The Static Balance to Dynamic Balance Relationship

As stated elsewhere on this web site, a much overlooked area in Rotor or blade balancing is the importance of the STATIC balance and why it is so important for the operator to have either the ability to either adjust or at least quantify the Static balance, in particular the SPAN MOMENT ARM. A recent paper presented to the American Helicopter Society Forum called Rotor Blade Static Balance –Art or Science, has provided the all important relationship between the Static balance and the Dynamic balance in terms most helicopter engineers & pilots will understand – in units of IPS equivalents.

Below is a synopsis of this important paper which provides a refreshingly simple correlation between Static and Dynamic balancing. A full version of the paper can be downloaded FREE. It discusses why, when discussing rotor or blade balancing in aviation, the two must be thought of together as the ENTIRE solution.

This is done easily by adopting a new philosophy to blade management. Thinking only of the Dynamic Balance solution will inevitably be a costly oversight for any operator now and in the future.

Correlation data is provided for commonly used helicopters and for some fixed wing propeller aircraft, Static balancing being equally important to BOTH rotor blade and propeller.

The Goal of all balancing is to reduce vibration

Final rotor blade track and balance is almost always performed dynamically. This process accommodates any cause for imbalance in the entire rotating system. Due to the high cost of operating a helicopter, and the limited authority available to the dynamic tuner, maintenance test flights and test flight hours can generally be reduced by performing off-aircraft, static balance of the rotor blades. Static balancing provides the dynamic balance technician with a set of rotor blades at the onset of dynamic tuning which are matched in terms of balance characteristics.

Until recently, this capability has only existed within OEMs and depot facilities.

Dynamic balancing is always the final solution

Static balancing procedures do not replace dynamic tuning procedures, but augments the dynamic tuning process. At the end of the day, it is the job of the Dynamic balance to remove ANY cause for imbalance in the entire rotating system.

These may be aerodynamic imperfections, engineering tolerances in bearings/mated surfaces/split cones/spline shafts etc, alignment (shaft/Transmission mounts/spars/etc), play and wear in the pitch change and dive link mechanisms, swash plate anomalies and other variables around the head & rotor assemblies – these sources of vibration are all lumped together under the term DYNAMIC Balance. These are traditionally what the DYNAMIC adjustment is designed to correct for. The OEM has given only enough authority to correct for imperfections in these areas with very little capacity – if any, to correct for deviations outside of these areas – particularly when wear, tolerances & imperfections all approach limits and require the maximum Dynamic adjustment to counter the “Dynamic” induced vibration.

In addition to the vibration sources indicated above, under the Current Blade Management the Dynamic balance must also counter any Static imbalance between the blades which may be present. Often, this static imbalance is often outside the scope of the existing Dynamic Balance Authority adjustment – hence the “rogue” blade is born.

In short, the Dynamic Balance must correct for BOTH Dynamic imbalance PLUS the residual STATIC imbalance.

Total Dynamic Blade Balance = Dynamic Imbalance Induced Vibration + Residual Static imbalance induced vibration.

To ensure sufficient DYNAMIC adjustment authority is available to counter the Total Dynamic Blade Imbalance” – the Static balance between all (at least opposite) blades must be maintained within reasonably close tolerances. To ensure total blade interchangeability within the fleet or to any position on the one head this same tight control of Static Span balance must be maintained across the entire blade population within the fleet.

Dynamic tuning is expensive to perform

Dynamic tuning requires the operation of the helicopter which is expensive. Costs vary from several hundred dollars to several thousand dollars per hour to operate.

Dynamic tuning is needlessly time consuming if span balance is not close

The rotor system must be physically balanced within reasonable limits before the blades can be reasonably dynamically balanced i.e Rotor Track & Balanced (RTB). Most rotor systems do not have enough dynamic weight authority to allow adequate adjustment when blade repairs or blade exchanges are made. The only solution currently available and accepted industry wide is to try and find “matched” sets of blades. Countless man-hours and aircraft down time are consumed in attempting to find these “matched” sets of blades.

The chart below shows the amount of weight that is available for dynamic weight adjustments for a sampling of aircraft:

Dynamic Weight Authority for Various Blades

The Car Tyre/Rotor Balancing Analogy

The automotive industry abandoned the bubble balance when the spin balancers were introduced. It is always possible to balance an auto wheel, providing the wheel is not bent, because of the relatively large amount of lead weight that can be added to the rim.

