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Helicopter vibration reduction techniques


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Helicopter vibration reduction techniques

  2. 2. Agendas Covered1. INTRODUCTION 1.1 Background and Motivation 1.2 Overview of helicopter vibration 1.3 Objectives2. LITERATURE REVIEW 2.1 Loads acting on a Helicopter in flight3. HELICOPTER VIBRATION REDUCTION METHODS 3.1 Passive helicopter vibration reduction 3.1.1 Blade design optimization 3.1.2 Main Rotor Gearbox Mounting Systems 3.1.3 Dynamic Response of the Fuselage 2
  3. 3. Agendas ......... (Continued)3.2. Active helicopter vibration reduction 3.2.1 Higher harmonic control 3.2.2 Individual blade control 3.2.3 Active Control of Structural Response (ACSR)3.3.Semi-active vibration reduction technology 3.3.1 Overview of semi-active vibration reduction concept 3
  4. 4. Agendas ..........(Continued) 3.3.3 Helicopter vibration reduction using semi-active approach 3.3.2 Comparison between active and semi-active concepts4. CONCLUDING REMARKS 4
  5. 5. CHAPTER 1INTRODUCTION Helicopters play an essential role in today’s aviation with unique abilities to hover and take off/land vertically These capabilities enable helicopters to carry out many distinctive tasks in both civilian and military operations. 5
  6. 6. Despite these attractive abilities,helicopter trips are usually unpleasantfor passengers and crew because ofhigh vibration level in the cabin.This vibration is also responsible for degradation in structural integrity as well as reduction in component fatigue life 6
  7. 7. decrease the effectiveness of onboard avionics or computer systems that are critical for aircraft primary control, navigation, and weapon systems Consequently, significant efforts havebeen dedicated over the last severaldecades for developing strategies toreduce helicopter vibration 7
  8. 8. A review the various techniques usedby different helicopter companies tocontrol helicopter vibrations ispresented here 8
  9. 9. 1.2 Overview of Helicopter Vibration Helicopter vibration generally originates from many sources; for example, transmission, engine, and tail rotor but most of the vibration comes primarily from the main rotor system, even with a perfectly tracked rotor. 9
  10. 10. Figure 1.1 shows a typical vibrationprofile of a helicopter, as a function ofcruise speeds,severe vibration usually occurs in two distinctflight conditions; 10
  11. 11. low speed transition flight (generally during approach for landing) and high-speed flight.the severe vibration level is primarilydue to impulsive loads induced by interactions between rotor blades and strong tip vortices dominating the rotor wake (Fig. 1.2) This condition is usually referred to as Blade Vortex Interaction (BVI) 11
  12. 12. Figure 1.2: Blade Vortex Interaction (BVI) schematic In moderate-to-high speed cruise, the BVI-induced vibration is reduced since vortices are washed further downstream from the rotor blades, and the vibration is caused mainly by the unsteady aerodynamic environment in which the rotor blades are operating. 12
  13. 13. The control of vibration is important for four main reasons:1. To improve crew efficiency, and hence safety of operation;2. To improve comfort of passengers;3. To improve the reliability of avionics and mechanical equipments;4. To improve the fatigue lives of airframe structural components Hence it is very important to control vibration throughout the design, development and in-service stages of a helicopter project 13
  14. 14. CHAPTER 3HELICOPTER VIBRATION REDUCTION METHODS3.1 Passive Helicopter Vibration Reduction Most of the passive strategies produce moderate vibration reduction in certain flight conditions, and only at some locations in the fuselage (such as, pilot seats or avionics compartments) The major advantage of the passive concepts is that they require no external power to operate However, they generally involve a significant weight penalty and are fixed in design, implying no ability to adjust to any possible change in operating conditions (such as changes in rotor RPM or aircraft forward14 speed).
  15. 15. Examples of these passive vibration reductionstrategies include tuned-mass absorbers, isolators, and blade design optimizations. tuned-mass absorbers Tuned-mass vibration absorbers can be employed for reducing helicopter vibration both in the fuselage and on the rotor system. The absorbers are generally designed using classical spring mass systems tuned to absorb energy at a specific frequency, for example at N/rev, thus reducing system response or vibration at the tuned frequency ( Fig. 3.1.1). 15
  16. 16. Figure 3.1.1: Frequency response of a dynamic system with and without an absorber In the fuselage, the absorbers are usually employed toreduce vibration levels at pilot seats or at locations wheresensitive equipment is placed. Without adding mass, an aircraft battery may be usedas the mass in the absorber assembly. 16
  17. 17. For example, a helicopter known as sea king uses its battery vibration absorber or the mass may be parasitic, as in certain models of the Boeing Vertol Chinook helicopter, where five vibration absorbers one in the nose, two under the cockpit floor and two inside the aft pylon are usedSea King battery vibration absorber Boeing-Vertol CH-47 "Chinook"17
  18. 18. A centrifugal pendulum type of absorber mounted onthe rotor blade is another type . This type of absorberhas been used on the Bolkow Bo 105 and Hughes 500helicopters Next Figure shows the Hughes installation whichconsists of absorbers tuned to the 3 and 5excitation frequencies for the four-bladed rotorversion, 18
  19. 19. 3.2. Active Helicopter Vibration Reduction Method Active vibration reduction concepts have been introduced with the potential to improve vibration reduction capability and to overcome the fixed-design drawback of the passive designs The majority of the active vibration reduction concepts aim to reduce the vibration in the rotor system, and some active methods intend to attenuate/reduce the vibration only in the fuselage 19
  20. 20. In general, an active vibration reductionsystem consists of four main components;sensors, actuators, a power supply unit,and a controller (Figure) Actuators Sensors Controlled Structure Controller The principle of operation is: based on the sensor input and a mathematical modelof the system, generates an anti vibration field, thatis, as closely as possible identical to the uncontrolledvibration field but with opposite phase 20
