RTB equipment hardware and software has all but plateaued in development.
Accelerometers have not changed greatly in many years. Cabling has
remained essentially unchanged. The “brains box” has
not changed a great deal in the last 10-12 years since the advent
of the modern PC chip. Only presentation and limited functionality
has really changed over the recent past – not the efficiency
of how we conduct RTB. Calculating RTB solutions does not take large
amount of computing power and the capability has long been up to
the task – simply a matter of packaging and which presentation
the operator finds more user friendly.
The current generation of trackers which are generally a line scan
camera based system are still reasonably accurate but may suffer
from occasional difficulties in locking on to a track in certain
light or infrared conditions. Some equipment doesn’t even
use this track information to determine an in-flight solution other
than to establish an initial “flat” track condition.
From this flat track condition, it begins the RTB exercise –
even though the camera is mounted on the helicopter and the illusion
of it using the in-flight track condition for the balance solution
is created. The final solution to obtain a smooth rotor system may
result in the blades requiring to be flown OUT of track to obtain
the smoothest ride. This is purely a function of individual blades
and rotor systems.
Reducing RTB Costs
Dynamic Rotor Track & Balance (RTB) has long been a significant
cost in maintaining helicopters. This cost is a factor of time consumed
in tracking and balancing which translates from wasted man hours,
flight hours and significant aircraft downtime resulting in the
aircraft being unavailable for task. Each RTB event can often be
anywhere between one and up to five or more days depending upon
the skill of the maintenance personnel, maintenance test pilot,
RTB equipment used and the condition of the blades.
It is being increasingly recognised that the greatest
cost savings to be gained from any HUMS system for the
least cost outlay is by obtaining savings in the RTB area. Defence
Science and Technology Organization (DSTO) Australia, who has conducted
several international HUMS conferences over recent years, has produced
a cost analysis program which can quantify savings and expected
cost benefits which may be had from adopting certain aspects of
condition monitoring. This program called HUMSSAVE enables dollar
values to be inserted against capital equipment costs, hardware
costs, man hours, aircraft operating costs etc to provide a dollar
value per aircraft operating hour for the expected savings (or otherwise)
of adopting certain elements of, or complete HUMS systems for specific
fleets and types of aircraft.
It is easily demonstrated that the most cost effective component
which provides the greatest and quickest return – is
reducing the amount of time spent on RTB.
The greatest savings in RTB are being provided by a number of options:
- Hardwiring: permanently installing the cabling
and accelerometers into every aircraft in the fleet so that a
RTB junction box & analyser can be walked on board and plugged
in to the existing cabling and an RTB exercise commenced.
- Permanent fit: In addition to permanently
hardwiring for cables & accelerometers, a junction box and
analyser is also permanently installed.
- Improved RTB Efficiency: Making existing systems
capable of working more efficiently to obtain a solution.
Hardwiring and Permanent Installations
of RTB equipment is a very capital expensive exercise not withstanding
the increased maintenance for replacement accelerometers, broken
connectors etc throughout their in-service life. With fully integrated
HUMS comes the added expense again after installation of the man
hours required in data downloading and processing in addition to
the inevitable “system upgrades”
through the life of the HUMS/RTB system.
Improved RTB Efficiency
The most significant potential saving in RTB to be had is by adopting
a simple, inexpensive measure which EVERY operator can do –
with almost no impact on his current way of operating. It is called
Span Moment Arm Control
(SMAC). It allows your exisiting RTB equipment to work more efficiently
and obtain a smooth ride sooner than traditionally has been the
It simply requires that instead of thinking ONLY dynamic RTB when
considering balancing a rotor blade, the WHOLE balance problem
must be considered – static & dynamic balance
and their inter relationship.
By doing this, the typical results to date is that the average
RTB exercise for a helicopter is in the order of 2-3 flights
before it is smooth – with minimal weight being required on
the hub to dynamically balance the head. For CH47, typically the
flights are down to as few as 4-5 flights before it is flown serviceable.
To help understand and de-mystify RTB, we must look at rotor
blade adjustments available, their purpose and who can play
with them. After reviewing Rotor Blade Adjustments that are available,
we must then look at the vibration likely to be encountered and
corrected by the dynamic RTB smoothing process.
Associated with, but separate from the RTB smoothing, N
per rev vibration needs to be discussed and looked into. These
are often classed as a medium frequency vibration but can often
be more fatiguing and damaging than the classical 1/rev vibration
typically thought of with RTB smoothing and helicopter flight.
