BSS AEROdynamics lecture including helicopter

Lecture is for Aircraft Maintenance Engineering Students in AERO DYNAMICS

1. PARMANAND SHARMA SMRITI MAHAVIDYALAYA, JODHPUR (Affiliated to Jai Narain Vyas University, Jodhpur) Estd . 2022 AERODYNAMICS: ROTARY WING AERODYNAMICS Prof.(Dr.) Bhagwat Singh Shishodia

2. 2 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Unlike fixed-wing aircraft, the helicopter’s main  airfoil  is the rotating blade assembly ( rotor ) mounted atop its  fuselage  on a hinged shaft (mast) connected with the vehicle’s engine and  flight  controls. In comparison to airplanes, the tail of a helicopter is somewhat elongated and the  rudder  smaller; the tail is fitted with a small antitorque rotor (tail rotor) . The landing gear sometimes consists of a pair of skids rather than wheel assemblies The fact that the helicopter obtains its lifting power by means of a rotating airfoil (the rotor) greatly complicates the factors affecting its flight, for not only does the rotor turn but it also moves up and down in a flapping motion and is affected by the horizontal or vertical movement of the helicopter itself. Unlike the usual aircraft airfoils, helicopter rotor airfoils are usually symmetrical . The chord line of a rotor, like the chord line of a  wing , is an imaginary line drawn from the leading edge to the trailing edge of the airfoil.

3. 3 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Structures The structures of the helicopter are designed to give the helicopter its unique flight characteristics. A simplified explanation of how a helicopter flies is that the rotors are rotating airfoils that provide lift similar to the way wings provide lift on a fixed-wing aircraft. Air flows faster over the curved upper surface of the rotors, causing a negative pressure and thus, lifting the aircraft. Changing the angle of attack of the rotating blades increases or decreases lift, respectively raising or lowering the helicopter. Tilting the rotor plane of rotation causes the aircraft to move horizontally. Figure 1 shows the major components of a typical helicopter

4. 4 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Forces Acting on Helicopter • Lift— opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil and acts perpendicular to the flightpath through the center of lift. • Weight— the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity. It opposes lift and acts vertically downward through the aircraft’s center of gravity (CG). • Thrust— the force produced by the power plant/ propeller or rotor. It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis. However, this is not always the case, as explained later. • Drag— a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.

5. 5 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Forces Acting on Helicopter

6. 6 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls Most helicopters the engine turns a shaft that connects to an input quill on the transmission; the main rotor mast comes straight out of the top of the transmission and the tailrotor driveshaft connects to an output quill 90 degrees out from the mast. Spinning the rotor which has an aerofoil section causes lift, allowing the helicopter to rise vertically or hover. There are many terms associated with rotary wing flight and it is important for a student to become familiar with them to understand the mechanics of rotary wing flight.

7. 7 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Control Function A helicopter has four controls:  collective pitch control, throttle control, antitorque control, and cyclic pitch control. 1) COLLECTIVE  PITCH CONTROL is usually found at the pilot’s left hand; it is a lever that moves up and down to change the pitch angle of the main rotor blades. Raising or lowering the pitch control increases or decreases the pitch angle on all blades by the same amount. An increase in the pitch angle will increase the angle of attack, causing both lift and drag to increase and causing the rpm of the rotor and the engine to decrease. The reverse happens with a decrease in pitch angle. Because it is necessary to keep rotor rpm as constant as possible, the collective pitch control is linked to the throttle to automatically increase power when the pitch lever is raised and decrease it when the pitch lever is lowered. The collective pitch control thus acts as the primary control both for altitude and for power. The collective changes the pitch of all of the rotor blades simultaneously and by the same amount, thereby increasing or decreasing lift

