HELICOPTER VIBRATION REDUCTION TECHNIQUES

A SEMINAR REPORT

Presented By: Haftamu Abraha

Feb. 2012

Agendas Covered1. 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

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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

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

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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

Agendas ..........(Continued)

3.3.3 Helicopter vibration reduction using semi-active approach

3.3.2 Comparison between active and semi-active concepts

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semi-active concepts

4. CONCLUDING REMARKS

CHAPTER 1

INTRODUCTION

Helicopters play an essential role in today’s aviation with unique abilities

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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.

Despite these attractive abilities, helicopter trips are usually unpleasant for passengers and crew because of high vibration level in the cabin.

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high vibration level in the cabin.

This vibration is also responsible for

degradation in structural integrity as well as

reduction in component fatigue life

decrease the effectiveness of onboardavionics or computer systems thatare critical for aircraft primarycontrol, navigation, and weapon

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control, navigation, and weaponsystems

Consequently, significant efforts havebeen dedicated over the last severaldecades for developing strategies toreduce helicopter vibration

A review the various techniques usedby different helicopter companies tocontrol helicopter vibrations ispresented here

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presented here

1.2 Overview of Helicopter Vibration

Helicopter vibration generally originates from

many sources; for example,

transmission,

engine, and

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engine, and

tail rotor

but most of the vibration comes primarily fromthe main rotor system, even with a perfectlytracked rotor.

Figure 1.1 shows a typical vibration profile of a helicopter, as a function of cruise speeds,

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severe vibration usually occurs in two distinct flight conditions;

low speed transition flight (generally during approach for landing) and

high-speed flight.

the severe vibration level is primarily due to

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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)

Figure 1.2: Blade Vortex Interaction (BVI) schematic

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Figure 1.2: Blade Vortex Interaction (BVI) schematic

In moderate-to-high speed cruise, the BVI-inducedvibration is reduced since vortices are washedfurther downstream from the rotor blades, and thevibration is caused mainly by the unsteadyaerodynamic environment in which the rotor blades

are operating.

The control of vibration is important for four main reasons:

1. To improve crew efficiency, and hence safety ofoperation;

2. To improve comfort of passengers;

3. To improve the reliability of avionics and mechanicalequipments;

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equipments;

4. To improve the fatigue lives of airframe structuralcomponents

Hence it is very important to control vibrationthroughout

the design,

development and

in-service stages of a helicopter project

CHAPTER 3

HELICOPTER VIBRATION REDUCTION METHODS

3.1 Passive Helicopter Vibration Reduction

Most of the passive strategies produce moderatevibration reduction in certain flight conditions, andonly at some locations in the fuselage (such as, pilotonly at some locations in the fuselage (such as, pilotseats or avionics compartments)

The major advantage of the passive concepts is thatthey require no external power to operate

However, they generally involve a significant weightpenalty and are fixed in design, implying no ability toadjust to any possible change in operating conditions(such as changes in rotor RPM or aircraft forwardspeed).

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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

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Tuned-mass vibration absorbers can be employedfor reducing helicopter vibration both in thefuselage and on the rotor system. The absorbersare generally designed using classical spring masssystems tuned to absorb energy at a specificfrequency, for example at N/rev, thus reducingsystem response or vibration at the tunedfrequency ( Fig. 3.1.1).

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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.

