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| − |
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| − | = Aerodynamics =
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| − |
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| − | [[File:Four-forces.png|left|thumb|
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| − | <figure id="fig:4forces"><caption>The Four Forces of Flight</caption></figure>
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| − | ]]
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| − |
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| − | === The Four Forces of Flight ===
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| − | All aircraft are governed by the four force of flight (<xr id='fig:4forces'/>): lift, weight, drag and thrust. When the opposing forces balance each other (lift against weight and thrust against drag) the aircraft will continue with in the same direction and velocity without acceleration. When two opposing forces are different, the aircraft will accelerate until it reaches equilibrium. It is the job of the pilot to control these forces in a way that results in a desired outcome.
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| − |
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| − | Here is a closer look at the four forces in a helicopter:
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| − |
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| − | *Weight - Weight is the force of gravity acting downward. Weight directly opposes lift and is constantly changing during flight as fuel is burned.
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| − | *Lift - Lift is produced by the main rotor system(s) and directly opposes weight. Lift is affected by multiple factors including the angle of attack of the rotor blades, the density of the air, the speed of air over the airfoil, etc.
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| − | *Thrust - Thrust is the horizontal force, usually in the forward direction, but for a helicopter, can be potentially in any direction. Thrust is controlled by angling the rotor disk so that a portion of the total thrust of the rotors is in a horizontal direction. Thrust directly opposes drag.
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| − | *Drag - Drag is the force that is produced by an object moving through the air. Drag is a result of air pushing against the body and all parts of the aircraft. Drag directly opposes thrust.
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| − |
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| − | === Lift ===
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| − |
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| − | [[File:Airfoil.png|right|thumb|
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| − | <figure id="fig:airfoil"><caption>Lift Produced by the Airfoil</caption></figure>
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| − | ]]
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| − | Every object in the atmosphere is surrounded by a has that exerts a static force of 2,116 pounds per square foot at sea level. When a rotor blade is not moving, that force is exerted equally over the entire blade surface, and thus no useful lift is produced. In a helicopter, this is done by forcing air over the blades through their rotation around a mast.
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| − |
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| − | <xr id='fig:airfoil'/> shows how lift is produced by an airfoil as it moves through the air. Lets take a look at some of the elements in the production of lift:
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| − |
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| − | *Relative Wind – Direction of movement of the air relative to the airflow. Usually opposite to the direction of movement.
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| − | *Wing Chord Line – The Line from the leading edge to the training edge of the wing.
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| − | *Angle of Attack (AOA) – The angle between the relative wind and the wing chord line. In a helicopter, raising the collective generally increases the AOA of the rotor blades.
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| − | *Resultant Force – The force acting on the airfoil as it moves through the air.
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| − | *Lift – The vertical component of the resultant force. Increases as AOA or the airspeed increases.
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| − | *Induced Drag – The horizontal component of the resultant force. Increases as AOA or the airspeed increases. A byproduct of lift that must be overcome by thrust in level flight. In a helicopter, induced drag is what results in the increased engine power required to maintain RPM as the AOA is increased.
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| − | More specifically, the lift produced by an airfoil (e.g., a rotor blade) is governed by the Lift Equation:
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| − | L = C<sub>L</sub>(1/2)ρv<sup>2</sup>S
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| − | The components of this equation are:
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| − |
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| − | * L - Total lift produced by the airfoil
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| − | * C<sub>L</sub> - The coefficient of lift
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| − | * ρ - The density of the air
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| − | * v - The speed of air over the airfoil
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| − | * S - The surface area of the airfoil
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| − |
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| − | The coefficient of lift is generally a function of the profile (edge view) of the airfoil, and the angle of attack. C<sub>L</sub> typically increases linearly with angle of attack up to a specific critical angle of attack. Once the critical angle of attack is exceeded, the coefficient of lift decreased rapidly as the airfoil enters aerodynamic stall.
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| − |
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| − | === Drag ===
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| − |
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| − | [[File:Total-drag.png|right|thumb|
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| − | <figure id="fig:drag"><caption>Drag vs Airspeed</caption></figure>
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| − | ]]
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| − |
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| − | Drag is the force that resists the helicopter and/or rotor blades as the move through the air. Drag must be overcome by the engine to turn the rotor. Drag always acts parallel to the relative wind and total drag is comprised of three types of drag: profile, induced and parasite.
