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Monday, November 1, 2010

Vibration Identification & Minimization 

Uncontrolled helicopter vibrations can literally shake a helicopter apart. Here’s how to stop that before it happens.

By Mark Robins

Helicopter vibration adversely affects rotor blade structural integrity, component life, firewalls, instrumentation, structural members, and operator comfort and control. Despite large reductions in vibration due to improved helicopter design, the current overall levels (approximately 0.05 to 0.1g) remain significantly higher than that of jet-engine aircraft, which are below 0.01g. Minimizing these vibration levels, while understanding that they can never be fully eliminated, is essential to ensure a helicopter’s safety and longevity.

Helicopter vibration is so serious that FAA recently announced its plan to have all U.S.-certified rotorcraft install or be serviced using some type of mechanical condition monitoring or HUMS (health and usage monitoring system). FAA prompted this action due to the number of helicopter crashes caused by mechanical component failures worldwide, including many that have resulted in fatalities. The European Union and the Intl Helicopter Safety Team (IHST) also are going ahead with this action due to a series of fatal accidents that could have been prevented using such technologies.

A rotor installation has a flight sensor and a velocimeter to measure vibration. Jim Fackler Dynamics

Why It Happens

Vibrations are inherent in all helicopters. Rotating components are the primary source of helicopter vibration. “By far the biggest vibration amplitude is seen in the main rotor N-per-revolution (RPM x number of blades) which increases with density altitude, airspeed and weight,” says Jim Fackler, sole proprietor of Jim Fackler Dynamics of Duarte, Calif. “Being an aerodynamic phenomena, its severity is logarithmic with airspeed. Next are N-per-revolutions from the tail rotor, then driveshafts because they cover large areas of the airframe and transmit inordinately to the airframe, followed by higher main rotor harmonics.”

“The primary causes of rotating machinery vibration are unbalance, misalignment, looseness and the excitation of structural resonances by rotating shaft running frequencies and their harmonics,” says Arun Menon, product manager of San Diego, Calif.-based Data Physics Corp. “Other sources of vibration can be bearing defects, insufficient lubrication and dirt entrapment between moving/rotating surfaces. Excessive vibration of main and tail rotors can cause the tail section of helicopters to be physically ripped from the main structural assembly, and result in catastrophe. Severe imbalance in the rotating parts also can lead to instabilities during flight making control difficult, if not impossible. Severe vibration can also affect avionics and navigation equipment onboard the fight deck.”

Unequal distribution of mass between two opposite rotor blades will create a forcing function when a rotor is spun. “A drive shaft can have irregular wall thickness or the weight of bolts may be slightly different, resulting in a 1/rev (1 x the rotating speed) vibration each time the heavy spot passes a fixed point in space,” says Dave Lilly, manager of technical applications at Honeywell Aerospace, which is working jointly with Diagnostic Solutions Inc. “Misalignments can cause both 1/rev and 2/rev (1 x and 2 x the rotating speed) vibrations but using modern installation and maintenance practices these are much rarer today.”

Aerodynamic vibration can be recurring, such as a single blade “lifting” or working harder or less harder than the others in the group. “These differences can be the result of mismatching the blade time, small differences in the airfoil, or such things as cumulative wear of the leading edge or repeated field repairs,” says Brian Hatcher, owner of Diagnostic Solutions. “Aerodynamic vibration of a transitional nature are usually attributed to airflow over irregular surfaces, such as wing stores, and can appear and disappear as the aircraft moves through ‘conditional’ flow patterns.”

Gears are notorious for vibration issues. Gear manufacturing defects, incorrect assembly tolerances and material defects are all major contributors of problems that can manifest in high vibration levels, Menon says. Gearbox vibration most often requires a gearbox overhaul or replacement. Gearbox or drive train vibrations can be harmful to a pilot. The most severe being pain, numbness, loss of tactile discrimination and dexterity.

To minimize vibration, a pendulous dampener in the nose of a Bell 222 dampens the main rotor. Tuning is done by adding or subtracting weights. Jim Fackler Dynamics
Rotor Track & Balance

Rotor track and balance (RT&B) smoothes and reduces vibrations in helicopters caused by the rotor system. RT&B attempts to bring each rotational component in the assembly with each other. According to Neil Seamon, senior applications engineer at Diagnostic Solutions, a number of portable and onboard systems help automate some or all of the process:

• Installation of the equipment (processor, vibration sensors at preselected locations for the task),

• Operation of the aircraft through a predetermined measurement or set of measurement points,

• Maintenance actions as a result of the measurements taken,

• Verification of the proper installation and efficacy of the adjustments,

• Repeat as necessary until the desired results are obtained.

