Rotor Blade Balancing – Rotor Track & Balance (RTB) & Helicopter Vibration
Rotor Blade Balancing
Balancing Helicopter Rotor Blades
Rotor Blade Balancing of a Helicopter is most commonly only thought of as being the Dynamic Rotor Blade balancing component of the complete Rotor Balancing Process. The Dynamic Blade Balancing process is referred to as Rotor Track and Balance or RTB. This is only one component of the entire rotor smoothing process and overall vibration analysis. Health Usage Monitoring systems (HUMS) have become more popular with operators and manufacturers over the last 20 years. It has not always been the case and was in fact quite difficult to get industry wide acceptance of the HUMS systems and the advantages that they provide when used, operated and managed correctly. Reducing main rotor and airframe vibration is a process and needs to be thought of as such. By only looking at one part of that process in isolation and conducting just that one exercise or component of the process, will lead to an aircraft that will feel uncomfortable to fly in, increased component wear and airframe fatigue including airframe structural cracking in the long term. Consideration must be given to the full spectrum of vibration that haunts a helicopter by the very nature of how it creates lift. This vibration ranges from Low, Medium and High frequency vibrations. Some are functions of the mechanical components and their rotation. Some are functions of the aerodynamic interaction of both the airframe and the rotors such as Nper rev vibration. Helicopter rotor smoothing to reduce all airframe vibrations is a methodical process. It includes not only Rotor Track and Balance (RTB) but also vibration analysis to tune Nper rev vibration absorbers to absorb rotor and airflow induced vibration but also to identify worn airframe components as well.
RTB Equipment Development
RTB equipment hardware and software has all but plateaued in development.
Accelerometers: Have not changed greatly in many years. There are two main types depending on the brand of Rotor Track and Balance equipment you are using. Some will use the basic accelerometer.
As the name implies, it measures pure acceleration in one direction or axis only. It uses an inertial mass as its sensor. The most common in use for vibration measurement is the piezoelectric type. This relies on the compression forces
that a small mass will impart on a crystal whose electrical properties will vary under compression. As the mass presses harder against the crystal under acceleration, it creates a varying and corresponding electrical output. This is proportional to the compression force applied. It normaly registers the impact force in terms of “g” force. This electrical acceleration signal is sent to the processor where it is integrated and applied to the algorithms within the processor to produce a solution.
The Velometer is in essence, an accelerometer but with a basic amplification & integration circuit built into each sensor’s base. This both amplifies the acceleration signal and integrates the basic electrical output from the accelerometer which is essentially a “g” force (or acceleration). It then converts it into a corresponding velocity signal or displacement reading, generally in terms of inches per second. Generally speaking, a Velometer can produce a more linear response over a far greater Frequency range than that of a accelerometer. It tends to be particularly good at the medium frequency range.
Cabling: has remained essentially unchanged. Most new helicopters, particularly multi-engined models, come with accelerometers/velometers and cabling already hardwired and installed. Often they are already fitted with onboard aquisition computer and blade tracker sensor/camera as well.
Processor: The “brains box” has not changed a great deal in the last 10-20 years since the advent of the modern PC chip. Only
presentation and limited functionality has really changed over the recent past – not the efficiency of how we conduct Rotor Track and Balance. Calculating RTB solutions does not take large amounts of computing power and the capability has long been up to the task. Manufacturers of RTB equipment offer variations between models which simply differ only in packaging, presentation & functionality. Or the cheapest price??
The decision depends on what the operator finds more user friendly and most suitable for there specific purpose. The only real product difference in essence, boils down to what the operator prefers in looks, functionality and “intuitive” interface. Much the same as the differences between buying phones and computers.
The only real improvement has been in the speed of the processor within the “black box”. This speed has helped to reduce the amount of time taken for inflight data aquistion and storage to memory. This has resulted in slight reductions in the time required for an aircraft to be maintained in level, unaccelerated flight (or given flight regime), while the processor samples the accelerometers /photo cells or camera data. The increased processor speed enables quicker sampling and recording of the nominated sample taken.
Most other improvements have been cosmetic, display & user interface orientated.
Blade Trackers: The current generation of blade trackers which are generally a line scan camera based system, are still reasonably accurate.
But they may suffer from occasional difficulties in locking on to a track in certain light or infrared conditions. Particularly if the sun or light is not favourable intensity or position. Some equipment don’t even use this track information to determine an in-flight solution after the initial track is established. It is often only used to establish an initial “flat” track condition only. After flight regimes are commenced, only weight/PCL/Tab/sweep are used and the track is not even considered. Hence, it is not unusual to have blade tips flying with a considerable
track split to achieve an overall smooth result.
From this flat track condition, begins the RTB exercise – even though the camera is mounted on the helicopter and the illusion of it using the in-flight track condition for the Rotor Track and Balance solution is created. The final solution to obtain a smooth rotor system may result in the blades requiring to be flown OUT of track to obtain the smoothest ride. This is purely a function of individual blades and rotor systems.
Contrary to popular belief often held, blade tips do not necessarily need to fly in the same plane in order for the entire rotor system to be balanced to a smooth condition.
However, the initial starting point for most Rotor Track & Balance exercises when starting from scratch, is to set the initial ground track to within approx 1/4 inch. This is normally what most manufacturers use as a start point. It is done by setting the Pitch Change Links/Rods to a nominal length normally provided in the maintenance manual. Then a ground run at flat pitch ground and then flight idle power setting is performed. The Track picture is taken. Either by the Camera in today’s technology or by strobe gun of reflective tape on the tips of the blades in the earlier generation RTB equipment. Or perform a “flag” track. On a KA32 with a coaxial rotor, they would use a strip of cardboard and provided by the tips have been chalked with different coloured chalk, slowly feed the cardboard into the rotating blade tips. This would leave witness marks on the cardboard to describe how far apart each blade was tracking on the ground at flight idle.
Adjustments Available for Establishing Ground Track: Use only Pitch Change Link/Rod to adjust individual blades pitch to change the tracking of the respective blades. Any changes to the PCL/R are generally measured in terms of “flats” i.e. flats of the turnbuckle nut which will vary the length of the PCL/R.
e.g 1/2 a flat or 2 flat adjustment.
Other PCL/Rs may be measured in “Notches” as the top of the PCL nut may be a castleated nut and may have “notches” which may facilitate a locking mechanism.
Blade Tabs are normally reserved for adjustment if track and vertical vibration increases with increasing airspeed.
Consult the respective maintenance manual for the correct adjustment for each particular model.
Reducing RTB Costs
Dynamic Rotor Track & Balance (RTB) has long been a significant cost in maintaining helicopters. This cost is a factor of time consumed in tracking and balancing. It translates directly from wasted man hours, flight hours and significant aircraft downtime resulting in the aircraft being unavailable for tasking or revenue flights. Each RTB event can often be anywhere between one and up to five or more days depending upon the skill of the maintenance personnel, maintenance test pilot, RTB equipment used and the condition of the blades.
It is being increasingly recognised that the greatest cost savings to be gained from any HUMS system for the least cost outlay is by obtaining savings in the RTB area. Defence Science and Technology Organization (DSTO) Australia, who has conducted several international HUMS conferences over recent years, has produced a cost analysis program which can quantify savings and expected cost benefits which may be had from adopting certain aspects of condition monitoring. This program called HUMSSAVE enables dollar values to be inserted against capital equipment costs, hardware costs, man hours, aircraft operating costs etc to provide a dollar value per aircraft operating hour for the expected savings (or otherwise) of adopting certain elements of, or complete HUMS systems for specific fleets and types of aircraft.