It has been a widely accepted practice throughout the helicopter and fixed wing community to treat the balancing of the rotor/propeller much like that of a car tyre. The aviation community was led to believe that static balance of rotor blades would no longer be necessary with the advent of the dynamic tuning capability. It was believed that the static balance step was unnecessary, thus saving money through its omission in the overall track and balance process. Only OEMs and depot facilities continued to perform static balance.

The difference in the aviation application is that precious little weight is available to adjust in most applications. The mechanic is often forced into trying to solve a rotor track and balance problem with the wrong resource, i.e. trim tab or pitch change instead of balance weight adjustment. This precipitates track problems and/or transfers a lateral vibration problem into a vertical vibration problem.

There is a fundamental but important difference between aviation and the auto industry. The auto industry has eliminated the requirement of the static bubble balance since the amount of weight which can be installed on a tyre rim is quite large (in percentage terms) by comparison to that available on a rotor/propeller.

It is also generally put on the rim, which is where the tyre Dynamic balance puts the correction weight as well. The distance from the centre of rotation to where the tyre rim weight is installed has considerable Span Moment or lever arm effect, which means that a relatively small weight will result in a large correctional force.

By comparison, the rotor/propeller puts its Dynamic correctional weights at the HUB of the system, which by comparison to a tyre, has a very small moment arm over which to act to correct for any imbalance. There is generally sufficient weight authority provided on a rotor/propeller system to correct only for a pure Dynamic imbalance. If the same weight adjustment is forced to correct for BOTH Dynamic & Static imbalance, a large percentage of the weight correction will be consumed in correcting the Static problem leaving inadequate adjustment authority remaining for the Dynamic imbalance to be corrected.

Maintenance Costs Reduction

With the tooling available today, the ability is now in the hands of the operator to make these corrections and significantly reduce the blade maintenance cost he has been forced to endure over all these years. If helicopter on-going operating costs are to be lowered and if manufacturers are indeed honest in their promotional material about reducing DOC’s, ALL operators should be insisting on this ability for all helicopter types and if the blades don’t currently allow tip weight access – re-engineer blades to allow it and ensure that all new aircraft released onto the market allow such access. For blades which do not currently allow access to tip weights, there are alternatives available to correct for Span Moment deviations. These can be gained by contacting us.

Span balance is the single biggest factor in rotor smoothing

Span static imbalance is the single greatest cause of vibration in a rotor system, and static imbalance is the quickest and easiest to resolve with the proper tool.

Manufactures and Depots have coveted the static balance process as “black magic”

All rotor blades are designed with numerous adjustments. Some are designed for Dynamic and some for Static balance purpose.

Tip weights are designed into EVERY blade. MOST blades allow access to tip weights as this is where STATIC imbalance must be corrected – not by using Dynamic correctional adjustments. Because of the very large lever arm (distance from center of rotation to placement of correctional weights), only small weight packages are required to correct for quite large Static imbalance.

ALL manufacturers should be encouraging the use of, and engineering their rotor blades to enable tip weight access. Operators should be insisting upon it in order to reduce Direct Operating Costs.

Shown below is the tip of an AH-64 Apache main rotor blade. The tip cap is removed and the weight packages are resting on top of the blade where they would be inserted upon assembly.

Apache AH-64 Main Rotor Blade Weight Packages

OEMs have cautioned that if a weight should be slung, the result would be catastrophic. This is an understandable and commendable caution – but no different from any other caution which accompanies any Dynamic Blade adjustment – Pitch Change Link, Hub weight or Tab. The reality is that current maintenance procedures allow for the disassembly and reassembly of most weight packages. The maintenance techniques associated with weight package adjustments are conventional. The absence of an adequate tool for static balancing was the limiting factor in preventing balance adjustments at maintenance facilities other than the OEMs and depot facilities.

That tool is now available.

The Static/Dynamic Balance Relationship

Dynamic tuning is presented in terms of acceleration

Vibration is represented in terms of acceleration. Acceleration is measured in “inches per second per second.” Because the rotor systems are rotating objects, the dynamicist typically defines vibration in terms of inches per second or “IPS” by simply removing the reference to the revolutions per second of the rotating object.

