A

Seminar ReportOn

HELICOPTER VIBRATION REDUCTION TECHNIQUES

By

DINU M R

DEPARTMENT OF MECHANICAL ENGINEERING

VALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING &TECHNOLOGY

PARIPPALLY,TRIVANDRUM- 691574[2012 – 2013]

A

Seminar ReportOn

HELICOPTER VIBRATION REDUCTION TECHNIQUES

In partial fulfillment of requirements for the degree of

Bachelor of Technology

In

Mechanical Engineering

SUBMITTED BY:

DINU MR

Under the Guidance of

Shyn CS

DEPARTMENT OF MECHANICAL ENGINEERINGVALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING &

TECHNOLOGYPARIPPALLY, TRIVANDRUM- 691574

[2012 – 2013]

CERTIFICATE

This is to certify that the Seminar entitled “HELICOPTER VIBRATION

REDUCTION TECHNIQUES” has been submitted by DINU M R under my

guidance in partial fulfillment of the degree of Bachelor of Technology in

Mechanical Engineering of Kerala University, Trivandrum during the academic

year 2012-2013 (Semester-VII).

Date:

Place:

Guide Head, Mechanical Department

SHYN CS SREERAJ PS

ACKNOWLEDGEMENT

Apart from the efforts of me, the success of this seminar depends largely on the encouragement

and guidelines of many others. I take this opportunity to express my gratitude to the people who

have been instrumental in the successful completion of this seminar.

I am extremely grateful to Prof. SREERAJ PS, HOD, Department of Mechanical Engineering, for

the guidance and encouragement and for providing me with best facilities and atmosphere for

the creative work.

I would like to thank my seminar guide, Mr. SHYN CS, Associate Professor, Department of

Mechanical Engineering, for the valuable guidance, care and timely support throughout the

seminar work. He has always a constant source of encouragement.

I thank all the staff members of our department for extending their cooperation during my

seminar.

I would like to thank my friends for their encouragement, which helped me to keep my spirit alive

and to complete this work successfully.

Dinu M R

PAGE INDEX

Topic Page No.

ABSTRACT1. INTRODUCTION2. OVER VIEW OF HELICOPTER VIBRATION3. HELICOPTER VIBRATION REDUCTION METHODS

3.1. PASSIVE HELICOPTER VIBRATION REDUCTION 3.2. ACTIVE HELICOPTER VIBRATION REDUCTION

3.2.1. HIGHER HARMONIC CONTROL(HHC)3.2.2. ACTIVE CONTROL OF STRUCTURAL RESPONSE(ASCR)3.2.3. SEMI-ACTIVE VIBRATION REDUCTION TECHNOLOGY

4. COMPARISON OF THREE TECHNIQUES4.1. PASSIVE TECHNIQUES

4.1.1. ADVANTAGES4.1.2. DISADVANTAGES

4.2. ACTIVE TECHNIQUES4.2.1. ADVANTAGES4.2.2. DISADVANTAGES

4.3. SEMI-ACTIVE TECHNIQUE4.3.1. ADVANTAGE

5. CONCLUSION

FIGURE INDEX

Figure Page No

2.1.vibration profile of a helicopter, as a function of cruise speeds

2.2. Blade Vortex Interaction (BVI) schematic

3.1. Frequency response of a dynamic system with and without an absorber

3.2.Boeing-Vertol CH-47 "Chinook"

3.3.Sea King battery vibration absorber

3.4.Parts of Vibration Reduction System

3.4. Concept of HHC

3.5. Individual Blade Control (IBC)

3.6.Individual Blade Control (IBC) systems

3.7.Basic concept of ACSR.

3.8.Application of ACSR to the Westland/Augusta Helicopter

5.1.Comparison of vibration levels

ABSTRACT

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. 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

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 A review the various

techniques used by different helicopter companies to control helicopter vibrations

is presented here

CHAPTER2

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.

Fig 2.1.vibration profile of a helicopter, as a function of cruise

speeds

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 impulsive loads induced by

interactions between rotor blades and strong tip vortices dominating the rotor

wake (Fig 2.2.) This condition is usually referred to as Blade Vortex Interaction

(BVI).

Fig 2.2. 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.

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 equipment’s;

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

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

speed).

Examples of these passive vibration reduction strategies include

Tuned-mass absorbers,

Isolators

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.).

Fig 3.1. 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 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

Fig 3.2.Boeing-Vertol CH-47 "Chinook"

Fig 3.3.Sea King battery vibration absorber

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 3 And 5 excitation frequencies for the four-

bladed rotor version.

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. In general, an active

vibration reduction system consists of four main components:

Sensors

Actuators

Power supply unit

Controller

Fig 3.4.Parts of Vibration Reduction System

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. 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)

Active Control of Structural Response (ACSR).

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

Fig 3.4. Concept of HHC

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. 3.5.).

Fig 3.5. 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.

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

Fig 3.6.Individual Blade Control (IBC) systems

3.2.2 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. Fig 3.7. Shows the

basic concept of ACSR.

Fig 3.7.Basic concept of ACSR.

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 model.

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 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)

Fig 3.8.Application of ACSR to the Westland/Augusta Helicopter

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) 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 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

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.

CHAPTER4

4. COMPARISON OF THE THREE TECHNIQUES

4.1 Passive Techniques

4.1.1 Advantages

Require No external power

4.1.2 Disadvantages

Significant Weight Penalty Fixed in Design-no ability to adjust to any

change in flight condition

4.2 Active Techniques

4.2.1 Advantage

Low weight Penalty

4.2.2 Disadvantage

Requirement for external power

4.3. Semi-active Technique

4.3.1 Advantage

like active-adapt to changing conditions

like passive- small power requirement

(Compared to active)

CHAPTER 5

CONCLUSION

Fig 5.1.shows a comparison of the vibration levels of the Westland W30

helicopter without a vibration reduction system, and when fitted with a Flexi

spring rotor head absorber, and an ACSR system.

Fig 5.1.Comparison of vibration levels

REFERENCES