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| − |
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| − | ==== Profile Drag ====
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| − | Profile drag is a mix of skin friction and form drag that results from turbulent airflow that is created as a structure moves through the air. Profile drag is a function of the smoothness of a surface, and the size/shape of the structure that protrudes into the relative wind. It is generally weakly correlated with airspeed.
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| − |
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| − | ==== Induced Drag ====
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| − |
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| − | Induced drag is generated by airflow circulation around the rotor blade as it creates lift. When an airfoil creates lift, some of the air is forced downward meaning that the relative wind is angled slightly above the horizontal plane. This results in a force that tends to slow the blades that increases as the angle of attack increases (i.e., with increased collective). Induced drag is the main reason that more engine power is required as collective is increased.
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| − |
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| − | ==== Parasite Drag ====
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| − |
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| − | Parasite drag is the drag produced by all the non-lifting surfaces as the helicopter moves through the air (i.e., cabin, rotor mast, tail, landing gear). Parasite drag generally increases as the square of the the airspeed, thus doubling airspeed results in a 4x increase in drag. This means that a 4x increase in thrust is required to achieve a 2x increase in speed.
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| − |
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| − | ==== Total Drag ====
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| − | Total drag is the total of profile, induced and parasite drag (see <xr id='fig:drag'/>). Since induced drag decreases with airspeed, while parasite drag increases, there is generally an airspeed at which the lift-to-drag ratio is minimized. This point is referred to as L/D<sub>MAX</sub> and typically defines the best climb airspeed of the aircraft. In the example shown here L/D<sub>MAX</sub> is approximately 50 knots meaning 50 knots would be the best climb speed.
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| − |
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| − | === The Rotor Disk ===
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| − |
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| − | ==== Relative Wind ====
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| − | There are two parts to the relative wind as it passes through the rotor system:
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| − |
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| − | * Horizontal part - caused by the blade moving through the rotor plane consisting of the speed of the rotating blade plus the forward airspeed.
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| − | *Vertical part - caused by air being moved down through the rotor system, plus any vertical movement of the helicopter. The vertical component is generally larger at slow airspeed.
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| − |
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| − | ==== Induced Flow ====
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| − |
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| − | As the pitch of the rotor blades increase, more air is forced down through the rotor system. This induces an downward flow of air which when combined with the horizontal motion of the blade through the air results in a relative wind that is angled downward relative to the path of the blade. This results in a decreased angle of attack, and this less lift. Induced flow is greatest during hover in no-wind conditions.
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| − |
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| − | ===== Ground Effect =====
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| − | Proximity to the ground (land, water, or over a prim) interrupts the downward movement of air, thus reducing the effect induced flow. The effect increased the closer your are to the surface, and diminishes as you increase height over the surface. Ground effect decreases the power needed to hover.
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| − |
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| − | ===== In Ground Effect (IGE) Hover =====
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| − | Hovering near a surface is referred to as an IGE (In Ground Effect) hover. Ground effect permits the relative wind over the rotor blades to be more horizontal thus increasing their efficiency. Ground effect is increased when hovering over smooth surfaces, and decreased over grasses and other rough surfaces.
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| − |
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| − | ===== Out of Ground Effect (OGE) Hover =====
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| − | When hovering outside the range of ground effect (generally above a rotor diameter), the helicopter is said to be in an Out of Ground Effect (OGE) hover. Since you lose the benefits of ground effect in an OGE hover, it requires more power to maintain altitude. At high density altitudes, or in helicopters with less power, it may not be possible to maintain an OGE hover in some cases. Performing an OGE hover also presents the risk of entering vortex ring state if the helicopter is allowed to descend at too great a speed. See [[#VORTEX RING STATE]] for more information on vortex ring state.
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| − |
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| − | === Hovering Flight ===
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| − |
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| − | Hovering is the most challenging part of flying a helicopter. This is due to the fact that the helicopter is inherently unstable and constant control inputs are needed to maintain a hover. Furthermore, all of these control movements interact with each other such that moving any one control usually requires moving another control as well. That said, the basic principle is relatively simple. The cyclic is used to eliminate horizontal drift, the pedals are used to maintain directional control, and the collective is used to maintain a constant height above the ground.
