Robert J. Bruckner

Glenn Research Center, Cleveland, Ohio

An Assessment of Gas Foil Bearing Scalability and

the Potential Benefi ts to Civilian Turbofan Engines

NASA/TM—2010-216732

August 2010

GT2010–22118
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Robert J. Bruckner

Glenn Research Center, Cleveland, Ohio

An Assessment of Gas Foil Bearing Scalability and

the Potential Benefi ts to Civilian Turbofan Engines

NASA/TM—2010-216732

August 2010

GT2010–22118

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Turbo Expo 2010

sponsored by the American Society of Mechanical Engineers

Glasgow, Scotland, United Kingdom, June 14–18, 2010
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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216732 1

An Assessment of Gas Foil Bearing Scalability and

the Potential Benefits to Civilian Turbofan Engines

Robert J. Bruckner

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Over the past several years the term oil-free turbomachinery 

has been used to describe a rotor support system for high 

speed turbomachinery that does not require oil for lubrication, 

damping, or cooling. The foundation technology for oil-free 

turbomachinery is the compliant foil bearing. This technology 

can replace the conventional rolling element bearings found in 

current engines. Two major benefits are realized with this 

technology. The primary benefit is the elimination of the oil 

lubrication system, accessory gearbox, tower shaft, and one 

turbine frame. These components account for 8 to 13 percent

of the turbofan engine weight. The second benefit that 

compliant foil bearings offer to turbofan engines is the 

capability to operate at higher rotational speeds and shaft 

diameters. While traditional rolling element bearings have 

diminished life, reliability, and load capacity with increasing 

speeds, the foil bearing has a load capacity proportional to 

speed. The traditional applications for foil bearings have been 

in small, lightweight machines. However, recent advancements 

in the design and manufacturing of foil bearings have 

increased their potential size. An analysis, grounded in 

experimentally proven operation, is performed to assess the 

scalability of the modern foil bearing. This analysis was 

coupled to the requirements of civilian turbofan engines. The 

application of the foil bearing to larger, high bypass ratio 

engines nominally at the 120 kN (~25000 lb) thrust class has 

been examined. The application of this advanced technology 

to this system was found to reduce mission fuel burn by

3.05 percent.

Introduction

A study has been performed to assess the scalability of 

compliant surface gas foil bearings and to quantify the 

potential benefits to civilian turbofan engines. The present 

study is grounded within the NASA Fundamental Aeronautics 

Program—Subsonic Fixed Wing Project and its focus is 

limited to what is termed “N+1” propulsion systems. These 

systems are representative of the next generation of civilian 

aircraft. In terms of the propulsion system, this implies a two 

spool, direct-drive turbofan engines having bypass ratios in the 

range of 8 to 12. Future studies will consider advanced 

propulsion systems in which a new design space is enabled by 

gas foil bearings. By considering clean sheet propulsion 

system designs that are not limited to current engineering 

design practices, enhanced benefits attributed to this bearing 

technology are expected in future system studies. Typically, 

gas foil bearings are used in much smaller turbomachines 

where the rotors are lighter, rotate faster, and where system 

auxiliary weights (such as the lubrication system) are a 

significant fraction of the overall system weight. However, 

advances in load capacity and scalability of the gas foil 

bearing warranted further study into larger systems. The gas 

foil bearing is the key technology in what has become known 

as “Oil-Free turbomachinery”.

Oil-Free turbomachinery, within the context of this study, is 

defined as high-speed rotating equipment operating without oil 

lubricated rotor supports, such as bearings, dampers, seals, and

other components required in conventional turbo machines. 

Through the elimination of the lubricating oil, rolling element 

bearings and the requisite supporting subsystems it is

envisioned that revolutionary improvements in performance,

efficiency, and reliability of turbomachinery propulsion 

systems can be realized. The enabling technology of Oil-Free

turbomachinery is the modern development of high speed, high

temperature, and high load capacity foil bearings.

