Rotor Track and Balance
The Rogue Rotor Blade
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 moment arm.
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 RTB knowledge.
- US Army Issued a Contract to collect data & identify/define CH47 RTB problems.
- Hypothesis by the Contractor – Chord Centre of Gravity Migrated Aft.
- 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 weighing systems.
Blade Problem Investigation
Chinook Data taken from Ft. Campbell KY & Ft. Carson CO
- 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
- Span 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 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. SPAN Moment Arm of Blade B = Db x Wb.
For a statically balanced head, the SPAN Moment Arms of all blades on that head should be equal (or at least within close tolerance).
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 a possible number of choices depending on the type of rotor head you are trying to balance:
a. Sweep the blade (teetering head fix …this partially varies the span moment arm by default)
b. Adjust PCL/tab to try and induce a different vertical component to mask the lateral problem (hence eventually will see a tangential move line which will NEVER correct the imbalance and will NEVER come within the ideal RTB vibration acceptable limits.
c. Accept the current Vibration levels?
d. Change the blade (which one??)
Do ANY of these points above sound familiar to YOU??? Have YOU seen this before?
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 and MUST be corrected – NOT the Dynamic adjustment stations.
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 DYNAMIC problem.
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
3. Traditional Static balance techniques (link to photo of UH1 balance)
Hopefully now you are beginning to see that these “traditional” methods were myths and legends and simply grasping at straws to try and find a fix for a problem which most people simply did not understand. Now we have the explanation – Span Moment Arm and the importance of controlling and knowing the Blade Static Balance/Span Moment arm in order to give the Dynamic Balance a chance to work.
The crux of the problem is the correction of Span Moment variation/migration using and consuming Dynamic lateral adjustment authority.
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.