Rotary-Wing Aircraft Assembly and Rigging PDF

AVIA-1035 1  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  The flight control units located in the flight deck of all helicopters are very nearly the same.  All helicopters have either one or tw...

AVIA-1035 1  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  The flight control units located in the flight deck of all helicopters are very nearly the same.  All helicopters have either one or two of each of the following: collective pitch control, throttle grip, cyclic pitch control, and directional control pedals.  Figure 2-22 Basically, these units do the same things, regardless of the type of helicopter on which they are installed; however, the operation of the control system varies greatly by helicopter model. 2  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  Figure 2-22 3  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  Rigging the helicopter coordinates the movements of the flight controls and establishes the relationship between the main rotor and its controls, and between the tail rotor and its controls.  Rigging is not a difficult job, but it requires great precision and attention to detail.  Strict adherence to rigging procedures described in the manufacturer’s maintenance manuals and service instructions is a must.  Adjustments, clearances, and tolerances must be exact. 4  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  Rigging of the various flight control systems can be broken down into the following three major steps:  1.Placing the control system in a specific position— holding it in position with pins, clamps, or jigs, then adjusting the various linkages to fit the immobilized control component. 5  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  2.Placing the control surfaces in a specific reference position—using a rigging jig, a precision bubble protractor, or a spirit level to check the angular difference between the control surface and some fixed surface on the aircraft.  Figure 2-23 6  Airframe 2-17 Rotary-Wing Aircraft Assembly and Rigging  Figure 2-23 7  Airframe 2-16 Rotary-Wing Aircraft Assembly and Rigging  3.Setting the maximum range of travel of the various components—this adjustment limits the physical movement of the control system.  After completion of the static rigging, a functional check of the flight control system must be accomplished. 8  Airframe 2-17 Rotary-Wing Aircraft Assembly and Rigging  The nature of the functional check varies with the type of helicopter and system concerned, but usually includes determining that:  1.The direction of movement of the main and tail rotor blades is correct in relation to movement of the pilot’s controls.  2.The operation of interconnected control systems (engine throttle and collective pitch) is properly coordinated. 9  Airframe 2-17 Rotary-Wing Aircraft Assembly and Rigging  3.The range of movement and neutral position of the pilot’s controls are correct.  4.The maximum and minimum pitch angles of the main rotor blades are within specified limits.  This includes checking the fore-and-aft and lateral cyclic pitch and collective pitch blade angles.  5.The tracking of the main rotor blades is correct. 10 Rigging – Eurocopter AS355N 11  Airframe 2-17 Rotary-Wing Aircraft Assembly and Rigging  6.In the case of multirotor aircraft, the rigging and movement of the rotor blades are synchronized.  7.When tabs are provided on main rotor blades, they are correctly set.  8.The neutral, maximum, and minimum pitch angles and coning angles of the tail rotor blades are correct.  9.When dual controls are provided, they function correctly and in synchronization. 12  Airframe 2-17 Rotary-Wing Aircraft Assembly and Rigging  Upon completion of rigging, a thorough check should be made of all attaching, securing, and pivot points.  All bolts, nuts, and rod ends should be properly secured and safetied as specified in the manufacturers’ maintenance and service instructions. 13  General 5-51 Helicopter Structures and Airfoils  The helicopter, as we know it today, falls under the classification known as rotorcraft.  Rotorcraft are also known as rotary wing aircraft, because instead of their wing being fixed like it is on an airplane, the wing rotates.  The rotating wing of a rotorcraft can be thought of as a lift producing device, like the wing of an airplane, or as a thrust producing device, like the propeller on a piston engine. 14  General 3-53 Helicopter Structures and Airfoils  The main parts that make up a helicopter are the cabin, landing gear, tail boom, powerplant, transmission, main rotor, and tail rotor.  Figure 5-81 15  General 5-51 Helicopter Structures and Airfoils  Figure 5-81 16  General 5-54 Helicopter Axes of Flight  Helicopters, like airplanes, have a vertical, lateral, and longitudinal axis that passes through the helicopter’s center of gravity.  Helicopters yaw around the vertical axis, pitch around the lateral axis, and rotate around the longitudinal axis.  Figure 3-90 shows the three axes of a helicopter and how they relate to the helicopter’s movement. 