Rotor Track & Balance Smoothing
Main Rotor Vibrations
Mechanics are primarily interested in vibrations felt during in-f1ight
or ground operations as felt in the cabin. Most vibrations are always
present in the helicopter at low magnitudes. It is when the magnitude
of any vibration increases that it becomes of concern. The main
problem is deciding when a vibration level has reached the point
of being excessive. Extreme low frequency, and most medium frequency
vibrations are caused by the rotor or dynamic controls. Various
malfunctions in stationary compartments can affect the absorption
of damping of the existing vibrations and increase the overall level
felt by the pilot. A number of vibrations are present which are
considered a normal characteristic of the machine. The N per revolution
(N/rev) vibration is the most prominent of these, with N+1/rev or
N-1/rev the next most prominent. There is always a small amount
of high frequency present. Flight experience is necessary to learn
the normal vibration levels. Even experienced pilots sometimes make
the mistake of concentrating on feeling one specific vibration and
conclude that the vibration level is higher than normal when actually
it is not, it seems so because the pilot is concentrating on it.
For simplicity and some sort of standardization vibrations are arbitrarily
divided into general frequencies as follows:
Extreme low frequency - Less than l/rev, Normally limited to suspended
transmission mount installations such as in the Bell helicopter
types pylon rock
Low frequency- l/rev or 2/rev type vibration
Med. Frequency- Generally 4/rev or 6/rev, commonly is a N/rev vibe
in multibladed systems
High frequency- tail rotor or faster
Extreme Low Frequency Vibration
Extreme low frequency vibration is virtually limited to pylon
rock. This is encounted with suspended transmission systems which
is typical in Bell helicopters such as the UH-1/B205 series.
Low Frequency Vibration
Low frequency vibrations, 1/rev and 2/rev are caused by the rotor
itself. l/rev vibrations are of two basic types, vertical
or lateral. A 1/rev is caused simply by one blade developing
more lift at a given point than the other blade develops at the
same point. A lateral vibration is caused by a spanwise unbalance
of the rotor due to a difference of weight between the blades, difference
in Span Moment Arms, the alignment of the CG of the blades with
respect to the spanwise axis which affects chordwise balance, or
unbalance of the hub or stabilizer bar. Rigidly controlled manufacturing
processes and techniques, eliminate all but minor differences between
blades, resulting in blades which are virtually identical. The minor
differences which remain will affect flight but are compensated
for by adjustments of trim tabs, pitch settings and Dynamic Balance
Smoothing of l/rev verticals is essentially a trial and
error process although most rotor heads behave reasonably predicatably.
Dynamic Balance Equipment manufacturers use a sample head which
is in a known good condition and “map” out the various
adjustments required and magnitude of change and in what direction
the changes occur. This is then amalgamated into a software package
(or “SmartCharts”) which can allow with reasonable reliability,
multiple adjustments and forecast the most likely result based on
the data base and experience the RTB Equipment manufacturer has
put in the software.. Once in a while it will be found to be impossible
to get two blades flying satisfactorily together and it will be
necessary to remove and replace one blade – this is more than
likely a span moment
arm problem and can easily be fixed by passing the blade over
a digital static blade balancer.
Until now this option has not been available. The only
option to date has been to swap blades and try and match sets of
blades to try and get an acceptably smooth aircraft. Often this
process has taken days and many wasted man hours, aircraft down
time, flying time and lost revenue.
Laterals. Should a rotor, or rotor component,
be out of balance, a 1/rev vibration called a lateral will be present.
Laterals existing due to an unbalance in the rotor are of two types;
spanwise and chordwise. Spanwise unbalance is caused by one blade
or hub being heavier than the other (i.e. an unbalance along the
rotor span) or the Span moment arm of one blade being different
from the other blade's. A chordwise unbalance means there is more
weight toward the trailing edge of one blade than the other.
Medium Frequency Vibration
Medium frequency vibrations at frequencies of 4/rev and 6/rev are
another inherent vibration associated with most rotors. In two bladed
teetering head systems, an increase in the level of these vibrations
is caused by a change in the capability of the fuselage to absorb
vibration, or a loose airframe component, such as the skids vibrating
at that frequency. The most common cause is loose skids caused by
worn, loose, or incorrect skid retaining straps.
In multi-bladed systems, medium frequency vibrations are normally
Nper rev and harmonics thereof. N being the number of rotor blades
in the particular system being examined. It is quite common to have
numerous absorption devices to minimise the effects of N per rev
vibration. N per rev will manifest itself as both a vertical and
lateral component. It's cause is discussed in depth in N per rev.
