Research and Development over the last decade
has now pinpointed the crux of the Rogue Rotor Blade problem
to be caused by Span Moment arm
migration throughout the life of the blade.
The “Rogue” (or difficult-to-balance) rotor
blade is not new. It has been a little understood phenomena, costing
the industry millions of dollars over the years.
The real reason for “rogue” blades has been
identified and explained for some time in aerodynamics
text books but not widely known by the average helicopter mechanic,
pilot or within the industry. The key to rogue blades
is controlling the span
Blade Problem History
The helicopter industry has lived with the rotor blade balance
problem for many years.
It was not until it became an operational issue and it began to
impact significantly on the US Army’s deployment capability
during the 1991 Gulf War that an investigation was formally undertaken.
Gulf War 1991
- Blade Problem became an Operational Issue
- Aircraft were regularly requiring 10-40 Flight hours to
complete a routine RTB – particularly CH47
- Initially thought to be a training problem and a lack of
- US Army Issued a Contract to collect data & identify/define
CH47 RTB problems.
- Hypothesis by the Contractor – Chord Centre of Gravity
- Assumed that paint being eroded from leading edges and
the accumulation of paint on the aft portion of the blade
with successive re-paints, the Chord CofG would migrate aft
thus creating tracking problems with the blade.
- Initial Aim of the contractor was to quantify
Chord Moment arm migration. A simple, accurate system was
devised. It was based around the already proven three point electronic
Blade Problem Investigation
Chinook Data taken from Ft. Campbell KY & Ft. Carson
- Over 200 blades measured
- span moments ranged from 700 in-lbs. light to 700 in-lbs.
heavy - equivalent to plus or minus 2 pounds at the tip
- There was not a static chord moment trend nor did chord variation
affect rotor vibration
- Moving 1.7 lbs Fwd
Aft, makes only 1 deg tab change difference. E.g. arbor balance
for teetering heads
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.
Blade Problem Conclusion
- Chord CofG is not an operational problem
CofG has significant operational variation
"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)
This quote from RAF/RAAF aerodynamics text book acknowledges the
importance of “strict” span moment control to maintain
INTERCHANGEABILITY within a population of rotor blades.
Span Moment variation doesn’t STOP blades from flying as
“Sets” – BUT it does seriously limit mixing or
interchangeability of blades within any given fleet.
SPAN MOMENT ARM control assures Blade INTERCHANGEABILITY within
a fleet or blade population.
Span Moment - Weight Distribution
Span Moment Arm is defined as the turning force exerted by the
mass of an object (eg blade) about a pivot point (eg the mast).
It is calculated by multiplying Mass of the object by the distance
the Center of Gravity is from the pivot point. It is generally measured
in inchlbs (typical US/Imperial units).
SPAN Moment Arm of Blade A = Da x Wa.
For a statically balanced head, the SPAN Moment Arms of
all blades on that head should be equal (or at least within
Da x Wa = Db x Wb
If Da x Wa > Db x Wb and the blades are rotated, a DYNAMIC
imbalance will immediately be manifested by a noticeable Lateral
vibration being indicated by the RTB equipment. Now the operator
must use DYNAMIC correction and weight adjustment to correct
this STATIC induced problem.
If Da x Wa > Db x Wb by a significant amount (eg one
blade has had repairs done, several coats of paint, trapped water
etc), the Lateral vibration will be so great that there will
be insufficient DYNAMIC weight adjustment to correct for
this STATIC imbalance condition. The RTB equipment will typically
adjust using Lateral weight corrections until the IPS level passes
through a tangential position and out the other side.
The Maintenance Manual will give you one of two choices:
a. Change the blade (which one??)
b. Accept the current Vibration levels.
By measuring and referencing each individual blade to the mast,
corrections to the tip weights can easily be calculated to ensure
Da x Wa = Db x Wb. This is where STATIC balance adjustments
SHOULD be corrected – NOT the Dynamic adjustment
If SPAN Moment Arms are therefore maintained within tight tolerance,
all blades will once again become interchangeable and full adjustment
authority of the Dynamic Adjustment station made available for any
Traditionally rotor blades were attempted to be matched by:
1. matching equal weights (mass) of rotor blades.
2. matching flight hours, eg 1000hr blades together etc
Traditional Static balance techniques (link to photo of UH1 balance)
The crux of the problem is the correction of Span Moment
variation/migration using and consuming Dynamic lateral adjustment
Dynamic RTB Lateral
Traditional Static balance on hanger floors are generally limited
to placing weight on the DYNAMIC balance weight station which is
generally located close to the hub.
If we use up the dynamic adjustment capability to counter a STATIC
span problem, we reduce the available lateral weight adjustment
for the Dynamic RTB. If the
vibration amplitude readings have tangential tendencies with unacceptably
high amplitude as indicated below, it is highly likely that there
is a Span Moment Arm problem.
The quick and easy fix for this is to pass the blades over a Static
Balance capable of measuring and quantifying the Span Moment Arm
of each blade. This can only quickly and accurately be done by using
a digital static balance tool.
This quickly and easily PROVES that there is either a Span
Moment Arm problem or a problem in the head.
This is the quickest and easiest way to troubleshoot the head and
blades to isolate and identify whether the problem lies in the blades,
the head or other rotating components on/around the head If a blade
is out of limits, the respective blade can be statically balanced
and the RTB re-commenced. NO BLADE SWAPPING would be required
as is currently common practice as directed by maintenance manuals
and experience to date. If the blades are within serviceable limits,
the problem must be in the head or the other rotating components
on/around the head or mast.
Typical Indications of Probable Span Moment Problems
The solution to the “rogue” blade problem or Span
Moment Arm migration is to:
- Measure, quantify and adjust the Span moment arm of each individual
blade at operator level.
- Adopt a simple NEW
blade management plan – incorporate routine STATIC balancing.
- Have access to a digital static
balance tool which any operator can easily use and which will
quickly quantify the blade’s mass, static Span & Chord
CofG and moment arms
- Correct any differences from OEM specification by adjusting
the STATIC span adjustment rather than forcing the Dynamic RTB
to try and correct by using the
dynamic lateral weight adjustments to correct for a STATIC problem.