Future Advanced Rotorcraft Drive System (FARDS) Full Scale Gearbox Demonstration

Future Advanced Rotorcraft Drive System (FARDS) Full Scale Gearbox Demonstration

ABSTRACT 

The Future Advanced Rotorcraft Drive System (FARDS) program focused on improving the performance and affordability of current aircraft drive systems. During the course of the program eighteen enabling technologies were developed to achieve these objectives within an existing helicopter transmission system. The transmission system was designed to enhance the Bell 407 light commercial aircraft configuration which has a similar configuration to the OH-58D Kiowa Warrior. Full scale main rotor gearbox testing was completed in 2016, which demonstrated many of these enabling technologies to a Technology Readiness Level (TRL) of 6. The demonstration testing was designed to closely follow the civil certification process (similar to military qualification testing), and included gear tooth pattern development, gear tooth bending fatigue, endurance, and three loss-of-lubrication tests. This testing successfully demonstrated a significant improvement in drive system technologies. The developed technologies are now ready to transition to future rotorcraft, such as Future Vertical Lift.

INTRODUCTION

The Future Advanced Rotorcraft Drive System (FARDS) program was a six-year program focused on eighteen technologies to improve drive system performance and affordability. Each of the eighteen enabling technologies developed under the FARDS program falls within these areas: gears (new materials and processing techniques), bearings (advanced configurations and materials), thermal management (cooling techniques and oil additives), drive shafts (new configurations), and fault detection technologies.

The eighteen technologies include the following:

Gear Technologies
1. C64 gear steel material
2. Cavitation peening
3. Optimized gear teeth geometry 4. Friction welding
5. Hybrid clutch

Bearing Technologies

6. Wave bearings
7. Hybrid Cronidor 30 bearings
8. Fully ceramic bearings
9. Fiber Reinforced Aluminum (FRA) bearing liners Drive Shaft Technologies
10. Hybrid Mast
11. 3X Supercritical Driveshaft
Thermal Management Technologies
12. Heat pipes
13. Nano-diamond oil
Condition Based Maintenance Technologies
14. Adaptable gearbox monitoring system
15. Fluid quality and debris monitor
16. Non-metallic debris monitor
17. Torsional anomaly detection system

18. Bolt tension detectionAs reported elsewhere, some of the FARDS technologies were demonstrated to a TRL6 individually. Examples include the condition based maintenance technologies, specifically the non-metallic debris monitor (Reference 1) and the direct measurement of bolt tension (Reference 2).

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Figure 1. OH-58D MRGB cross-section. Figure 2. FARDS MRGB demonstrator cross-section.

Some of the technologies were integrated into a tail rotor drive shaft TRL 6 demonstration (Reference 3). This paper focuses on the full-scale testing of the technologies identified for the main rotor gearbox (MRGB). The main goal for the FARDS MRGB gearbox demonstration was to evaluate several enhanced design features incorporated into a Bell 407 MRGB. The 407 MRGB is similar to the OH- 58D Kiowa Warrior MRGB, but the main difference is that the Bell 407 power rating is 642 HP versus 550 HP for the OH-58D.

FARDS MRGB TECHNOLOGIES

Of the eighteen FARDS technologies, ten were relevant for MRGB demonstrator testing. Of these ten, three technologies were not included in the MRGB demonstrator. The wave bearing technology was excluded due to its longer development schedule, and manufacturing inconsistencies in the hybrid mast technology eliminated its consideration from the demonstration. Lastly, component testing showed that the cavitation peening technology was similar in performance to traditional shot peening, and was therefore excluded.

The seven technologies that made up the demonstrator testing include:

C64 Gear Material
Optimized Gear Teeth Geometry
Friction Welding
Hybrid Cronidor 39 Bearings
Fiber Reinforced Aluminum (FRA) liners Heat Pipes
Nano-Diamond Oil

In addition to the individual FARDS technologies, an optimization of the MRGB housing and gearbox configuration was performed to accommodate the new FARDS components and to reduce weight. The FARDS gearbox housing was designed to fit within the existing OH- 58 and M407 aircraft installation footprint. A cross-section is shown in Figure 1. For this housing design, the formerly separate mid case was incorporated into the main rotor gearbox. The ring gear became scalloped around bolt interfaces, and the entire MRGB case was machined out of a lighter magnesium alloy billet (previously aluminum sand casting) with corrosion protection exterior coating. Bearing re-arrangements and other design changes were also made to the gearbox. Figure 2 displays a cross-section of the modified FARDS MRGB demonstrator. All of the external component attachments, including the mounting pylons, remained the same. The careful redesign made it possible to test the new MRGB in the existing M407 MRGB test stand in the Bell Drive Systems Test lab (DSTL) located at Grand Prairie, Texas. Figure 3 shows the fabricated components involved in the test program.

MRGB DEMONSTRATOR TESTING

The FARDS MRGB development consisted of three phases: design, manufacturing and testing. During the conceptual design phase, an initial look at the technologies and how the improvements impact the gearbox system was performed. The technologies were assessed for both capability and risk, and were integrated into a gearbox layout for the final demonstrator configuration.

Bell Helicopter, with support from outside suppliers and technology development subcontractors, manufactured and assembled the MRGB demonstrator gearboxes. To support the testing, four complete assemblies were fabricated as three test units and one spare. Major components (like gearing) were produced in sufficient quantities to support replacements during fatigue testing and to enable gear tooth pattern development. Two additional gear sets were produced to support loss of lube tests.

The MRGB demonstrator test plan closely followed the civil certification process (similar to military qualification testing) for the entire gearbox. Successful testing of all individual components within the gearbox resulted in a TRL 6 determination of each technology. Tests performed on the demonstrator include:

  •   Development Bench Test
  •   Gear tooth bending fatigue test
  •   Endurance Test
  •   Loss-of-lubrication test with FARDS MRGB including PEEK bearing cages
  •   Loss-of-lubrication test with FARDS MRGB including steel bearing cages
  •   Loss-of-lubrication test with FARDS MRGB including steel bearing cages without heat pipesMRGB DEVELOPMENT BENCH TESTThe gearbox demonstrator development bench test was conducted in three successive phases. The first phase of testing included: fit and function, oil quality determination, and gearbox test stand shakedown. This phase verified proper operation of the gearbox and lubrication system. The second phase of testing included gear tooth contact pattern development, conducted at full speed and load. Pattern development testing ensures that acceptable load contact patterns exist and that contact area growth with increasing load is within desirable limits. Gear tooth pattern development also includes a comparison of the spiral bevel tooth loading analysis vs. the measured tooth contact pattern under load. Consistency between the two is confirmed in Figure 4. The third phase of testing included thermal testing of the gearbox internal components. These tests concentrated on verifying the performance of the gearbox and lubrication system under extreme conditions and allowed for further evaluation of component performance.

Once all parts were fabricated, the gearbox was assembled, successfully demonstrating fit/function elements. The gear tooth contact pattern development test resulted in acceptable load contact patterns that were within the desired limits when the contact area grew at higher loads. The gears went through testing without any re-grinding of gears during pattern development, showing consistency between the design/analysis and manufacturing of the gears.

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*Nano-diamond oil not shown

Friction welded gearshaft

Hybrid Cronidor 30 bearings

Fiber Reinforced Aluminum (FRA) bearing liners

Heat Pipes

Optimized gear teeth geometry

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C64 Gear steel

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Figure 3. FARDS demonstrator gearbox components.

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