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Revision as of 21:57, 14 August 2021
- 1 Introduction to the Helicopter
- 2 Aerodynamics
- 2.1 The Four Forces of Flight
- 2.2 Lift
- 2.3 Drag
- 2.4 The Rotor Disk
- 2.5 Hovering Flight
- 2.6 Forward Flight
- 2.7 Autorotation
- 3 Helicopter Flight Controls
- 4 Helicopter Systems
- 4.1 Airframe
- 4.2 Main Rotor
- 4.3 Flight Instruments
- 4.4 Engines
- 4.5 Transmission System
- 4.6 Fuel System
- 4.7 Hydraulics
- 4.8 Electrical Systems
- 4.9 Fuel Systems
- 4.10 Stability Augmentation Systems
- 4.11 Anti-Icing Systems
- 5 Weight and Balance
- 6 Helicopter Performance
- 7 Ground Preperation
- 8 Basic Flight Maneuvers
- 9 Advanced Flight Maneuvers
- 10 Helicopter Emergencies and Hazards
- 11 Navigation
1 Introduction to the Helicopter
A helicopter is an aircraft that is lifted and propelled by one or more rotors, each consisting of two or more rotor blades. Helicopters are one of two classes of rotorcraft along with gyrocopters. Unlike fixed-wing aircraft, helicopters are capable of vertical flight giving them great versatility to fly into small confined areas that are not accessible to most other aircraft.
While early prototype helicopters were built early in the 20th century, Igor Sikorsky is credited with producing the first production helicopter, the R-4 in 1942. It was the R-4 that set the standard of using one main rotor and a tail rotor in helicopter design.
The unique capabilities of the helicopter -- specifically its ability to takeoff and land vertically, and to hover for an extended period of time -- enable the helicopter to perform tasks no other aircraft can handle. Helicopters have found use in transportation, construction, firefighting and search and rescue.
1.2 Rotor System
The rotor system is the rotating part of the helicopter that generates lift. A rotor system may be mounted horizontally, to produce vertical lift as in the main rotor; or vertically, as in a tail rotor, to counter torque from the main rotor or provide yaw control of the aircraft. Helicopters come in a variety of rotor configurations as described in the following sections.
1.2.1 Single Main Rotor
Most helicopters use a single main rotor with a single tail rotor to counteract thrust. The tail rotor is required because the main rotor produce a torque, or twisting force, as it rotates under power which causes the aircraft to want to turn in the opposite direction of the blades. The tail rotor is usually mounted on a boom of some sort to position it out from under the main rotor. The tail rotor produces sideways thrust that counter-acts the torque from the main rotor.
For most US-made helicopters, the main rotor turns counter-clockwise when viewed from above, but Russian-made helicopters and some European helicopters have clockwise rotating rotor systems. Throughout this manual, we will assume counter-clockwise rotating blades which produce a torque making the helicopter tend to yaw right. For helicopters with clockwise rotating rotor systems, this yaw, and thus the actions needed to counteract it, would be reversed.
1.2.2 Tandem Rotor
Tandem rotor helicopters have two main counter-rotating main rotor systems, usually mounted one in front of the other. Since the rotors turn in opposite directions, the torque from each rotor cancels each other, thus eliminating the need for a tail rotor. This allows all of the engine power to be used to generate thrust, unlike a standard single-rotor design where some of the power must be diverted to drive the tail rotor. Yaw control in a tandem rotor helicopter is achieved by vectoring the front and back rotors in opposite directions.
1.2.3 Coaxial Rotor
A coaxial rotor helicopter has counter-rotating blades stacked on top of each other. Like the tandem system, the torque from each rotor cancels the other eliminating the need for a tail rotor. Yaw control in a coaxial helicopter can be achieved by altering the amount of collective pitch in the upper versus the lower rotor system. This causes the torque from the two systems to become unbalanced allowing the helicopter to turn.
1.2.4 Intermeshing Rotor
In an intermeshing design, the two counter-rotating rotors are mounted in such a way that the rotor blades intermesh with each other, often in a side-by-side configuration. The blades are usually mounted at a slight angle relative to each other, and are synchronized so as to prevent the blades from colliding with each other.
1.3 Controlling Flight
A helicopter has four primary flight controls:
- Antitorque pedals
The cyclic is a stick that can be moved like a joystick forward and back, and left and right to control the direction of thrust from the main rotor. The cyclic is usually a stick mounted on the floor between the pilot's legs, though some helicopters such as the Robinson models use a T-stick cyclic mounted in the center of the cabin with a T-bar on the top.
The control is called the cyclic because it varies the pitch of the rotor blades as a function of their angle with the rotor mast over the course of a revolution. This results in an unequal lift/thrust from the blade is it rotates causing the rotor disk to tilt in the direction the pilot pushes the cyclic. When the rotor disk tilts, it diverts some of its total thrust to a horizontal component giving the helicopter a thrust in the direction the cyclic was pushed.
The collective is located to the left of the pilot, and is usually a lever that can be raised and lowered. The collective increases the pitch of the blades "collectively" around the entire rotation of the blades thus increasing the total thrust developed by the rotor system. The collective is usually used to control altitude in a helicopter.
