Rotor Track and Balance  – Blade Balancing – Blade Management

Rotor Track and Balance – Blade Balancing – Blade Management

Blade Management

Traditional Rotor Track and Balance and rotor blade management has always focused on the Dynamic Track and Balance (RTB) to get a flying “set” of blades. Unfortunately, most helicopter operators and indeed some manufacturers, believe that flying “sets” of blades is the only acceptable way of managing a rotor blade population. This is being done in an effort to remove the possibility or to at least control the occurrence of “rogue” rotor blades. Rogue rotor blades are generally caused by span moment arm migration among a fleet of blades. This can be remedied by performing an accurate static blade balance with tip weight adjustments – providing you have the tooling.

By changing emphasis to Static Balance Blade management followed by Dynamic Rotor Track and Balance, rotor blades will become far more interoperable and remain in service for far longer period of time than currently is the case. Blades will not be returned needlessly to overhaul centres because “they will not fly with other blades”. They will be managed instead at operator/hanger level.

Maintenance manuals have identified for many years the limitation of this process. They have clearly stated that if the Rotor Track and Balance (RTB) results in the move line passing tangentially to the zero axis – this is the best you can reasonably expect leaving you with one of two options:

a. You must accept this result and the resultant vibration level, or
b. change a blade.

– the blades will not fly together.

This phenomena is often typified by charts as similar to those below.

Laterals

Verticals

The first question which should have been answered many years ago is: WHY won’t these blades fly together?

WHY?:          These blades will not fly together primarily because of a mismatched Span Moment Arm.

A new approach to Blade Management needs be adopted based on the control and correction of Span Moment Arm migration of operational blades at operator level if improvements to Rotor Track and Balance (RTB) efficiency are to be obtained.

Current/Traditional Blade Management

The traditional approach never looks at the entire balance solution. It only ever uses the Dynamic RTB to balance a rotor. The solution must incorporate BOTH Static  & Dynamic balance to achieve an efficient result.

Traditionally blades were attempted to be matched by a number of administrative methodologies such as matching hours flown on each blade, matching serial numbers or a combination of both. Whatever method, more often than not, it boiled down to a time consuming trial and error process to find a matched set of blades that would fly together.

On multi-bladed systems, static balance is only ever done properly when new or when the blade is sent for a major overhaul at an approved blade repair centre which has a physical master blade.

On teetering heads such as the UH1, operators perform a simple “see-saw” or pivot balance. The limitations of this procedure are listed elsewhere on this site. The result of this method however is that it only promotes the use of “sets” of blades – it does not provide a means of making all blades interchangeable. This method of static balancing also is limited by using the Dynamic adjustment weights/stations to correct for a static problem – thus reducing the amount of dynamic adjustment available to solve dynamic problems.  

This technique will ultimately create the “rogue” rotor blade….this is part of the problem that has consistently plagued the industry and created such needless cost, frustration and waste of both time and money.

Red: Static/Moment Balance Correction Weights
Blue: Dynamic RTB Lateral Correction Weights

The traditional blade management process allows the Span Moment Arm of individual blades to migrate – UNDETECTED. This results in mismatched blades.

Typical Span Moment Arm Migration

Lets follow the path of this migration and how it reduces the dynamic weight adjustment progressively to the point where blades become unflyable with other blades.

Teetering Head

• See-Saw Balance of Hub PLUS blades.
• Static Span Correction using DYNAMIC Adjustment.
• Results in LESS Dynamic Lateral adjustment for RTB .
• Blades may or may not fly together.
• If Blades fly successfully together, in-service migration of Span Moment Arm will occur.
• Successive RTB exercises tries to compensate through life of blades using ONLY Dynamic Adjustments.
• Ultimately will not fly together as Span Moment migrates to a point which is greater in magnitude than the Dynamic Adjustment by itself is capable of compensating for.

Articulated/Semi/Rigid

• Initial Static Balance against Master
• Initial RTB when new blade fitted to an existing head (providing the Span Moment Arm of the other blades on the head have a similar Span Moment – or else the “New” blade will show up immediately as a “Rogue” blade).
• Undetected in-service Span Moment migration
• RTB tries to compensate on successive RTB exercises through the life of any set of blades using Dynamic Lateral adjustment (Hub Weight)
• Ultimately will not fly together as Span Moment migrates to a point which is greater in magnitude than the Dynamic Adjustment by itself is capable of compensating for – now a decision must be made to accept the existing vibration level or change a blade – BUT which one?

Do any of the above scenarios sound familiar to your own experiences? If so, you may want to consider adopting a New Blade Management system and routine Static Balancing on the hangar floor.

• Why Does Span Moment Arm Migrate?
• How can we adjust Span Moment arm?

Traditional static balance procedure

• Overhaul venue statically balances to a “Master” Blade
  • Blades go to Overhaul venue
    • Costly – in both Money & Man Hours
    • Time out of Service – Spares & increased inventory holdings
    • Damage risk – during removal/installation and transport & Handling
    • Provides Mass only of the blade – Current weighing procedures do not provide the customer with value of individual Span or Chord Moment arms or CofG – check out your blade paperwork next time you get a new or overhauled blade – check your current blade paperwork.
• Master Blade Must Be Maintained
  • Costly – requires calibration every 2 years for some blades
  • Deviation from Design Specifications or the “Golden” Master Blade a likelihood.