8. 8 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Control Function 2 ) THE  THROTTLE  CONTROL is used in conjunction with the collective pitch control and is an  integral  part of its assembly. The throttle control is twisted outboard to increase rotor rpm and inboard to decrease rpm. The engine throttle control is located on the hand grip at the end of the collective.  3) THE CYCLIC PITCH CONTROL , The cyclic is the control “stick” located between the pilot’s legs. It can be moved in any direction to tilt the plane of rotation of the rotor blades. This causes the helicopter to move in the direction that the cyclic is moved. As stated, the foot pedals control the pitch of the tail rotor blades thereby balancing main rotor torque. The cyclic changes the angle of the swash plate which changes the plane of rotation of the rotor blades. This moves the aircraft horizontally in any direction depending on the positioning of the cyclic

9. 9 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Control Function 4 ) THE ANTITORQUE CONTROLS are pedals linked to operate a pitch change mechanism in the tail rotor gearbox. A change in pedal position changes the pitch angle of the tail rotor to offset torque. As torque varies with every change of  flight  condition, the pilot is required to change pedal position accordingly. The antitorque control does not control the direction of flight. The large rotating mass of the main rotor blades of a helicopter produce torque. This torque increases with engine power and tries to spin the fuselage in the opposite direction. The tail boom and tail rotor, or antitorque rotor, counteract this torque effect. Controlled with foot pedals, the countertorque of the tail rotor must be modulated as engine power levels are changed. This is done by changing the pitch of the tail rotor blades. This, in turn, changes the amount of countertorque , and the aircraft can be rotated about its vertical axis, allowing the pilot to control the direction the helicopter is facing.

10. 10 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Similar to a vertical stabilizer on the empennage of an airplane, a fin or pylon is also a common feature on rotorcraft . Normally, it supports the tail rotor assembly, although some tail rotors are mounted on the tail cone of the boom. Additionally, a horizontal member called a stabilizer is often constructed at the tail cone or on the pylon. A Fenestron is a unique tail rotor design which is actually a multiblade ducted fan mounted in the vertical pylon. It works the same way as an ordinary tail rotor, providing sideways thrust to counter the torque produced by the main rotors. [Figure 9] A Fenestron or “fan-in-tail” antitorque system. This design provides an improved margin of safety during ground operations

11. 11 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing NOTAR  ("no tail rotor") is a  helicopter  system which avoids the use of a  tail rotor . The system uses a fan inside the tail boom to build a high volume of low-pressure air, which exits through two slots and creates a  boundary layer  flow of air along the tailboom utilizing the  Coandă effect . The boundary layer changes the direction of airflow around the tailboom , creating thrust opposite the motion imparted to the  fuselage  by the torque effect of the main rotor. Directional yaw control is gained through a vented, rotating drum at the end of the tailboom , called the direct jet thruster. Advocates of NOTAR assert that the system offers quieter and safer operation than a traditional tail rotor. 1 Air intake 2 Variable pitch fan 3 Tail boom with Coandă Slots 4 Vertical stabilizers 5 Direct jet thruster 6 Downwash 7 Circulation control tailboom cross-section 8 Anti-torque lift

12. 12 PARMANAND SHARMA SMRITI MAHAVIDYALAYA MAIN ROTOR SYSTEM Root: The inner end of the blade where the rotors connect to the blade grips. Blade Grips: Large attaching points where the rotor blade connects to the hub. Hub: Sits atop the mast, and connects the rotor blades to the control tubes. Mast: Rotating shaft from the transmission, which connects the rotor blades to the helicopt Control Tubes: Push \ Pull tubes that change the pitch of the rotor blades. Pitch Change Horn: The armature that converts control tube movement to blade pitch Pitch: Increased or decreased angle of the rotor blades to raise, lower, or change the direction of the rotors thrust force. Jesus Nut: Is the singular nut that holds the hub onto the mast. (If it fails, the next person you see will be Jesus).

13. 13 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The S wash P late assembly has two primary roles:        Under the direction of the  collective  control, the swash plate assembly can change the angle of both blades simultaneously. Doing this increases or decreases the lift that the main rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude.        Under the direction of the  cyclic  control, the swash plate assembly can change the angle of the blades individually as they revolve. This allows the helicopter to move in any direction around a 360-degree circle, including forward, backward, left and right . .