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| − |
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| − | ==== Translating Tendency ====
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| − |
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| − | [[File:Trans-tend.jpg|right|thumb|
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| − | <figure id="fig:transtend"><caption>Translating Tendency</caption></figure>
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| − | ]]
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| − |
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| − | The primary purpose of tail rotor is to counteract torque from the main rotors. However, a side effect of this is a slight sideward force that would push the helicopter to the right if left uncorrected. This is referred to as "translating tendency" and is illustrated in <xr id='fig:transtend'/>. The counter-clockwise rotating main rotor induces a torque trying to turn the helicopter to the right. The tail rotor pushes air through it to the left, thus creating a thrust to the right. Since this thrust is away from the center of gravity, this causes a counter-clockwise torque (balancing against the torque from the main rotor), and a horizontal component that acts to push the helicopter to the right.
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| − |
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| − | To compensate for translating tendency, the controls on the helicopter are often rigged so that there is a slight left tilt of the main rotor when the cyclic controls are centered. This left force from the main rotor balances the right force from the tail rotor. A side effect of this is that the left skid will hang slightly lower then the right one in a hover.
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| − |
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| − | ==== Pendular Action ====
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| − |
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| − | Since the helicopter is suspended from a single point well above the center of mass, it can have a tendency to rock like a pendulum. This is called pendular action and can be exaggerated by poor control technique. When a pilot begins overcontrolling, the pendular action can rapidly increase causing the helicopter to rapidly depart controlled flight. For this reason, it is important for the pilot to use small and smooth control inputs when flying a helicopter.
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| − |
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| − | ==== Gyroscopic Precession ====
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| − |
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| − | The spinning blades of a helicopter act like a gyroscope. One of the properties of gyroscopes is gyroscopic precession. Gyroscopic precession causes a spinning object to act 90 degrees in the direction of rotation from where a force is applied. For this reason, the controls on a helicopter are rigged such that the greatest blade pitch occurs 90 degrees before the highest point of the rotor disk.
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| − |
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| − | As an example, consider a helicopter with counter-clockwise rotating blades. Pushing forward on the cyclic will cause the highest blade pitch to occur at the 9 oclock position on the left side of the helicopter. Gyroscopic precession will cause the back of the rotor disk at the 6 oclock position to tilt up, this vectoring some of the thrust in the forward direction.
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| − |
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| − | ==== Torque ====
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| − |
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| − | In single main rotor helicopters, torque is produced by the main rotor whenever the helicopter is under power. In most aircraft, the main rotor turns counter-clockwise when viewed from above, resulting in the body of the helicopter having a tenancy to turn clockwise (to the right) the more power you use. As a result, the pilot must apply right pedal to compensate. This is particularly evident when pulling collective to lift into a hover which requires the most amount of power. Conversely, right pedal must be applied when reducing power/collective. A few aircraft (e.g., Russian and French) have main rotors that turn clockwise when viewed from above. In these aircraft, the effect is reversed and left pedal is needed to compensate as collective is increased.
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| − |
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| − | In aircraft with tandem rotors such as the Chinook, or in aircraft with coaxial rotors such as the Hiller UH-4, the rotors are counter-rotating and thus torque is canceled.
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| − |
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| − | === Forward Flight ===
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| − |
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| − | [[File:Forward-flight.png|right|thumb|
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| − | <figure id="fig:forward-flight"><caption>Forces in Forward Flight</caption></figure>
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| − | ]]
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| − |
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| − | In straight-and-level, unaccelerated forward flight, lift equals weight and thrust equals drag. If lift drops below weight, the helicopter will descend until the forces are in balance again. The same will occur if lift increases above weight. Similarly if thrust increases above drag, forward airspeed will increase thus increasing drag until it exactly balances the new higher thrust.
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| − |
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| − | <xr id="fig:forward-flight"/> shows the four forces of a helicopter in forward level flight at constant speed. When the rotor disk tilts forward, the total main rotor thrust (shown by the blue line) is angled forward perpendicular to the rotor disk. In this state, the main rotor thrust can be decomposed into a vertical (green) and a horizontal (red) component. Since we are in level unaccelerated flight, we know that the vertical component (i.e., lift) must equal weight, and the horizontal component (thrust) must equal drag.
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| − |
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| − | As the rotor disk tilts forward more, the horizontal component increases resulting in higher forward airspeed. But since the lift must equal weight in order to maintain altitude, it is necessary to increase the main/rotor thrust as this happens. This is normally done with increased collective and is why more collective is required as forward speed is increased.
