FINDING OIL-LOSS SOLUTIONS FOR ROTORCRAFT DRIVES
by Stephen Berkebile, Jason Fetty, Robert F. Handschuh, and Brian Dykas
PACIFIC OCEAN (June 7, 2016) Sailors assigned to the Blackjacks of Helicopter Sea Combat Squadron (HSC) 21 conduct maintenance on an MH-60S helicopter on the flight deck of hospital ship USNS Mercy (T-AH 19) in preparation for the first mission stop in Timor Leste. The Sailors are also known as maintainers, and are responsible for the upkeep of the aircraft before and after flight. Deployed in support of Pacific Partnership 2016, Mercy is scheduled to visit Timor Leste, the Republic of the Philippines, Vietnam, Malaysia and Indonesia. Medical, engineering and various other personnel embarked aboard Mercy will work side-by-side with partner nation counterparts, exchanging ideas, building best practices and relationships to ensure preparedness should disaster strike. (U.S. Navy photo by Mass Communication Specialist 2nd Class Hank Gettys/Released)
A helicopter’s drive system converts the high rotational speed from engine output shafting into the lower rotational speed and higher torque required by the main and tail rotors for flight, with accompanying changes in shafting orientation. Transmission gearboxes within the drive system contain gears and bearings that are subjected to punishing loads and contact stresses as they transmit several thousand horsepower. Proper supply of oil within the gearboxes is critical to the continuing function of the drive system under these strenuous internal conditions during flight. If this lubricant supply is compromised, degradation in the drives will rapidly lead to loss of power and a forced landing, or worse.
THE PROBLEM OF PROPER LUBRICATION
Oil in a power transmission or gear box serves multiple purposes. The primary roles are lubrication of contacting surfaces and temperature regulation by removal of heat. Under normal operating conditions, an extremely thin film of oil separates gear and bearing surfaces, prevent- ing direct metal-to-metal contact, reducing friction, and allowing the components to operate through billions of cycles. Unfortunately, even with the reduced friction, contact between highly loaded components at high speeds in the gearbox causes substantial heat generation. One way to dissipate this heat, however, is through advection by the gearbox oil. Under normal rotorcraft operations, this gearbox oil is recirculated and cooled to keep heat generation low and remove excess heat.
An oil-out condition occurs when the primary oil flow to a gearbox is interrupted. This condition may result from a ballistic impact or any other event that blocks, impedes, or removes the oil supply to transmission components. Loss of the primary oil flow can result in an immediate or rapid failure of the drive system due to the reduced heat dissipation, increased friction (resulting in additional heat generation), and material degradation in the highly loaded gear and bearing contacts. Thermal growth in components leads to a decrease in gear backlash, which eventually causes binding and thermal runaway of these components. Seized gears can prevent the rotors from turning, so autorotation to a safe landing is not always possible in the event of loss of lubrication.
These events affect not just military aircraft, but civil aircraft as well. Recent transmission oil-loss incidents have caused emergency landings and fatalities. These incidents include a 2008 Sikorsky S-92 incident in which a transmission oil loss caused a forced emergency landing, a 2009 Sikorsky S-92 incident in which a transmission oil-loss event caused an aircraft crash and resulted in 17 fatalities, and a 2012
Eurocopter EC 225 incident in which a lubrication system failure caused a forced emergency landing [1, 2].
LOSS-OF-LUBRICATION PERFORMANCE
Army rotorcraft drive systems are subject to loss-of-lubrication design certification requirements, as described in ADS-50-PRF, which states that the drive systems are required to operate after loss of primary oil flow for a minimum of 30 minutes at cruise conditions (approximately 50% power rating) [3]. However, the Army desires the ability to run for a longer period of time after a loss-of-lubrication condition. Future platforms are planned to have longer endurance and range capabilities, requiring corresponding improvement in loss-of-lubrication perfor- mance to enable long-distance exit from hostile areas, ideally exceeding half-mission range.
The certification requirements of modern rotorcraft have resulted in oil-out performance of transmissions receiving considerable attention, and the rotorcraft community has increased scrutiny of this survivability aspect over the past few years as a result of the high-profile mishaps. Accordingly, government and industry organizations have increased research and development in transmission oil-out behavior. Improving the oil-out survivability of the existing fleet is a great challenge, since modifying the gearbox and constructing new drop-in gearboxes both require extensive flight certification beyond the design and prototyping, and with no guarantee that the requirement will be achieved. In some cases, the 30-minute requirement has been met by the addition of secondary emergency lubrication systems external to the gearbox. However, secondary systems add complexity and weight to the vehicle, especially if they contain their own lubricant supply, and thus they are not always feasible.
