Most helicopter operators think only of Rotor
Track & Balancing (RTB) or Dynamic Balancing when blade balancing
When thinking about Blade Balancing, we need to think in terms
of both Static and Dynamic Balance. These two elements
of Blade Balancing must be bought together under a complete –
but simple blade
In order to understand the ENTIRE Balance problem, we need to look
at the Rotor Blade Adjustments that are available
and the Blade Manufacturing Process.
Blade Manufacturing Process
A rotor blade is normally manufactured around a spar surrounded
by some composite material or "filler" to provide the
aerofoil shape. In modern blades this is normally either a metal
or nomex honeycomb or a foam cell filler material.
This is then protected by a hard skin made of either aluminium,fibreglass
or carbon fibre with protective metal abrasion strips bonded to
the leading edges to provide protection from particle erosion.
Typical Rotor Blade Construction
These blades for reason of lightweight construction have many "air
cells" inside the blade which can give rise to trapped
water problems. This can be particularly common in some
blades more than others depending upon skin porosity, particularly
when the blade is at rest for extended periods and subjected to
daily environmental changes. The stretching and contraction of the
blade’s skin (particularly woven reinforced composite skins)
due to the flex in the blade caused by gravity while at rest, can
exacerbate water ingress into blade cores. Trapped Water
has the potential to dramatically alter the RTB characteristics
of a blade - particularly those that are susceptible to the trapped
The nature of the construction requires a large element of hand
made content. As much as tolerances are adhered to, there will inevitably
be variations in weight and distribution of that weight across the
blade - i.e. variations in CofG - both chord
and span wise.
The composite nature of construction has also greatly increased
the tolerance and capability to repair a wide range of damage to
rotor blades. This increased ability to repair blades at operator
level has also increased the need for maintaining the static
balance – but in particular the Span
Moment Arm. Up until now there has not been a tool which could
accurately re-reference rotor blades back to OEM specifications
efficiently at operator level.
To compensate for these small variations in Span
and Chord CofG due to manufacture and repair, scope
for adjustment must be allowed for. This is done by allowing for
adjustment in both static span
and chord moment arms.
Mass is not supercritical but obviously has prescribed limits within
which it must fall or else there will be inadequate adjustment for
the span moment arm in particular. But the mass distribution especially
in Span, must be maintained within close limits to the original
specification if you wish to have all your rotor blades totally
interchangeable with minimal
time spent on RTB.
Once the blade is manufactured, it is statically balanced. This
process measures mass, Span CofG/moment arm and Chord CofG/Moment
Arm. Till now, the tool used for this purpose is a basic lever/fulcrum
and pivot balance assembly device, similar as the one pictured.
Its principal of operation is not too different from the old kitchen
scales used by our grand parents. It is generally very cumbersome,
sensitive to environmental changes necessitating a dedicated environmentally
controlled room, free from drafts or breezes, and takes considerable
time for the balance apparatus to stabilize after any adjustment
in order to achieve a successful result. In addition, it requires
some master or reference device against which to compare the blade
mass, span & Chord CofG/moment arm in question.
This master Reference device is known as the "Master
Blade". This system requires an entire environmentally
controlled room to ensure wind, temperature and humidity variations
do not adversely effect the reference system and to ensure some
degree of repeatability of accuracy.
This process is generally very slow and labour intensive - often
taking anywhere from 2-8 hours to measure and adjust one blade.
Dynamic Blade Adjustments
In simplistic terms, the following are blade adjustments which
operator’s can currently use.....
PCL – Pitch
Change Link or Rods.
TAB - Tab is generally
used to correct for track splits or 1:1 vertical vibration which
increases with IAS.
Stations located normally close to or on the hub itself.
Product Weight in SOME Helicopters
- can be used to alter climb/dive tendencies. It is adjustment of
DYNAMIC Chord CofG or Moment Arm
Traditionally the Static Chord and Span weight adjustments have
only been able to be adjusted by OEMs or approved blade repair facilities
as these facilities were the holders of the “Master
These adjustments are primarily used to adjust the rotor blade
and set the Static Span
and Chord Moment Arms/ CofG
to that of the original design specifications. This gives the blade
a good “start” point for the Dynamic balance and ensures
the RTB has every chance of success in the quickest possible time.