One adjustment made during RT&B involves mass imbalance, such as one blade ‘weighing’ more or less than its opposite. “This is generally solved by adding/removing weights to the root or tip in order to bring the group into harmony,” says Seamon. “Sometimes referred to as balancing, this is generally done to remove vibration in the lateral, or ‘in-plane’ [with respect to the rotor disk] vibration.”

Additionally, some aerodynamic tuning is possible on most modern rotor blades through small, semi-rigid tabs installed on the trailing edge of the blades. These are usually bent, using a specialized tool, up or down as needed to make very small adjustments to the aerodynamics of a specific blade. This makes it more closely resemble the others in the rotor system. These most often are adjusted to reduce the vertical vibrations, or the “hop.” Depending upon the aircraft design, there are also adjustable pitch links or control rods, which affect the relationship of the individual blade to the rotating swashplate and affects both the lateral and vertical vibration axis. There also may be additional weights moved within a blade (forward and aft) for additional tuning for specific faults.

Most RT&B focuses on the 1/rev, or the force that is imparted once per revolution of the rotor system or rotating assembly. “An additional source of vibration is typically referred to as N/rev, where N equals the number of rotor blades,” says Lilly. “These are caused by a number of factors such as the ‘work’ of each individual blade and the downward force each blade exerts on the fuselage as it passes over. Several aircraft employ some form of N/rev absorber and a comprehensive RT&B program will encompass the tuning of, or at least or assessing the health of these devices.”

Manufacturers have become very good at producing matching blades, but some small variations are bound to occur. These variations can include chord and span balance and torsional flex. Even if the manufacturer produces matched blades, differences can occur during normal use and repair. These small differences can have an aerodynamic effect on the way the blade flies and thus on its track and balance. Sometimes a blade that is slightly out of track is creating the same amount of lift as its neighbors and thus the same balance. “Even with all these factors, the history of blade track and balance has ingrained a sense of comfort in the pilot and operators when the aircraft is well tracked,” Hatcher says.

Amplitude and Phase

To fully and successfully examine and minimize helicopter vibration, the entire helicopter must be analyzed. Vibration analysis measures amplitude and phase to balance rotating machinery. Amplitude is the severity of the vibration. Phase is the relationship between peak signal and a reference on the rotor. The phase change rate indicates the presence of a critical speed (rotor natural frequency excitation speed) or imbalance in relation to a timing pulse that occurs during rotation. Criticals are marked by a change in phase as the rotating part is accelerated through its resonant critical. Once all of these parameters are found and analyzed, the amplification factor and/or damping associated with a particular mode can be determined.

“Manufacturers (not operators) typically yellow arc critical operating modes,” says Fackler. “It’s done in flight test before and during certification. No one operates at criticals, or better said, they won’t for long. Some other helicopters have main rotor RPMs to be avoided.”

Dynamic signal analyzers are instruments that measure the vibration amplitudes and phase as a function of operating speed for machinery. “They are integral in the analysis of systems like helicopters and other aircraft,” Menon says. “Dynamic signal analyzers can offer a variety of tools for total diagnostics of helicopter systems and components including acoustic signature analysis, vibration analysis with both time and frequency domain tools, synchronous averaging, modal and operating deflection shape analysis, order analysis and demodulation analysis.”

Meters & More Meters

An accelerometer is the most common type of transducer used to measure vibration. A typical accelerometer hooked to a signal conditioner produces a voltage that is directly proportional to the instantaneous acceleration of the accelerometer transducer. This variable signal is produced using the piezoelectric effect. A small mass acts on the piezoelectric crystal creating a charge proportional to the force applied. Since the mass and crystal experience no rubbing and very little movement, the accelerometer has the advantage of long life and a high-frequency range.

A velometer is a transducer that measures the instantaneous velocity of vibration. This is accomplished with a velocity output accelerometer. This is an accelerometer with a built-in circuit that integrates the acceleration signal into a velocity signal, or coil-based velometer. In the case of the former, a signal conditioner must be used to provide a proportional voltage output although proportional to velocity in this case, not acceleration, Menon says.

The coil-based velometer uses a metal coil moving through a magnetic field producing a voltage proportional to the velocity of the coil. The principal at work is the Hall Effect. This type of sensor can be big enough to not require signal conditioning, and can have a sensitivity that is accurate down to lower frequencies than a typical accelerometer. The primary disadvantage can be mechanical wear since there are moving parts involved.

Spectral Analysis

With advances in sensor technologies, spectral analysis of helicopter components is now possible. Similar to ultrasound monitoring in medicine, these new digital mechanical microsensors convert vibrations across a spectrum of frequencies into a signal that can be remotely monitored using wireless transmitters.

