Traditional blade
management has always focused on the Dynamic RTB to get a flying
“set” of blades. Maintenance manuals have identified
for many years the limitation of this process. They have clearly
stated that if the 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 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 tat would fly together.
On multi-bladed systems, static balance is only ever done when
new or when the blade is sent for a major overhaul at an approved
blade repair center which has a master blade.
On teetering heads such as the UH1, operators perform 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.
 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.
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 an 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 New 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 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.
Periodicity
After the initial introduction and resetting of a fleet of blades
back to OEM specifications, this be maintained by periodic checking.
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.
New Blade Management Summary
- Develop an efficient, easy Static Balance capability for the
operator – a virtual Master Blade.
- Incorporate Static Balance 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
For more advantages, see Free Rotor
Blade Management download. |