It ensures that the Dynamic Balance only has to correct for engineering
tolerances, wear & tear in the dynamic components of the rotor
system (mast, transmission, head, pitch change mechanism, etc) and
any small aerodynamic variations.
Span moment is critical to maintain close specifications if
fleet wide blade Interchangeability is desired
to be maintained. If Span Moment arm is NOT maintained within close
tolerances, Span Moment Arm migration
will occur. If the Span CofG differs significantly between individual
blades, the Angular Momentum will vary with the distance from the
mast for the same given localized change in mass (e.g. a repair,
extra paint or trapped water). A mismatch of blade CofG (Span Moment
Arm) will produce a significant lateral vibration which under our
current blade management practices, can only be corrected by adjustment
of DYNAMIC weight adjustments by the operator while conducting
a RTB. If this difference in Span CofG (Span Turning Moment or moment
Arm) is corrected by adjusting the tip weights, this frees
up the DYNAMIC weight adjustment for use by the
Dynamic RTB process.
Span moments are normally adjusted by weights located in the tip
of the Blade.
Static Chord Adjustment
The purpose of Static Chord adjustment is to balance the blade
in the chord wise direction. If the CofG is located aft of ideal,
the rotor blade would tend to climb since the turning moment that
the aft located CofG would produce would tend to make the nose climb.
If the CofG is forward of the ideal CofG, the turning force created
by the blade mass distribution about the chord would be a nose down
tendency, making the blade dive.
The static chord moment is done either at the hub or root end
of the blade (UH1, B206, B412/212,) or by changing the distribution
of span adjustment weights together with Chord weights located out
at either the tip or the hub end (CH47, UH60) or combination of
Dynamic Chord Adjustment
Dynamic Chord Adjustment
is used to control blade track should blades split as pitch (via
the collective) is varied.
To take into account these variations and changes to climbing/diving
tendencies with increasing RPM or pitch changes, the distribution
of the Static Chord weights will be done to try and achieve blades
which will fly the same track.
This can be done either on the aircraft (eg Bell 412 with Product
Weights) or on a Whirltower.
Reason for Restricting Adjustment
Download our .PDF Document Blade
Most operators could not afford the capital cost of a dedicated
room, set of scales and master blades for the occasional static
Now with the availability of accurate, portable digital weighing
systems, there is no reason that the static balance and
adjustment of rotor blades should not return back to the
operator – just as he has been doing with the Dynamic balance
solution. This capability will result in significant reduction
cost of maintenance directly related to Rotor Track
The successful Static Balance consists of the ability to
accurately measure and adjust any a rotor blade by
comparison to the OEM design specifications for any particular blade
for both mass, Span and Chord CofG & Moment Arm ie mass distribution.
This is done by using manufacturer’s Static
Span Adjustment and Static Chord Adjustment.
A static balance is rarely done by most helicopter operators.
This is because blades can only be successfully statically balanced
to obtain interchangeability by comparing
them to a Master Blade normally only
held by certified blade overhaul facilities and OEMs. This methodology
has its own limitations as revealed in AMCOM
The traditional “see-saw” Picture of UH1 balance or
pivot balance of a teetering head done on the hanger floor has significant
limitations. The main one being it only compares one operational
blade with another operational blade i.e. one blade of unknown characteristics
to another unknown blade. It does not compare an operational blade
with a “master” blade or OEM specifications. Therefore
it cannot provide fleetwide interchangeability of blades - only
match “sets” of blades.
There is a widespread lack of understanding of WHAT is important
in the static balance. Most assume it is the equalization of mass
– it is really the equalization of mass DISTRIBUTION
– in particular the control of the Span CofG or the control
of SPAN MOMENT ARM within reasonable limits
to ensure maximum dynamic adjustment remains for the dynamic RTB.