Nomenclature

D Diameter of journal, mm [in.]

D Load capacity coefficient, mN/(mm3

krpm) [lbf/(in.

3

krpm)]

N rotational speed, kilo-revolutions per minute (krpm)

W Load supported by foil bearing, N [lbf]

Subscripts

pl preload

t total

dw deadweight

Compliant Foil Bearings

A foil bearing is a self-acting hydrodynamic device capable 

of separating rotating components of turbomachinery by a film 

of air or other gaseous “lubricant”. The benefits of such 

bearings to high speed turbomachinery can be quite 

significant. Among these benefits are reduced machine weight 

due to the elimination of the oil systems, removal of the 

traditional diameter—rotational speed limit (DN) imposed by 
NASA/TM—2010-216732 2

rolling element bearings, and a synergistic use of working fluid 

as a lubricant enabling a contaminant free working fluid.

However in order to realize these benefits, foil bearing must be 

able to support the required machine load and operate reliably 

and without burden to machine performance.

A generic foil journal bearing is shown in Figure 1. It 

consists of a rigid, stationary shell that contains a compliant 

foundation. This compliant foundation can take different 

forms, but is usually portrayed as a thin layer of corrugated 

metal referred to as the bump foil. A smooth foil, known as the 

top foil, is then fastened to one edge of the bump foil. A 

rotating shaft fits inside the top foil. Foil bearings typically 

operate without static clearance between the stationary top foil 

and rotating shaft. In fact there is often a preload force 

between the two parts. This static preload is unique to foil 

bearings and presents a technical challenge for both machine 

startup as well as analytic modeling. At a certain speed known 

as the lift off speed, the hydrodynamic gas film force 

overcomes the preload force and complete fluid film 

lubrication is established.

The design of an oil-free turbomachine is typically not an 

objective in and of itself. Rather, current oil-free 

turbomachines are a result of other requirements such as 

autonomous maintenance-free operation, zero working fluid 

contamination, or speed requirements beyond current bearing

technology. 

One of the earliest published works on the idea of a foil 

bearing, using oil as the lubricant, was published in 1953 by

H. Blok of the Technical University at Delft in Holland

(Ref. 1). In this work Blok describes an experimental 

apparatus in which a foil is wrapped halfway around a journal 

and loaded via a system of weights and pulleys. The results of 

friction and load tests are presented with theoretical equations 

representing the accommodation of the foil membrane to the 

hydrodynamic pressure.

The foil gas bearing initially found application in the field 

of magnetic tape storage media where the tape behaved like a 

foil and it was also desirable that there be little or no dry 

sliding friction between the tape and other solid surfaces. Air 

bearing hydrodynamics of flying read-write heads continues to 

be a major technology investment for the hard disc drive 

industry. The next major application of foil bearings occurred 

in the 1970s with the air cycle machines on large commercial 

aircraft. These machines provide air conditioning and cabin 

pressurization for high altitude flights. Of paramount 

importance in the operation of the air cycle machines is to 

provide contaminant free air to the occupants of the vehicle.

The foil air bearing enabled the removal of the oil system, the 

most serious threat to air contamination.

The ability to operate efficiently for long periods of time 

with minimal maintenance was also enabled by the application 

of foil bearings. A parallel development that occurred during 

this time period was the closed Brayton cycle for space power 

generation. The Brayton Rotating Unit (BRU) and the miniBRU were NASA funded development projects that relied 

upon, and advanced the state of the art of gas foil bearing 

technology. The 1980’s saw the application of foil bearings to 

turbocompressors. Again, the characteristic of no process fluid 

contamination was paramount. In addition, long life cryogenic 

capabilities were important for this application (Ref. 2). Foils 

bearing supported liquid rocket turbo-pumps have also been 

demonstrated.