17  General 5-54 Helicopter Axes of Flight  Figure 5-90 18  General 5-54 Helicopter Axes of Flight  All three axes will intersect at the helicopter’s center of gravity, and the helicopter pivots around this point.  Notice in the figure that the vertical axis passes almost through the center of the main rotor, because the helicopter’s center of gravity needs to be very close to this point. 19  General 5-54 Control Around the Vertical Axis  For a single main rotor helicopter, control around the vertical axis is handled by the anti-torque rotor (tail rotor) or from the fan’s airflow on a NOTAR type helicopter.  Like in an airplane, rotation around this axis is known as yaw.  The pilot controls yaw by pushing on the anti-torque pedals located on the cockpit floor, in the same way the airplane pilot controls yaw by pushing on the rudder pedals. 20  General 5-54 Control Around the Vertical Axis  To make the nose of the helicopter yaw to the right, the pilot pushes on the right anti- torque pedal.  When viewed from the top, if the helicopter tries to spin in a counterclockwise direction because of the torque of the main rotor, the pilot will also push on the right anti-torque pedal to counteract the main rotor torque.  By using the anti-torque pedals, the pilot can intentionally make the helicopter rotate in either direction around the vertical axis. 21  General 5-54 Control Around the Vertical Axis  The anti-torque pedals can be seen in Figure 5-91.  Some helicopters have a vertical stabilizer, such as those shown in Figures 5-90 and 5-92.  In forward flight, the vertical stabilizer creates a force that helps counteract the torque of the main rotor, thereby reducing the power needed to drive the anti-torque system located at the end of the tail boom.  Three Axes of Flight (4min) 22  General 5-54 Control Around the Vertical Axis  Figures 5-90 23  General 5-55 Control Around the Vertical Axis  Figures 5-92 24  General 5-54 Longitudinal and Lateral Axes  Movement around the longitudinal and lateral axes is handled by the helicopter’s main rotor.  Axes = plural form of axis  In the cockpit, there are two levers that control the main rotor, known as the collective and cyclic pitch controls.  The collective pitch lever is on the side of the pilot’s seat, and the cyclic pitch lever is at the front of the seat in the middle.  Figure 5-91 25  General 5-55 Control Around the Longitudinal and Lateral Axes  Figure 5-91 26  General 5-54 Control Around the Longitudinal and Lateral Axes  When the collective pitch control lever is raised, the blade angle of all the rotor blades increases uniformly and they create the lift that allows the helicopter to take off vertically.  The grip on the end of the collective pitch control is the throttle for the engine, which is rotated to increase engine power as the lever is raised.  On many helicopters, the throttle automatically rotates and increases engine power as the collective lever is raised. 27  General 5-54 Control Around the Longitudinal and Lateral Axes  The collective pitch lever may have adjustable friction built into it, so the pilot does not have to hold upward pressure on it during flight.  The cyclic pitch control lever, like the yoke of an airplane, can be pulled back or pushed forward, and can be moved left and right.  When the cyclic pitch lever is pushed forward, the rotor blades create more lift as they pass through the back half of their rotation and less lift as they pass through the front half. 28  General 5-55 Control Around the Longitudinal and Lateral Axes  The difference in lift is caused by changing the blade angle (pitch) of the rotor blades.  The pitch change rods that were seen earlier, in Figures 5-82 and 5-83, are controlled by the cyclic pitch lever and they are what change the pitch of the rotor blades.  The increased lift in the back either causes the main rotor to tilt forward, the nose of the helicopter to tilt downward, or both.  The end result is the helicopter moves in the forward direction. 29  General 5-52 Control Around the Longitudinal and Lateral Axes  Figures 5-82 30  General 5-52 Control Around the Longitudinal and Lateral Axes  Figures 5-83 31  General 5-55 Control Around the Longitudinal and Lateral Axes  If the cyclic pitch lever is pulled back, the rotor blade lift will be greater in the front and the helicopter will back up.  If the cyclic pitch lever is moved to the left or the right, the helicopter will bank left or bank right.  For the helicopter to bank to the right, the main rotor blades must create more lift as they pass by the left side of the helicopter.  Just the opposite is true if the helicopter is banking to the left. 32  General 5-55 Control Around the Longitudinal and Lateral Axes  By creating more lift in the back than in the front, and more lift on the left than on the right, the helicopter can be in forward flight and banking to the right.  In Figure 5-92, an Agusta A-109 can be seen in forward flight and banking to the right.  