In a helicopter’s main rotor gearbox, a frequency is generated for each gear and bearing turning. An individual gear set will generate a specific vibration profile (teeth x RPM). “A malfunctioning helicopter gearbox will show an unusual spectral profile, like a murmur in your heart,” says Mark Rose, helicopter pilot of 35 years and manager of Alpine Lift Helicopters, Albany, Ore. “A recent accident involving a multi-million dollar helicopter and loss of life due to not refilling the tail rotor gearbox could have been prevented. A low-cost, on-board, temperature/vibration reporting system would have warned the crew in time to land.”

Spectral analysis closely monitors helicopter components for any change that may occur in the normal cycle. It may help detect if a premature failure is about to occur. These failures can include cracks, loose attachments or bearing failures. In actual operation, spectral analysis can report and detect vibrations and/or temperatures that exceed normal levels so the cockpit crew can instantly be alerted via the cockpit-mounted computer display. This wireless system assists the crew in making critical decisions about continuing flight before a catastrophic in-flight failure occurs. “These systems are required to be FAA accepted but the guidelines are broad to attract wider use, not to deter use or creativity,” Rose says.

Absorption & Isolation

Vibration absorbers can help provide a smooth and safe helicopter ride. If a sinusoidal force acts on an undamped mass-spring system and the forcing frequency equals the natural frequency of the mass-spring system, the response is infinite. This is called resonance, and it can cause vibration and other severe problems helicopters.

“When an absorbing mass-spring system is attached to a main system mass and the resonance of the absorber is tuned to match the main system, the motion of the main mass may be significantly reduced at its resonance frequency,” Menon says. “Thus, the energy of the main mass is apparently ‘absorbed’ by the tuned dynamic absorber.”

There are both dynamic and active vibration absorbers. “Dynamic absorbers are like shock struts, dampers and elastomeric bearings,” says Rose. “Active types are electric-based anti-vibration or absorption panels that may be computer controlled. Eurocopter has experimented with anti-slap trailing edges in rotor blades that are electrically activated to reduce flight noise in different flight configurations.”

Fackler says there are basically two types of active absorbers. The first are those that cancel by applying an equal force opposite the offending vibration, typically sprung masses that are tuned to the helicopters N-per rev at its operating frequency. The second are those absorbers that accumulate energy and return it later in the operating cycle: bifilars and lead lag dampers.

Similar to absorption techniques, isolation removes the transmission path for vibration energy, so if the main rotor and transmission are isolated from the airframe the airframe will not be susceptible to vibration from these components. Isolation floating or soft mounting allows the “big pieces” to shake a bit without directly transmitting that motion to the cabin. “This is not a total solution however, as vibration in those components can still lead to local failure resulting in loss of power and/or control,” cautions Menon.

Hatcher agrees that since this force is now being taken up by the isolation mounts themselves, they must become an item for periodic inspection and/or replacement. “The role of the isolation mechanism simply becomes the ‘collection point’ for the vibration, and therefore must be closely monitored for degradation,” he cautions.

Predictive Mx

Vibration analysis is a very important part of a helicopter’s predictive maintenance program. Predictive maintenance can prevent breakdowns, unscheduled shutdowns, operational instabilities and expensive repairs.

“Predictive maintenance programs monitor vibration and other system parameter levels over time and track their rate and extent of deviation from acceptable baseline levels,” says Menon. These types of programs are available commercially and have features that simplify the data comparison effort. Ultimately, a predictive maintenance program’s main goal is to identify the failure level of the component and remove it before a preset threshold.

Predictive maintenance can be handled in-house or by external consultants. Menon believes internal analysis teams should complete these programs because of knowledge of equipment, regular and frequent access to equipment and regular dialog with equipment operators and pilots.

Fackler believes it takes both. “I’ve found the operators know their machines the best, but often lack the detail skills in vibration management,” he says. “Outside consultants are for the most part engineers that feel obliged to find something whether it is there or not. Both have vested interests. If you are looking for a company or person to administer a vibration program, look for someone that has a track record in finding the problem before it is torn down and found. Many analysts point to how their systems identified a problem only after finding the problem post mortem then going back through the data to prove the data.”

Once vibration has been analyzed and discovered, it’s time to tighten, balance, align, lubricate, weight, trim, remove, replace, repair and correct. “There will never be vibration cessation,” says Fackler. “There will be identification of vibration outside the norm which could trigger a maintenance action that might not otherwise be taken. While there are general rules about helicopter dynamics, each aircraft is unique and it is best to look at the aircraft against itself first, then against fleet averages. Pragmatically speaking, helicopters are made to earn a living, not test to death. When you reach a return on the testing—if you reach a return on the testing—you are doing pretty good.”

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