In the late 1990s the first commercially available oil-free 

turbogenerators became available and the first oil-free diesel 

engine turbocharger was demonstrated. These machines also 

used foil bearings. A significant change in the nature of foil 

bearing occurred with this application. Until the 1990s 

commercial foil bearing applications had been limited to low 

temperature and low load capacity applications so that the 

bearing surfaces could be coated with soft, low friction 

polymer-based materials to minimize the dry rubbing friction 

on startup, shutdown, and overload conditions. The 

introduction of micro-turbogenerators for distributed power 

generation was the first high temperature, high load capacity 

application of foil bearings (Ref. 3).

More recently a limited life single spool turbojet engine in 

the 1.3 kN (300 lb) thrust class range has been demonstrated 

(Ref. 4). And finally, the emergence of high speed permanent 

magnet motors has changed the paradigm of large industrial air 

handling equipment. This new paradigm eliminates the large, 

expensive, and difficult to maintain gearboxes that have 

coupled low speed electric motors to high speed fluid 

machines in the past. Companies in the United States and 

South Korea have brought direct-drive compressors and 

blowers for factory service air and wastewater treatment plants 

to market. The rotor support technology of choice for these 

machines has been foil bearings to further capitalize on the 

maintenance free system.

Applications that are on the horizon include the 

commercialization of the automotive turbocharger, space 

nuclear power generation, and aviation turbofan engines. Each 

of these applications requires the characteristics of high 
NASA/TM—2010-216732 3

temperature, highly loaded foil bearings. Many significant 

challenges must be overcome to meet the demands of these 

future applications. The key challenges that must be 

understood include; the extent to which the foil bearing is 

scalable in terms of structure, gas dynamics, and 

rotordynamics, the identification of high speed limits that may 

exist for foil bearings, and the experimental verification of foil 

thrust bearing performance.

The application of foil bearings to small and mid-range 

aviation turbofan engines can enable substantial savings in the 

design and operation of a vehicle designed with such engines.

A propulsion system study (Ref. 5) on a 50 passenger 

commercial regional jet and a notional 10 passenger 

supersonic business jet concludes that engine weight can be 

reduced by as much as 26 percent. This is a significant benefit 

to a machine as complex and mature as commercial turbofan 

engine. The application of foil bearings to these engines could 

achieve nearly a 3 percent reduction in mission fuel burn and 

NOx emissions for both of these applications. Additionally, an 

oil-free turbofan engine is a much simpler machine that is less 

likely to experience unanticipated problems, simpler to 

maintain, and more affordable to operate. 

Technology State of the Art

Heshmat (Ref. 6) presents an excellent summary of the 

gas foil journal bearing commercial state-of-the-art. In this 

publication he lists the journal bearing diameter size used in 

various air cycle machines for both military and commercial 

aircraft. Bearings up to 50 mm in diameter have been 

commercially available for several decades. Recently 

improvements in foil bearing technology have occurred in 

terms of temperature capability as well as in size. The air cycle 

machines cited in Heshmat (Ref. 5) typically operate at 

temperatures below 500 °F, where soft polymer tribological 

coatings are used to overcome instances of dry sliding friction.

However, for the gas turbine engine, a higher temperature 

capability is required. The NASA series of high temperature 

plasma sprayed materials has overcome this issue with 

demonstrated performance up to 1500 °F. Additionally the size 

class of commercially available foil journal bearings has 

increased to the 80 mm range. In the laboratory foil bearings 

have been demonstrated up to 150 mm in diameter and DN 

values of 4.03×106 mm-rpm (Ref. 7). Large radial foil seals, an 

offspring of foil bearing technology, have been demonstrated 

to DN value in excess of 4.03×106 (Ref. 8). Although these 

large foil bearings are not yet at the same state-of-the-art as the 

50 to 75 mm diameter bearings of air cycle machines, these 

demonstrations provide evidence that the physics of foil 

bearings is scalable to these high surface speeds. Additionally, 

these demonstration show that traditional machine design 

practices can accommodate the tolerances required by foil 

bearings. Although foil bearings and seals have been 

demonstrated in these applications, their performance in terms 

of load capacity and rotordynamic stability has not yet been 

experimentally verified at these conditions.

