One critical sub system in any vertical lift aircraft is the main transmission. Apart from incremental advances most transmission performance characteristics, such as power density, reliability, noise and speed, in rotary wing aircraft have remained relatively unchanged in the last several decades. The most revolutionary vertical lift aircraft flying today is the V-22 Osprey. This aircraft was designed in the 1980s by Bell and Boeing and first flew in 1989. That makes the technology in this aircraft over 20 years old.
The military engagements in the last decade have highlighted the important role of vertical lift aircraft in countering the asymmetric threats that we face now and into the future. Yet without major advances in rotary wing aircraft transmission technology, it is very likely that our vertical lift capabilities in the year 2020 and beyond will not be very dissimilar from our current capabilities. In the last year or so, I have made a concerted effort to bring this situation to the attention of senior personnel in the military. Concurring with the assessment and with their assistance, we have made some small advances in developing programs to address these future needs of the vertical lift industry. As these projects become established it is my expectation that they will provide “grist” for future topics for this newsletter, with the sponsor’s permission off course.
Suren B. Rao
The introduction of high temper resistant steels for aircraft gear boxes has been happening for some time now. Sponsored by a group of companies, the Aerospace Bloc of the Gear Research Institute has been very active conducting contact fatigue, bending fatigue and scoring resistance tests to establish the fatigue properties of these materials. This data would enable gear design engineers to utilize these materials in their designs.
Several of these new steels also are corrosion resistant. Further, superfinishing utilizing the isotropic superfinishing (ISF) process has also become ubiquitous in many aircraft gear applications for enhanced fatigue life. Consequently, the comparative corrosion resistance of these steels and the impact of the ISF process on corrosion resistance is of significant interest to the sponsors.
In an unusual contractual arrangement REM Surface Engineering agreed to sponsor a project to comparatively evaluate the corrosion resistance of AISI 9310 (VIM VAR), AMS 6308, Pyrowear 675, CSS42L, with and without superfinishing, against hardened stainless steel 440C. REM also agreed that the results of this effort could be disseminated to the members of the Aerospace Bloc, with no restrictions, in lieu of utilizing test specimens belonging to the Bloc. Publishing of the results was also recommended.
A significant part of the effort was in establishing the test procedures. The standard ASTM B 117 – 95 test for corrosion was considered too aggressive to yield meaningful results for the aircraft gear industry. A new test procedure including specimen preparation was developed and scrutinized by the Bloc. After much debate a test procedure that meets their individual needs was defined. The project is underway as this newsletter is being written.
Figure 1: Corrosion specimen in salt environment
Figure 1 shows a test specimen suspended over a salt solution and figure 2 shows all the test specimens being subjected to a heating cycle in a furnace. After that the test specimens are visually examined for the extent of corrosion. Other parameters to quantify corrosion will also be explored and established.
Education and Training
The above described effort is typical of the type of project that an undergraduate student can undertake to broaden his/her experimental, gear related background.
Figure 2: Corrosion specimens in the heating cycle
The Gear Research Institute is a non profit corporation. It has contracted with the Applied Research Laboratory of The Pennsylvania State University to conduct its activities, as a sponsor within the Drivetrain Technology Center. The Gear Research Institute is equipped with extensive research capabilities. These include rolling contact fatigue (RCF) testers for low- and high-temperature roller testing, power circulating (PC) gear testers for parallel axis gears with a 4-inch center distance (testers can be modified to accommodate other center distances), single tooth fatigue (STF) testers for spur and helical gears, gear tooth impact tester, and worm gear testers with 1.75 and 4-inch center distances. Extensive metallurgical characterization facilities are also available at Penn State in support of the Gear Research Institute. For further details on our testing capabilities please go to the Drivetrain Technology Center website or call Dr. Suren Rao, Managing Director, at (814) 865-3537.