1.3.3 Antitorque Pedals
Antitorque Pedals are located on the floor in the same position rudder pedals would be in a fixed-wing aircraft. They are typically controlled with the feet and move together, pushing one causes the other to move forward and vice-versa. In a standard single main rotor helicopter, the anti-torque pedals control the pitch of the tail rotor, and thus the thrust from the tail rotor. This causes the helicopter to yaw left or right in the direction on which the pedal was pressed.
Helicopters are designed to operate at a specific RPM, or a narrow range of RPMs. As the pilot increases or decreases the collective, the amount of engine power needed to maintain the target RPM will increase or decrease. In single engine helicopters, the throttle is usually a twist grip mounted on the collective. As the pilot increases collective, they simultaneously roll on throttle to increase engine power. When the pilot decreases the collective, they roll off throttle to prevent the rotors from overspeeding.
Many helicopters have systems to assist the pilot in controlling the throttle as described below.
A correlator is a mechanical linkage that automatically adds throttle when the collective is increased, and reduces throttle when the collective is lowered. The adjustments from a correlator are usually approximate and must be fine tuned by the pilot.
A governor an electronic device that measures the rotor RPM, and increases or decreases the throttle through an electric servo to maintain a specific RPM. In helicopters equipped with a governor, the governor can be engaged through a switch, or may come on automatically when the RPM enters a specific range. In all cases, it is possible for the pilot to manually override the governor by manually moving the throttle.
1.4 Flight Conditions
2.1 The Four Forces of Flight
All aircraft are governed by the four force of flight (Figure 1): 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.
Here is a closer look at the four forces in a helicopter:
- Weight - Weight is the force of gravity acting downward. Weight directly opposes lift and is constantly changing during flight as fuel is burned.
- 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.
- 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.
- 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.
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.
Figure 2 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:
- Relative Wind – Direction of movement of the air relative to the airflow. Usually opposite to the direction of movement.
- Wing Chord Line – The Line from the leading edge to the training edge of the wing.
- 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.
- Resultant Force – The force acting on the airfoil as it moves through the air.
- Lift – The vertical component of the resultant force. Increases as AOA or the airspeed increases.
- 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.
More specifically, the lift produced by an airfoil (e.g., a rotor blade) is governed by the Lift Equation:
L = CL(1/2)ρv2S
The components of this equation are:
- L - Total lift produced by the airfoil
- CL - The coefficient of lift
- ρ - The density of the air
- v - The speed of air over the airfoil
- S - The surface area of the airfoil
The coefficient of lift is generally a function of the profile (edge view) of the airfoil, and the angle of attack. CL 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.
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.
2.3.1 Profile Drag
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.
2.3.2 Induced Drag
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.
2.3.3 Parasite Drag
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.
2.3.4 Total Drag
Total drag is the total of profile, induced and parasite drag (see Figure 3). 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/DMAX and typically defines the best climb airspeed of the aircraft. In the example shown here L/DMAX is approximately 50 knots meaning 50 knots would be the best climb speed.
2.4 The Rotor Disk
2.4.1 Relative Wind
There are two parts to the relative wind as it passes through the rotor system:
- Horizontal part - caused by the blade moving through the rotor plane consisting of the speed of the rotating blade plus the forward airspeed.
- 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.
2.4.2 Induced Flow
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.
22.214.171.124 Ground Effect
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.
126.96.36.199 In Ground Effect (IGE) Hover
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.
188.8.131.52 Out of Ground Effect (OGE) Hover
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.
2.5 Hovering Flight
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.
2.5.1 Translating Tendency
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 Figure 4. 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.
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.
2.5.2 Pendular Action
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.
2.5.3 Gyroscopic Precession
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.
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.
2.6 Forward Flight
2.6.1 Airflow in Forward Flight
184.108.40.206 Retreating Blade Stall
220.127.116.11 Dissymmetry of Lift
2.6.2 Translational Lift
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.
18.104.22.168 Effective Translational Lift (ETL)
3 Helicopter Flight Controls
4 Helicopter Systems
4.2 Main Rotor
4.2.1 Rotor Types
22.214.171.124 Semirigid Systems
126.96.36.199 Rigid Systems
188.8.131.52 Fully Articulated Systems
4.2.2 The Swash Plate Assembly
4.2.3 The Freewheeling Unit
4.2.4 The Rotor Brake
4.3 Flight Instruments
4.3.1 Airspeed Indicator
4.3.4 Engine Instruments
4.4.1 Piston Engines
4.4.2 Turbine Engines
4.5 Transmission System
4.5.1 Main Rotor Transmission
4.6 Fuel System
4.6.1 Fuel Supply System
4.6.2 Engine Control System
4.6.3 Carburetor Ice
4.6.4 Fuel Injection
4.8 Electrical Systems
4.9 Fuel Systems
4.10 Stability Augmentation Systems
4.11 Anti-Icing Systems
5 Weight and Balance
6 Helicopter Performance
7 Ground Preperation
8 Basic Flight Maneuvers
8.1 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.
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.
8.3 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.
8.4 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).
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.
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.
8.7 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.
8.8 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.
9 Advanced Flight Maneuvers
9.1 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.
9.2 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.
9.3 QUICK STOP
9.4 SHALLOW APPROACH and RUN-ON LANDING
9.5 SLOPE LANDINGS
10 Helicopter Emergencies and Hazards
10.1 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.
10.2 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.
10.3 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.
10.4 RETREATING BLADE STALL
10.5 LOW ROTOR RPM and ROTOR STALL