It is easily demonstrated that the most cost effective component which provides the greatest and quickest return – is reducing the amount of time spent on RTB.
The greatest savings in RTB are being provided by a number of options:
- Hardwiring: permanently installing the cabling and accelerometers into every aircraft in the fleet so that a RTB junction box & analyser can be walked on board and plugged in to the existing cabling and an RTB exercise commenced.
- Permanent fit: In addition to permanently hardwiring for cables & accelerometers, a junction box and analyser is also permanently installed. This is increasingly becoming standard on new model helicopters. But there is still a very large population of legacy, older helicopters around the world.
- Improved RTB Efficiency: Making existing systems capable of working more efficiently to obtain a solution. This is by far the simplest and easiest, and cheapest efficiency ANY helicopter operator can achieve with miniaml effort and cost. This is what RWAS is trying to provide through education and this web site.
Hardwiring and Permanent Installations of RTB equipment is a very capital expensive exercise. Remember, you are paying for a complete system PER AIRCRAFT. Not just one unit to do an entire fleet. It is very capital intensive if retrofitting such systems.
Not withstanding the increased maintenance for replacement accelerometers, broken connectors etc throughout their in-service life, the fleet operator must also engage either a subcontracted service or additional staff to analysis the routinely downloaded data. With fully integrated HUMS comes the added expense after installation of the man hours required in both data downloading and processing in addition to the inevitable “system upgrades” through the life of the HUMS/RTB system.
Improved RTB Efficiency
The most significant potential saving in RTB to be had, is by adopting a simple, inexpensive measure which EVERY operator can do – with almost no impact on his current way of operating. It is called Span Moment Arm Control (SMAC). It allows your exisiting Rotor Track and Balance equipment to work more efficiently and obtain a smooth ride sooner than traditionally has been the case.
It has the unique and supremely important advantage of making ALL the blades completely interchangeable thorughout the fleet. It eliminates “rogue” rotor blades.
It simply requires that instead of thinking ONLY dynamic RTB when considering balancing a rotor blade, the WHOLE balance problem must be considered – static & dynamic balance and their inter relationship.
By doing this, the typical results to date is that the average Rotor Track and Balance exercise for a helicopter is in the order of 1-3 flights before it is smooth – with minimal weight being required on the hub to dynamically balance the head. For CH47, typically the flights are down to as few as 4-5 flights or less, before it is flown serviceable.
To help understand and de-mystify Rotor Track and Balance, we must look at rotor blade adjustments available, their purpose and who can play with them. After reviewing Rotor Blade Adjustments that are available, we must then look at the vibration likely to be encountered and corrected by the dynamic RTB smoothing process.
Associated with, but separate from the RTB smoothing, N per rev vibration needs to be discussed and looked into. These are often classed as a medium frequency vibration but can often be more fatiguing and damaging than the classical 1/rev vibration typically thought of with RTB smoothing and helicopter flight.
Rotor Track & Balance Smoothing
Types of Vibrations in Helicopters
Main Rotor Vibrations
Mechanics are primarily interested in vibrations felt during in-flight or ground operations as felt in the cabin. Most vibrations are always present in the helicopter at low magnitudes. It is when the magnitude of any vibration increases that it becomes of concern. The main problem is deciding when a vibration level has reached the point of being excessive.
Normally the accepted IPS level for most models of helicopters is around the 0.20 – 0.25IPS. This is normally regarded as the upper acceptable serviceable limits. This figure may vary a little depending upon models and manufacturers. Consult you Maintenance Manual for the prescribed limits for your helicopter. Larger helicopters may have a slightly larger tolerance for example a CH47 Chinook is 0.40 IPS. For any Rotor Track and Balance exercise, the ideal goal is to aim for the smoothest ride possible. Ideally below 0.10 IPS if possible throughout the flight regimes.The reason for aiming for the least vibration level is that it will reduce the harmful effects of large amplitude vibration on damaging airframe, avionics and crew fatigue through its service life. It will also provide to longer time in service before another RTB and “tweek” is required due to dynamic component fair wear & tear.
Extreme low frequency, and most medium frequency vibrations are caused by the rotor or dynamic controls. Various malfunctions in stationary compartments can affect the absorption, damping or masking of the existing vibrations and increase the overall level felt by the pilot. A number of vibrations are present which are considered a normal characteristic of the machine. The N per revolution (N/rev) vibration is the most prominent of these, with N+1/rev or N-1/rev the next most prominent. There is always a small amount of high frequency present. Flight experience is necessary to learn the normal vibration levels for any model of helicopter. Each model has its own signatures and vibration levels. Even experienced pilots sometimes make the mistake of concentrating on feeling one specific vibration and conclude that the vibration level is higher than normal when actually it is not. It simply seems so because the pilot is concentrating on it.
For simplicity and some sort of standardization, vibrations are arbitrarily divided into general frequencies as follows:
Extreme low frequency – Less than l/rev, Normally limited to suspended transmission mount installations such as in the Bell helicopter types pylon rock, loose skids/mounts, loose cargo doors vibrating in their tracks, hoist mounts, loose fitting externally fitted role equipment.
Low frequency- l/rev or 2/rev type vibration – Generally rotor blade induced or hardware associated with the rotor system. A function of Nr.
Med. Frequency- Generally 4/rev or 6/rev, commonly is a N/rev vibe in multibladed systems
High frequency- tail rotor or faster, Hydraulic pumps, cooling fans, etc
Very Low Frequency Vibration – <1/rev
Extreme low frequency vibration was not uncommon in teetering head systems.
Those with Main Transmissions with spring/rubber bearing mounted or dampened systems and was commonly called pylon rock. This is encounted with suspended transmission systems which is typical in Bell helicopters such as the UH-1/B204/205/212/206 series. Rigid mounted transmission common on most fully articulated or rigid rotor systems.
Other sources of very low frequency vibration can be from:
1. Loose skid mounts, – check mounts, rubber cushion absorbers, spring tensions etc
2. Loose cargo doors vibrating in the airflow in the tracks, – check sliders for wear
3. Loose mounts of role equipment – eg Night suns, FLIR ball turrets, external stores, hoists, etc
4. Loose/worn Lift Link bearings (some transmissions on suspension mounts eg UH1
Adjustments Available : Tighten any loose attachments, supports, replace worn bushings, spring mounts or broken mounts or absorbers.
Low Frequency Vibration – 1/Rev Vertical & Lateral RTB
This is the “classic” Rotor Track & Balance. Low frequency vibrations, 1/rev and 2/rev are caused by the rotor itself. 1/rev vibrations are of two basic types, vertical or lateral. Smoothing of 1/rev verticals is essentially a trial and error process although most rotor heads behave reasonably predictably. See below in the description for the Making of RTB Equipment for a better description of the predictability for correcting rotor system imbalance. Modern Rotor Track & Balance equipment is pretty much automated.
As discussed elsewhere on this site, a basic polar chart to determine corrections can be manually made by mechanics skilled in RTB and vibration analysis. This is called “going back to basics”.
However, most equipment will provide a solution reasonably quickly and reliably. Most will give a multi move adjustment which will consist of both available Dynamic adjustments for a vertical adjustment and also for a lateral adjustment as well. This is a common capability of most electronic computerised RTB equipment nowadays.
Traditionally in the earlier days when Flag tracking or the early days of strobex tracking guns and rudimentary accelerometers, it was not uncommon to do only one adjustment per flight. First the Laterals. Then the Verticals. This was in order to keep the whole procedure very methodical and monitor the effect that every adjustment made on the rotor system.
