AIAA JOURNAL
`Vol. 52, No. 5, May 2014
`
`Aeropropulsion for Commercial Aviation in the Twenty-First
`Century and Research Directions Needed
`
`Alan H. Epstein∗
`Pratt & Whitney, East Hartford, Connecticut 06108
`
`DOI: 10.2514/1.J052713
`
`Two driving imperatives of 21st century commercial aviation are improving fuel consumption and reducing
`environmental impact. The research important to aeropropulsion’s advancing these goals is shaped both by physics of
`the design space and by design choice. As fuel becomes increasingly more expensive, engine architectures and design
`details evolve to reflect the new balance between engine fuel consumption, weight, and manufacturing and
`maintenance costs. The evolution of engine architectures changes the relative value of specific technologies. The
`engines of the future will be advanced gas turbines due to their superior fuel burn at the aircraft level. They will be
`fueled by sustainable liquid hydrocarbons. Both the thermal and propulsive efficiency of the gas turbine can be
`significantly improved. The need to improve propulsive efficiency has driven engine bypass ratio up, to 12 recently,
`and higher in the future. This is a different, less familiar design space than the 5 to 8 bypass ratio, which characterized
`the last 40 years of engine experience. Realignment of research priorities is required to address 21st century
`challenges, such as the knowledge needed to realize efficient engines at very small core sizes. The new challenges open
`up new opportunities for both designers and researchers.
`
`I.
`
`Introduction
`
`I T HAS been more than 70 years since the flight of the first jet
`
`airplane and over 50 years since the first successful commercial
`jet airliner,
`the Boeing 707, entered service. Reflecting R&D
`investments of tens of billions of dollars over this period, the jet
`engine has improved enormously: efficiency up by three times,
`power to weight ratio up by of two to four times, and reliability and
`life improved 100 200 times and 5 10 times,
`respectively.
`The turbofan jet engine is now the aeropropulsion system of choice.
`It is appropriate now to ask how much further jet engines can be
`improved. Will continued investments here be fruitful, and if so, what
`should they be? In a broader sense, the gas turbine is now the aircraft
`engine of choice because of its high efficiency, low weight, low
`emissions, and extraordinary reliability. How much longer will this
`continue?
`This paper considers these questions with the aim of identifying
`and prioritizing research paths relevant for advancing aeropropul
`sion. There are very diverse applications for airplanes, including
`commercial, military, and general aviation. Commercial aviation is
`focused on the transportation of people and goods and represents the
`majority of the economic value that aircraft bring to the world. It is
`also responsible for the majority of the environmental impact of
`aviation and most of the business revenue associated with aviation
`engines. This discussion is thus focused on commercial aviation.
`
`Needs and opportunities peculiar to military applications or general
`aviation are not considered here.
`
`II. Defining Aeropropulsion
`Aircraft propulsion can be considered as consisting of two
`necessary elements. The first is a motor to convert stored energy to
`mechanical power, typically in the form of a rotating shaft. The
`second is the conversion of mechanical power into propulsive power.
`Excluding rockets, to date we have identified only two methods
`of propelling an airplane: flapping wings or spinning a propeller. The
`flapping of wings has not been notably successful for airplanes and
`so may be safely neglected here. Indeed,
`theoretical analysis
`suggests that flapping is less efficient than a propeller in converting
`mechanical power into propulsive power [1]. A propeller may be
`operated in free air, installed in a duct to produce a jet and called a fan,
`or canted to the flight direction and called a rotor (as in a helicopter).
`Herein, we will adopt the term propulsor as referring to a device
`which converts shaft power to propulsive power,
`inclusive of
`propellers, fans, and rotors.
`Propulsors are turned by motors: internal combustion in the old
`days, gas turbines for the past half century. Recently, there has been
`consideration of using electric motors, so care must be taken
`to distinguish between power and energy. Power and energy
`requirements for a wide variety of land, sea, and air vehicles are
`
`Alan H. Epstein is Vice President of Technology and Environment at the Pratt & Whitney Division of United
`Technology Corporation. He leads Pratt & Whitney’s efforts to identify and evaluate new methods to improve engine
`performance, fuel efficiency, and environmental impact. He also provides strategic leadership in the investment,
`development, and incorporation of technologies that reduce the environmental impact of Pratt & Whitney’s
`worldwide products and services. Before joining Pratt & Whitney, Dr. Epstein was the R. C. Maclaurin Professor of
`Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT) and the Director of the MIT Gas
`Turbine Laboratory. He currently holds an appoint there as Professor Emeritus. Dr. Epstein is a member of the U.S.