Gas foil bearings exhibit a unique operating space based on 

their characteristics. Figure 2 presents the general operating 

space of a gas foil bearing in terms of rotational speed (DN) 

and specific load. This plane is initially subdivided by the load 

capacity coefficient, which is a ray originating from the origin 

of the graph. The region above and to the left of this ray 

represents operation of a bearing beyond its load capacity in a 

mode of dry sliding and rubbing. Hydrodynamic operation of a 

gas foil bearing in this region is not possible. The first region 

encountered below the load capacity ray is a region of mixed 

lubrication. In this region the specific power loss is quite high 

and catastrophic failures occur quickly and without warning.

For these reasons it is not recommended that foil bearings 

operate in a steady-state mode while in this region. The 

remaining two regions are the minimum power loss region and 

the lightly loaded region. Gas foil bearing operation in these 

two regions is characterized by low specific power loss and the 

ability to recover from overload and special event conditions.

Operation below and to the right of the power loss minimum 

region is the recommended design range for gas foil bearings.

These characteristics have been experimentally verified by 

Radil (Ref. 9) and have been coined the ‘foil bearing 

performance map’.

Foil gas bearings are typically constructed from several 

layers of sheet metal foils from which they derive their name.

Under static conditions the top foil is preloaded against the 

shaft, Wpl, such that there is no static clearance or eccentricity 

between the stationary and rotating parts. This is a 

fundamental difference between the foil bearing and rigid 

hydrodynamic bearings and gives rise to additional parameters 

required to characterize these bearings. The static preload and 

dry sliding friction coefficient become important parameters in 

foil bearing characterization. Additionally, the design 

generation or load capacity coefficient is a third parameter that 

characterizes a foil journal bearing. Dellacorte (Ref. 10)

identifies a design classification scheme that correlates 

compliant foundation complexity with bearing load capacity.
NASA/TM—2010-216732 4

A generation I design consists of a uniform stiffness 

foundation and is typical of early foil bearings. Generation II 

bearings vary the compliant foundation in one direction, for 

example in the axial or circumferential direction.

Generation III bearings vary the compliant foundation in at 

least two directions. DellaCorte goes on to quantify the load 

capacity of foil journal bearings according to Equation (1). In 

the steady state performance testing of foil bearings great care 

is taken to minimize all applied loads other than the preload 

and deadweight load such that the total load is known very 

accurately. The performance coefficient is typically 0.08, 0.19, 

and 0.27 mN/(mm3

krpm) [0.3, 0.7, and 1.0 lbf/(in.

3

krpm)] for 

generation I, II, and III bearings, respectively.

Wt = D (L x D) (D x N) (1)

Where,

Wt = Wpl + Wdw + Wdynamic + Wthermal +.....

Great care must be taken in using Equation (1) in both 

generating the data used to calculate the performance 

coefficient and in applying the equation for bearing design. At 

the authors laboratory experimental test rigs are balanced well 

beyond the ISO standards for high speed machinery in order to 

minimize the dynamic loads. Temperatures are also 

continuously monitored during the test to verify that steady 

state conditions are reached prior to data acquisition. The 

converse is also true for dynamic characterization of foil 

bearings. There currently is a dearth of experimental data in 

the open literature on dynamic stiffness and damping 

coefficients for foil bearings. Additionally, the data that does 

exist is somewhat inconsistent. The cause for this 

inconsistency is the lack of reporting on the non-controlled 

loads encountered during these dynamic tests.

Sizing Analysis

The most challenging bearing location in a gas turbine 

engine is the high pressure turbine bearing. This bearing can 

be located either between the high pressure and low pressure 

turbine, thus requiring an additional structural frame in the 

engine, or between compressor and turbine on the high spool 

(under the combustor). In the following analysis the latter 

location is assumed. This location within the gas turbine 

engine is particularly difficult to service with lubricating oils.