Progress has recently been made in the design of a new transmission, with the Augusta-Westland AW189 main gear box lasting more than 50 minutes during its certification [4]. Augusta-Westland’s holistic approach in combining materials and design elements demonstrates that improvement is possible with a new design. Nonetheless, developing solutions that can be applied to new and existing systems with little or no internal modification would also be desirable.
PAST AND CURRENT EFFORTS
Over the last 15 years, loss-of-lubrication research has been conducted at NASA Glenn Research Center on a test facility originally intended to conduct surface contact fatigue experiments on gears (see Figure 1). NASA’s initial work in this area produced some inconsistent results [5], so a larger test series was conducted that concentrated on making the test section of the gearbox more like that of a high-speed, aerospace gearbox [6]. During this series of approximately 60 tests, many configurations were assessed, including materials, shrouding, and gear designs. This work resulted in establishment of a loss-of-lubrication test procedure and rig configuration with considerably better repeatability during testing in terms of time-to-failure and facility temperatures recorded.
Figure 1 High-Speed Spur Gear Rig for Component- Level Evaluation
Once the test procedure and facility setup were established, enhanced testing was initiated, including determining the temperatures of the gear teeth during operation. In one study, gears were instrumented with thermocouples connected via slip rings to have on-component data during normal and loss-of-lube conditions, which has been useful in understanding gear bulk temperatures as compared to the static temperature measurements made in the gear shrouding. A recent development has been the addition of high-speed infrared imaging system to measure full-field temperatures of the gears while in operation (10,000 RPM) [7]. An example of the infrared temperature measurement of the spur gears during operation is shown in Figure 2. The temperature of the gear teeth is critical during failure, as can be seen in the glowing of gears seconds before failure in Figure 3.
Figure 2 Spur Gear Temperature During Loss of Lubrication Measured by IR Emission
Figure 3 Spur Gears Operating After Oil Shutoff Glowing From Frictional Heating Shortly Before Failure
Current and future work by the U.S. Army Research Laboratory and NASA combines approaches from both macroscopic and microscopic viewpoints and includes experimental work as well as efforts in modeling and simulation. One of the key unknown parameters is the heat generation while in oil-out conditions. Work directed toward a better understanding of how this parameter impacts loss-of-lubrication performance is needed to advance and improve loss-of-lubrication behavior. Penn State University has developed a multiphysics simulation that couples computational fluid dynamics and tribology (the study of friction and wear) modelling using heat generated [8]. This Army- supported project continues to advance the fidelity of the simulation.
Another unknown is the detailed progression of microscopic material degradation during failure. Insight into these chemical and physical mechanisms will inform materials selection in the future.
As a part of these efforts, a current Joint Aircraft Survivability Program (JASP) project seeks to establish and rapidly transition an improved portfolio of technologies to increase transmission performance after the loss of the primary lubrication system, with the ultimate goal of increasing vehicle survivability and extending this period to at least half-mission duration. Many concepts to reduce heat generation, increase heat rejection, increase material tolerance to higher temperatures, and increase material resistance to damage have been consid- ered and assessed over the past 2 years. This collaboration between the U.S. Army, U.S. Navy, and NASA was created to accelerate the work under way among various government and industry organizations and to join broad expertise and laboratory resources in a more unified, joint effort. The overall objectives are to:
- Identify and screen promising candidate technologies falling within the realm of lubricant science and tribological surfaces.
- Determine the effectiveness of screened technologies at the component level in spur gear testing.
- Perform a full-scale loss-of-lubrication experiment on an H-60 transmission, incorporating screened technologies as appropriate, to demonstrate improved performance when compared to a baseline.
- Record data from a suite of instrumentation to provide critical data that will be useful for verification and validation of emerging multiphysics simulation tools.
Choosing technologies to implement requires consideration of several factors, such as cost, ease of implementation, and compatibility between the technologies. Although operating the flight-qualified full transmission allows freedom to include system-level and mature approaches, the intent is to develop cutting-edge enabling technologies with potential for implementation in multiple platforms within a 3- to 5-yr time period. With this goal in mind, the technologies selected for final implementation are intended to be widely applicable across platforms, even if those technologies will require further qualification and optimization efforts before use in a vehicle.
FINDING LOSS-OF-LUBE TECHNOLOGIES
The approach that we have taken to identify and develop promising loss-of-lubrication technologies is designed to rapidly move those technologies along the Technology Readiness Level (TRL) path. The approach begins with screening emerging technologies using coupon- level methods. Then those showing promise are implemented at the component level, followed by a final selection and simultaneous evaluation of a suite of technologies at the system level. In this way, the tradeoffs between simpler and inexpensive coupon evaluation can be balanced with the greater fidelity of the more complex and costly gear evaluation. The approach is nevertheless ambitious considering the desired schedule and distributed scope.