The rotor blade in the rear and the one on the left are both in an upward raised position, meaning they have both experienced the condition called flap. 33  General 5-55 Control Around the Longitudinal and Lateral Axes  Figure 5-92, Agusta A-109 34 Control Around the Longitudinal and Lateral Axes  Agusta A-109 35  General 5-55 Control Around the Longitudinal and Lateral Axes  Some helicopters use a horizontal stabilizer, similar to what is seen on an airplane, to help provide additional stability around the lateral axis.  A horizontal stabilizer can be seen on the Agusta A-109 in Figure 5-92. 36  General 5-56 Hovering  For a helicopter, hovering means that it is in flight at a constant altitude, with no forward, aft, or sideways movement.  In order to hover, a helicopter must be producing enough lift in its main rotor blades to equal the weight of the aircraft.  The engine of the helicopter must be producing enough power to drive the main rotor, and also to drive whatever type of anti-torque system is being used. 37  General 5-56 Hovering – Ground Effect  The ability of a helicopter to hover is affected by many things, including whether or not it is in Ground Effect, the Density Altitude of the air, the available power from the engine, and how heavily loaded it is. 38  General 5-56 Hovering  Fig 5-93, CH-53 is seen in a hover, with all the rotor blades coning up as a result of creating equal lift. 39  General 5-56 Forward Flight  In the early days of helicopter development, the ability to hover was mastered before there was success in attaining forward flight.  The early attempts at forward flight resulted in the helicopter rolling over when it tried to depart from the hover and move in any direction.  The cause of the “rollover” is what we now refer to as Dissymmetry of Lift.  When a helicopter is in a hover, all the rotor blades are experiencing the same velocity of airflow. 40  General 5-56 Forward Flight  When the helicopter starts to move, the velocity of airflow seen by the rotor blades changes.  For helicopters built in the United States, the main rotor blades turn in a counterclockwise direction when viewed from the top.  Viewed from the top, as the blades move around the right side of the helicopter, they are moving toward the nose; as they move around the left side of the helicopter, they are moving toward the tail. 41  General 5-56 Forward Flight  When the helicopter starts moving forward, the blade on the right side is moving toward the relative wind, and the blade on the left side is moving away from the relative wind.  This causes the blade on the right side to create more lift and the blade on the left side to create less lift.  Figure 5-94 shows how this occurs.  In Figure 5-94, blade number 2 would be called the advancing blade, and blade number 1 would be called the retreating blade. 42  General 5-56 Forward Flight  Figure 5-94 43  General 5-56 Forward Flight  The advancing blade is moving toward the relative wind, and therefore experiences a greater velocity of airflow.  The increased lift created by the blade on the right side will try to roll the helicopter to the left.  If this condition is allowed to exist, it will ultimately lead to the helicopter crashing. 44  General 5-56 Blade Flapping Flight  To solve the problem of dissymmetry of lift, helicopter designers came up with a hinged design that allows the rotor blade to flap up when it experiences increased lift, and to flap down when it experiences decreased lift.  When a rotor blade advances toward the front of the helicopter and experiences an increased velocity of airflow, the increase in lift causes the blade to flap up. 45  General 5-56 Blade Flapping Flight  This upward motion of the blade changes the direction of the relative wind in relation to the chord line of the blade, and causes the angle of attack to decrease.  The decrease in the angle of attack decreases the lift on the blade.  The retreating blade experiences a reduced velocity of airflow and reduced lift, and flaps down.  By flapping down, the retreating blade ends up with an increased angle of attack and an increase in lift. 46 Blade Flapping Flight 47  General 5-56 Blade Flapping Flight  The end result is the lift on the blades is equalized, and the tendency for the helicopter to roll never materializes.  The semi-rigid and fully articulated rotor systems have flapping hinges that automatically allow the blades to move up or down with changes in lift.  The rigid type of rotor system has blades that are flexible enough to bend up or down with changes in lift. 48  General 5-57 Blade Flapping Flight  Advancing Blade and Retreating Blade Problems. As a helicopter flies forward at higher and higher speeds, the blade advancing toward the relative wind sees the airflow at an ever increasing velocity.  Eventually, the velocity of the air over the rotor blade will reach sonic velocity, much like the critical Mach number for the wing of an airplane.  When this happens, a shock wave will form and the air will separate from the rotor blade, resulting in a high-speed stall. 