The threshold of acceptable Dynamic vibration for most helicopter components is generally 0.2 ips.

Static balancing is presented in terms of moment

Static balance is represented in terms of the first moment, or a weight times the arm. In the United States the common unit of measurement for the first moment, which can occur in the span or chord orientation, is expressed in “inch-pounds.” This is very similar to the torque convention of pound-inches. A moment is simply an expression of torque.

How does dynamic compare to the static balance?

Dynamic tuning equipment is programmed with coefficients that describe the vibration change that will result by adding one gram of weight to a predefined location. These coefficients are generally described conveniently in ips / gram. By knowing where the dynamic weight package is located in a rotor blade, the conversion from ips to in-lbs is simple. The following chart describes the relationship between ips and in-lbs for the AH-64 Apache main rotor blade.

ips to in-lb conversion for the AH-64 MRB

The same technique can be applied to any rotor or propeller blade. The chart below displays the relationship between the vibration expressed in IPS and the static balance characteristic expressed in inch-pounds for several other systems:

ips per in-lb for various blades

Digital Static Balance - Repeatability

The repeatability of the digital static balance system is easy to test. It takes only four minutes to make a blade measurement and the answer is displayed digitally on the operator console. It is easy to measure the same blade numerous times, day after day, and determine the repeatability. The following chart displays the repeatability of one such digital fixture for various rotor and propeller blades.

Accuracy and Repeatability in terms of IPS

It is important to note that the only balance ultimately important is the dynamic balance result from the dynamic track and balance process.

The accuracy of the individual blade measurements for a ship set of blades is not important in itself, as long as all of the blades in the ship set are virtually the same.

Accuracy is important in sustaining the store of spare blades so that all blades are interchangeable, thus eliminating the necessity to static balance all of the blades in a ship set whenever one or more blades are repaired or replaced. Even though there have been “Master Blades” utilized that do not meet the engineering static balance specification, that has been OK when all of the blades in a ship set were made to look close enough, or virtually the same as the same master blade (i.e. using the same reference system), to allow dynamic tuning success.

Because accuracy is difficult to define, and it is not actually as important as the repeatability for producing a low vibration solution, it is prudent to look at repeatability first.

The repeatability of a static balance fixture is expressed in terms of the resolution of the fixture for a given rotor blade. This resolution is expressed in inch-pounds and will vary for different rotor blade models. Multiplying the demonstrated repeatability of the balance fixture by the factors that were developed previously, the repeatability of the balance fixture can be presented in terms of ips, which is the measure better understood by the pilot and maintainer.

The following chart expands the AH-64 example and derives the repeatability of the digital static balance fixture in terms of ips:

Conversion of Repeatability from In-Lbs to ips for AH-64 MRB

This same conversion has been applied to the other sample blades presented earlier in the following chart:

Repeatability Expressed in In-Lbs and ips for Various Blades

Another way to look at the impact and importance of static balance is to relate the maximum acceptable vibration level of 0.2 ips to an equivalent static imbalance in terms of moment and described in in-lbs.

It is a simple matter to divide the “ips/in-lb” factor derived previously into the maximum acceptable vibration level of 0.2 ips. The following chart presents those results:

In-Lbs Occurring at 0.2 ips for Various Blades

It is obvious from the chart that the demonstrated repeatability of the static balance fixture equates to a fraction of the final acceptable vibration level (0.2ips) after dynamic tuning. Knowing that the rotor and propeller blades are the single greatest contributor to system imbalance, it is easy to see why the dynamic tuning process is so greatly simplified after the blades have all been statically balanced. The dynamic tuner is now able to fine-tune the system because the most significant cause for imbalance has already been eliminated.

Real World Facts

Data has been captured on thousands of rotor blade static measurements. Data from two of the U. S. Army’s primary aircraft are displayed in the following series of charts.

This first chart displays the results of initial measurements of over 1100 different AH-64 main rotor blades. The manufacturer’s specification for span moment is 24,300 in-lbs. As the chart shows, blades ranged in value from 24,000 in-lbs to 24,500 in-lbs.

The equivalent of 0.2 ips has been superimposed as control limits on this chart to show just what 0.2 ips looks like in relation to the static balance argument for the population of Apache blades measured. In the case of the Apache, the equivalent of 0.2 ips is 62 in-lbs.