14. 14 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The swash plate assembly consists of two plates -- the  fixed  and the  rotating  swash plates -- shown above in blue and red, respectively.        The  rotating swash plate  rotates with the drive shaft (green) and the rotor's blades (grey) because of the links (purple) that connect the rotating plate to the drive shaft.        The  pitch control rods  (orange) allow the rotating swash plate to change the pitch of the rotor blades.        The angle of the  fixed swash plate  is changed by the control rods (yellow) attached to the fixed swash plate.        The fixed plate's control rods are affected by the pilot's input to the  cyclic and collective controls .        The fixed and rotating swash plates are connected with a set of  bearings  between the two plates. These bearings allow the rotating swash plate to spin on top of the fixed swash plate v . .

15. 15 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The cyclic is a control stick that tilts the rotor disk in any direction — forward, backward (or aft), left, or right. The helicopter moves in the direction that the pilot pushes the cyclic. The pilot holds the cyclic in his or her right hand. The cyclic is very sensitive and the pilot usually cannot let go of it during flight. In every helicopter the cyclic is operated with the right hand A helicopter has three flight control inputs. The cyclic stick, or just cyclic, is used during hovering to direct the helicopter to the left, right, front or back. During forward flight the cyclic is used to initiate turns or up and down pitches. With the pedals the helicopter can be turned along the yaw axis while hovering and with the collective the pilot controls ascent and descent during hovering and, together with the cyclic, the speed during forward flight (Fig 1).

16. 16 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The cyclic is a control stick that tilts the rotor disk in any direction — forward, backward (or aft), left, or right. The helicopter moves in the direction that the pilot pushes the cyclic. The pilot holds the cyclic in his or her right hand. The cyclic is very sensitive and the pilot usually cannot let go of it during flight. In every helicopter the cyclic is operated with the right hand. The left and right, forward and aft control. It puts in one control input into the rotor system at a time through the swash plate. It is also known as the "Stick". It comes out of the centre of the floor of the cockpit, and sits between the pilots legs. It is operated by the pilots right hand. The cyclic changes the angle of attack of the main rotor's wings unevenly by tilting the swash plate assembly. On one side of the helicopter, the angle of attack (and therefore the lift) is greater .  

17. 17 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The collective is a control stick that changes the angle of the main rotor blades. Pulling the collective up increases the angle of attack, which increases lift and moves the helicopter up . The collective is beside the pilot’s seat and the pilot holds it in his or her left hand . The up and down control. It puts a collective control input into the rotor system, meaning that it puts either "all up", or "all down" control inputs in at one time through the swash plate. It is operated by the stick on the left side of the seat, called the collective pitch control. It is operated by the pilots left hand. The collective lets you change the angle of attack of the main rotor simultaneously on both blades Collective

18. 18 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The throttle controls the amount of power that the engine produces . It is a twistable handle that is on the end of the collective , where the pilot holds it. As the pilot pulls up the collective, he or she usually has to increase the throttle . In many helicopters, this is done automatically for the pilot, but the pilot can adjust it if he or she needs to. The antitorque pedals control the amount of thrust the tail rotor produces . If there was no tail rotor, a helicopter with only one main rotor would spin in the direction opposite the main rotor blades. The tail rotor prevents this from happening by pushing the helicopter’s tail . By using the pedals, the pilot can control the rotation of the helicopter to the left or right. The pedals are on the floor and the pilot controls them with his or her feet . These are not rudder pedals, although they are in the same place as rudder pedals on an airplane. A single rotor helicopter has no real rudder.

19. 19 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls The Tail Rotor The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction. This is called  TORQUE REACTION The tail rotor is used like a small propeller, to pull against torque reaction and hold the helicopter straight. By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor through a gearbox. When using the tail rotor trying to compensate the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate. The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon.

20. 20 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls: Dissymmetry of LIFT One cannot begin to talk about the mechanics of helicopters until the problems associated with rotary wing aerodynamics are understood. When the first rotary wing pioneers started trying to make a helicopter fly, they noticed a strange problem. The helicopters rotor system would generally work just fine until one of two things happened: Either the aircraft began to move in any given direction, or it experienced any sort of wind introduced into the main rotor system. Upon either of these events, the rotor system would become unstable, and the resultant crash would usually take the life of the brave soul at the controls. The question then was; Why does this happen? The answer is what we refer to today as "Dissymmetry of lift".