The computerised Rotor Track and Balance equipment has made RTB a much more efficient exercise but innovation has plateaued in this field over the last 20+ years with minimal improvement in efficiencies.
If troubles are encounted, then some basic trouble should be investigated this may include:
1. Accelerometer/Velometer/optical tracker – Mounted correct way up
2. Cables – connected to the correct channels
3. Accelerometer/Velometer – serviceable? Is it calibrated/working?
4. Making adjustments in the correct sense?
5. Worn components – Main Rotor – PCL bearings/play, swashplate play, worn slider sleeve, scissor arms/bearings, tab settings, etc
6. If still cannot get the dynamic adjustment to work, then that really leaves only one other thing. Span Moment Arm Migration.
Basic 1/rev Adjustments – Vertical
WHY? : A 1/rev vertical vibration is generally caused simply by one blade developing more lift at a given point of rotation than the other blade/s develops at the same point. Ideally, blades normally will flap to equality to try and equalise lift being produced across the disc. This is admirably demonstrated in a video available to view through our downloads.
But any one blade may produce a differing amount of lift at the same point of rotation e.g. at the 3 o’clock position, than the other/s blade/s.
This will give a distinct, apparent “Vertical” bounce. If large enough, it will be quite noticeable by pilots/crew. The easy way to see it, is to look across the cockpit and observe the knees/body of the other pilot. If you are both “bouncing” vertically at the same time, you are experiencing a true “vertical” vibration. This is caused by one blade creating a significant different amount of lift than the other/s at the same point of rotation and transmitting this energy equally to the entire fuselage via the rotor head/Tx in the vertical plane.
If you look across the cockpit and you see the Left Hand seat pilot (or parts thereof), apparently “bouncing” up and down at 180deg out of phase from the RHS pilot, you are experiencing a “Lateral” vibration. You will see the LHS pilot rising vertically in his seat as the RHS pilot is descending…..180 degrees out of phase. Because the lateral imbalance is applied across the fuselage (not vertically), it results in one side of the aircraft going down while the opposite side is going up.
Rigidly controlled manufacturing processes and techniques, eliminate all but minor differences between blades, resulting in blades which are virtually identical. The minor differences which remain will affect flight but are compensated for by adjustments of trim tabs, pitch settings and Dynamic Balance weights. The manufacturer creates these adjustment stations and features to fine tune the rotor system in order to minimise vibration due to small dissimilarities between blades and wear and tear in the dynamic components.
This is what the “Dynamic” adjustments are designed for. They are designed to correct for SMALL differences in blades, the manufacturing process, the wear and tear of rotating components (blade erosion, bearings, bushings, play, tabs, Transmission mounts wear, Swashplate bearings and liners, scissor arms, etc). It enables the operator to keep the machine relatively smooth throughout the life of the blades/Transmission/Head.
You will note that the “Dynamic” adjustments are located close to the hub of the rotor system (except tab). This provides a relatively SMALL effect when a change of either Pitch Change Link/Rod (for Vertical vibe) or weight adjustment (for Lateral vibe). Because these Dynamic Adjustment stations are all close to the hub, relatively large weight and PCL adjustments have only small overall effect. This is because the moment arm over which they operate is only small. It makes for an easier and more manageable adjustment process.
The Trim tab however is normally located either midspan or on the outer span of the rotor blade. Some blades may indeed have 3 or more tabs. One located on the inner span, another mid-span. and a third located on the outer span.
Vertical Adjustments Available:
Pitch Change Rod/Link (PCL): The PCL is used to initially “set” the initial track to a start point prior as the very first thing to be achieved while doing the initial ground runs.
OEMs like to have a start point from which to commence the whole procedure.
Since new blades are normally quite closely matched from manufacture and after factory static balance, it is assumed that they should all pretty much fly together if all blades are flying in the same tip path plain. This makes it a good assumption for a start point.
Any changes to the PCL/R are generally measured in terms of “flats” i.e. flats of the turnbuckle nut which will vary the length of the PCL/R.
e.g 1/2 a flat or 2 flat adjustment.
Other PCL/Rs may be measured in “Notches” as the top of the PCL nut may be a castleated nut and may have “notches” which may facilitate a locking mechanism.
Consult the respective maintenance manual for the correct adjustment for each particular model.
Normally, most manufacturers like the track-split to be no more than about 1/4inch at Flat Pitch Ground (Flight Idle RPM) before commencing the Rotor Track and Balance air exercise. It is often common practice for most Maintenance test pilots to “set” the track a little by applying just a little collective to stabilise the track for this. This is the start point.
When the aircraft is lifted to the hover, the Rotor Track and Balance equipment will determine the vertical components and lateral component. Normally it is desirable to try and fly as many of theflight regimes that the particular RTB equipment and maintenance manual for your particular helicopter calls for. The more Flight Regimes that can be measured, the more comprehensive the solution can be and greater chance of a successful solution in a shorter possible time.
It will give a better picture as to whether only PCL is to be used or perhaps Tab may be required if the
vertical component is increasing with IAS.
This of course is limited by what the crew determine to be a safe-to-fly vibration level. You should only ever take the aircraft to a flight regime that is deemed comfortable and not to fly the aircraft in regimes where the vibration is excessive and potentially damaging to airframe or components.
Trim TAB : TAB is used generally if the vertical IPS level increases with an increasing IAS. The Trim tab is normally located either midspan or on the outer span of the rotor blade. Some blades may have a number of tabs. One located on the inner span, another mid-span. and a third located on the outer span. The Bell412 is one such with 2 x Trim Tab stations. The RTB equipment will provide specific Tab stations to adjust depending upon how the vertical vibe manifests itself with increasing IAS.
A big danger with some rotor blades is over bending the Tabs. Large deflections on Trim Tabs can lead to these adjustments washing out over in-service use due to airflow and airspeed. Tabs can develop into “Soft Tabs” which also can loose their adjusted angle with routine use. They can also get work hardened if over used and potentially crack. A TAB Bending Tool for suitable for your particular blades is certainly a worthy investment. This takes out the guess work and makes Rotor Track and Balance a more predictable exercise.
Basic 1/rev Adjustments – Lateral
WHY?: A lateral vibration is caused by a spanwise imbalance of the rotor system. This may be due to a difference in mass or imbalance of the hub or stabilizer bar due to faulty installation. A slight misalignment of the Transmission/mast/split cones/stabiliser bars/mast mounted vibration absorbers/grease in blade grips/contaminated blades, other slight imperfections in the Transmission/rotor head assembly or more commonly, Span Moment Arm of a rotor blade (will cover later).
On Articulated rotor heads, blade dampers can be a source of a lateral imbalance. If the damper is unserviceable (failed internally) or underserviced (incorrect oil level), it will alter the dampening characteristics every revolution as lift & drag across the blade varies as dissymmetry of lift between the advancing and retreating cycle of each blade coupled with airspeed. This action will cause the blade to Lead & Lag excessivley. This action, if the blade can physically move more fore & aft excessively due to a malfunctioning damper, effectively changes the relationship of the individual blade’s span CofG in relation to the centre of the Mast/Hub. it brings the effective CofG closer to the Centre of rotation when compared to that of the other, “normal” blades. This small change in the faulty damper’s blade span CofG is enough to cause a lateral imbalance every revolution.