However, since the gas foil bearing utilizes the secondary 

airflow that naturally exists in this location and requires no 

special services such as oil supplies and scavenge, it is 

advantageous to consider such cantilevered turbine designs for 

gas foil bearings. Figure 3 illustrates a typical and notional oilfree rotordynamic layout of a 2-spool turbofan engine.

Preliminary sizing of a gas foil bearing is accomplished by the 

application of the rule of thumb load capacity equation by 

DellaCorte as presented above. However the sizing calculation 

is accomplished by recasting so that an applied load and liftoff speed is known and the bearing size is calculated. This

analysis has been conducted on the current fleet of commercial 

gas turbine engines to determine the size of foil bearing 

required to support the gas generator rotor in an overhung 

turbine design. Rotor mass, design rotational speed, and starter 

cut-off speed are used as inputs to this analysis. It has been 

assumed that bearing lift-off speed should occur at 20 percent

of design speed for the high spool. By sizing the bearings for 

lift off at 20 percent of design speed a factor of safety of 5 is 

implied at design speed. This design margin is consistent with 

maneuver and special event loads that must be accounted for 

in aircraft engine design. Figure 4 illustrates the sizing point 

and operating point on a specific foil bearing map. The data 

presented in Figure 4 is typical of a third generation foil 

bearing operating with a 7 kPa preload pressure and 50 kPa 

deadweight load.

In the sizing analysis, bearing aspect ratios (L/D) of 1.0 and 

1.5 are examined. The shorter bearings are more appropriate 

for supercritical shafting in which the bearings are subject to 

angular mis-alignment during startup, while the longer 

bearings typically exhibit better steady state performance.

Results are plotted against low pressure turbine output power 

as the size class of the engine. In such a comparison both
NASA/TM—2010-216732 5

turbofan, turbojet, and turboshaft engines can be compared.

Results of gas generator rotor weight, rotor speed and foil 

bearing size versus engine power output are summarized in 

Figure 5.

As mentioned in the previous section, the current state-ofthe-art for gas foil bearing technology includes bearing in the 

size class from 50 to 75 mm in diameter are in widespread 

commercial use in air cycle machine, microturbines, and high 

speed electric motors. This size class of foil bearing has been 

commercialized at both low and high temperature bearing 

cavity conditions. According to the analysis performed above 

current foil bearing technology can be used for gas turbine 

engines up to 7.5 MW. In the range of 75 to 100 mm in 

diameter, gas foil bearings have received a wealth of research 

and development activity over the past several years. Although 

there are no current commercial applications in this size class, 

the technology is well positioned for applications to gas 

turbine engines up to 15 MW. The largest gas foil bearing that 

has ever been designed and tested had a diameter of 150 mm.

This size bearing would be able to support the gas generator 

rotor of a gas turbine engine that produces 25 MW of shaft 

power. Due to the asymptotic nature of the above analysis, gas 

foil bearing technology must only be grown from a 150 mm 

diameter bearing to a 250 mm diameter bearing in order to be 

applicable to even the largest of aeropropulsion engines 

available today.
NASA/TM—2010-216732 6

A similar bearing sizing analysis was performed for the low 

pressure turbine (LPT). The results are presented in Figure 6.

The trend line for the LPT bearing shows a similar trend as 

that for the gas generator rotor with the distinction that larger 

diameter bearings are required at this location. This is largely 

due to the low shaft speed of the LPT. For the 25 MW engine 

size a 200 mm diameter bearing would provide adequate load 

carrying capability. Although this is beyond the scale of 

demonstrated foil bearing size, there are no indications that 

this is impossible to achieve with this technology. Furthermore 

if one examines the high speed LPT point in Figure 6 and fairs 

in a trend line to the origin, then a 150 mm diameter bearing 

would be able to support the LPT of a 25 MW engine. The 

high speed LPT point corresponds to an LPT that has been 

designed for a geared turbofan having a reduction gear ratio of 

3:1 between the LPT and fan. This result indicates that 

advanced high pressure ratio geared turbofan engines are more 

suitable candidate engines for oil-free technology than the 

conventional direct drive architectures.