AH-64 MRB Span Moments with 0.2 ips Control Limits

This is a powerful chart. First, it impresses the viewer with the dramatic reality of how drastically the rotor blade static balance characteristics change over time and with use by observing the wide spread of span moment arm distribution.

Secondly, it clearly answers the question as to why rotor tuning can take so long to complete. The dynamic tuner only has 105 in-lbs worth of weights to adjust on the Apache. Dynamic balance is impossible if the span moments of opposing blades differ by more than 105 in-lb e.g. if one blade was 52.5 in-lbs “lighter” than an ideal blade (Span Moment Arm of 24,000in-lbs), and if the opposite blade was 52.5 in-lbs heavier than an ideal blade, there would be insufficient dynamic adjustment authority to dynamically balance the blades.

Thirdly, efforts to statically balance this rotor blade to an accuracy less than 3 in-lbs has greatly diminishing benefit. The resolution of this balance fixture for the AH-64 main rotor blade is 3 in-lbs. In terms of vibration, that amounts to just 0.0097 ips! The blue “tram tracks” represents the 0.2 ips value in in-lbs either side of the “ideal” blade.

The following chart is even more powerful as it shows the dramatic concentration of blade span moment arm around the “ideal” 24,000 in-lbs required for a AH-64 blade. This has been accomplished by using a digital balance tool and resetting the tip weights at operator level. All the blades which fall within the blue “tram lines” would be capable of successfully being dynamically balanced because they are within the 105 in-lb dynamic authority of the dynamic adjustment weights.

AH-64 MRB Span Moments after Static Balance

Note: This data has been taken from a number of US Army Apache units. The “before” data is taken as the first reading registered on the Balance Fixture. The “after” balance data is taken as the last recorded reading for the same serial numbered blade. The blades may or may not have had weight adjustments done by the final reading, hence the small number of blades with an apparent deviation in span moment arm. No “filtration” of the data has been performed to create an illusion of a miracle result – it is simple representation of the data as received from user operators.

The next chart depicts first measurement results for the population of Black Hawk main rotor blades reported. Here, the 0.2 ips equivalent happens to be equal to 32.5 in-lbs and has been superimposed on the chart. The manufacturer’s specification for span moment is 35,418 in-lbs. The range of moments for the Black Hawk is approximately 600 in-lbs. Because the dynamic authority for the Black Hawk is limited to 117 in-lbs, the Black Hawk maintainer has the same problem as the Apache maintenance officer. When the opposing blades differ by more than 117 in-lbs, dynamic balance cannot be achieved.

UH-60 MRB Span Moments with 0.2 ips Control Limits

By adjusting the purpose built tip weights to bring the Span Moment arms back to a narrow range well within the “tram track” range of the Dynamic Adjustment, then the blades become totally interchangeable again and the Dynamic RTB a simple and quick process as was originally intended.

This same pattern will be followed by all blade populations for every helicopter and propeller world wide – the problem is Span Moment Arm migration – the problem is world wide.

Conclusion

Digital static balance technology delivers results quickly, consistently, reliably, inexpensively. Many blades can be balanced on a digital balancer in the time it takes to balance one blade using conventional methods. The reduction in maintenance man-hours to balance the blades, the reduction in man-hours to dynamically balance the blades, the reduction of operational flight hours to dynamically balance the blades, and the reduction in expenses to support the static balance infrastructure (no master blades) all add up to a tremendous cost reduction in maintenance downtime and expense.

The United States Army now employs dozens of Universal Static Balance Fixtures within all levels of maintenance, depot, intermediate and unit. The United States Army has saved millions of dollars in reduced rotor blade returns to the manufacturers and depot repair facilities. The United States Army has saved millions of dollars in reduced maintenance test flight expenses by cutting associated activity in half.

Similar savings are available to commercial operators with the cooperation of the FAA and/or the Original Equipment Manufacturers (OEMs).

Rotor blades are an expensive asset and maintenance of those assets is critical to helicopter performance. Maintenance facilities stand to collectively save millions of dollars by adopting the “virtual master blade” in place of the conventional techniques requiring master blades.

To read more detail of this paper and the effects of using digital static technology in the US Army helicopter fleet, download our free copy of “Rotor Blade Static Balance – Art or Science” as presented to the American Helicopter Society in May 2003.