21. 21 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls: Dissymmetry of LIFT What "Dis-Symmetry of lift" means is, when the rotor system is experiencing the same conditions all around the perimeter of the rotors arc, all things are equal, and the system is in balance. Once the system experiences a differential in wind speed from any angle, it becomes unbalanced, and begins to rotate. Take for instance forward flight. Imagine a two bladed rotor system spinning at 100 MPH. Once the aircraft moves forward, it begins to change this balance. If we travel 10 MPH forward, then the forward moving, or advancing rotor blade, is experiencing 110 MPH of wind speed, and the rearward, or retreating blade, is experiencing only 90 MPH of wind speed. When this happens, we get an unbalanced condition, and the advancing blade experiencing more lift wants to climb, while the retreating blade experiences less lift and wants to drop. This is where we get the term "Dis-Symmetry of lift". The lift is not symmetrical around the entire rotor system.

22. 22 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls: Dissymmetry of LIFT How do we compensate for this situation? We compensate by allowing the rotor to flap. By allowing the advancing blade to flap upward, and the retreating blade to flap downward, it changes the angle of incidence on both rotor blades which balances out the entire rotor system. As you can see in this simple graphic, there are a few ways to allow for blade flapping. One is to allow the blades to flap on hinges (Articulated rotor system). Another way is to have the whole hub swing up and down around an internal bearing called a trunion (Semi-rigid rotor system). Unfortunately, we can not compensate completely for dis-symmetry of lift by using blade flapping. Once the aircraft gets to a certain airspeed, and the rotor had flapped as much as it possibly can, then "Retreating blade stall" may be experienced. In retreating blade stall, the retreating blade can no longer compensate for dis-symmetry of lift, and the outer portions of the blade will "Stall". This situation, when not immediately recognized can cause a severe loss of aircraft controllability. Most have resigned themselves to slower airspeeds for their aircraft, at a lower cost and less maintenance.

23. 23 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter Controls: Dissymmetry of LIFT

24. 24 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter A erodynamic Terminology Rotor : The rotating component of a helicopter that provides lift. Blade : The individual wing-like sections of the rotor. Main Rotor : The primary rotor responsible for lifting the helicopter off the ground. Tail Rotor : A smaller rotor mounted at the tail of the helicopter, which provides anti-torque to counteract the main rotor's torque. Swashplate : A mechanism that changes the pitch of the rotor blades, allowing for control of the helicopter's movement. Collective Pitch : The angle of attack of all rotor blades collectively, controlled by the pilot to control the helicopter's vertical movement. Cyclic Pitch : The varying angle of attack of the rotor blades as they rotate, controlled by the pilot to control the helicopter's lateral and longitudinal movements. Autorotation : A state where a helicopter descends with little or no engine power, and the rotor is being driven by the upward flow of air through it. Pitch : The tilt of the rotor blades relative to their rotational plane. Yaw : The rotation of the helicopter around its vertical axis. Roll : The rotation of the helicopter around its longitudinal axis. Pitch Axis, Yaw Axis, Roll Axis : The three axes of rotation that define the movement of an aircraft. Angle of Attack : The angle between the chord line of an airfoil and the oncoming air. Dissymmetry of Lift : The unequal lift distribution on the advancing and retreating sides of the rotor disk, addressed by blade flapping and feathering. Ground Effect : The increase in lift and efficiency a helicopter experiences when it is close to the ground.