There is now sufficient evidence to suggest that the industry
has neglected this misunderstood area at great cost to itself in
terms of high Direct Operating Costs. These excessive DOC's are
created wasted man hours, flight hours and aircraft downtime bought
about by trouble shooting RTB problems and attempting to match "sets"
of blades caused by Span Moment Arm migration.
To assist in understanding the importance of the Static Balancing
process, it is important to understand the Blade Manufacturing
process and what Blade Adjustments are
available, the purpose of these adjustments and who traditionally
has been responsible to do these adjustments.
The “golden’ master blade is simply a physical representation
of the mathematical ideal rotor blade i.e. ideal mass, Span CofG/Moment
Arm(mass x Length) and ideal Chord CofG/Moment Arm(mass x Length)
– nothing more.
How is a Master Blade made?
Download our Static Balance PDF Document
Master blades are hand made blades or sometimes even metal beams
which have the same mathematical key features (within certain tolerances)
of the original engineering specifications. These key features being
Mass, Span CofG/Moment Arm and ideal Chord CofG/Moment Arm. The
Master blade does not have to by aerodynamically perfect or capable
of flying on a rotor head. The Master Blade is simply a Reference
device by which operational blades may be compared with to determine
variation from the design specification. It's purpose is similar
to that of the steel or brass counterweight used in the old style
“scales-of-justice” style of balance systems.
Master Blade Deviation
A study has been conducted into Master blade deviation with surprising
results. This study was carried out as a joint civil and US Army
study. The results of this in itself should be sufficient reason
for the helicopter industry to seek and insist for an urgent alternative
to replace the current static blade balance methodology. See “AMCOM
Report” in Downloads.
Master Blade Calibration
“Master” blades must be returned to the OEM periodically
(every 2 years is common) for “calibration” –
at a commensurate cost of course. The idea of the static balance
is to get the production blade as close to the original design specifications
of Mass, Span CofG/Moment Arm and Chord CofG/Moment Arm. Mass can
easily be measured by load cells and compared to the ideal specifications.
The Moment Arm (Span or Chord) is simply the Centre of Gravity (distance)
x blade mass. These are all mathematical quantities and can easily
be done by any basic PC and accurate load sensing device with the
appropriate software. After all, this is how most aircraft have
been weighed and fore/aft & lateral CofG determined
for some time.
Download our Whirl Tower PDF Document
The whirl tower is only used by OEMs who believe in “Pre-Track”
blades. This concept can really only work if flying new or near
new sets of blades within an operating fleet. It will rarely work
on mixed age blades due to Span Moment
Arm migration. – so why pre-track to begin with? Suffice
to say that armed with a good digital static balance tool which
can accurately measure and quantify blade mass, Span CofG/Moment
arm and Chord CofG/Moment Arm, together with any reasonable commercially
available RTB equipment, it is doubtful if ANY blades need to be
put on a whirl tower – the helicopter is the whirl tower.
What is its Purpose?
Download our Static Balance PDF Document
The purpose of the whirl tower is to produce production blades
which should theoretically fly together with the absolute minimum
- if any RTB (assuming a near perfect transmission/head & hub
The Problem - is that this is only true generally for the very
first time a helicopter is flown ex-factory or in the remote chance
that a complete set of brand new blades are purchased and installed
on the same head. ……why is this so?
Dynamic Chord Moment Adjustment
Download our Whirl Tower PDF Document
Once a static balance is completed at the factory, the blade is
then put on the whirl tower and spun against a whirl tower master
blade. The whirl tower master blade and the production blade are
spun up to operating RPM and pitch applied. Track is monitored for
splits. If the production blade flies higher than the “master”,
the static chord weights are adjusted forward to provide more nose-down
moment of inertia to keep the blades flying in similar track.
The reverse is true if the blade track dives against the master
blade. This similar process is used on the Bell 412 during a regular
RTB exercise when adjusting the “Product Weights”.