  21. 21. If these two vibration fields (the uncontrolled and theactuator generated) were identical in amplitude andhad exact the opposite phase, then the addition of thetwo fields would lead to complete elimination of thevibrations levelsAlso, the controller can be configured to adjust itselffor any possible change in operating conditions usingan adaptive control scheme.The most commonly examined active vibrationreduction strategies include Higher Harmonic Control (HHC), Individual Blade Control (IBC), and Active Control of Structural Response (ACSR). 21
  22. 22. 3.2.1 Higher Harmonic Control (HHC)The main objective of this concept is to generate higher harmonicunsteady aerodynamic loads on the rotor blades that cancel theoriginal loads responsible for the vibrationThe unsteady aerodynamic loads are introduced by adding higherharmonic pitch input through actuation of the swash plate athigher harmonics The rotor generates oscillatory forces which cause the fuselageto vibrate. Transducers mounted at key locations in the fuselagemeasure the vibration, and this data is analyzed by an onboardcomputerBased upon this data, the computer generates, using optimalcontrol techniques, signals which are transmitted to a set ofactuators 22
  23. 23. Figure 3.2.2 shows diagrammatically the concept of HHC 23
  24. 24. Conventionally, the swash plate is used to providerotor blade collective and first harmonic cyclic pitchinputs (1/rev), which are controlled by the pilot tooperate the aircraft.In addition to the pilot pitch inputs, the HHC systemprovides higher harmonic pitch inputs (for example;3/rev, 4/rev, and 5/rev pitch inputs for a 4-bladedrotor) through hydraulic or electromagnetic actuators,attached to the swash plate in the non-rotating frame( Fig. 3.2.3). 24
  25. 25. 3.2.2 Individual Blade Control (IBC) The main idea of IBC is similar to that of HHC (generating unsteady aerodynamic loads to cancel the original vibration), but with a different implementation method. Instead of placing the actuators in the non- rotating frame (HHC concept), the IBC approach uses actuators located in the rotating frame to provide, for example, blade pitch, active flap, and blade twist inputs for vibration reduction. 25
  26. 26. Schemetics of Individual Blade Control(IBC) systems are shown below: (a) blade pitch, (b) active flap, and (c) blade twist controls 26
  27. 27. 3.2.3 Active Control of Structural Response (ACSR) Unlike the HHC and IBC techniques that are intended to reduce the vibration in the rotor system, ACSR approach is designed to attenuate the N/rev vibration in the fuselage, and is one of the most successful helicopter vibration reduction methods at the present time Vibration sensors are placed at key locations in the fuselage, where minimal vibration is desired (for example, pilot and passenger seats or avionics compartments) Depending on the vibration levels from the sensors, an ACSR controller will calculate proper actions for actuators to reduce the vibration.
  28. 28. The calculated outputs will be fed toappropriate actuators, locatedthroughout the airframe, to produce thedesired active forcesFigure 3.2.5 shows the basic concept ofACSR. 28
  29. 29. The basis of ACSR is that, if a force F is applied to astructure at a point P and an equal and opposite force(the reaction) is applied at a point Q, then the effectwill be to excite all the modes of vibration of thestructure which possess relative motion betweenpoints P and Q This requirement for relative motion in the modalresponse between the points where the actuator forcesare applied is an essential feature of ACSR. Commonly used force actuators include electro-hydraulic Piezoelectric, and inertial force actuators Extensive studies on ACSR system have beenconducted analytically and experimentally. 29
  30. 30. Recently, the ACSR technology has been incorporatedin modern production helicopters such as the WestlandEH101 (Fig. Application of ACSR to the Westland/Augusta Helicopter) Hydraulic Supply Composite Compliant Titanium Element Lug End ACSR Actuator • sa Steel downtube 30
  31. 31. 3.3. Semi-active Vibration Reduction TechnologySemi-active vibration reduction concepts aredeveloped to combine the advantages of both purelyactive as well as purely passive concepts. Like purely active concepts, semi-active conceptshave the ability to adapt to changing conditions,avoiding performance losses seen in passive systemsin “off-design” conditionsIn addition, like passive systems, semi-active systemsare considered relatively reliable and fail-safe, andrequire only very small power (compared to activesystems) 31
  32. 32. Semi-active strategies achieve vibration reduction bymodifying structural properties, stiffness or damping,of semi-active actuatorsSemi-active vibration reduction concepts have alreadybeen investigated in several engineering applicationsbut only very recently has there been any focus onusing them to reduce helicopter vibrationMajor differences between active and semi-activeconcepts are their actuators and associatedcontrollers.Active actuators generally provide direct active force,while semi-active actuators generate indirect semi-active force through property modification.There are several advantages for using the semi-active concepts over the active concepts: 32
  33. 33. power requirement of the semi-active approachesis typically smaller than that of the activemethodsB/c active actuators generate direct force toovercome the external loads acting on thesystem, while semi-active actuators only modifythe structural properties of the system 33
  34. 34. Comparison Of the three Techniques1. Passive Techniques Advantages Require No external powerDisadvantages Significant Weight Penalty Fixed in Design-no ability to adjust to any change in flight condition 34
  35. 35. 2. Active Techniques Advantage Low weight Penalty Disadvantage Requirement for external power3. Semi-active TechniqueAdvantage like active-adapt to changing conditions like passive- small power requirement (compared to active) 35
  36. 36. CHAPTER 4: CONCLUDING REMARKS Figure 4.1 shows a comparison of the vibration levels of the Westland W30 helicopter without a vibration reduction system, and when fitted with a Flexispring rotor head absorber, and an ACSR system 36
  37. 37. 37