`National Academy of Engineering and is a Fellow of AIAA and of the American Society of Mechanical Engineers. He
`received his B.S., M.S., and Ph.D. degrees from MIT in aeronautics and astronautics.
`
`Presented as Paper 2013-0001 at the AIAA Aerospace Sciences Meeting, Grapevine, TX, 7 10 January 2013; received 4 April 2013; revision received 17 October
`2013; accepted for publication 31 October 2013; published online 28 March 2014. Copyright © 2013 by United Technologies Corporation. Published by the
`American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier
`pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 1533-385X/14 and $10.00 in
`correspondence with the CCC.
`*Vice President, Technology and Environment, 400 Main Street, M/S 162-24. Fellow AIAA.
`
`901
`
`Downloaded by UNITED TECHNOLOGIES CORP on May 23, 2016 | http://arcaiaaorg | DOI: 102514/1J052713
`
`GE v. UTC
`Trial IPR2016-00952
`
`UTC-2007.001
`
`

`
`902
`
`EPSTEIN
`
`cells from chemical energy in fuel, from chemical energy in batteries,
`or from solar cells on the vehicle. The latter is impractical on this
`planet for transport aircraft given the level of solar irradiance falling
`on the Earth. This irradiance is insufficient to support aircraft wing
`loading above 20 lb∕ft2 (98 kg∕m2), far below that needed for all but
`the slowest speed flight. Thus, we can safely rule out solar cells
`powering commercial airplanes. This means that aircraft of the
`future, as those of the past, must be fueled.
`Batteries are a different approach to energy storage and present
`their own challenges. The theoretical energy density of lithium
`chemistry is about 10% that of kerosene. When batteries are
`engineered with current technology for such practical considerations
`as safety, performance over a wide temperature range, and long life,
`their energy density is 10% of the theoretical maximum and thus only
`about 1% that of kerosene. Thus, even a several hundred percent
`improvement in battery technology would still leave batteries many
`times inferior to hydrocarbon fuels in terms of energy density.
`Furthermore, the weight decrease during flight of fueled aircraft is an
`important factor in establishing aircraft range (the Breguet range
`equation [5]). Fuel weight decreases during a mission but battery
`weight does not, implying an additional penalty for a battery
`powered vehicle. Given the previous considerations, pending the
`discovery of as yet unknown battery chemistry, it is unlikely that
`batteries will replace fuel on commercial aircraft.
`To significantly improve its climate change impact, aviation must
`reduce both the amount of fuel burned on each flight and the net CO2
`produced by that fuel. This means a switch from fossil fuel. Currently,
`hydrocarbon fossil fuel serves as both the energy source and the
`energy storage medium on the airplane. This must change. Rather
`than depend on fossil energy, aviation must move to a sustainable
`energy source such as solar, wind, or nuclear. Whatever the energy
`source, the previous discussion implies that energy is best delivered
`to and stored on the aircraft as a liquid hydrocarbon. Current focus is
`on capturing solar energy in the form of renewable biofuels. Here, the
`CO2 exhausted by the engine is that absorbed from the atmosphere by
`plants or algae. With current technology, the growing, processing,
`and transportation of fuel produces an amount of CO2 somewhat less
`than that in the engine exhaust, and so the net reduction from a biofuel
`is greater than 50% [6]. Biofuel supply chain technology should be
`able to improve this considerably.
`Many ground and flight tests have shown that drop in biofuels are
`technically feasible, and a blend of up to 50% of a biofuel is now
`approved for use on commercial aircraft. The fuels approved to date
`are in relatively short supply and expensive. One reason is that they
`use expensive feedstock, basically vegetable oil. With current crop
`yields and processes, the net efficiency of the conversion of solar
`energy to jet fuel in this manner is only about 0.05%, implying that
`there is considerable room for improvement. Improvement requires
`research in such areas as increasing crop yields, new or modified
`organisms engineered for biofuel production, and new processes
`suited to low cost feedstock. Promising avenues include cellulosic
`biomass, algae, and halophytes. Also, as society greens, the CO2
`overhead associated with the growth, processing, and transportation
`of biofuels should improve.