25. 25 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Helicopter A erodynamic Terminology Retreating Blade Stall : A condition where the retreating blade of a rotor disk reaches too high of an angle of attack, leading to a loss of lift. Forward Flight : The helicopter's movement in the forward direction. Autorotation : A maneuver in which a helicopter descends without engine power, utilizing upward airflow through the rotor to maintain rotation. Translational Lift : The increased lift experienced by a helicopter as it begins to move forward. Helicopter Fuselage : The main body of the helicopter, housing the cockpit, passengers, and cargo. Rotor Head : The assembly that connects the rotor blades to the helicopter and allows them to move. Feathering : The ability to change the pitch angle of a rotor blade to reduce drag. Flapping : The up-and-down movement of rotor blades as they rotate. Pylon : A structure that connects the main rotor to the helicopter's fuselage. Retreating Blade : The rotor blade that is moving in the opposite direction of the helicopter's forward flight. Advancing Blade : The rotor blade that is moving in the same direction as the helicopter's forward flight. Inboard Section : The part of the rotor blade closer to the rotor hub. Outboard Section : The part of the rotor blade farther from the rotor hub. Centrifugal Force : The outward force experienced by objects rotating in a circular path. Retreating Blade Stall: A condition where the retreating blade reaches too high of an angle of attack, leading to a loss of lift.

26. 26 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Airframe The airframe, or fundamental structure, of a helicopter can be made of either metal or wood composite materials, or some combination of the two. Typically, a composite component consists of many layers of fiber-impregnated resins, bonded to form a smooth panel. Tubular and sheet metal substructures are usually made of aluminum, though stainless steel or titanium are sometimes used in areas subject to higher stress or heat. Airframe design encompasses engineering,  aerodynamics , materials technology, and manufacturing methods to achieve favorable balances of performance, reliability, and cost. Fuselage As with fixed-wing aircraft, helicopter fuselages and tail booms are often truss-type or semimonocoque structures of stress-skin desig n. Steel and aluminum tubing, formed aluminum, and aluminum skin are commonly used. Modern helicopter fuselage design includes an increasing utilization of advanced composites as well. Firewalls and engine decks are usually stainless steel . Helicopter fuselages vary widely from those with a truss frame, two seats, no doors, and a monocoque shell flight compartment to those with fully enclosed airplane-style cabins as found on larger twin-engine helicopters. The multidirectional nature of helicopter flight makes wide-range visibility from the cockpit essential. Large, formed polycarbonate, glass, or plexiglass windscreens are common .

27. 27 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Landing Gear or Skids As mentioned, a helicopter’s landing gear can be simply a set of tubular metal skids. Many helicopters do have landing gear with wheels, some retractable. Powerplant and Transmission The two most common types of engine used in helicopters are the  reciprocating engine  and the  turbine engine . Reciprocating engines, also called piston engines, are generally used in smaller helicopters. Most training helicopters use reciprocating engines because they are relatively simple and inexpensive to operate.

28. 28 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Turbine Engines Turbine engines are more powerful and are used in a wide variety of helicopters. They produce a tremendous amount of power for their size but are generally more expensive to operate. The turbine engine used in helicopters operates differently than those used in airplane applications . In most applications, the exhaust outlets simply release expended gases and do not contribute to the forward motion of the helicopter. Because the airflow is not a straight line pass through as in jet engines and is not used for propulsion , the cooling effect of the air is limited. Approximately 75 percent of the incoming airflow is used to cool the engine. Power is provided to the main rotor and tail rotor systems through the freewheeling unit which is attached to the accessory gearbox power output gear shaft. 

29. 29 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Transmission The  transmission system  transfers power from the engine to the main rotor, tail rotor, and other accessories during normal  flight conditions . The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. The freewheeling unit, or autorotative clutch, allows the main rotor transmission to drive the tail rotor drive shaft during autorotation. Helicopter transmissions are normally lubricated and cooled with their own oil supply. A sight gauge is provided to check the oil level. Some transmissions have chip detectors located in the sump. These detectors are wired to warning lights located on the pilot’s instrument panel that illuminate in the event of an internal problem. Some chip detectors on modern helicopters have a “burn off” capability and attempt to correct the situation without pilot action. If the problem cannot be corrected on its own, the pilot must refer to the emergency procedures for that particular helicopter.