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| − |
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| − | [[File:Forward-flt-airflow.png|left|thumb|
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| − | <figure id="fig:Forward-flt-airflow"><caption>Airflow in Forward Flight</caption></figure>
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| − | ]]
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| − | ==== Airflow in Forward Flight ====
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| − |
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| − | In forward flight, the forward speed of the aircraft combines with the rotor tip speed to determine the speed of the relative air seen around the rotor disk. For example, consider the situation shown in <xr id='fig:Forward-flt-airflow'/>. In this example, the airspeed at the rotor tip in a hover is 300 knots, and the aircraft is moving forward at 100 knots. Since the aircraft is moving forward at 100 knots, there is a 100 knot relative wind from the opposite direction. Assuming a helicopter with counter-clockwise rotating blades, the blade on the right side at point A (called the "advancing blade") is moving forward into the relative wind, so the net relative wind it see will be the 300 from its rotation, plus the 100 from the oncoming air from forward motion. This results in a 400 knot relative wind at the blade tip at point A. On the left side of the rotor disk at point B (called the "retreating blade"), the blade is moving backwards. The relative wind seen by the blade at point B will be the 300 knots from the blade rotation, minus the 100 knots for the forward speed. This means the relative wind at point B will be 200 knots.
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| − |
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| − | ===== Dissymmetry of Lift =====
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| − |
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| − | Dissymmetry of Lift is the apparent unequal lift on the two sides of the rotor disk in forward flight due to the unequal airflow over each half of the disk. Referring back to the example in <xr id='fig:Forward-flt-airflow'/>, we see that the advancing blade sees a relative wind of 400 knots, while the retreating blade sees a relative wind of 200 knots. Recall from [[#Lift|Section 2.2 - Lift]], that lift is proportional to the square of the relative wind. Since the relative wind on the advancing blade is double that on the retreating blade, that would suggest the lift on the right side of the rotor disk should be 4 times that on the left side. If this were true, the helicopter would roll violently to the left as forward speed were increases. Since this does not happen, there must be some other mechanism at play.
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| − |
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| − | What is actually happening, is blade flapping. A hinge at the rotor hub, called the flapping hinge, allows each blade to move up and down. Blades will flap up on the advancing side, and down on the retreating side. The upward motion of the blade on the advancing side causes the angle of attack on that side to decrease, thus reducing the lift. Similarly, the downward motion on the retreating side increase the angle of attack, this increasing the lift on that side. The flapping of the blades allow the forces to balance exactly, equalizing the lift around the rotor disk. This normally happens automatically without any input from the pilot.
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| − |
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| − | ===== Retreating Blade Stall =====
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| − |
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| − | As the speed of the helicopter increases more, the relative wind over the retreating blade becomes smaller and smaller. At some point, there will not be enough airflow over the retreating blade to maintain lift, and the blade will stall. This is called retreating blade stall. When the aircraft enters retreating blade stall, it will begin to lose lift and roll, sometimes violently, to the left. The pilot must react quickly with aft cyclic to reduce the airspeed and stop the stall. Retreating blade stall is what generally determines the never exceed speed (Vne) in helicopters and is usually marked with a red line on the airspeed indicator. The pilot should not that the red line represents Vne at sea level. The never exceed speed generally decreases with altitude.
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| − |
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| − | ==== Translational Lift ====
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| − |
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| − | When a helicopter is in a hover or at a low speed, airflow through the disk is at a right angle to the rotor plane, and the rotors must provide all induced airflow through the rotors. As airspeed is increased, some of the induced flow through the rotors begins to be provided by the forward speed of the helicopter. The net result of this is increased lift at no additional cost in power. The largest increase occurs at around 10 to 20 mph and is referred to as ETL (Effective Translational Lift). As an helicopter passes through ETL, it requires significantly less power to maintain altitude, and more power is available for climbs. This is also the reason that less power is required in cruise flight than for hovering. It is also part of the reason that a normal takeoff involves a short ground run to accelerate past ETL before climbing out.
