This chapter reviews relevant background to commercial aircraft propulsion and aircraft–propulsion integration in general, describes the current state of the art, and suggests promising research directions for integrating aircraft and propulsion technologies in order to reduce energy consumption and thus aircraft CO2 emissions. The discussion is focused on subsonic airliner propulsion. Considerations particular to other types of aircraft such as general aviation, supersonic transports, or military vehicles are outside the committee’s purview and are thus not covered.
The power needed to propel an aircraft increases at more than the airspeed squared. Thus, high powers and large energies are needed to fly at high speeds over long distances. Relative to ground vehicles, it takes an enormous amount of energy and power to move a large commercial aircraft at high speeds across a continent or ocean (see Figure 2.1).1 Thus, the value of high efficiency and high specific energy increases with speed and range.
The maximum power required for an aircraft is the power at takeoff; it scales with aircraft weight. The interrelated requirements that determine the power levels are too complex to discuss here, but they include considerations of runway length, airport elevation and ambient temperatures,2 climb rate, and cruise efficiency.
The energy use and power required during different segments of a flight are illustrated in Figure 2.2 for a current single-aisle aircraft with seating for 150-180 passengers. The particular mission shown is for a load of 150 passengers with fuel on board to fly a 1,000 nautical mile (nm) mission. The maximum range of this aircraft is over 3,000 nm. At the maximum range, the total energy required for cruise is about three times that required for a 1,000 nm flight. Future improvements in aircraft, engines, and air traffic management could reduce the power and energy requirements noticeably, but the trends and order of magnitude will remain similar.
Fuel efficiency has always been a primary design criterion for commercial aircraft since it is an important determinant of aircraft range, size, and economics. Overall, the fuel burn per seat mile of gas turbine–powered commercial aircraft has been reduced by 70 percent since service started in the 1950s, at an average rate of about
1 The power and energy needs of aircraft are driven by their size, speed, and range. Compared to an automobile, a large commercial aircraft has a much greater passenger capacity (100-400 passengers), traveling at perhaps 10 times an automobile’s speed and at up to 20 times the range while still delivering a fuel efficiency per passenger mile that is comparable to automobiles at average passenger loads.
2 Airports at high elevations and high ambient temperatures are more challenging.
2 percent per year since 1970 (see Figure 2.3). About half the gain has been the result of improvements to the airplane, the rest to the engine.
For the purposes of this study, it is useful to consider aircraft propulsion as consisting of three interdependent elements: an energy storage system, a motor to produce shaft power from that stored energy, and a propulsor to convert shaft power to propulsive power. Each is discussed briefly below.
In current aircraft, energy is stored in the form of a liquid hydrocarbon fuel, which is burned with air in the engines. Other liquid or gaseous fuels such as hydrogen or natural gas can be considered, but they are not in use at this time and, as discussed in Chapter 5, they are not likely to be ready for operational use within the 30-year time frame addressed by this report. Fuels that are gases at room temperature incur relatively high penalties in terms of required volume and tank weight that may offset their potential technical advantages, not considering issues of infrastructure, production, and distribution.
The energy storage system on current commercial aircraft typically consists of fuel tanks, valves, sensors, and transfer pumps and piping to move fuel to the engines. Most fuel is stored in the wings. This arrangement has several advantages: (1) the fuel is located at about the center of gravity of the aircraft to minimize the shift in center of gravity as fuel is burned, (2) the wing’s structural weight is reduced because the fuel weight partially offsets the bending moment produced by wing lift, and (3) no space useful for payload is lost to fuel. Also, the wings are “wet,” that is the skin of the wings is also the wall of the fuel tanks, so that little additional weight is needed to contain the liquid fuel. Very long-range aircraft, in which the fuel weight may comprise more than 40 percent of the gross takeoff weight, may require additional tanks in the fuselage or tail.
