The document discusses various techniques for reducing vibration in helicopters, including passive, active, and semi-active methods. Passive techniques like tuned mass absorbers and blade design optimization provide moderate vibration reduction but with a significant weight penalty. Active concepts like higher harmonic control and individual blade control generate unsteady loads to cancel vibrations, but require external power. Semi-active systems modify structural properties using small amounts of power. The most successful current method is active control of structural response, which places actuators throughout an airframe to reduce vibrations measured by sensors.

1. HELICOPTER VIBRATION REDUCTION TECHNIQUES
A SEMINAR REPORT
Presented By: Haftamu Abraha
Feb. 2012

2. 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

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. 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
4

5. 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

6. 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

7. 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

8. A review the various techniques used
by different helicopter companies to
control helicopter vibrations is
presented here
8

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. 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

11. 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

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. 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. 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 forward14
speed).

15. 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

16. 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

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 used
Sea King battery vibration absorber Boeing-Vertol CH-47 "Chinook"17

18. 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,
18

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. 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

21. 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

22. 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
22

23. Figure 3.2.2 shows diagrammatically the concept of HHC
23

24. 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).
24

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. Schemetics of Individual Blade Control
(IBC) systems are shown below:
(a) blade pitch, (b) active flap, and (c) blade twist controls
26

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. 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.
28

29. 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 been
conducted analytically and experimentally.
29

30. 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 Titanium
Element Lug End
ACSR Actuator
• sa
Steel downtube
30

31. 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

32. 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:
32

33. 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
33

34. 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
34

35. 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

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