HELICOPTER VIBRATION REDUCTION TECHNIQUES
A SEMINAR REPORT Presented By: Haftamu Abraha Feb. 2012
Agendas Covered 1. INTRODUCTION 1.1 Background and Motivation 1.2 Overview of helicopter vibration 1.3 Objectives 2. LITERATURE REVIEW 2.1 Loads acting on a Helicopter in flight 3. 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
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
Agendas ..........(Continued) 3.3.3 Helicopter vibration reduction using semi-active approach 3.3.2 Comparison between active and semi-active concepts 4. CONCLUDING REMARKS
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CHAPTER 1
INTRODUCTION
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
Despite these attractive abilities, helicopter trips are usually unpleasant for passengers and crew because of high vibration level in the cabin. This vibration is also responsible for
degradation in structural integrity as well as reduction in component fatigue life 6
decrease the effectiveness of onboard avionics or computer systems that are critical for aircraft primary control, navigation, and weapon systems
Consequently, significant efforts have been dedicated over the last several decades for developing strategies to reduce helicopter vibration 7
A review the various techniques used by different helicopter companies to control helicopter vibrations is presented here
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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
Figure 1.1 shows a typical vibration profile of a helicopter, as a function of cruise speeds,
severe vibration usually occurs in two distinct flight conditions; 10
low speed transition flight (generally during approach for landing) and high-speed flight.
the severe vibration level is primarily due 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
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
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
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CHAPTER 3 HELICOPTER VIBRATION REDUCTION METHODS 3.1 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 forward 14 speed).
Examples of these passive vibration reduction strategies 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
Figure 3.1.1: Frequency response of a dynamic system with and without an absorber
In the fuselage, the absorbers are usually employed to reduce vibration levels at pilot seats or at locations where sensitive equipment is placed. Without adding mass, an aircraft battery may be used as the mass in the absorber assembly. 16
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 used
Sea King battery vibration absorber
Boeing-Vertol CH-47 "Chinook"17
A centrifugal pendulum type of absorber mounted on the rotor blade is another type . This type of absorber has been used on the Bolkow Bo 105 and Hughes 500 helicopters Next Figure shows the Hughes installation which consists of absorbers tuned to the 3Ω and 5Ω excitation frequencies for the four-bladed rotor version,
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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
In general, an active vibration reduction system 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 model of the system, generates an anti vibration field, that is, as closely as possible identical to the uncontrolled vibration field but with opposite phase 20
If these two vibration fields (the uncontrolled and the actuator generated) were identical in amplitude and had exact the opposite phase, then the addition of the two fields would lead to complete elimination of the vibrations levels Also, the controller can be configured to adjust itself for any possible change in operating conditions using an adaptive control scheme. The most commonly examined active vibration reduction strategies include Higher Harmonic Control (HHC), Individual Blade Control (IBC), and Active Control of Structural Response (ACSR). 21
3.2.1 Higher Harmonic Control (HHC)
The main objective of this concept is to generate higher harmonic unsteady aerodynamic loads on the rotor blades that cancel the original loads responsible for the vibration The unsteady aerodynamic loads are introduced by adding higher harmonic pitch input through actuation of the swash plate at higher harmonics The rotor generates oscillatory forces which cause the fuselage to vibrate. Transducers mounted at key locations in the fuselage measure the vibration, and this data is analyzed by an onboard computer Based upon this data, the computer generates, using optimal control techniques, signals which are transmitted to a set of actuators
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Figure 3.2.2 shows diagrammatically the concept of HHC
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Conventionally, the swash plate is used to provide rotor blade collective and first harmonic cyclic pitch inputs (1/rev), which are controlled by the pilot to operate the aircraft. In addition to the pilot pitch inputs, the HHC system provides higher harmonic pitch inputs (for example; 3/rev, 4/rev, and 5/rev pitch inputs for a 4-bladed rotor) through hydraulic or electromagnetic actuators, attached to the swash plate in the non-rotating frame ( Fig. 3.2.3).
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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 nonrotating 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
Schemetics of Individual Blade Control (IBC) systems are shown below:
(a) blade pitch, (b) active flap, and (c) blade twist controls 26
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.
The calculated outputs will be fed to appropriate actuators, located throughout the airframe, to produce the desired active forces Figure 3.2.5 shows the basic concept of ACSR.
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The basis of ACSR is that, if a force F is applied to a structure at a point P and an equal and opposite force (the reaction) is applied at a point Q, then the effect will be to excite all the modes of vibration of the structure which possess relative motion between points P and Q This requirement for relative motion in the modal response between the points where the actuator forces are 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 conducted analytically and experimentally.
been
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Recently, the ACSR technology has been incorporated in modern production helicopters such as the Westland EH101 (Fig. Application of ACSR to the Westland/Augusta Helicopter) Hydraulic Supply Composite Compliant Element
Titanium Lug End • sa
ACSR Actuator
Steel downtube
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3.3. Semi-active Vibration Reduction Technology Semi-active vibration reduction concepts are developed to combine the advantages of both purely active as well as purely passive concepts. Like purely active concepts, semi-active concepts have the ability to adapt to changing conditions, avoiding performance losses seen in passive systems in “off-design” conditions In addition, like passive systems, semi-active systems are considered relatively reliable and fail-safe, and require only very small power (compared to active systems) 31
Semi-active strategies achieve vibration reduction by modifying structural properties, stiffness or damping, of semi-active actuators Semi-active vibration reduction concepts have already been investigated in several engineering applications but only very recently has there been any focus on using them to reduce helicopter vibration Major differences between active and semi-active concepts are their actuators and associated controllers. Active actuators generally provide direct active force, while semi-active actuators generate indirect semiactive force through property modification. There are several advantages for using the semiactive concepts over the active concepts: 32
power requirement of the semi-active approaches is typically smaller than that of the active methods B/c active actuators generate direct force to overcome the external loads acting on the system, while semi-active actuators only modify the structural properties of the system
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Comparison Of the three Techniques 1. Passive Techniques Advantages Require No external power
Disadvantages Significant Weight Penalty Fixed in Design-no ability to adjust to any change in flight condition
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2. Active Techniques Advantage Low weight Penalty Disadvantage Requirement for external power 3. Semi-active Technique Advantage like active-adapt to changing conditions like passive- small power requirement (compared to active) 35
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
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