30. 30 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Rotary System Main Rotor System The rotor system is the rotating part of a helicopter which generates lift. The rotor consists of a mast, hub, and rotor blades. The mast is a cylindrical metal shaft that extends upwards from and is driven, and sometimes supported, by the transmission. At the top of the mast is the attachment point for the rotor blades called the hub . The rotor blades are then attached to the hub by any number of different methods. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. There are three basic classifications: rigid, semirigid, or fully articulated.

31. 31 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Rigid Rotor System The simplest is the rigid rotor system. In this system, the rotor blades are rigidly attached to the main rotor hub and are not free to slide back and forth (drag) or move up and down (flap). The forces tending to make the rotor blades do so are absorbed by the flexible properties of the blade. The pitch of the blades, however, can be adjusted by rotation about the spanwise axis via the feathering hinges.

32. 32 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Semirigid Rotor System The semirigid rotor system in Figure Below makes use of a teetering hinge at the blade attach point. While held in check from sliding back and forth, the teetering hinge does allow the blades to flap up and down. With this hinge, when one blade flaps up, the other flaps down

33. 33 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Fully Articulated Rotor System Fully articulated rotor blade systems provide hinges that allow the rotors to move fore and aft, as well as up and down . This lead-lag, drag, or hunting movement as it is called is in response to the Coriolis effect during rotational speed changes . When first starting to spin, the blades lag until centrifugal force is fully developed. Once rotating, a reduction in speed causes the blades to lead the main rotor hub until forces come into balance. Constant fluctuations in rotor blade speeds cause the blades to “hunt.” They are free to do so in a fully articulating system due to being mounted on the vertical drag hinge. One or more horizontal hinges provide for flapping on a fully articulated rotor system . Also, the feathering hinge allows blade pitch changes by permitting rotation about the spanwise axis. Various dampers and stops can be found on different designs to reduce shock and limit travel in certain directions .

34. 34 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Fully Articulated Rotor System Flapping is caused by a phenomenon known as dissymmetry of lift. As the plane of rotation of the rotor blades is tilted and the helicopter begins to move forward, an advancing blade and a retreating blade become established (on two-bladed systems) . The relative windspeed is greater on an advancing blade than it is on a retreating blade. This causes greater lift to be developed on the advancing blade, causing it to rise up or flap. When blade rotation reaches the point where the blade becomes the retreating blade, the extra lift is lost and the blade flaps downward. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100 knots, the relative windspeed on the advancing side is 400 knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymetry of lift

35. 35 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing The relative wind is the direction of the wind in relation to the airfoil. In an  airplane , the flight path of the wing is fixed in relation to its forward flight; in a helicopter, the flight path of the rotor advances forward (to the helicopter’s nose) and then rearward (to the helicopter’s tail) in the process of its circular movement. Relative wind is always considered to be in parallel and opposite direction to the flight path. In considering helicopter flight, the relative wind can be affected by the rotation of the blades, the horizontal movement of the helicopter, the flapping of the rotor blades, and wind speed and direction. In flight, the relative wind is a combination of the rotation of the rotor blade and the movement of the helicopter. Like a  propeller , the rotor has a  pitch angle , which is the angle between the horizontal plane of rotation of the rotor disc and the chord line of the airfoil. The pilot uses the  collective  and cyclic pitch control (see below) to vary this pitch angle. In a fixed-wing aircraft, the  angle of attack   ( the angle of the wing in relation to the relative wind) is important in determining  lift . The same is true in a helicopter, where the angle of attack is the angle at which the relative wind meets the chord line of the rotor blade.

36. 36 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Angle of attack and pitch angle are two distinct conditions. Varying the pitch angle of a rotor blade changes its angle of attack and hence its lift. A higher pitch angle (up to the point of stall) will increase lift; a lower pitch angle will decrease it. Individual blades of a rotor have their pitch angles adjusted individually. Rotor speed also controls lift—the higher the revolutions per minute (rpm), the higher the lift. However, the pilot will generally attempt to maintain a constant rotor rpm and will change the lift force by varying the angle of attack. As with fixed-wing aircraft, air  density  (the result of air temperature, humidity, and pressure) affects helicopter performance. The higher the density, the more lift will be generated; the lower the density, the less lift will be generated. Just as in fixed-wing aircraft, a change in lift also results in a change in  drag . When lift is increased by enlarging the angle of pitch and thus the angle of attack, drag will increase and slow down the rotor rpm. Additional power will then be required to sustain a desired rpm. Thus, while a helicopter is affected like a conventional aircraft by the forces of lift,  thrust , weight, and drag , its mode of flight  induces  additional effects .