Ideal Dynamic Chord CofG – Track remains
relatively constant as collective is increased i.e. angle of incidence
Aft Dynamic Chord CofG - Nose climb tendency as
collective is increased. Corrected by moving some of the Chord CofG
weights forward to move the CofG more forward. This reduces the
Forward Dynamic Chord CofG - Nose dive tendency
as collective is increased. Corrected by moving some of the Chord
CofG weights rearward to move the CofG more aft. This reduces the
Span Moment Arm Control
What is Span Moment Arm Control?
Span Moment Arm Control is the periodic monitoring and correction
when required, of the Static Span CofG of a rotor blade to ensure
that maximum Dynamic weight adjustment is made available to correct
for any Dynamic 1 /rev vibration which may reasonably develop during
any one RTB exercise.
Span Moment Arm Control is essential if fleetwide interchangeable
rotor blades is desired with the minimum of time wasted on RTB.
The importance of Span Moment arm control has been known for some
Unfortunately, the importance of SPAN MOMENT ARM control has either
been lost or neglected over the years.
“Spanwise balance is adjusted during manufacture by
balancing individual trailing edge skin section and by balance
weights fitted at the outboard end of the spar. The strict weight
control and static and dynamic balancing which the
blades receive during manufacture permit interchangeability
of individual blades”. (Ref: Principles of Flight, Helicopter
AP3456A Dec 83)
Span Moment Arm Migration
What causes Span Moment Arm Migration? Span Moment Arm Migration
occurs on EVERY rotor blade during its operational
life. It commences as soon a the blade is put onto the rotor head
and is exposed to everyday operational environment including abrasive
leading edge erosion, leading edge pitting (from salt water and
rain exposure), trapped moisture from rain and condensation, blade
painting or touchups, repairs to dents or damage and unauthorized
tampering or incorrect adjustment of the static balance weights.
Many of these factors are beyond the control of the everyday operator.
Till now, the operator would simply use the Dynamic Balance (RTB)
procedure to try and correct for any vibration or blade balance
problem. The RTB would be repeated until either it was deemed acceptably
smooth ride (often only marginally serviceable) or as a last resort
would pick a blade to swap out in an effort to try and get a “matched
set” of blades which would fly smoothly together. If the swapped
out blade could not be balanced on another head, it would be sent
to the OEM or blade repair venue for repair. Here, the OEM/repairer
more often than not would simply rub the blade back to skin (since
it had probably received several coats of paint in its life time),
repaint it, then statically balance it. The effective part of the
entire process being simply the static balance.
Now with the advent of digital static balancing, the operator can
STATICALLY balance the blades at his level without sending the blades
to the OEM or costly repair venue.
Span Moment Arm Migration
- Master Blade Reference(Link to Norms paper) system variations
as described below
- Field or Unit Painting/Surface Finish
- Field Repairs/Unauthorised Adjustments on Blades
- Blade wear/erosion
- Water Ingress, Trapped water
Master Blade Reference Variations
Variations up to the equivalent of 85% dynamic adjustment authority
– Link to a page which shows extract from Norms paper demonstrating
the extent of variation – possibly use slides from PPT show:
- Field or Unit Painting/Surface Finish
- Complete re-spray versus Touch-up by only partially rubbing
back paint and applying an even coat over entire blade surface……blade
weight increases unevenly across blade surface and thus slowly
migrates Span CofG outboard toward the tip(relative the hub).
To attempt to equalize this effect, a “new” blade
would be needed to have weight added to it to equalize the
Span CofG – or tip weight removed off the old blade
as the added weight of the accumulating paint pushes the Span
CofG outboard . Traditionally on teetering heads, this weight
is added in the Blade Retention bolts or the same adjustment
point as the DYNAMIC RTB balance uses to correct for laterals.
- Field Repairs/Unauthorised Adjustments on Blades
- Small mass change along the chord makes much less
change in Chord moment arm than does the same mass change
if made along the span.