This small change in Blade Span CofG due to blade Lead & Lag, is the same principle that is sometimes used on teetering heads to adjust for lateral vibrations. It is the technique called “varying the sweep” – or adjusting the drag brace. It is a common method to try and adjust for a lateral imbalance which is actually caused by the change in mass distribution due to other reasons.
Sweeping the blade is usually resorted to after the more conventional and correct method of adjustment, i.e. adding/subtracting weight to the hub, has been exhausted or there is insufficient room for any more weight to be added to the weight pocket station.
This problem should really be addressed and corrected by altering the blade’s static balance adjustment – the TIP weights. But because manufacturers have denied the average operator and service organisation from playing with TIP weights, RTB equipment manufacturers have created another way to partially make up for this deficiency in product support.
Most mechanics who have done Rotor Track and Balance on a teetering head would have come across the the polar charts and the use of sweep to compensate for a lateral vibe. He will also have come across the “clock angle correction” used as well. This is used when move-lines are not going to move as they should. When it looks like it will clearly pass tangentially, and not toward the ideal middle of the clock chart in order to “zeroise” or reduce the IPS level. So a clock angle correction is placed on it and more often than not, this requires blade sweep.
What is “sweeping the blade” actually doing? It is reducing the Blade Span Moment arm for that one blade. It is effectively making that blade’s Span CofG closer to the centre of rotation. It is the same effect as removing weight from the TIP Weights – which is what we really should be doing if the manufacturers would let us.
The best flight regime to determine a faulty damper is with the helicopter in a steady Rate of Descent. About 60Kias or 80Kias and 1,000ft/min. It MUST be a steady state descent with Airspeed and ROD held constant. This will highlight a lateral caused by a faulty damper.
The most common persistent Lateral imbalance is generally because of a difference in Span Moment Arms of the blade/blades. The alignment of the span CofG of each of the blades with respect to their span axis.
Contrary to popular belief, the physical difference in mass of each rotor blade is rarely if ever the problem. It is the Distribution of Mass of each blade. It is the difference between individual blade Span Moment Arms which is the most common problem. This is why every helicopter engineer and pilot has experienced “rogue” rotor blades. Blades which cannot be flown with other rotor blades and made to dynamically balance. Or being forced to fly certain “sets” of blades. Limiting which blades can be flown on certain individual helicopters. Forcing operators to hold a needless quantity of blades as spares. Needlessly having aircraft unserviceable awaiting for a “new” blade to come because another blade could not fly with existing fleet blades. Endless swapping of blades and the wasted man hours associated with this activity, just to find a “Set” of blades that will fly together.
The importance of Span Moment Arm as far as Lateral Vibration cannot be over stated. As an example, a AW139 was test flown and RTB serviceable to less than 0.05 IPS Lateral in all regimes. It was stored in an open hanger and not flown for some 2 weeks. The open hanger had an abundance of steel rafters overhead. These rafters were ideal for the local bird community to roost in of a night time and take shelter. The rafters were positioned is such a way that the favoured roosting rafters were located directly above 2 x blades of this particular Helicopter. After 2 weeks the aircraft was ground run. On start up the Lateral Vibration was very noticeable. Luckily the pilot doing the ground run also did the RTB Test Flight 2 weeks earlier and KNEW that this is not right. When the onboard HUMS system was consulted to quantify the Lateral reading, it was now indicating 0.45IPS at flat pitch Flt Idle. After shutdown, the crew had a thought and inspected the upper surfaces of the blades.
Below is what they discovered. Two blades contaminated with bird pooh. The other 3 blades reasonably clean. Mass Distribution is critical to control Lateral vibrations.
Common Causes Of Span Moment Arm Migration – Lateral 1/Rev Vibration
1. Dirty Blades. As discussed above. Uneven dirt and grime accumulation can cause lateral imbalance changes creating a significant and noticeable lateral vibration in the 1/rev range.
2. Uneven paint jobs/layers. Common on outer portions of blades due to routine paint touch-up jobs. Repetitive overspray gets thicker toward mid-chord and toward the trailing edge while the Leading Edges constantly gets eaten away. This has a cumulative effect and migrates the span moment arm when compared to original specs.
3. Local blade repair, dent repair, bog, sealant added, etc. It all alters the mass distribution along the span….over a large lever arm from the centre of rotation.
a. Some blades are notoriously porous. The Fibreglass weave can allow capillary action to occur especially if blade surface erosion has exposed the skin in any way. If a porous paint finish is used on the blades, this also allows molecular access, capillary movement and Nett absorption of water into the honeycomb core structures. Once it is inside, it is very difficult to remove unless special procedures are utilised.
b. Exposure to Large Diurnal temperature changes. Warm moist (high humidity eg tropics/maritime environment) coupled with cooler nights is conducive for saturated air to be absorbed through the porous weave of the blade skins. Once inside, the cooler night time temperatures causes condensation inside the honeycomb or cellular structures inside the core of the blade. This also leads to a Nett gain in condensed water trapped inside the blade. This can add a deal of mass along the span of the blade. Because of the large moment arm over which this may operate, it has a dramatic and often noticeably large effect when dynamically balancing blades. This trapped water can be removed. But required overhaul facilities to do this. Old techniques used by OEMs were to literally drill holes in the blade skins and leave for considerable time. This was a very time consuming, costly and inefficient way of rectifying this problem. Most OEMs simply returned the blades back to the original condition. One company has developed a unique and vastly more efficient method of removing trapped water/fluids from blade structures. They also go one better than OEMs by using their own “skin sealing” procedures which reduces the porosity of the blade skins thus reducing and even largely eliminating the recurrence of capillary ingress of water.
c. Trapped water in upper pockets in blade surfaces. Some blades have flush weight pockets on the upper surfaces of blades. E.g B412, Aw139, Bk117 etc. It is not uncommon to have the seal of these pockets compromised and allow water to seep into the pockets. The operator has no way of knowing except by the increased lateral vibration and by undoing the pockets and physically inspecting them. But then this means you also break the seal…if it is actually in tact. So you then have to reseal it and again run the real potential risk of compromise.
5. Blade erosion. Some blades erode quite badly. It is dependant upon the environment, but dusty, sandy environments create a highly erosive environment and is harsh on rotor blade leading edges. Bell206 blades have notoriously “soft” leading edges and flying in tropical environments in areas of high rainfall with large drop size encounted in tropical thunderstorms, the Leading edges get severely pitted and eroded. If a salt water factor is added to this equation, quite jaggered and uneven pitting can be experienced. This leads to significant Span Moment arm migration. If one of this blades is tried to be flown with a brand new blade, it will be a near impossible task.
SOLUTION: The ANSWER to ALL the ABOVE problems and to keep the rotor blade on the hub for longer and still be able to fly these blades with ALL other blades, including NEW blades?
Lateral Adjustments Available:
Weight: Adding weight to assigned weight stations. These are often co-located in the hollow, blade retention bolts. Or specific weight stations located on the blade spindles or hub area, with maximum weight load capabilities where weight packages/washers are attached. They are normally located close to the hub. This allows for reasonable amount of mass adjustment for a small, but predictable effect. It has traditionally been a classic mistake to also perform static balance adjustment on teetering heads using the same Dynamic Adjustment stations to correct for a Static balance problem. This generally has resulted in running out of weight adjustment capability due to the pockets getting filled up.
