The document summarizes various helicopter vibration reduction techniques. It discusses passive techniques like tuned mass absorbers which reduce vibration at specific frequencies. Active techniques like Higher Harmonic Control (HHC) and Active Control of Structural Response (ACSR) generate forces to cancel vibrations. Semi-active techniques adapt to changing conditions while requiring less power than active systems. Passive techniques have weight penalties while active/semi-active techniques require external power but can adjust to different flight conditions. ACSR has been successfully incorporated in helicopters to significantly reduce vibration levels.

1. A
Seminar Report
On
HELICOPTER VIBRATION REDUCTION
TECHNIQUES
By
DINU M R
DEPARTMENT OF MECHANICAL ENGINEERING
VALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING
&TECHNOLOGY
PARIPPALLY,TRIVANDRUM- 691574
[2012 – 2013]

2. A
Seminar Report
On
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 ENGINEERING
VALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING &
TECHNOLOGY
PARIPPALLY, TRIVANDRUM- 691574
[2012 – 2013]

3. 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

4. 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

5. PAGE INDEX
Topic Page No.
ABSTRACT
1. INTRODUCTION
2. OVER VIEW OF HELICOPTER VIBRATION
3. 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 TECHNIQUES
4.1. PASSIVE TECHNIQUES
4.1.1. ADVANTAGES
4.1.2. DISADVANTAGES
4.2. ACTIVE TECHNIQUES
4.2.1. ADVANTAGES
4.2.2. DISADVANTAGES
4.3. SEMI-ACTIVE TECHNIQUE
4.3.1. ADVANTAGE
5. CONCLUSION

6. 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

7. ABSTRACT

8. 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 vibrationA review the various
techniques usedby different helicopter companies tocontrol helicopter vibrations
ispresented here

9. 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
fromthe 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 distinctflight conditions;low speed
transition flight (generallyduring approach for landing) andhigh-speed flight.The
severe vibration level is primarilydue toimpulsive loads induced by
interactionsbetween rotor bladesand strong tip vortices dominating therotor wake
(Fig 2.2.)This condition is usually referred to as Blade Vortex Interaction (BVI).

10. Fig 2.2. 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 the
Vibration is caused mainly by the unsteadyaerodynamic environment in which
the rotor bladesare operating.
The control of vibration is importantfor 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 mechanicalequipment’s;
4. To improve the fatigue lives of airframe structuralcomponents
Hence it is very important to control vibrationthroughoutthe design, development
andin-service stages of a helicopter project

11. 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, pilot
Seats or avionics compartments)
The major advantage of the passive concepts is thatthey require no external
power to operateHowever, 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).
Examples of these passive vibration reductionstrategies include
 Tuned-mass absorbers,
 Isolators
 Blade design optimizations.
 Tuned-mass absorbers
 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 tuned frequency ( Fig 3.1.).

12. 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 wheresensitive equipment is placed.Without
adding mass, an aircraft battery may be usedas the mass in the absorber assembly.
For example, a helicopter known as seaking uses its battery vibration absorberor
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

13. Fig 3.2.Boeing-Vertol CH-47 "Chinook"
Fig 3.3.Sea King battery vibration absorber

14. 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.
3.2. Active Helicopter Vibration Reduction Method
Active vibration reduction concepts have beenintroducedwith the potential to
improve vibrationreduction capability andto overcome the fixed-design drawback
of thepassive designsthe majority of the active vibration reduction concepts 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
 Power supply unit
 Controller
Fig 3.4.Parts of Vibration Reduction System

15. 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. 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.
The most commonly examined active vibrationreduction 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 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.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

16. Fig 3.4. Concept of HHC
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. 3.5.).

17. Fig 3.5. 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.Instead of placing the actuators in the nonrotatingframe
(HHC concept), the IBCapproach uses actuators located in the rotatingframe to
provide, for example, blade pitch,active flap, and blade twist inputs for
vibrationreduction.
Schematics of Individual Blade Control(IBC) systems are shown below:

18. 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 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 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, located
throughout the airframe, to produce thedesired active forces. Fig 3.7.Shows the
basic concept ofACSR.

19. Fig 3.7.Basic concept of ACSR.
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 model.
Response 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

20. Extensive studies on ACSR system have beenconducted analytically and
experimentally.Recently, the ACSR technology has been incorporatedin modern
production helicopters such as the WestlandEH101 (Fig. Application of ACSR to
the Westland/Augusta Helicopter)
Fig 3.8.Application of ACSR to the Westland/Augusta Helicopter

21. 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,
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 on using them to
reduce helicopter vibration.
Major differences between active and semi-activeconcepts are their
actuators and associatedcontrollers.Active actuators generally provide direct

22. active force,while semi-active actuators generate indirect semi activeforce
through property modification.There are several advantages for using the semi
activeconcepts 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.
CHAPTER4

23. 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

24.  like active-adapt to changing conditions
 like passive- small power requirement
(Compared to active)
CHAPTER 5

25. CONCLUSION
Fig 5.1.shows a comparison of the vibrationlevels of the Westland W30
helicopter withouta vibration reduction system, and when fittedwith a Flexi spring
rotor head absorber, and anACSR system.
Fig 5.1.Comparison of vibration levels
REFERENCES