- Moment of Inertia/Angular momentum is related
to the distance out from the hub (Span) that the Span
CofG is considered to act - chord has very little moment arm.
By comparison, the distance along the Span from the hub has a
far greater effect on Moment of Inertia and Angular Momentum than
any repair or change across the chord. The distance from the Chord
CogG to any change in mass is far smaller than any changes
in the Span.
Blade wear/ erosion
Salt Water and heavy rain pitting on Bell blades, Sand and hard
particle erosion in dry desert environments.
Water Ingress, Trapped water
Through leaks and condensation. CH47 very susceptible due to surface
finish. Has a similar effect as a blade repair. This topic is worthy
of further discussion and will be elaborated on at a future time.
Typical Span Moment Arm Control – Current Practice
If a set of new blades were added to the one head and flown for
1500 hours, chances are all these same blades would undergo similar
paint erosion, leading edge erosion, and re-painting of the blades
at major overhauls. It is highly likely that this “set”
of blades are dynamically well balanced – but if one were
to replace either of these blades after say 1500 hours of flying,
we would generally find great difficulty in dynamically balancing
this new “set” of blades together. It may eventually
be done successfully, but often it will take several days and many
flights often having to go back and start over again – this
is a costly exercise. More often than not, the “new”
blade is unable to fly with old blades and must be removed and replaced
with another (generally a blade with similar hours on it i.e. similar
layers of paint) before the “set” will successfully
fly. The new blade now sits in the corner of the hanger –
often called “a Rogue” blade. It is
common to see blades returned to overhaul simply to weigh against
a master blade and more often than not, simply to be sent straight
back to the user with the ensuing cost of the static balance and
associated costs of transport, damage risk in travel, and increased
number of blades required in inventory to cover this needless removal
from service of a perfectly serviceable blade………Does
this sound familiar to your company or organization????
The Problem is
not with the new blades – it is with your fleet blade population
or blade inventory. Their Span Moment Arms have migrated to
the point where they will fly together as sets – but will
be impossible to fly with “new” blades which are bought
in as replacements.
The Solution is
simple - implement a new blade
management plan which incorporates routine Span Moment Arm
The Blade Problem
is the undetected Span Moment Arm migration to the extent of consuming
a significant, if not the majority of Dynamic Lateral balance correction
weight capability. This is done in the effort to correct for a STATIC
problem - but uses the DYNAMIC correction adjustment to achieve
the balance. Thus leaving little DYNAMIC weight adjustment authority
remaining to correct for legitimate Dynamic vibration induced by
serviceable engineering tolerances and wear limits in the rotor,
hub, mast, transmission and transmission mounts. Hence the Dynamic
RTB has little chance of success due to the Span Moment Arm migration
which has taken place.
The problem is simple. Span Moment Arm migration changes the Span
CofG gradually and continually over the life of an operational blade.
If this Span Moment migration goes undetected and uncorrected, it
will deviate so much from the OEM specifications that any new or
reconditioned blade, will have insufficient DYNAMIC weight adjustment
capability to counter the STATIC span problem PLUS the Dynamic RTB
Lateral correction which the hub weights are designed to correct
for – the blades will be unable to fly smoothly together.
The “rogue” blade is born!
Operational Span Moment/Dynamic Authority
Below is the typical life cycle of in-service rotor blades demonstrating
the gradual Span Moment Arm Migration which the typical rotor blade
will be subjected to during its in-service use. It is divided into
a teetering and articulated/semi or rigid head.
- See-Saw Balance of Hub PLUS blades
- Static Span Correction using DYNAMIC Adjustment
- LESS Dynamic Lateral adjustment for RTB
- Blades may not fly together
- In-service migration of Span Moment Arm
- RTB tries to compensate
- Ultimately blades will not fly together
When blades are statically balanced on the hanger floor using a
pivot or ball balance, the correction weight is added to the DYNAMIC
lateral weight adjustment station. This reduces the Dynamic weight
adjustment available for legitimate Dynamic lateral unbalance. Reduced
Dynamic Weight capability increases likelihood of mismatched blades
- i.e. insufficient authority remaining to correct dynamic in-balance.