Teetering Heads – Sweep: Blade sweep is an added method used for 2 bladed teetering rotor heads. It is a method which by default, alters Span CofG or Moment Arm. Some people believe it is the effect on the chord CofG. This has minimal effect because the chord moment arm is so small. On the other hand, a small change in the Span Moment Arm has a substantial effect for such a small change in mass distribution. This is because of the lever arm over which the Span Moment Arm has affect. It is several feet or meters giving a much greater total change or effective Moment. Whereas the Chord moment arm only operates over a matter of inches or Millimetres. A much smaller moment arm by comparison providing a very small effective Moment. This is why blade sweep came into use. To provide another “tool” in armoury of RTB for Lateral vibrations on teetering heads. It is being used as an unofficial adjustment to tip weights without actually adjusting tip weights….but it does have a small effect on the Blade Span Moment arm i.e a small effect on the Span Moment Arm….just not as effective as if you adjusted the TIP Weights.
The REAL “tool” should be the ability to adjust TIP weights to correct for Span Moment Arm problems. To keep the Span Moment Arm of every blade within reasonable tolerances of the OEMs design criteria. This would make Lateral & Vertical Vibration control by RTB a very easy and straight forward process. You would be assured of the FULL range of DYNAMIC adjustment authority to correct for any misalignment, bearing wear & tear, erosion, trapped water, etc.
TIP Weights: These as the most effective but least used “tool” available to keep our blades Laterally balanced and by default, assist in controlling the vertical balance. The TIP Weights should be used ONLY in conjunction with a recognised DIGITAL Static Balance Tool.
They are used to accurately maintain Span Moment Arm within designated OEM tolerances. By doing this on a routine basis, it ensures FULL authority is available for DYNAMIC Lateral weight adjustment packages to actually work before running out of authority. If the DYNAMIC Lateral balance runs out of authority with insufficient adjustment available, you will see the move lines of the Polar Chart moving tangential to the origin and failing to get within and acceptable ride for vibration levels. You have a “rogue” blade in the making. Or a blade shuffle to try and find one that will fly with the others in one set. You now condemn yourself to limiting your blade population to flying in pre-designated sets – This Does not have to be the case. There is an easy solution.
Unfortunately most civil helicopter manufacturers are reluctant to let operators and maintenance facilities to adopt TIP Weight Adjustment. This is probably because up till now, all OEMs have only ever thought of physical Master Blades as a means of adjusting TIP Weights. With the arrival of the USBF in late 1990’s, this has now changed. Unfortunately though, OEMs are still reluctant to approve the use of the USBF on their blades…for what ever reason. Many “commercial” Helicopter types/models have had their blades adjusted using the USBF. This has been done under the coverage of various military organisations around the world who have provided the Airworthiness Authority to use the USBF on these helicopters.
The USBF is in widespread use throughout many militaries and even OEMs around the world. It should be in wide spread use with civil operators and maintenance organisations around the world. It would provide SIGNIFICANT cost and time savings in Static balance of rotor blades. Blades would rarely have to go back to OEM. “Sets” of blades would be a thing of the past. Massive savings in freight costs. In saved man hours. In reduced blade inventories to be held.
Massive industry savings. Industry simply has to put pressure on OEMs for approval to use this tool on their blades to release the massive savings it WILL deliver. Simply ask Colombia Helicopters what they think of their USBF and how it has performed over the years.
IF RTB Corrections Don’t Appear to Work?:
If the RTB Vibration corrections do not result in the desired correction, then alternative causal factors must be investigated. Start with:
- Have ALL the RTB corrections been applied in the correct sense?
- Is the RTB hardware installed correctly? Accel/Velometers/Cabling connections.
- Airframe components –
- Transmission – mounts/swashplate/scissor arm bushings/bearings/Lift Links
- PCR/L bearings – condition/wear
- Elastomeric bearings – condition
- Greased heads – equal grease?
- Stabiliser bars
- Head mounted Vibration absorbers/bearings/bushings
- Span Moment Arm…Static balance of ALL blades. Consider:
- Trapped water – upper surface weight pockets.
- Damaged paint surfaces – leaking water through porous skins.
- Build up of multi layers of paint toward the Mid-chord line if blade tip erosion paint touch up is carried out routinely in abrasive environments.
- local blade repairs – bog, additional sealant added to the blade or blade tip joints.
- Physical blade erosion – pitting, missing/altered blade surface profile from erosion .
SOLUTION: – using a Digital Static balance Tool. Use Tip weights to adjust….re-do RTB.
Whirl Towers: Some manufacturers think that if they statically balance a blade and then put the blade on a whirl tower, then this will make all blades throughout the world interchangeable and able to fly easily with each other. In an ideal world this would be true. However we are not living – or flying in an ideal world. They call this procedure “pre-track” of the blade. They set the Blade Trim Tabs against a master blade on the whirl tower.
The reality is, using a whirl tower for production blades is a waste of time. A whirl tower can really only be justified for experimental and research data and mapping of experimental, research or new blade designs for aerodynamics and structural responses. There is extremely few, if any reasons why a production blade should require the services of whirl tower. Unless the dynamic integrity of some repair were to be in question. in which case, you would have to ask the validity of the OEMs own repair manual, procedures and instructions.
Think about the procedural process if a production blade is “pre-tracked” against a physical “master” blade. They would first set the trim tabs and PCR/Ls to nominal. Then on a static whirl tower, they would spin it against a Master Blade. They should first set the track against the Master Blade using PCL/R. They should then apply Collective pitch to approximate a hover power setting. This is to check the climbing/Diving characteristics of the blade as pitch is increased against the master blade. To correct any divergence in tracking climbing/diving properties, the Chord Moment Arm (weight adjustment close to the blade root generally, or maybe the distribution for the tip weight packages) should be adjusted to stabilise the track tendency with pitch application. They may then further adjust the Trim Tabs to fly the blade in reasonable track and 1/rev Vertical – after adjusting the PCL/R to also a nominal setting against the master blade.
However, the big limitation with the Whirl Tower and therefore the effectiveness of pre-setting tab settings, is that a Whirl tower can only ever simulate a static condition or hover condition.
It is not experiencing ANY relative wind other than rotational velocity i.e. it can only ever simulate a hover condition. Not a 60kt/80kt/120kt/135kt or Vne condition. TAB on a blade is there to primarily control the tip path plane & lift variation of the blade as Airspeed increases. This cannot be done on a static whirl tower.
The only thing that could reasonably be achieved is to preset reasonably accurately, the Static Chord Moment arm and adjust the Chord Moment Arm to ensure that the track of the blade does not climb or dive when collective is applied and pitch is increased.
But this same adjustment is already effectively being done by operators of Bell412 when they adjust the “Product Weights” in their blades. The se alter the dynamic chord moment arm. It is done at operator level. It is done using standard RTB equipment. This in turn has a direct affect on the tracking qualities of the blade as collective pitch is applied from flat-pitch ground to the Hover.
Again, once these blades become “production blades”, both the Span and the Chord Moment arms are purely mathematical engineering values. they DO NOT NEED to be spun on a whirl tower. It can easily, quickly and CHEAPLY be done on a Digital Balancing Tool.
Making RTB Systems: Dynamic Balance Equipment manufacturers (Chadwick Helmuth, Helitune, ACES, RADS, etc), use a sample head which is in a known, good condition to “map” out the various adjustments required and magnitude of change and in what direction the changes occur. They do this like all good artillery men. They will make one adjustment at a time and measure the effect. They will vary this by known, set amounts. One step at a time noting the changes. One large adjustment and one small adjustment. This methodical technique “maps” that particular model of rotor head and enables “polar” charts” to be made at each flight regime. Because the manufacturing tolerances of every model helicopter are very tightly controlled, the same adjustments are very reliably repeatable.