49  General 5-57 Blade Flapping Flight  As the helicopter’s forward speed increases, the relative wind over the retreating blade decreases, resulting in a loss of lift.  The loss of lift causes the blade to flap down and the effective angle of attack to increase.  At a high enough forward speed, the angle of attack will increase to a point that the rotor blade stalls.  The tip of the blade stalls first, and then progresses in toward the blade root.  High AOA stall 50  General 5-57 Blade Flapping Flight  When approximately 25 percent of the rotor system is stalled, due to the problems with the advancing and retreating blades, control of the helicopter will be lost.  Conditions that will lead to the rotor blades stalling include high forward speed, heavy gross weight, turbulent air, high-density altitude, and steep or abrupt turns. 51  General 5-57 Autorotation  The engine on a helicopter drives the main rotor system by way of a clutch and a transmission.  The clutch allows the engine to be running and the rotor system not to be turning while the helicopter is on the ground, and it also allows the rotor system to disconnect from the engine while in flight, if the engine fails.  Having the rotor system disconnect from the engine in the event of an engine failure is necessary if the helicopter is to be capable of a flight condition called autorotation. 52  General 5-57 Autorotation  Autorotation is a flight condition where the main rotor blades are driven by the force of the relative wind passing through the blades, rather than by the engine.  This flight condition is similar to an airplane gliding if its engine fails while in flight.  As long as the helicopter maintains forward airspeed, while decreasing altitude, and the pilot lowers the blade angle on the blades with the collective pitch, the rotor blades will continue to rotate. 53  General 5-57 Autorotation  The altitude of the helicopter, which equals potential energy, is given up in order to have enough energy (kinetic energy) to keep the rotor blades turning.  As the helicopter nears the ground, the cyclic pitch control is used to slow the forward speed and to flare the helicopter for landing.  With the airspeed bled off, and the helicopter now close to the ground, the final step is to use the collective pitch control to cushion the landing. 54  General 5-57 Autorotation  From the time of loss of power, you have (in most helicopters) a little under 2 seconds to drop the collective all the way. This completely flattens the blades so that they are producing very little drag and not slowing down. If you do not drop collective in time, the blades will stall to an unrecoverable point where your helicopter turns into a rock. 55  General 5-57 Autorotation  As soon as the collective is dropped, the helicopter will start to descend quickly, this isn’t dangerous in itself, so long as you do something before you hit the ground. The air getting pushed up through the rotor disk as the helicopter falls will actually keep the blades spinning at the appropriate speed (in small helicopters sometimes it will even speed them up a bit.)  Now you are falling a couple hundred feet per minute and you need to quickly find a field or other open area to land in, the bigger and flatter the better. When the helicopter is about 200 feet off of the ground you will gently start to flare back, bringing the nose of the helicopter up. The goal here is to reduce your forward speed to zero by the time you are about 10 or so feet off the ground. You will then level the helicopter, and pull full collective to cushion the landing. The rotor disk will have just enough energy to generate some lift to soften the landing but will quickly slow down and you will land. If done properly, the helicopter can touch down just as smooth as a normal landing. 56  General 5-57 Autorotation  Figure 5-96, a Bell Jet Ranger is shown approaching the ground in the final stage of an autorotation.  Autorotation  More Autorotations 57 AVIA-1035 58  Airframe 2-17 Autogyro  An autogyro is an aircraft with a free-spinning horizontal rotor that turns due to passage of air upward through the rotor.  This air motion is created from forward motion of the aircraft resulting from either a tractor or pusher configured engine/propeller design.  Figure 2-24 59  Airframe 2-17 Autogyro  Figure 2-24  Autogyro - Take off and Landing (2:04) 60  Airframe 2-17 Single Rotor Helicopter  An aircraft with a single horizontal main rotor that provides both lift and direction of travel is a single rotor helicopter.  A secondary rotor mounted vertically on the tail counteracts the rotational force (torque) of the main rotor to correct yaw of the fuselage.  Figure 2-25 61  Airframe 2-17 Single Rotor Helicopter  Figure 2-25 62  Airframe 2-18 Dual Rotor Helicopter  An aircraft with two horizontal rotors that provide both the lift and directional control is a dual rotor helicopter.  The rotors are counter-rotating to balance the aerodynamic torque and eliminate the need for a separate anti-torque system.  Figure 2-26 63  Airframe 2-18 Dual Rotor Helicopter  Figure 2-26 64