`
`IV. Motors to Power Propulsors
`Energy will continue to be supplied to and stored on aircraft as
`liquid fuel, but will the gas turbine continue as the device of choice to
`convert that energy to shaft power? Other candidates might be fuel
`cells powering electric motors, different
`thermodynamic cycles
`(Otto, Rankin, Sterling, etc.), or some hybrid combination. This
`question can best be addressed by considering why gas turbines are
`the current motor of choice, physical constraints and limitations, and
`metrics by which aircraft motors are now and will be assessed. To
`potentially replace an existing approach, a new approach must be
`significantly better than the incumbent or at least appear to be. The
`metrics by which these are evaluated include efficiency, weight,
`safety and reliability, emissions, and cost.
`
`Fig. 1 Energy and power of air, land, and sea vehicles.
`
`shown in Fig. 1. Clearly, large aircraft flying long distances at high
`speed require prodigious amounts of both power and energy.
`
`III. Energy Sources and Energy Storage
`Will we use the same fuel in the future as we use now? Engineering
`criteria for jet aircraft fuel selection changed little in the 20th century.
`In the last decade, increased concern for the environment, climate
`change in particular, has added a new imperative for aviation:
`reduction in greenhouse gases, especially CO2. Thus, it is prudent to
`consider whether we will continue to use the same aircraft fuels in the
`21st century as we did in the 20th. Current jet fuel is chemically
`similar to kerosene. The technical attribute of fuel most important to
`airplane design and performance is energy density, both gravimetric
`and volumetric. Cost and emissions are very important as well, with
`additional concerns of thermal stability, lubricity, etc. Over the past
`70 years, research on improved fuels has yielded relatively minor
`gains, mainly in slightly increased density (JP 10) and thermal
`stability (JP 7, JP 8 ‡ 100).
`The energy density and energy cost for a variety of “fuels” are
`shown in Table 1 [2]. In terms of room temperature liquids, Jet A has
`the highest energy density and lowest cost. Although the gravimetric
`energy density of methane is close to that of Jet A, and hydrogen is 2.7
`times greater than Jet A; these are gases at room temperature and thus
`must be stored as cryogenic liquids or at high pressure. The weight of
`high pressure containment makes the latter option impractical given
`tank materials available today. Cryogenic storage as liquid is possible
`but introduces many questions including routine handling and safety,
`especially in accidents. Liquid hydrogen has less than 10% the
`volumetric density of Jet A. For equivalent onboard energy, liquid
`hydrogen fuel requires storage volume 10 times greater than today’s
`liquid fuel with a concomitant increase in aircraft weight, drag, and
`energy consumption. This suggests that liquid hydrogen is not an
`attractive fuel for high speed aircraft, a lesson first learned in the
`1950s [3]. Hydrogen might have a role for low speed surveillance
`applications when persistence is a dominant design criterion [4].
`Combustion motors derive their energy from chemical energy
`stored in fuel. Electric motors need electric power. Conceivably, this
`can be generated by combustion motors driving generators, by fuel
`
`Fuel type
`
`Table 1 Gravimetric (GED) and volumetric energy density
`(VED) and cost of liquid fuels
`GED, MJ∕kg VED, MJ∕l Cost, $∕MJ
`0.3
`0.3
`0.03
`0.6
`0.6
`170
`14
`20
`0.29
`38
`35
`0.26
`44
`36
`0.018
`0.005
`45
`19a
`0.44
`117
`8.3a
`
`Li battery (rechargeable)
`Li Battery (primary)
`Honey
`Goose fat
`Kerosene (Jet A)
`Natural gas
`Hydrogen
`
`aVolume of liquid only, not accounting for cryotank.