The resultant test data is analysed. This is then amalgamated into a software package or “SmartCharts”, “characteristics”, “personality” or whatever unique term each manufacturer deems to call these programs/software. This then allows with reasonable reliability, multiple adjustments and ability to forecast the most likely result based on the data base and experience the RTB Equipment manufacturer has put in the software.
With the new computer-based systems, “smart” charts and algorithms are created to expedite the RTB process.
A good mechanic could make his own correction chart himself for ANY rotor/propeller system if he understands the basics behind balancing rotors or propellers.
Rogue Blade: Once in a while it will be found to be impossible to get one or more blades flying satisfactorily together and it will be necessary to remove and replace one blade – this is more than likely a Span Moment Arm problem and can easily be fixed by passing the blade over a digital static blade balancer.
Until now this option has not been available. The only option to date has been to swap blades and try and match sets of blades to try and get an acceptably smooth aircraft. Often this process has taken days and many wasted man hours, aircraft down time, flying time and lost revenue.
Medium Frequency Vibration
Medium frequency vibrations at frequencies of 4/rev and 6/rev are another inherent vibration associated with most rotors. In two bladed teetering head systems, an increase in the level of these vibrations is caused by a change in the capability of the fuselage to absorb vibration, or a loose airframe component, such as the skids or panels vibrating at that frequency. The most common cause is loose skids caused by worn, loose, or incorrect skid retaining straps, bushings, bolts or clamps.
In multi-bladed systems, medium frequency vibrations are normally Nper rev and harmonics thereof. N being the number of rotor blades in the particular system being examined. For example, a four bladed system would have ann inherent 4/rev as an Nper Rev. A five bladed system – 5/rev. It is quite common to have numerous absorption devices to minimise the effects of N per rev vibration. N per rev will manifest itself as both a vertical and lateral component. It’s cause is discussed in depth in N per rev section below.
High Frequency Vibration
High frequency vibrations can be caused by anything in the helicopter that rotates or vibrates at a speed equal to or greater than that of the tail rotor. This includes many unusual situations such as hydraulic line buzzing, or starter relay buzzing, to loose elevator linkage at the swash plate horn, loose elevator, or tail rotor balance and track.
The most common and obvious cause: is generally related to the Tail Rotor, Tail Rotor Drive shaft/s, hanger bearings, intermediate or T/R gearbox. In general, it is relatively easy to do a quick ground run and check for Tail Rotor balance.
But the first thing is to check for something quick and easy. Try a thorough inspection on all the Tail Rotor pitch change mechanism, trunnion bearings, and all rotating components on the T/R assembly for wear and excessive play.
Next, uncover and check all Tail Rotor drive shaft sections for FOD (tools, rags, lockwire and ANYTHING that shouldn’t be there in a drive shaft of flight control run area.
Next, check ALL bearings and support structures for cracks, looseness and play.
Then after satisfied there is nothing OBVIOUS that could be causing a High Freq vibe, install RTB gear to the Tail Rotor & conduct a Tail Rotor Track and balance. If you can, also STRONGLY recommend to take a spectrum sufficient to cover the frequency/RPM range off all the componentary on your model of helicopter, to see if there are any abnormal peaks.
Other Causes: Hydraulic pumps, generators, Cooling fans, Aircon, Engines output shafts, intermediate shafts, etc
The best way to try and isolate these problems is to do a vibration Spectrum and look for peak values at specific RPM. If any peak values are observed, consult a vibration Order Sheet which will advise what components are rotating at those RPM where the peak values are observed. This will help isolate the potentially offending equipment.
Sometimes a “roving’ accelerometer/velometer is extremely beneficial and useful tool. It helps to track down insidious vibrations felt by the pilot in the flight controls, cockpit floor, instrument panel or crew in the cabin structure. By walking around the aircraft with a “free” accelerometer and looking at the spectrum, this can reveal peaks in one particular part of the airframe and not be detected elsewhere. This is because the accelerometer/velometer will only detect vibration magnitude if it is aligned to the axis of the vibration. The accel/Velometer, will only detect the “displacement” the vibration (an out-of-balance rotating wheel/disc) is causing if it is aligned to that plane of rotation. Imagine if the accel/velometer is aligned 90deg perpendicular to the plane in which the rotation is occurring i.e parallel to the axis of rotation of the rotating component. It will not detect the amplitude in the plane of rotation because the displacement caused by the vibration is occurring in line with the plane of rotation NOT the axis of rotation.
TroubleShooting – Coming Soon
Below are some case studies of aircraft vibration problems which have been encountered. There is of course many more examples. Further examples are available for download Free from the download page.
Nper Rev vibration is a function of the number of blades within the rotor system. For example a 4 bladed system would have an inherent 4 per rev freq, a 5 bladed system, a 5 per rev and so on. Associated with these, are the harmonics of the N per – the N-1 & N+1 rev vibes. A site (NASA) explains these quite effectively.
Nper Rev vibration is broken into both an Nper vertical and an Nper lateral component.
Some people often believe that N per rev vibration is a function of the RTB or rotor smoothing exercise. This is incorrect. The N per smoothing is a separate operation but is often carried out at the same time or in conjunction with an RTB exercise for obvious reasons that the vibration equipment is already fitted. It is true that the smoother a rotor system is, then it tends to unmask the Nper rev vibration such that they appear a lot worse to the aircrew inside even though no adjustments were done to the absorbers. This is because the higher levels of 1 per rev vibration tends to mask the N per rev vibration. As the 1 per rev levels are reduced toward zero, the N per rev vibe becomes APPARENTLY more noticeable. If you were to take actual IPS level readings, you would see that the actual IPS magnitude has not altered – merely become more noticeable.
For us to more fully understand the reduction of N per rev vibration, it is best we discuss the causes of N per rev.
Causes of N per Rev
Many theories have been espoused. It is probably the most avoided question when it comes to asking about helicopter vibration. Try asking your friendly helicopter manufacturer, representative or your RTB equipment manufacturer.
The common explanation used is that it “is an inherent vibration within the rotor system”. The question remains – What CAUSES it? “Inherent” does not give a cause – without a cause, merely a brush off.
Nper Rev is noticeable in two distinct, but different flight regimes:
- High Speed: It increases generally as the Helicopter increases forward speed particularly approaching cruise speed and beyond. Increasing markedly as it approaches Vne.
- Low Speed: It increases markedly during an approach or deceleration as Airspeed reduces through approx 45KIAS down to approx 15KIAS. This also coincides with the onset by vibrations caused by the reduction/loss of Translational Lift and the ingestion of rotor tip vortices. The loss of translational lift & tip vortex re-ingestion vibes interact with the Nper Rev vibe and make for quite a rough deceleration experience in some aircraft (Aw139 being particularly noticeable).
Nper rev and its associated harmonic derivative frequencies are product of two main sources.
1. Rotor Blade flapping to equality. The greatest contributor of Nper Rev vibration with increasing airspeed, is the action of the rotor blade flapping to equality to equalize lift across the disc throughout its rotation. It is caused by energy transfer from rotor blade to airframe via the transmission. This energy is created in the rotor in the form of a “whiplash” action induced along the blade as the tip of the blade reverses direction of travel. This change in direction of the tip is caused by the ever-changing creation of lift as the local angle-of-attack is constantly altering as the local relative wind ever-changes with constantly varying local airflows throughout the rotation of the rotor blade. These changes are most prevalent with increasing Airspeed. Hence most compensating devices are tuned for best effect at cruise or high IAS.
This can be observed in hub mounted camera view through downloads.