37. 37 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Basic Operations In a helicopter, the total lift and thrust forces generated by the rotor are exerted perpendicular to its plane of rotation . When a helicopter hovers in a windless condition, the plane of rotation of the rotor (the tip-path plane) is parallel to the ground, and the sum of the  weight  and drag forces are exactly balanced by the sum of the thrust and lift forces. In vertical flight, the components of weight and drag are  combined  in a single vector that is directed straight down; the components of lift and thrust are combined in a single vector that is directed straight up. To achieve forward flight in a helicopter, the plane of rotation of the rotor is tipped forward. (It should be understood that the helicopter’s rotor mast does not tip but rather the individual rotor blades within the plane of rotation have their pitch angle varied.) For sideward flight, the plane of the rotation of the rotor is tilted in the direction desired. For rearward flight, the plane of the rotation of the rotor is tilted rearward. Because the rotor is powered, there is an equal and opposite  torque  reaction, which tends to rotate the fuselage in a direction opposite to the rotor. This torque is  offset  by the tail rotor (antitorque rotor) located at the end of the fuselage. The pilot controls the thrust of the tail rotor by means of foot pedals, neutralizing torque as required.

38. 38 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing

39. 39 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing There are other forces acting upon a helicopter not found in a conventional aircraft. These include the  gyroscopic  precession effect of the rotor — that is, the dissymmetry of lift created by the forward movement of the helicopter, resulting in the advancing blade having more lift and the retreating blade less. This occurs because the advancing blade has a combined speed of the blade velocity and the speed of the helicopter in forward flight, while the retreating blade has the difference between the blade velocity and the speed of the helicopter. This difference in speed causes a difference in lift—the advancing blade is moving faster and hence is generating more lift. If uncontrolled, this would result in the helicopter rolling . However, the difference in lift is  compensated  for by the 1) blade flapping and by 2) cyclic feathering (changing the angle of pitch).

40. 40 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing Flapping is caused by a phenomenon known as dissymmetry of lift. As the plane of rotation of the rotor blades is tilted and the helicopter begins to move forward, an advancing blade and a retreating blade become established (on two-bladed systems). The relative windspeed is greater on an advancing blade than it is on a retreating blade. This causes greater lift to be developed on the advancing blade, causing it to rise up or flap. When blade rotation reaches the point where the blade becomes the retreating blade, the extra lift is lost and the blade flaps downward. Because the blades are attached to a rotor hub by horizontal flapping hinges, which permit their movement in a vertical plane, the advancing blade flaps up, decreasing its angle of attack, while the retreating blade flaps down, increasing its angle of attack. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100 knots, the relative windspeed on the advancing side is 400 knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymetry of lift

41. 41 PARMANAND SHARMA SMRITI MAHAVIDYALAYA Elementary rotary wing This combination of effects equalizes the lift. (Blades also are attached to the hub by a vertical hinge, which permits each blade to move back and forth in the plane of rotation. The vertical hinge dampens out vibration and absorbs the effect of acceleration or deceleration.) In addition, in forward flight, the position of the cyclic pitch control causes a similar effect, contributing to the equalization of lift. Other forces acting upon helicopters include C oning , the upward bending effect on blades caused by centrifugal force; Coriolis effect , the acceleration or deceleration of the blades caused by the flapping movement bringing them closer to (acceleration) or farther away from (deceleration) the  axis  of rotation; and Drift , the tendency of the tail rotor thrust to move the helicopter in hover.

42. 42 PARMANAND SHARMA SMRITI MAHAVIDYALAYA THANK YOU