`
`Downloaded by UNITED TECHNOLOGIES CORP on May 23, 2016 | http://arcaiaaorg | DOI: 102514/1J052713
`
`UTC-2007.002
`
`

`
`EPSTEIN
`
`903
`
`A. Thermal Efficiency
`The limit to the ideal thermal efficiency of Brayton cycles such as a
`gas turbine is readily estimated at about 80% for flight in the lower
`stratosphere. How close to that theoretical maximum these devices
`can practically reach is not as simple a question. In the gas turbine
`industry, there are several definitions of efficiency that are defined for
`different uses. Koff [7] defined the thermodynamic efficiency of the
`core as the fluid power available at the core exit divided by the heat
`added from the fuel’s chemical energy and plotted that against the
`propulsive efficiency times the transmission efficiency (transmission
`includes the losses in the turbine driving the fan, the fan itself, the fan
`duct, etc.). This is shown in Fig. 2 at cruise and illustrates the progress
`to date. The product of the two efficiencies is shown as arcs, which
`represent the total efficiency of the conversion of chemical energy in
`the fuel to propulsive power. Since Whittle’s first engine, this
`thermodynamic efficiency has improved from about 10% to over
`50%. When weight and drag are not an issue, as in ground based
`power plants, then gas turbine combined cycle plants (a gas turbine
`whose exhaust heat runs a steam cycle) can now deliver efficiencies
`above 60%. Propulsive efficiency has improved as well, from 50 to
`70%. Overall, gas turbine aeroengine total efficiency has climbed
`from 10% to almost 40%.
`Koff’s definition of thermal efficiency is useful for comparing
`among jet engines. Another useful definition of gas turbine thermal
`efficiency for comparing with other engines or motors is one used for
`turboprops that accounts for all of the core fluid power as shaft power
`deliverable to a propulsor, designated here as “motor efficiency”. The
`evolution of commercial aircraft gas turbine motor efficiency is
`shown in Fig. 3. This efficiency has improved by about 16 points over
`four decades and now approaches 55%. (The considerable scatter
`implies that thermal efficiency has not always been the primary
`design driver.) By contrast, diesel engines now range from 30 to 50%
`motor efficiency, with the higher efficiencies at the largest sizes,
`10 60 MW [8]. A practical advantage of diesels over gas turbines in
`
`Fig. 2 Core thermal and propulsive efficiencies for commercial aircraft
`engines.
`
`Fig. 3 Evolution of commercial turbofan motor efficiency.
`
`some applications is that diesels retain relatively more of their peak
`efficiency at part power. Because most transport aircraft engines are
`designed for peak efficiency at cruise, where most of the fuel is
`burned, this attribute has much less importance for airplanes than for
`ground vehicles or power generation.
`Current fuel cells combine H2 and O2 to generate electricity. How
`the H2 is generated varies widely. If the fuel cell is to operate from a
`complex hydrocarbon fuel, then the definition of efficiency must
`include all of the reforming processes that convert the fuel into H2.
`Current ground power generation systems [9] operate at about 40%
`overall efficiency. In a practical aviation application, the efficiency
`implications of the electric motors, drive train, and their cooling
`would need to be considered as well, consistent with the definition of
`motor efficiency.
`In summary, modern, large gas turbine engines are the most
`efficient devices in service to convert hydrocarbon chemical energy
`to mechanical power. They are by no means mature, and so
`considerable improvement in efficiency can result from focused
`research, as is discussed later.
`
`B. Weight
`Airplanes are all about weight, and so airplane engines must be as
`well. The Wright brothers built their own engine out of aluminum for
`just this reason, even though aluminum was a very expensive material
`in those days. Thirty seven years later in 1940, American technical
`luminaries were very skeptical of the concept of gas turbines for this
`same reason [10]:
`
`“The gas turbine could hardly be considered a feasible
`application to airplanes mainly because of complying with
`the stringent weight
`requirements imposed by aero
`nautics : : : The present internal combustion engine used in
`airplanes weighs about 1.1 pounds per horsepower, and to
`approach such a figure with a gas turbine seems beyond the
`realm of possibility with existing materials.”
`
`This report was issued a year after the first jet plane had flown in
`Germany, unbeknownst to the authors. The designers of the German
`engine used air cooling to circumvent “ : : : the realm of possibility
`with existing materials”. This illustrates both the role that materials
`play in determining engine weight and the skill of engineers
`and designers in circumventing what scientists may regard as
`fundamental barriers, such as material properties.
