8083-21A ed.). FAA. 2012. pp. 2–18 – 2–19.
External links[edit]
- Article by Paul Cantrell
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