For example, a helicopter known as sea king uses its battery vibration absorber

or the mass may be parasitic, as in certainmodels of the Boeing Vertol Chinookhelicopter, where five vibration absorbers

one in the nose,

two under the cockpit floor

and two inside the aft pylon are used

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and two inside the aft pylon are used

Sea King battery vibration absorber Boeing-Vertol CH-47 "Chinook"

A centrifugal pendulum type of absorber mounted on

the 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 3Ω and 5Ωexcitation frequencies for the four-bladed rotor

version,

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version,

3.2. Active Helicopter Vibration Reduction Method

Active vibration reduction concepts have beenintroduced

with the potential to improve vibrationreduction capability and

to overcome the fixed-design drawback of the

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to overcome the fixed-design drawback of thepassive designs

The majority of the active vibration reductionconcepts aim to reduce the vibration in the rotorsystem,

and some active methods intend to attenuate/reducethe vibration only in the fuselage

In general, an active vibration reductionsystem consists of four main components;sensors, actuators, a power supply unit,and a controller (Figure)

Actuators Sensors

ControlledStructure

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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

Controller

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 levels

Also, the controller can be configured to adjust itselffor any possible change in operating conditions usingan adaptive control scheme.

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an 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).

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 vibration

The unsteady aerodynamic loads are introduced by adding higherharmonic pitch input through actuation of the swash plate athigher harmonics

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higher 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 onboardcomputer

Based upon this data, the computer generates, using optimalcontrol techniques, signals which are transmitted to a set ofactuators

Figure 3.2.2 shows diagrammatically the concept of HHC

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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).( 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 tocancel the original vibration), but with adifferent implementation method.

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different implementation method.

Instead of placing the actuators in the non-rotating frame (HHC concept), the IBCapproach uses actuators located in the rotatingframe to provide, for example, blade pitch,active flap, and blade twist inputs for vibrationreduction.

Schemetics of Individual Blade Control (IBC) systems are shown below:

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(a) blade pitch, (b) active flap, and (c) blade twist controls

3.2.3 Active Control of Structural Response (ACSR)

Unlike the HHC and IBC techniques that are intendedto reduce the vibration in the rotor system, ACSRapproach is designed to attenuate the N/rev vibrationin the fuselage, and is one of the most successfulhelicopter vibration reduction methods at the presenthelicopter vibration reduction methods at the presenttime

Vibration sensors are placed at key locations in thefuselage, where minimal vibration is desired (forexample, pilot and passenger seats or avionicscompartments)

Depending on the vibration levels from the sensors, anACSR controller will calculate proper actions foractuators to reduce the vibration.

The calculated outputs will be fed toappropriate actuators, locatedthroughout the airframe, to produce thedesired active forces

Figure 3.2.5 shows the basic concept ofACSR.

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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.

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Commonly used force actuators include

electro-hydraulic

Piezoelectric, and

inertial force actuators

Extensive studies on ACSR system have been

conducted analytically and experimentally.

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)

• sa

Steel downtube

CompositeCompliantElement

TitaniumLug End

ACSR Actuator

Hydraulic Supply

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3.3. Semi-active Vibration Reduction Technology

Semi-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,

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have the ability to adapt to changing conditions,avoiding performance losses seen in passive systemsin “off-design” conditions

In addition, like passive systems, semi-active systemsare considered relatively reliable and fail-safe, andrequire only very small power (compared to activesystems)

Semi-active strategies achieve vibration reduction bymodifying structural properties, stiffness or damping,of semi-active actuators

Semi-active vibration reduction concepts have alreadybeen investigated in several engineering applicationsbut only very recently has there been any focus onusing them to reduce helicopter vibration

Major differences between active and semi-activeconcepts are their actuators and associated

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Major 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:

power requirement of the semi-active approachesis typically smaller than that of the activemethods

B/c active actuators generate direct force toovercome the external loads acting on thesystem, while semi-active actuators only modifythe structural properties of the system

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Comparison Of the three Techniques

1. Passive Techniques

Advantages

Require No external power

DisadvantagesDisadvantages

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 Requirement for external power

3. Semi-active Technique

Advantage

like active-adapt to changing conditions

like passive- small power requirement (compared to active)

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CHAPTER 4: CONCLUDING REMARKS

Figure 4.1 shows a comparison of the vibrationlevels of the Westland W30 helicopter withouta vibration reduction system, and when fittedwith a Flexispring rotor head absorber, and anACSR system

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ACSR system

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