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| − | ===== Effective Translational Lift (ETL) =====
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| − |
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| − | === Autorotation ===
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| | | | |
| | = Helicopter Flight Controls = | | = Helicopter Flight Controls = |
overview
1 Master Contents
2 Helicopter Flight Controls
3 Helicopter Systems
3.1 Airframe
3.2 Main Rotor
3.2.1 Rotor Types
3.2.1.1 Semirigid Systems
3.2.1.2 Rigid Systems
3.2.1.3 Fully Articulated Systems
3.2.2 The Swash Plate Assembly
3.2.3 The Freewheeling Unit
3.2.4 The Rotor Brake
3.3 Flight Instruments
3.3.1 Airspeed Indicator
3.3.2 Altimeter
3.3.3 VSI
3.3.4 Engine Instruments
3.4 Engines
3.4.1 Piston Engines
3.4.2 Turbine Engines
3.5 Transmission System
3.5.1 Main Rotor Transmission
3.5.2 Clutch
3.6 Fuel System
3.6.1 Fuel Supply System
3.6.2 Engine Control System
3.6.3 Carburetor Ice
3.6.4 Fuel Injection
3.7 Hydraulics
3.8 Electrical Systems
3.9 Fuel Systems
3.10 Stability Augmentation Systems
3.11 Anti-Icing Systems
4 Weight and Balance
5 Helicopter Performance
6 Ground Preperation
7 Basic Flight Maneuvers
7.1 PICKUP TO HOVER
Picking up to a hover is performed by slowly raising the collective until the helicopter becomes light on the skids. Pedal should be applied to stop any rotation, and cyclic should be applied to stop any lateral or forward/back motion. Considerable left pedal may be required in the pick-up to counteract torque from the main rotor.
7.2 HOVERING
There are two types of hovering. In Ground Effect (IGE) hovering occurs in ground effect within a few meters of a surface (land, water, or a flat prim) and Out of Ground Effect (OGE) hovering occurs at altitude away from any surface. While hovering in general requires more power (collective) than other phases of flight, OGE hovering requires significantly more power than IGE hovering, and may be impossible depending on how heavily loaded the helicopter is.
Cyclic inputs in a hover should be small and controlled taking into account delay between input and the reaction of the helicopter. Use collective to control hight. It is important to learn to anticipate the reaction of the helicopter in response to an input. Over-controlling is a common problem for the beginning helicopter pilot. Forward hover taxi is performed by a very slight forward positioning of the cyclic. In general all hover taxiing should be performed with no more than a dot-width of movement from the center on the cyclic control display.
7.3 SET DOWN FROM HOVER
Setting down from a hover essentially the reverse of a pickup. Slowly lower collective letting helicopter settle. As collective is lowed, right pedal may be necessary due to the reduction in torque.
7.4 NORMAL TAKEOFFS
Normal takeoffs should begin in a hover with an adequate obstacle free area ahead of the helicopter. From a hover, apply forward cyclic and allow helicopter to accelerate forward in ground effect. At about 30 mph, the helicopter will achieve ETL (Effective Translational Lift) allowing the helicopter to climb (see Section 8 for details on ETL). Some brief aft cyclic at 40 mph may be necessary. Control airspeed with forward cyclic to maintain a best climb airspeed of about 50 mph. Applying too much forward cyclic will reduce climb performance. Raise collective to increase rate of climb, but do not exceed the maximum manifold pressure of 29 inches (red line).
7.5 CRUISE
In cruise flight, use collective to control altitude, forward/back cyclic to control airspeed, and left/right cyclic for turns. Use the yaw string attached to the front of the bubble to help in coordinating turns. When the yaw string is straight up, the helicopter is aligned with the direction of movement. If the yaw string is leaning to one side, apply pedal in the direction you want it to move. For example, if the yaw string is leaning to the right, apply left pedal until it is vertical. The inclinometer (the black ball) on the HUD can be used in lieu of the yaw string. When using the inclinometer, pedal should be applied on the side to which the ball moves. Think "step on the ball". The VSI (Vertical Speed Indicator) can be used as a reference to help maintain a constant altitude. As with any helicopter, aggressive and abrupt control inputs should be avoided.
7.6 TURNS
Turns should be made primarily with the cyclic, using the yaw string to maintain coordination with the pedals as described above. In very steep turns, it may be necessary to increase collective slightly in the turn.