`Since the early days of turbofan development, commercial
`turbofan power to weight ratios have improved by a factor of 4 or
`more, to 9 hp∕lb (15 kW∕kg). In contrast, a 10 60 MW diesel
`engine is more than 400 times heavier. Part of this weight difference is
`a result of aeroengine applications favoring light weight over low cost
`and thus embracing relatively expensive materials such as titanium.‡
`However, the most important factor influencing the relatively low
`weight of a gas turbine is that the average air velocity through a gas
`turbine is very much higher than that through other combustion or
`electrochemical (fuel cells) motors. At the same thermal efficiency,
`motors consume the same fuel and thus need the same air for
`combustion. To first order, the motor with the higher average through
`flow velocity will have the smaller cross section and weight. For a
`consistent comparison, the weight of an electrochemical motor must
`include the complete fuel cell system, electric drive train, cooling
`system, and structure needed to produce shaft power at all altitudes.
`On a weight basis alone, fuel cells appear to be highly unattractive for
`commercial aircraft propulsion.
`
`C. Emissions and Noise
`Since the 1960s, both the chemical emissions and the noise of jet
`engines have been regulated to improve well being around airports,
`with regulations becoming increasingly stringent over time. Noise
`
`‡To reduce weight, the Wright brothers used aluminum for their first engine.
`Aluminum was then 30 times more expensive than steel. Titanium is now
`about 30 times more expensive than steel.
`
`Downloaded by UNITED TECHNOLOGIES CORP on May 23, 2016 | http://arcaiaaorg | DOI: 102514/1J052713
`
`UTC-2007.003
`
`

`
`904
`
`EPSTEIN
`
`has been the bane of aviation from its inception over 100 years ago
`[11] and continues to be so to this day. Takeoff and landing noise in
`the immediate vicinity of an airport is regulated; cruise noise is not.
`Lack of viable noise reduction technology has been a recognized
`barrier to the introduction of commercially viable supersonic
`transportation since the 1960s.
`Currently, oxides of nitrogen (NOx), particulates, and unburned
`hydrocarbons are regulated during landing and takeoff. Gas turbines
`inherently produce much less NOx than internal combustion (IC)
`engines, so that IC engines need considerable exhaust treatment such
`as catalytic converters. Fuel cells that operate from hydrogen or
`methanol produce no regulated emissions. Fuel cells that internally
`produce hydrogen from hydrocarbons, such as solid oxide cells or
`fuel reformers, operate at higher temperature and may produce NOx
`and unburned hydrocarbons, but this area has yet to see much study.
`
`D. Reliability and Maintainability
`Reliability and maintainability are important measures of airplane
`engine value. The first
`influences safety, while both influence
`operating cost. One measure of reliability is in flight shutdown
`(IFSD) rate. This metric has improved dramatically, by a factor of
`200, over the past 50 years; see Fig. 4. Extended operations requires
`an IFSD rate better than 0.020 shutdowns per 1000 h of operation.
`Today’s state of the art (SOA) is better than 0.002. Time between
`overhauls and time on wing are useful measures of maintainability.
`These, too, have improved from 400 800 h in the days of the large
`piston engines to 6000 14,000 h today. Now, engines may stay on the
`wing seven to 10 years before they need be removed for overhaul.
`
`E. Engine Economics: Cost, Price, and Value
`Engine related costs are one of the most important factors affecting
`the economics of aircraft ownership and airline operations, and so
`these costs are an important consideration in engine selection.
`Engines account for about 15 20% of the list price of a new aircraft,
`over 50% of the maintenance cost, and of course, they determine the
`amount of fuel burned. Therefore, operators’ cost is always a major
`design criterion for engine designers. Researchers often do not
`consider product cost because of the difficulty of connecting it to
`engineering fundamentals and the paucity of available data.
`For many decades, fuel was significantly less than $1 per gallon.
`One widely used airline cost measure is cash airplane related
`operating cost (CAROC), which includes fuel, airframe and engine
`maintenance, crew costs, fees, and ground handling but excludes
`capital related costs. Figure 5 shows the spot jet fuel price over the
`past 20 years and illustrates wide body aircraft operating cost as a
`function of that fuel price. At $0.50 per gallon, the fraction of
`CAROC attributable to engines is 22%. This rises to 60% at $4 per
`gallon. In the past five years, fuel has been as high as $5 per gallon. An
`extrapolation of CAROC to prices well above the historical record is
`shown in Fig. 6, suggesting that high fuel prices may overwhelm
`other considerations. Prediction of future aircraft fuel prices is well
`beyond the capability of this author. However, if fuel prices continue
`
`Fig. 5 Fuel price and wide-body airplane cash operating cost in then-
`year USD.