High Frequency Vibration
High frequency vibrations can be caused by anything in the helicopter
that rotates or vibrates at a speed equal to or greater than that
of the tail rotor. This includes many unusual situations such as
hydraulic line buzzing, or starter relay buzzing, to the most common
and obvious causes; loose elevator linkage at the swash plate horn,
loose elevator, or tail rotor balance and track.
TroubleShooting - Coming Soon
Below are some case studies of aircraft vibration problems which
have been encountered. There is of course many more examples. Further
examples are available for
download Free from the download page.
Nper Rev Vibration
N per Rev vibration is a function of the number of blades within
the rotor system. For example a 4 bladed system would have an inherent
4 per rev freq, a 5 bladed system, a 5 per rev and so on. Associated
with these, are the harmonics of the N per – the N-1 &
N+1 rev vibes. A site (NASA) explains these quite effectively.
Some people often believe that N per rev vibration is a function
of the RTB or rotor smoothing exercise. This is incorrect.
The N per smoothing is a separate operation but is often carried
out at the same time or in conjunction with an RTB exercise for
obvious reasons that the vibration equipment is already fitted.
It is true that the smoother a rotor system is, then it tends to
unmask the Nper rev vibration such that they appear a lot worse
to the aircrew inside even though no adjustments were done to the
absorbers. This is because the higher levels of 1 per rev vibration
tends to mask the N per rev vibration. As the 1 per rev levels are
reduced toward zero, the N per rev vibe becomes APPARENTLY more
noticeable. If you were to take actual IPS level readings, you would
see that the actual IPS magnitude has not altered – merely
become more noticeable.
For us to more fully understand the reduction of N per rev vibration,
it is best we discuss the causes of N per rev.
Causes of N per Rev
Many theories have been espoused. It is probably the most avoided
question when it comes to asking about helicopter vibration. Try
asking your friendly helicopter manufacturer, representative or
your RTB equipment manufacturer.
The common explanation used is that it “is an inherent
vibration within the rotor system”. The question remains
– What CAUSES it? “Inherent” does not give
a cause – without a cause, merely a brush off.
Nper rev and its associated harmonic derivative frequencies
are product of two main sources.
1. Rotor Blade flapping to equality. The greatest
contributor is the action of the rotor blade flapping to equality
to equalize lift across the disc throughout its rotation. It is
caused by energy transfer from rotor blade to airframe via the transmission.
This energy is created in the rotor in the form of a “whiplash”
action induced along the blade as the tip of the blade reverses
direction of travel. This change in direction of the tip is caused
by the ever-changing creation of lift as the local angle-of-attack
is constantly altering as the local relative wind ever-changes with
constantly varying local airflows throughout the rotation of the
rotor blade. These changes are most prevalent with increasing Airspeed.
Hence most compensating devices are tuned for best effect at cruise
or high IAS.
2. Airframe interference. Airframe interference is also
a contributor but is really only prevalent at slow Airspeed. It
is caused by the absorption of energy from the “pulse”
of air that each blade pushes downward as it passes over a large,
flat surface. This surface may be a large flat stabilator, an empennage
(these are mostly rounded and semi-aerodynamic however), winglets,
or upper structure covering the roof/transmission area. The roof
and transmission area is close to the root of the blades which generally
has far less “pulsed” downwash than the mid-span to
tip portion of the blade. A lot of the inboard portion of the blade
is producing little lift if any through most of its flight regime
unless in the hover. It is often either stalled or in flow reversal
for a good portion of the flight envelope. It can be seen that airframe
interference will really only be of any consequence at slow speed
flight or in the hover.
These causes are discussed in greater depth in RTB,
available in the Free downloads.
N per revs are broken into both an Nper vertical and an
Nper lateral component. Depending the aircraft make/model, the
severity of each component varies. The severity determines whether
the OEM will provide dampening for both vertical & lateral components
or just dampening for the most noticeable or predominant component.
Most OEMs have a lateral dampening device incorporated in the head
with some cabin absorber arrangement to reduce the vertical components.
S70 Head mounted Bifilars
AS350 head mounted damper
Isolation mounts isolate specific components from transferring
energy back to the airframe eg isolating Stabilators, Nodal beams
to isolate the transmission from the airframe (Bell206L), Spring
suspension systems for transmission mounts (Bell 206, 205/212)
Elastometric Vibration Absorber