Undetected Span CofG migration since accurate measurement of Span
Moment not done at operator level.
Blades fleetwide will ultimately end up being quite diverse in
Span Moment arm and therefore not interchangeable from one aircraft
to another – they MAY fly as sets – BUT definitely will
not be interchangeable fleetwide.
- Initial Static Balance against “Master” at factory
or overhaul facility
- Undetected in-service Span Moment migration
- RTB tries to compensate using Dynamic Lateral adjustment (Hub
- RTB continues to compensate until insufficient Lateral adjustment
remaining to smooth the aircraft
- Blades will not fly together
Undetected migration of Span Moment Arm causes blades to become
individual and not inter-changeable:
Accurately measure, adjust and track Span Moment arms on blades
at the lowest possible level – operator.
Static Span BalanceWeights:
Till Now - only OEM or Depot adustable.
Small weight change - has big effect on Span Moment arm.
Span imbalance across a head if individual blade Span CofG migrates
inward (eg blade repair performed on the inboard section of blade).
Same imbalance if corrected by using the DYNAMIC adjustment
stations at the hub requires a lot of added mass to correct
the STATIC imbalance.
Same imbalance if corrected by using the Static Span Adjustment
stations or Tip weights requires far less weight, is more
effective, and ensures 100% authority remains at the DYNAMIC hub
weight adjustment stations to correct for any Dynamic Lateral imbalance
which may arise during RTB.
Very accurate means of maintaining blade within original design
- Span Moment
- Chord Moment
- Mass - not a big issue
As indicated by existing aerodynamics
text books, maintaining Span Moment arms - maintains blade interchangeablity.
MASTER BLADE Comparative Balanced Blades
The following is a summary of results taken from an official investigation
of the accuracy of the Master Blade technology. A full report is
available as a FREE download.
Sampling was done on UH60 blades which had all been either freshly
overhauled or brand new blades. They therefore had all been weighed
against various “Master” blades at either approved overhaul
venues or at the Manufacturer.
- Problem – UH60
- New Blade 35,361 in-lbs
- IAI Overhauled 35,345 in-lbs
- IAI Overhauled 35,361 in-lbs
- CCAD Overhauled 35,442 in-lbs
- Manufacture’s Specification for Span Moment Arm 35,418
- It can be seen from even this small sampling, that the largest
deviation between the “lightest” and “heaviest”
blade is some 97 in-lbs.
- UH60 - has only 117in/lbs of dynamic adjustment available i.e.
if one blade had no hub weight on the Lateral weight adjustment
station and the blade directly opposite had maximum weight installed,
the maximum change would only be to correct for a 117 in-lb imbalance.
- Therefore under our current Blade management practices, we would
have consumed most of the DYNAMIC Lateral weight adjustment (97
in-lbs worth) to correct for a STATIC problem (which the tip weights
are designed to do). Leaving only 20 in-lbs remaining to correct
for any true DYNAMIC imbalance of the rotor system.
- The blades will NOT fly together as a set until this Static
Span imbalance is corrected.
The UH60 has potentially 85% of authority consumed between “new”
blades straight out of the box on the same head.
Blades tend to get heavier in service. Therefore the real problem
arises when a New blade out of the box is put against an existing
blade which was known to be flying on the same head before matched
with the new blade.
Span Moment arm differences are manifested in the Dynamic balance
when the lateral (and often the vertical) vibration has a large
amplitude and cannot be reduced to an acceptable level. This is
caused by the older “heavier” blade trying to be flown
with a New, spec blade.
The first action by maintenance generally, is to:
a. Replace new blade, or
b. Replace old blade???
Generally, everyone elects to replace the New blade - because they
know the older blade was flying successfully before the replacement
blade was put on.
In fact the U/S blade is the old blade - its span moment has migrated
over time to be widely divergent from the original design Specifications.
results are seen across different helicopter types. The problem