`
`Fig. 6 Wide-body cash operating cost as a function of fuel price in 2012
`USD (“other” costs include flight crew,
`insurance, and landing,
`navigation, and ground fees).
`
`above $3 per gallon, then the historical balance among operating
`costs remains disrupted, and fuel consumption will continue as the
`overriding economic concern.
`The cost of manufacturing a jet engine and its list price scales with
`engine sea level static (SLS) thrust. Figure 7 shows an estimate of the
`list price per unit of thrust of commercial jet engines over an order of
`magnitude in engine size. Prices range from about $200 to $400 per
`pound of thrust. The smaller engines are more expensive because
`items such as an electronic fuel control are needed independent of
`engine thrust. At the very high thrust size, mechanical scaling is
`unfavorable such that engine weight per unit thrust rises. To keep the
`weight of large engines under control, more expensive construction is
`used, such as hollow metallic or composite fan blades. Also, the
`largest engines power very long range aircraft, which are most
`sensitive to fuel price, so that a reduction in fuel burn may offset an
`increase in engine cost to the owner.
`
`Fig. 4 Evolution of aero engine reliability.
`
`Downloaded by UNITED TECHNOLOGIES CORP on May 23, 2016 | http://arcaiaaorg | DOI: 102514/1J052713
`
`UTC-2007.004
`
`

`
`EPSTEIN
`
`905
`
`Fig. 7 Bare engine list price per pound of thrust.
`
`The economics of aircraft manufacturing constrain the fraction of
`the airplane cost that aircraft manufacturers have been willing to
`allocate to the engines. Cross plotting engine list price with airplane
`list price reveals that the engines on current commercial aircraft are
`about 15 to 22% of the price of the airplane. This ratio has been
`constant since at least the 1980s.
`The price of new aircraft has been constrained by competition, the
`availability of used aircraft, and airline economics. Since the mid
`1960s, corrected for fuel price variation, new technology has dropped
`the operating cost of narrow body aircraft by 30 ∼ 40%, but net
`aircraft selling prices have remained constant. Technology has added
`value but this value has not been recovered by aircraft manufactures
`and their engine suppliers in the form of higher prices. Therefore,
`innovations and technology that raise engine production cost are
`avoided by industry. This suggests that new technology should avoid
`adding net cost to manufacturing an engine.
`
`V. Aircraft Engines of the Future
`Appropriate metrics for aircraft engines are efficiency, weight,
`emissions, noise, and reliability. In all of these, the large aircraft gas
`turbine is unmatched, with no successor on the horizon. Thus,
`hydrocarbon fueled, Brayton cycle driven propulsors appear to be
`the most promising approach for commercial aeropropulsion over the
`next few decades. What this means for specific aeropropulsion
`research directions is dependent on application, engine thrust class,
`and design choices. Indeed, the interplay between the clever designer
`and the insightful researcher is perhaps the least appreciated dynamic
`in propulsion. Design approaches can determine the relative value of
`a research topic. Designers can obviate, or at least delay, the need for
`fundamental understanding. The World War II German designers
`who used turbine air cooling in the Jumo 004 because they did not
`have access to high temperature materials illustrate this point.
`Another example is that a fundamental understanding of nacelle drag
`is much more important for a high bypass ratio turbofan than for a
`turboprop of the same thrust because the turboprop’s much smaller
`nacelle is a relatively minor factor in propulsive performance. Thus,
`the relative importance of a technology is often very dependent on
`design approaches and engine architecture. The converse is true as
`well; a good designer designs from strength and eschews approaches
`that are poorly understood.
`One example of how design approach can influence research
`directions concerns takeoff and landing approach noise. The exhaust
`jet has been the major takeoff noise source, and so it has been the
`focus of considerable research effort and resulting literature since the
`1960s. Although this research has resulted in greater understanding
`of the physical processes involved, it has not resulted in significant jet
`noise reduction technology. Nevertheless, jet noise is no longer the
`dominant noise source. Figure 8 illustrates the relative magnitude and
`direction of important turbofan engine noise sources as they have
`evolved over 40 years. This evolution resulted from technologies that
`have enabled low fan pressure ratios and the resulting high bypass
`ratios (BPRs). For the most modern designs in the 10 to 12 BPR
`range, the jet exhaust velocity is reduced so that it is largely irrelevant
`
`Fig. 8 Turbofan noise source evolution.
