Rotor Track and Balance – Blade Balancing
Static & Dynamic Blade Balancing
Most helicopter operators think only of Rotor Track & Balancing (RTB) or Dynamic Balancing when helicopter blade balancing is considered.
When thinking about Helicopter Blade Balancing, we need to think in terms of both Static and Dynamic Balance. These two elements of helicopter Blade Balancing must be bought together under a complete – but simple blade management plan.
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.
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 water phenomenon.
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 dynamic balance blade adjustments which operator’s can currently use…..
⇓ – PCL Pitch Change Link or Rods.
⇒ – Tab is generally used to correct for track splits or 1:1 vertical vibration which increases with IAS.
⇐ –Weight Stations located normally close to or on the hub itself.
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 Blades”.
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.
Static 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.
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 both methods.
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
Most operators could not afford the capital cost of a dedicated room, set of scales and master blades for the occasional static balance.
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 a significant reduction in current cost of maintenance directly related to the Dynamic Rotor Track and Balance.
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 i.e. mass distribution.
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 Report.
The traditional “see-saw” 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 equalisation 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 by 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 “Static Balancing -Art or Science” (link) or “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 assembly).
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?
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 increased.
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 climbing tendency.
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 diving tendency.
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 time.
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)
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.
- Master Blade Reference system variations as described below.
- Field or Unit Painting/Surface Finish
- Field Repairs/Unauthorised Adjustments on Blades
- Blade wear/erosion
- Water Ingress, Trapped water
Download our “Weight and Balance Measurement and Control For Helicopter Rotor Blades” PDF Document
1.Master Blade Reference Variations
- Variations up to the equivalent of 85% dynamic adjustment authority between the master blade population. These observations and investigations of these variations were presented to the Society of Allied Weight Engineers (SAWE) on 5 June 2000. It provided surprising variations in what was assumed to be tightly controlled & widely accepted weight reference system. This will be discussed further below. Download the complete Free SAWE presentation.
2. 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.
3. 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.
4. 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 CofG 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 only fly together as sets – but will be impossible – or extremely difficult at best, to fly with “new” blades which are bought in as replacements.
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 imbalance.
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 weight)
- RTB continues to compensate until insufficient Lateral adjustment remaining to smooth the aircraft
- Blades will not fly together
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 specs:
- 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 in-lbs
- 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.
Similar results are seen across different helicopter types. The problem is universal.