Practical Demonstration: (Hi Speed Nper Rev). The Vertical and Lateral component of the flapping blade can be well demonstrated by using either a garden hose or a rope with ring/shackle tied at one end.
Hammer a stake or peg into the ground and the place the shackle on the rope over the peg. Then hold the rope tight against the peg. Now, raise the end of the rope with your arm and then reverse the direction downwards quickly inducing a wave to travel down the rope to the peg. Do this 2-3 times or more for greater effect. What do you see?
- The ring will move vertically up and down the stake/peg. This is the vertical component – same as a flapping rotor blade.
- The ring will also cause the peg/stake to move sideways. If down long enough, this will actually loosen the peg and it will fall out of the ground. This is the Lateral component – same as a flapping rotor blade.
Where did this energy to cause this Vertical and Lateral vibration from?: For the rope – it has come from your arm flicking the rope. For the rotor blade it has come from direction reversal caused by blade flapping to equality every revolution as the airflow reverses across the disc with increasing airspeed. The faster the helicopter travels, the greater the amplitude of flapping, the greater the energy sent back to the mast/transmission. The greater the Nper Rev vibration as airspeed is increased because the dissymmetry of lift gets greater.
2. Airframe interference. The greatest contributor of Nper Rev vibration at slow airspeed, (predominantly on an approach or when decelerating below approx. 45Kias), is the airframe interference of the rotor blade downwash with the horizontal, flat plate surfaces of helicopter.
Airframe interference is really only prevalent at slow Airspeed. It is ostensibly only a vertical vibration. There is little, to no lateral component since it is caused by “pulses” of air coming vertically down from above the aircraft. It is caused by the absorption of energy from the “pulse” of air that each blade pushes downward as it passes over a large, flat, horizontal surface. This surface may be a large, flat stabilator, an empennage (these are mostly rounded and semi-aerodynamic however), winglets, large external stores, strakes/airdam or upper structure covering the roof/transmission area. The roof and transmission area is close to the root of the blades which generally has far less “pulsed” downwash than the mid-span to tip portion of the blade. A lot of the inboard portion of the blade is producing little lift if any through most of its flight regime unless in the hover. It is often either stalled or in flow reversal for a good portion of the flight envelope. It can be seen that airframe interference will really only be of any consequence at slow speed flight below approx 45KIAS down to the hover.
This same down wash on horizontal flat plate surfaces (Flat Plate Drag) is also a contributing factor to why so many helicopters will sit quite nose high/tail low in the hover or approaching the hover while decelerating. This is particularly noticeable on the AW139. The Rotor Downwash pushing vertically downwards on the large, fixed surface area of the horizontal stabiliser creates a considerable Downwards force pushing the tail down/nose high. This can be as high as 15-20deg NU approaching the hover. It will rapidly reduce when the airspeed reduces to near zero and the rotor wash becomes vertical and clears the horizontal stabiliser.
If the aircraft is peddle turned such that the wind blows the rotor wash well clear of the stabiliser, the hover attitude flattens considerably to 4-5deg NU.
Some helicopter designers/manufacturers use design techniques to lessen the effect of the Low Speed Airframe Interference Nper Rev vibration and downwash effects on these surfaces to lessen pitch attitude changes. See below in “Nper Rev Design Prevention Features” for further descriptions.
These causes are discussed in greater depth in RTB, available in the Free downloads.
Practical Demonstration: Low Speed N Per Rev. This demonstration requires you to enter a swimming
pool. Standing in the pool with your lower body immersed in water. Take your open hand with a flat palm – like a policeman’s stop signal. Extend your arm straight downwards against your body or upper thigh. The purpose is to now swing your extended arm with open, flat palm smartly past your upper thigh passing only 1-2inches from your upper thigh.
Feel the pulse of water that is created by your open palm (rotor blade!). This is fluid dynamics. The same applies to air. This is “pulse” that your upper thigh feels is exactly what the horizontal, flat plate surfaces of the helicopter feels every time a blade passes over it when the rotor wash is becoming toward the vertical flow and is not being streamlined by Airspeed.
This is Low Speed N Per Rev vibration.
Nper Rev Design Prevention Features.
Minimising N Per Rev Vibrations.
Nper Rev smoothing is NOT a function of Rotor Track & Balance. The RTB does NOT correct or smooth Nper Rev vibration. It is common that pilots & crew will “feel” the Nper rev more, the smoother or lower the IPS level of the 1 /rev Vertical & Lateral vibes as they reduce while performing the RTB.
Once the RTB or Main rotor is smoothed, then the Nper Rev can be tuned. Some aircraft will require an airborne tuning e.g UH60, Bell412 Frahm Damper, etc. Some can be done on the ground e.g Aw139 using manual excitation.
Each aircraft maintenance manual will stipulate how to perform a tuning for the Nper Rev dampers installed in each particular model. Each model will have different types of dampers and therefore will have different techniques and requirements to tune each respective damper.
Minimising Nper Rev vibrations must be thought of in two distinct flight regimes:
1. High Speed – Cruise and up to Vne. These normally try to dampen and absorb both the Vertical and the Lateral components. Devices can be either blade, hub, cabin or cockpit mounted. Or a combination of these.
2. Low Speed – usually 45Kias and below during deceleration on approach is the normal flight regime. generally only a vertical component due to being caused by rotor wash.
Typical Dampening Devices & Absorber Devices.
Nper revs are broken into both an Nper vertical and an Nper lateral component. Depending the aircraft make/model, the severity of each component varies. The severity determines whether the OEM will provide dampening for both vertical & lateral components or just dampening for the most noticeable or predominant component – usually the vertical. Most OEMs have a lateral dampening device incorporated in the head with some cabin absorber arrangement to reduce the vertical components.
Vertical Nper Rev (High Speed Nper Rev vibe):
- Cabin Absorbers – roof mounted (eg UH60, S92) or floor mounted (egAw139)
2. Cockpit Absorbers – In the nose area in the nose floor area (eg UH60) or behind the Instrument panel (eg Bell412)
These Nper Rev Cabin and cockpit absorbers were originally fixed masses which vibrated on spring or pendulous mounts to enable the mass to vibrate freely to the resonant frequency. They can be tuned by varying the masses to ensure maximum amplitude at the same RPM as the rotor. This vibrating mass would counter the Nper Rev that was generated by the Rotor blades.
Tuning Cabin/Nose Absorbers
Tuning Vibration Absorbers is done normally by using the standard RTB equipment but with it selected to “SPECTRUM”. This measures Amplitude of the accelerometer/velometer against RPM. The accelerometer location is normally defined by the maintenance manual in order to provide the optimum signal to enable mass adjustment to tune the absorber.
Some helicopters (Aw139) can be tuned on the ground. This is a relatively easy process and is recommended to be checked periodically. All Nper Rev absorbers/dampers vibrate with tremendous energy. The magnitude of the movement of these absorbers is impressive to see. All nuts/bolts/bushes/bearings and fixing devices should be checked routinely and periodically. Any binding, wear in bushes/bearings/looseness, etc, will change the resonant frequency and impeded the effectiveness of the absorber to do its job. This will result in a rougher ride than need be, with the ancillary wear on the airframe and avionics.
This Tuning is done statically in the hanger. No engines need to be operating. The bushes, mounting bolts and mass security bolts need periodic checking for security. The resonant frequency of the Mass damper needs to be tuned so that its maximum frequency after manual excitement, corresponds to the RRPM x No. of blades. Eg for the AW139, 5 x 296RPM (= 1480RPM for the AW139). After “exciting”, or banging the absorber mass with a hammer, the Nper Rev absorber should resonate and register max deflection at 1470-1480RPM.