System Level Benefits

The benefits of compliant gas foil bearing are applied to a 

large commercial propulsion system. The notional 120 kN 

thrust high bypass ratio turbofan engine is shown in Figure 7.

This size engine typically has a thrust to weight ratio of 5,

placing its bare engine weight at approximately 2300 kg. Oilfree turbomachinery is primarily a high temperature and 

lightweight structure. The most obvious systems benefit lies in 

the weight reduction that is achievable by the elimination of 

the oil and lubrication system. Figure 8 shows these weight 

trends in terms of bare engine weight. The previous study of 

oil-free turbomachinery on aviation propulsion systems was 

focused on smaller engines. In this size class the weight of the 

oil and lubrication system is a larger fraction of the complete 

engine system. A similar trend is noted here as in the foil 

bearing sizing study. Just as in the bearing sizing study where 

the bearing size does not increase dramatically beyond the

15 MW power turbine size, the trend of lubrication subsystem 

weight does not change appreciably beyond the 2500 kg 

engine. The implication of these trends is that if oil-free 

technology can buy its way onto a 2300 kg engine, then the 

benefits would also be favorable for the larger engines 

provided that bearing scaling issues do not limit the 

technology in this application. In addition to the elimination of 

the oil and lubrication system oil-free technology weight 

savings can also be realized through the elimination of the 

tower shaft and accessory gearboxes. The remaining 

accessories can be driven by dedicated, small electric motors.

The relocation of the accessories also reduces the nacelle 

diameter and drag. The cantilevered turbine design enables the 
NASA/TM—2010-216732 7

removal of a turbine frame, which comes with a substantial 

weight savings. Finally, performance benefits are achieved by 

reducing the tip clearances in the aero components. The tighter 

tip clearances are achieved in two ways. First by designing the 

shafting to have a first bend mode beyond the operating range.

This is possible because the gas foil bearing can accommodate 

larger and more rigid shafting. The second method of tip 

clearance reduction is by eliminating the squeeze film dampers 

associated with current rotordynamic designs of ball bearing 

supported gas turbine engines. Sensitivity factors that relate 

these distinct benefits to mission fuel burn for a typical 

commercial mission are applied. The results are shown in 

Table 1. The application of oil-free turbomachinery 

technology to the commercial high bypass ratio turbofan 

engine in the 120 kN thrust class engine can reduce mission 

fuel burn by 3.05 percent. While not necessarily a large 

number, this benefit is restricted to applying an advanced 

technology to an existing system. When the advanced rotor 

support technology is applied to the engine centerline, a “clean 

sheet” design is usually warranted.

TABLE 1.—SUMMARY OF MISSION

FUEL BURN REDUCTIONS

Benefit Fuel burn savings,

percent

Weight of oil 0.11

Weight of lube system 0.20

Weight of tower shaft 0.06

Weight of accessory GB 0.38

Combine turbine frames 0.47

Reduced nacelle area 0.04

HPC tip clearance 0.43

HPT tip clearance 0.56

LPT tip clearance 0.81

Total 3.05

Summary

Compliant surface gas foil bearings have been assessed with 

respect to large high bypass ratio turbofan engines in terms of 

their scalability and their potential systems benefits. The 

current state-of-the-art for commercial foil bearing is in the

50 to 75 mm diameter size class. Bearings up to 150 mm in 

diameter have been demonstrated to DN levels over 4 million.