Figure 4 Coupon-Level Evaluation of Technologies With a Ball-on-Disc Tribometer
Figure 5 Coupons Demonstrating Different Surface Finishing Technologies
The coupon-level evaluation is conducted at the U.S. Army Research Laboratory on a high-speed tribometer, an instrument that allows for precise control over contact conditions between two objects (such as the ball and disc shown in Figure 4). The ball-and-disc tribometer can be used to simulate specific gear or bearing conditions and make a rapid determination of experimental parameters, such as lubricant properties, alloy properties, or the smoothness of the surface finish, as demonstrated in Figure 5. When the lubricant supply is stopped, one can extract a time-to-failure, as shown in Figure 6.
Figure 6 Time-to-Failure for Coupon Evaluation of Standard (MJII, A555) and Novel Lubricants (UES, SP3, AHTL)
The component-level evaluation occurs on the spur gear rig at NASA Glenn Research Center, shown in Figure 1. The results of operation without oil can be seen in the glow of the gear teeth caused by frictional heating in Figure 3 and in the gears after failure shown in Figure 7. During these evaluations, we have taken care to ensure the conditions during coupon-level screening match well to those of the spur gear rig, and, as a result, the agreement between the two methods has been good. We anticipate similar results in the full transmission test to be conducted at the Patuxent River Naval Air Station in 2017.
Figure 7 Spur Gears After a Complete Loss-of- Lubrication Evaluation
Thus far, we have selected an aerospace lubricant with a phosphonium ionic liquid additive, developed under a Small Business Innovation Research (SBIR) program, a method of mirror-finish polishing called superfinishing, and the integration of hybrid bearings (containing silicon nitride ceramic rollers and Cronidur 30 steel rings) as the most promising technologies for integration into the full transmission test that can realistically be implemented within the scope of this project. Each of these technologies has demonstrated increased time-to-failure during our coupon- and component-level evaluation. The causes for why these technologies are providing an improvement in oil-out time are still being investigated.
Furthermore, work continues on identifying, understanding, and optimizing loss-of-lubrication technologies sponsored by parent organiza- tions, and this list may evolve before final implementation in the full transmission. For example, gear coatings developed under another SBIR project are currently being investigated. Beyond the anticipated outcome of the full transmission test and the identification of several promising technologies (and determination of candidates that should not be used), much has been learned about the details of the physical processes during a loss-of-lubrication event. The data collected at all three levels (coupon, component, and system) will be used to direct the advance of models and simulations for design and in the identification of promising directions for future material and lubricant development.
ACKNOWLEDGMENTS
The authors would like to acknowledge others who have made, and continue to make, significant contributions to the current JASP effort. These individuals include Radames Colon-Rivera, Kevin Radil, Mark Riggs, Nikhil Murthy, Kelsen LaBerge, Timothy Krantz, and Eric Hille.
ABOUT THE AUTHORS
Dr. Stephen Berkebile conducts research at the U.S. Army Research Laboratory in tribology and lubrication sciences. He currently leads several projects studying lubricants, coatings, and materials for loss of lubrication and increasing power density, as well as the basic chemical and physical processes at work in high-speed contacts between materials. Dr. Berkebile has a Ph.D. and an M.S. in physics from Karl-Franzens-Universität Graz/University of Graz and a B.A. in physics and German from Manchester College.
Mr. Jason Fetty is an aerospace engineer at the Aviation Development Directorate – Aviation Applied Technology Directorate. He has
worked in research and development of drive system technologies for Army rotorcraft since 2001, including participating in several major 6.3 drive system efforts, as well as multiple efforts involving gearbox loss-of-lubrication performance. Mr. Fetty has a B.S. in mechanical engineering from Virginia Tech.
Dr. Robert F. Handschuh is the Chief of the Rotating and Drive Systems Branch at NASA Glenn Research Center and the former leader of the Drive Systems Team. He has more than 30 years of experience with NASA and Department of Defense rotorcraft drive system analysis and experimental methods conducting research in high-speed gearing including windage, loss-of-lubrication technology, and hybrid gearing. Dr. Handschuh has a Ph.D. in mechanical engineering from Case Western Reserve University and a M.Eng. in mechanical engineering from the University of Toledo.
Dr. Brian Dykas leads the research in propulsion drives at the U.S. Army Research Laboratory. He has more than 10 years of research experience in the tribology of propulsion mechanical components. Dr. Dykas holds a Ph.D. in mechanical engineering from Case Western Reserve University.
References
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