Active Nper Rev Tuning: Newer generation Dampers/Absorbers have variable length supports on which these masses vibrate. By varying the length of the vibrating arm, generally by moving the mass length ways along the beam, it varies or automatically tunes the Damper to counter the Nper Rev vibration as it is detected by the accelerometer. The input to this is via accelerometers which may already be installed in conjunction with on-board RTB and HUMs systems. This is processed and from the processor a signal drives the mass along the spring arm to change it resonant frequency.
3. Head or Blade mounted Pendulous Dampers – Pendulous dampers help absorb both Vertical and lateral vibrations. They rely upon the pendulous mass to absorb energy that flapping blade has sent back down to the Rotor Head/Transmission. This can be shown in a video of B105 rotor head in flight.
4. Transmission Isolation Mounts – these may be spring, rubber or elastomeric. Their design is such that they isolate the transmission from the fuselage, effectively stopping or significantly reducing the Nper Rev vibration transferring to the fuselage. The most common of these are seen on the Bell product line. This approach avoids the need to cancel or counteract the Nper vibration, but rather allows the transmission to absorb all the vibration and isolates it from being transferred to the airframe and crew.
Lateral Nper Rev Dampers (High Speed Nper Rev vibe).
- Head Mounted Mass Harmonic Absorbers – These can be a freely mounted heavy mass which is secured in a mount assembly but with a large degree of freedom to move around and find its harmonic point of neutrality. It then absorbs lateral movement induced by energy travelling back to the rotor head as result of the flapping blades. These are commonly called Bifilars on Sikorsky aircraft but are also design features fitted to other types of aircraft including Mil8 and others.
2. Mast Harmonic Damper – A mass built into the top of the central mast supported on a flexible spring system (As350) or bendable rod system to enable the mass to vibrate and resonate to dampen Nper Rev vibration. Eg AW139 Mast mounted damper for laterals.
3. Pendulous Dampers – Mounted on each blade and free to pivot around a bearing (see above). It swings out due to centripetal force and is free to move independently to absorb both vertical and lateral energy transmitted along the blade. This dampens and lessens the energy that eventually reaches the airframe and makes for a smoother ride lessening the Nper Rev vibration. They help dampen both Lateral & Vertical. These are fixed mass and not tuneable But bearings should be checked for bearing serviceability and freedom of movement for correct function.
Vertical Nper Rev (Low Speed Nper Rev vibe):
Low Speed Nper Rev Vibrations are created by the impact of the “pulses” of air from every blade as it passes over the horizontal, flat plate, surface areas of the helicopter. It only happens at low airspeed as it reduces below approx 45Kias. At this point the rotor wash no longer streamlines with the relative airflow into a horizontal slipstream as the airspeed diminishes. The rotor wash becomes more vertical until the helicopter is in the full zero speed hover. At this point the Rotor wash will be truly vertical (plus or minus wind). It will only ever be truly vertical in zero wind conditions and stabilised in a hover.
There are various design features which are employed to minimise or dampen Low Speed Nper Rev vibration.
The main design features include:
- Isolation Absorbers or mounts – elastomeric or spring mounts.
- Stream lining of Large horizontal surface area – eg stabilators
- Reducing the Large Horizontal flat plate drag surface areas
- Keeping Large horizontal surface area out of the Low Speed Rotor Wash
1. Isolation Absorbers.
Isolation mounts isolate specific components from transferring energy back to the airframe eg isolating Stabilators, Nodal beams to isolate the transmission from the airframe (Bell206L), Spring suspension systems for transmission mounts (Bell 206, 205/212). The UH60/S70 series uses elastomeric bearings/absorbers in the mounting of the large, horizontal stabilator (below). If you are lucky enough to fly formation with BlackHawk helicopters, especially in the hover, you will observe just how much vibration and movement the Stabilator displays. It is very impressive how much energy these absorbers prevent from being transferred to the fuselage.
Elastometric Vibration Absorber.
The Bell412 also has a similar absorber but a spring in its sink elevator design. During each pre-flight, the pilot should check the spring tension on the sink elevator by applying gentle pressure chord wise on the sink elevator. This spring provides tension to assist in keeping the sink elevator aligned with increasing airspeed. It also acts as “dampener” at low speed to help absorb the 4per Rev Low Speed vibration as it decelerates on approach. The sink elevator has spring dampened freedom of movement to vibrate and be dampened by this built-in spring force. This prevents, or cushions the 4/rev vibration from being transmitted to the airframe providing a relatively smooth ride through low speed deceleration.
2. Streamlining of Large Horizontal Surface Areas.
An effective means of reducing the Large Flat plate horizontal surface areas presented by horizontal stabilisers is to have them automated, so that their angle of incidence varies through the flight envelope. By so doing, the flat plate surface “streamlines” to align itself with the relative airflow of the rotor wash with the varying airspeeds. In particular, between 45-110kts IAS. This enables these large flat plate surface areas to offer minimal flat plate drag to the relative airflow of the rotor and slipstream. It minimises Nper Rev vibration and maintains a relative flat and constant pitch attitude due to minimal rotor downwash vertical down force on the stabilator trying to push the tail down and the nose up. This type of design philosophy is in use on both the Uh60 & AH64.
In these types of systems, the stabilator is driven through an automatic flight control computer with various inputs which include IAS, collective position, lateral accelerometers which provides signals to electrical jacks to drive the stabilator to the optimum position in order that it may streamline automatically to offer the least resistance to vertical down-flow and maintain as level a pitch attitude as possible throughout its flight regime.
3. Reduction of Large Flat Plate Horizontal Surface Area.
Newer designs are beginning to realise that instead of creating very expensive and maintenance intensive systems such as Automatic Flight Control computers and fly-by-wire horizontal stabilisers, making the horizontal flat plate surface area presented to the Rotor-wash smaller, is cheaper and often as effective way of reducing the Low Speed Nper Rev and the Nose high pitch associated with Rotor wash drag on Horizontal surfaces. The AW139 has a bad reputation for both a very aggressive 5/rev Low speed vibration when decelerating on an approach and also a very nose high attitude during the deceleration approach/finals phase of flight. This is because of the Horizontal Flat Plate Surface areas presented by both the Stabiliser and also the air dam effect of the full length Coanda strake on the left side of the Tail Boom of the Aw139. (See pics below). The Aw189 was designed after the Aw139. It can be seen that a significant large area of the stabiliser has been reduced, offering considerable less flat plate drag to the Rotor-wash below 45Kias. Similarly, it can be seen that the Coanda Strake has been considerably reduced. A good 1-2mtrs of strake from the base of the tail boom has been designed out. This again adds to the smaller horizontal profile and air-dam effect to the rotor wash than the Aw139 design. As result, the Aw189 has less 5/rev at Low speed and has a lower nose up attitude below 45Kias when decelerating on approach making it easier for pilot control.
4. Keeping Large Flat Plate Horizontal Surface Areas out of Rotor Wash.
Other design bureau’s design their large flat plat surface areas – primarily the stabilisers, either as a considerably smaller horizontal profile or they locate them high up on the fin out of the way of any Rotor downwash. This removes the stabilator out of the Low Speed Nper Rev issue all together. This design feature is seen on the CH53, S61, AS332, EC225, etc
RWAS hopes sincerely this has been informative. Please feel free to provide feedback in order that we can continue to improve this content.
Like the lads below – “have a good one”!.