Based on current rules of thumb for foil bearing lift-off speed, 

this size bearing would be able to support the core of a large 

turbofan engine. There are no “show-stoppers” in terms of foil 

bearing scalability, however, development work would need to 

be accomplished to elevate the technology readiness of this 

size foil bearings. Benefits of oil-free technology on the large 

turbofan engine include weight reductions, drag reductions, 

and aerodynamic performance benefits. Combining these 

benefits with sensitivity factors to relate all benefits to mission 

fuel burn shows a 3.05 percent fuel burn reduction compared 

to the current state-of-the-art. However, additional, less 

quantifiable benefits are also achieved with the adoption of foil

bearing technology, namely oil-free turbomachines tend to be 

more reliable, safer, and require less maintenance than 

tradition oil lubricated machines.

References

1. Blok, H. and VanRussom, J., “The Foil Bearing—A New 

Departure in Hydrodynamic Lubrication,” Lubrication 

Engineering, 1953.

2. Suriano, F.J., “Gas Foil Bearing Development Program,” 

U.S. Airforce Report #AFWAL-TR-81-2095.

3. Klaass, R.M., “The Quest for Oil-Free Gas Turbine 

Engines”, SAE International 3055th ser. 2006-01, 2006.

4. Heshmat, H., Tomaszewski, M., and Walton J. F., “Small 

Gas Turbine Engine Operating With High-Temperature 

Foil Bearings,” Paper GT2006-90791, presented at the 

International Gas Turbine Institute Turbo Expo 

Conference, Barcelona, Spain, June 2006.

5. Bruckner, R.J., “A Propulsion System Analysis of Oil Free 

Turbomachinery for Aviation Turbofan Engines,” AIAA–

2004–4189, 2004.

6. Heshmat, H., “Operation of Foil Bearings Beyond the 

Bending Critical,” Transactions of the ASME, Vol. 122, 

January 2000.

7. Heshmat, H., Xu, D.S., “Experimental Investigation of 

150 mm Diameter Large Hybrid Foil/Magnetic Bearing,”

Proceedings of the International Gas Turbine Congress, 

Tokyo, Japan, ITC2003Toyko-TS—017, 2003.

8. Proctor, M., “Compliant Foil Seal Investigations,”

NASA/CP—2004-212963, 2004.

9. Radil, K., DellaCorte, C, “A Three-Dimensional Foil 

Bearing Performance Map Applied to Oil-Free 

Turbomachinery,” Army Research Laboratory Technical 

Report, ARL–TR–4473, April 2009.

10. DellaCorte, C., Valco, M.J., “Load Capacity Estimation of 

Foil Air Journal Bearings for Oil-Free Turbomachinery 

Applications,” NASA-TM-209782.
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5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Bruckner, Robert, J. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 561581.02.08.03.15.12 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17309 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2010-216732 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 07 

Available electronically at http://gltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Over the past several years the term oil-free turbomachinery has been used to describe a rotor support system for high speed turbomachinery 

that does not require oil for lubrication, damping, or cooling. The foundation technology for oil-free turbomachinery is the compliant foil 

bearing. This technology can replace the conventional rolling element bearings found in current engines. Two major benefits are realized 

with this technology. The primary benefit is the elimination of the oil lubrication system, accessory gearbox, tower shaft, and one turbine 

frame. These components account for 8 to 13 percent of the turbofan engine weight. The second benefit that compliant foil bearings offer to 

turbofan engines is the capability to operate at higher rotational speeds and shaft diameters. While traditional rolling element bearings have 

diminished life, reliability, and load capacity with increasing speeds, the foil bearing has a load capacity proportional to speed. The 

traditional applications for foil bearings have been in small, lightweight machines. However, recent advancements in the design and 

manufacturing of foil bearings have increased their potential size. An analysis, grounded in experimentally proven operation, is performed to 

assess the scalability of the modern foil bearing. This analysis was coupled to the requirements of civilian turbofan engines. The application 

of the foil bearing to larger, high bypass ratio engines nominally at the 120 kN (~25000 lb) thrust class has been examined. The application 

of this advanced technology to this system was found to reduce mission fuel burn by 3.05 percent.

15. SUBJECT TERMS

Foil bearings; Turbomachinery; Turbofan engines; Lubrication systems; Damping; Cooling 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

13 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

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