New Blade Management

With the identification of the cause of Rogue Blades, it is now clear that a new approach to blade balancing or more precisely – Blade Management needs to be adopted.

The traditional inefficiencies and cost can no longer be afforded by an industry where capital costs and Direct Operating Costs are very high with a disproportionate pressure to keep charge-out rates very low forcing very thin profit margins. Defence and Government air wings are not immune from the drive for “more bang for the buck”. Shrinking budgets worldwide coupled with the accompanying political pressure necessitates that these organizations think more commercially and seek more flying hours per maintenance dollar.

New Blade Management Procedure

We have always treated Static & Dynamic balancing as two totally different, unrelated exercises – they should not be.

A successful Dynamic balance is very dependant upon a good Static balance – most importantly – the Span Moment being maintained within a reasonable tolerance of the ideal “Spec” figure if blade interchangeability is to be assured and if the full benefit of the dynamic adjustments are to be provided to counter any dynamic problems.

This New Management procedure does not advocate that a static balance needs to be done every time a Rotor Track and Balance (RTB) is done.

It is based on a periodic static balance to particularly check Span Moment Arm, whenever scheduled routine maintenance is carried out in the existing servicing schedule – no additional maintenance is called for. It is recommended at routine maintenance on or about every 500-600flt hrs (or after blade repair/painting). This will ensure Span Moment Arm Control within tight limits and enable trouble free RTB and remove “rogue Blades” from your fleet and inventory blade stock ensuring total blade interchangeability across your fleet.

For a comprehensive Blade Management plan see the download page.

Potential Cost Savings

The US Army is projecting a savings of more than U$1.0M per year at Corpus Christi Army Depot alone by using this technology. See Redstone US Army Cost Savings Free Download for details.

A cost benefits analysis spreadsheet is available free, to assist in quantifying potential savings.

Converting to a New Blade Management System

See the Rotor Blade Management Free download.

Converting from a Traditional Blade Management plan to a Static Balance Blade Management Plan is simple, easy and straight forward. Cost is minimal in capital outlay – this cost easily recovered with the reclaiming of even one rotor blade. There is effectively no man hour cost since in the mature Static Balance Blade management Plan, static balancing is done during scheduled maintenance when blades are already off the aircraft. The Static Balance itself takes no more than 10 minutes per blade.

Tooling
Strategic positioning of tools ensures that blades can easily be passed over a static balance on a routine basis. Avion/Avtask has without doubt, the best main rotor blade and tail rotor blade static balance tools in the industry. Their tools are in widespread use throughout the US Army and many other global Defence Forces. Civil operators include Columbia Helicopters. These tools are ideal for operators of civil CH47, UH60, UH1 and many other models of helicopters.

Periodicity
After the initial introduction and resetting of a fleet of blades back to OEM specifications, this be maintained by periodic checking. This period is normally set to align with an already prescribed scheduled aircraft service. The period is determined by assessing the rate of span moment arm migration that the fleet’s rotor blades may experience. This is largely a factor of the environment in which the blades/helicopters are exposed or operated in. For example, a sandy, dusty, highly erosive environment may require more regular periodic checks such as every 500hrs due to regular paint touch ups in the field and blade erosion.   Similarly, operations in rainy, wet environments where aircraft are left standing exposed to high rainfall may suffer from trapped water problems, depending upon the model of blade and its susceptibility to trapped water.

Blade Admin/Paperwork

Significant simplification of blade paperwork and administration is achieved. Individual tracking of blades to achieve a “flyable” set is eliminated. Any blade out of the box should fly with existing in-service blades. Paperwork is summarized by computer generated printouts detailing the date, initial blade results, subsequent adjustments performed and the final blade mass, Span & Chord Moment arms and CofG. Potentially large numbers of administrative man hour savings are achievable. 

This simplification of blade paperwork allows for easy introduction of a Static Balance Blade management system with reduced man hours for maintenance control personnel. 

New Blade Management Summary

• Develop an efficient, easy Static Balance capability for the operator – a virtual Master Blade.
• Incorporate Static Balance and Static Balance Blade Management Plan in routine maintenance
• Blades become interchangeable again
• RTB made more efficient, Reduced Maintenance costs.
• Removes “guess work” from RTB and the time consuming trial & error approach to matching blades which as been traditionally adopted.

Advantages of Operator Static Balancing

• Records Span/Chord Moment Arm & Mass
• Allows all blades to be interchangeable again – minimal records and administration required
• Used as an onsite diagnostic tool should aircraft become difficult to dynamically balance – identifies quickly if it is a blade or a head/aircraft problem
• Greatly reduced time spent on RTB
• Increased aircraft operational availability

Try it and reap the benefits, financial rewards and competitive edge. 

For more advantages, see Free Rotor Blade Management download.

CH47 Columbia helicopter