April, 2011
Numerous hazards associated with aircraft vibrations continue to inconvenience helicopter owners and operators around the world, and operators have seen their aircraft components fail prematurely, long before their designated Time Before Overhaul (TBO). But despite the often fatal risks involved, most solutions try to significantly resolve vibration- related problems. To date, attempts only monitor vibration data in order to alert the user of a danger and possibly predict useful life of the aircraft's dynamic components. None of these actually target in eliminating the problem from the source, and such methods only result in higher costs and less reliable aircraft.
Vibrations occur because drive-train components such as the driveshaft, rotor blades, grearboxes, etc, are balanced independently at the manufacturer level and delivered to the assembly line as a "balanced component." But when these parts are assembled on to the aircraft, the entire aircraft drive-train is no longer balanced as "one unit". Therefore, vibrations travel along the entire aircraft drive-train and fuselage. During flight itself, torque is forced onto the drive-train components and vibrations at one area of the aircraft is then transmitted to other areas!
The only accepted method in fixing these inherent vibrations is to eliminate the root cause: less vibration is better than more vibration. It makes the simplest and most basic sense, as one of the leaders in the development of advanced rotary wing innovations, Shake'd Technologies, Inc. knows very well. In response, the New- York based company has effectively developed a breakthrough solution: Customized Dynamic Balancing (CDB). Unlike other methods, CDB approaches the entire aircraft condition as a whole, emphasizing the importance of analyzing the dynamics of the entire drive-train and the interaction between its interdependent parts, rather than by its separate components. As such, the process efficiently and immediately (see graph) targets vibration-related issues that endanger the aircraft, from precautionary landings and forced landings to accidents and other incidents as reported by helicopter owners. "CDB will mitigate these types of risks and hazards while increasing crew confidence in the machine," Eli Navon, CEO of Shake'd Technologies, Inc. said.
The CDB technology was developed to specifically eliminate the damaging effects of inherent drivetrain vibrations, using a proprietary set of algorithms. CDB algorithms analyze the vibration data it collects to determine the specific solution for each individual aircraft. In this way, each drivetrain is custom-balanced within its own vibration profile using the solution given. This is accomplished using CDB computer vibration analysis tools that gather requisite vibration information from the drivetrain components.
The process relies on accelerometers temporarily placed in designated locations along the drivetrain that measure vibration levels and other required data. The aircraft is then run on the ground as measurements are taken. The computer analyzes the gathered information, calculates a solution, and displays it to the technician. The technician follows the solution instructions and performs the adjustments to the drivetrain. As soon as the necessary solutions are applied, a verification run is performed to confirm the reduction in vibration. The sensors and special cables are then removed and the aircraft is returned to service. The entire process takes an average of two hours for the initial run. Subsequent runs could take as little as 30 minutes. Depending on the aircraft platform, CDB recommends the users to apply the solution every 200 flight hours to ensure constant operation with very low vibration levels!
CDB application is done during a ground run because minimum torque is transmitted through the aircraft's driveshafts, and at a lower rate when the aircraft is on the ground. The vibration data therefore gives a more precise vibration profile, and eliminates the differences of changing torque loads on drive-train components in flight. Torque levels transmitted while airborne are constantly changing due to flight conditions, which greatly affect vibration levels and distort the measured vibration levels, resulting in a misleading picture of the drive-train vibration profile.
For example, when vibration data is collected in the regular spectrum through an accelerometer at a certain hanger bearing area, a set of vibration levels is seen. But when the pilot increases the collective, these vibration levels dramatically decrease. When the shaft transmits a torque, it creates a twist on the component. The twist of the shaft then changes the position/location of the mass unbalance point/position. Usually, the shafts are unbalanced at many locations and at random quantities. These unbalanced points change locations along the component depending on the torque levels transmitted through the component.
"The many abilities of the CDB technology are all very unique. These include main rotor tracking and balance that allow the aircraft to be serviceable after only one test flight in the air," Navon said. "Additionally, once the solution is modified to work with a specific platform, it is able to examine the health condition of each gear mesh in all gearboxes and report unusual component behavior to the user."
CDB is very useful in identifying and troubleshooting issues beyond the drive train while avoiding the costly 'guesswork' that maintainers usually resort to in trying to find the root cause/source of the problem. Current vibration monitoring systems do not have the ability to pinpoint the real location or cause of a deficiency, leading to drastically wasted time and resources. The common theme to "solving" vibration concerns is to perform special maintenance inspections and subsequently replace components, a method which is both costly and ultimately ineffective.
CBD maintains a strict Preventive Maintenance Process, which keeps the vibration deterioration rate very low and moves component lifetime closer to its scheduled TBO. The longer a dynamic component remains on-wing, the lower the risks and operating costs. In addition, CDB supports the Condition Based Maintenance (CBM) concept by providing Reliability, Availability, and Maintainability (RAM) data that supports predictive maintenance practices based on actual conditions of dynamic components vice unscheduled maintenance, thereby reducing extensive aircraft on ground events.
Another important attribute of the CDB is its ability to integrate (in full or in part) with other health and usage monitoring systems. Since the hardware requirements for the CDB process are mostly common and readily available, the critical part lies in its software, and so integration efforts are directed towards software engineering. "It is envisioned that monitoring systems such as HUMS and MSPU could be modified to collect vibration data from the additional locations that CDB requires. The collected data would then be passed to the CDB system via a digital storage device (digi-stick), or other similar means, and then loaded into the technician's computer for processing." Navon said.
An Experimental Test Pilot has reportedly said of the CDB, "We did the process on two of our AH 64 Apaches. I have flown both and can tell you that it has made an obvious difference. Not only is the vibration way down, it seems that the noise in the cockpit is quieter. We are impressed." Truly a remarkable feat in the field of rotary wing vibrations, CDB has been successfully utilized by an international fleet of rotary wing aircraft for over three years. Today, Customized Dynamic Balancing is fully developed and proven to be very effective in reducing costs and increasing aircraft availability, safety, mission readiness and logistics supportability.