`
`to takeoff noise, which is now dominated by fan noise. Thus, research
`on jet noise is no longer warranted for this purpose. On approach and
`landing, the engine noise is now less than that of the airframe in some
`cases, suggesting that noise researchers may be wise to focus mainly
`on fan and airframe noise.
`In light of this background, the following sections consider the
`current state of the art and speculate on future design directions and
`the research necessary to realize them. Although predicting the future
`is an inexact art, thermodynamics is quite clear. We know that
`improved thermal efficiency will demand higher cycle pressures and
`temperatures, improved component efficiency, and reduced cooling
`and secondary air. We know that increasing propulsor efficiency
`requires low pressure ratio propulsors with low drag nacelles and
`perhaps variable geometry blading or exhaust nozzles. All of this
`must be accomplished at weights and overall costs that do not
`outweigh the advantages of improved efficiency. We know where we
`must go with some clarity. How to get there requires research.
`
`A. Propulsors
`Commercial aircraft built over the last 50 years have been gas
`turbine powered, and either turbofan or propeller propelled. At the
`most basic level, the differences are the total fan pressure ratio (FPR)
`produced across the rotor (FPR) and whether the rotor operates in a
`duct or in free air. The pressure ratio determines the propulsor exhaust
`velocity and therefore the propulsive efficiency. It also sets the
`propulsor diameter. For example, at the 25,000 30,000 lb takeoff
`thrust level, a currently flying turbofan engine with a FPR of 1.7 has a
`rotor diameter of about 1.6 m. Reducing the FPR to 1.2 at constant
`thrust grows the rotor diameter to 2.3 m. A two rotor, contra rotating
`propeller is 4.3 m in diameter, while a single rotation propeller needs
`a 5.2 m diameter to produce the same thrust. Clearly, engineering
`considerations for these configurations may be different in detail.
`We define propulsion efficiency, as is commonly done for
`propellers, as the thrust power delivered to vehicle (thrust FNfan times
`flight velocity V0) divided by the mechanical power input to the shaft,
`SHPfan. Figure 9 shows the variation in fan stream propulsive
`efficiency, nPfan, with fan pressure ratio, FPR, at a flight Mach
`number of 0.80 [12]. Three curves are shown: the ideal relation
`between FPR and propulsive efficiency (“ideal” solid line), a curve fit
`to practical designs for which the overall propulsor geometry was
`optimized for each pressure ratio (“actual” dashed line), and a curve
`
`Fig. 9 Propulsive efficiency scales with fan pressure ratio.
`
`Downloaded by UNITED TECHNOLOGIES CORP on May 23, 2016 | http://arcaiaaorg | DOI: 102514/1J052713
`
`UTC-2007.005
`
`

`
`906
`
`EPSTEIN
`
`with fixed component losses in the fan stream. For these purposes,
`FPR is defined as the total pressure ratio across the rotor and stator of
`a fan and across both rotors of a contra rotating prop. Ducted
`configuration total pressure losses (subsequently referred to as
`“loss”) include those from the rotor, stator, inlet, duct, and nozzle, as
`discussed next. Effects of external nacelle drag are not included in
`this plot. Propeller losses are principally in the rotors and, in the case
`of single rotation props, residual swirl. Propeller rotor adiabatic
`efficiencies are well below those of ducted fans, but the overall
`propulsive efficiency is higher, mainly due to lower exhaust velocity
`but also because there is no inlet, stator, duct, or nozzle to add their
`losses. Note that the diameter of the propulsor must grow as FPR
`drops to maintain constant thrust.
`To illustrate the importance of internal component losses on ducted
`propulsor design, the line labeled “fixed component losses” in Fig. 9
`is an example of impractical designs. On this curve, as FPR is reduced
`from the reference design point; the propulsive efficiency first
`increases due to reduced fan nozzle exhaust velocity. At a sufficiently
`low FPR (about 1.4 in this example), this benefit is overwhelmed by
`the fixed internal component losses (inlet, rotor, stator, duct, leakage,
`and nozzle), and so propulsive efficiency begins to drop.
`For practical designs (Fig. 9, dashed line), the dominant loss
`mechanisms change with FPR. Figure 10 shows the percentage
`change in net thrust, Fnet, attributable to different los