7.7 NORMAL LANDINGS
Begin a normal landing by reducing air speed to bout 50 mph and lowering the collective to maintain an approximately 500 foot per minute descent. Choose a target spot for your landing, keeping that spot at approximately the same place on the window. As you approach your touchdown spot, slowly decrease airspeed with back cyclic, and descent rate with up collective. Maintain above 30 mph until just before touchdown to avoid vortex ring state. As you transition to a hover, you will need additional up collective and left pedal to compensate for the torque.
7.8 AIR TAXI
An air taxi is used to move quickly from one point to another on an airport. Begin an air taxi like a normal takeoff, but lower collective slightly to maintain a height of 10 to 20 meters over the airport while pitching with cyclic for approximately 50 mph. To exit an air taxi and land, apply back cyclic, down collective and ride pedal as necessary. Then make a normal landing to hover.
8 Advanced Flight Maneuvers
8.1 MAX PERFORMANCE TAKEOFFS
If takeoff is from a confined area, the max performance takeoff procedure can be used. Begin with helicopter on the ground and collective full down. Raise collective until manifold pressure is at 29 inches (red line) while applying slight forward cyclic to begin forward movement. When clear of the obstacle apply additional forward cyclic. Once ETL is reached at about 30 mph, continue climb as normal.
8.2 STEEP APPROACHES
A steep approach can be used when landing is to a confined area. Follow the same procedure as for a normal landing, but slow to 30 mph and maintain that speed through the descent.
8.3 QUICK STOP
TBD
8.4 SHALLOW APPROACH and RUN-ON LANDING
TBD
8.5 SLOPE LANDINGS
TBD
9 Helicopter Emergencies and Hazards
9.1 PRACTICE AUTOROTATIONS
The recommended altitude for practice autorotations is 500 feet AGL. Recommended procedures are as follows:
- Set up a stabilized constant altitude approach to the runway at 500 feet AGL, then just before crossing the runway threshold, lower collective to minimum, roll throttle to idle, and apply right pedal as necessary to maintain coordination (use yaw string for reference). Throttle can be rolled to idle by clicking on the "idle" box under the throttle slider in the HUD. The box will turn red to indicate you have enabled "idle lock" which will lock the throttle in the idle position and prevent it from coming on with further collective adjustments (disengage idle lock by clicking the box again, or clicking anywhere on the throttle slider. For a runway that starts at a sim edge, you can set up the autorotation before crossing the sim boundary.
- Maintain a forward speed of 50-60 mph during the descent. This speed can be adjusted up or down slightly in order to land at a specific spot, but should be within this target range before entering the flare.
- At approximately 30 to 50 feet AGL, begin a cyclic flare by pulling back on the cyclic.
- At approximately 10 feet AGL, level the helicopter with cyclic, and begin pulling up on the collective to cushion the landing.
Timing of the flare and collective input is critical. Since throttle is locked in idle, RPM will being to decay as soon as collective is pulled. Pulling too soon will result in RPM decaying while still airborne with the helicopter dropping to the ground. Pulling too late will result in a hard landing.
9.2 PRACTICE HOVERING AUTOROTATIONS
Practice hovering autorotations from a medium height hover. The apply the following procedures:
- Click the "idle lock" button on HUD to lock throttle in idle position.
- Apply immediate right pedal to hold heading and maintain collective until helicopter begins to sink
- Apply full up collective as helicopter begins to sink
- Lower collective once helicopter is fully on the ground
The goal should be to set down softly with little or no change in direction.
9.3 VORTEX RING STATE
Vortex ring state, sometimes called "settling with power", is a dangerous condition that can occur when a helicopter is descending into its own downwash. Essentially a vortex ring system engulfs the rotors and they fail to produce lift. Once in vortex ring state, increases in power in an attempt to slow the descent will only make the condition worse, thus actually increasing the descent rate. If the condition is allowed to develop too far, or you are too close to the ground, it may be impossible to recover. You are at danger for vortex ring state when all three of the the following conditions hold:
- Descent rate greater than 300 feet/minute
- Airspeed less than 30 mph
- More than 50% power
If you have alerts turned on, your helicopter will alert you when you are in vortex ring state. To recover from vortex ring state, you should lower the collective and apply forward cyclic to regain airspeed. However, the best practice is to avoid it in the first place.
9.4 RETREATING BLADE STALL
TBD
9.5 LOW ROTOR RPM and ROTOR STALL
TBD
10 Navigation
TBD