Aircraft weight and drag penalties incurred by energy sources less dense than jet fuel (e.g., hydrogen and natural gas) will need to be accounted for when comparing designs using alternative energy sources. For example, given the low density of hydrogen, the drag and weight increase from the tanks needed for cryogenic liquid H2 offset the gain in energy density for high-speed aircraft. Similarly, battery-powered concepts in which the batteries occupy the freight compartment need to be compared with aircraft having similar net payload capabilities.
Fueled energy system weight change during flight is another important consideration, strongly influencing aircraft range or the takeoff weight needed to achieve a fixed range. Because current aircraft typically lose 10-40 percent of the initial weight as fuel is burned, the net propulsive energy (i.e., the energy supplied to the vehicle by the propulsor) that is needed to keep the vehicle aloft decreases during a mission, allowing flight at higher altitude, which further reduces drag. By contrast, the weight of a closed battery system such as Li-ion stays constant during a flight, so the system would require more total energy than a fueled system all else being equal. An air battery system such as Li-air would actually gain weight during a mission, requiring additional energy. The impact of the aircraft weight change increases with aircraft range, as illustrated in Figure 2.4 for a generic single-aisle aircraft. At approximately 3,000 nm range, a non-weight-changing (battery-powered) aircraft with the same starting weight uses 12-13 percent more energy than one that reduces weight when burning fuel to supply the energy. This difference is much less significant for short ranges than for long-range aircraft.3
The choice of motor to convert energy into shaft power must be compatible with the form of the energy stored. Energy may be stored electrochemically as in a battery, in which case an electric motor is an obvious device to convert that energy to shaft power. An electric motor would also be used if a fuel cell were employed to provide electrical power from fuel. Flight-weight, reliable, electric motors and the requisite drive electronics, cabling, and cooling do not currently exist in the sizes required for commercial aircraft propulsion. The prospects for these technologies are discussed in Chapter 4.
Gas turbine engines now power essentially all commercial aircraft. While light aircraft may use spark-ignition or diesel motors to reduce cost, commercial aircraft all use gas turbine engines owing to the combination of low weight, high efficiency, and very high reliability. Current aircraft gas turbines are optimized for fossil fuel, but alternative synthetic fuels from a variety of sustainable feedstock have recently been certified for commercial use. Turbines are also capable of burning gaseous fuels, given suitable modifications to the fuel and combustion systems. Fuels are discussed at length in Chapter 5. Aircraft gas turbine engines certified for crewed aircraft are available in sizes from about 300 to 90,000 kilowatt (kW).
The engineering metrics most important for the motor driving the propulsor are the specific weight and the efficiency. The specific weights of aircraft gas turbines up to the shaft driving the propulsor (thus comparable to an electric motor) range from about 12 to 23 kW/kg when accessories such as fuel pumps and controls are included. Certified, flight-weight electric drives at the size required for large commercial aircraft (larger than 1 MW) do not now exist; 250 kW size drives and power electronics have been demonstrated with specific weights of 1.5-2 kW/kg. The motor efficiency of large commercial aircraft gas turbines in service (defined as shaft power produced divided by fuel energy flow in) is now about 55 percent. With a typical propulsor efficiency of 80 percent, total fuel-to-propulsor efficiency is about 45 percent. As discussed in Chapter 4, the propulsive power for a turboelectric system would also flow through a generator, motor, power electronics, cabling, and cooling, which have a combined efficiency of about 80 percent, resulting in a net turboelectric motor efficiency of about 45 percent and a total fuel-to-propulsor efficiency of about 35 percent. Thus, reductions in CO2 emissions for a turboelectric propulsion system will depend on factors other than motor efficiency. This is also discussed in Chapter 4.
However it is produced, shaft power is converted to propulsive power with a “propulsor” having either a ducted or an unducted configuration. When the propulsor consists of two contra-rotating propellers in tandem, it is sometimes referred to as an “open rotor.” Propulsive efficiency is defined as the propulsive power delivered to the aircraft (which is equal to thrust times airspeed) divided by the shaft power input to the propulsor. All else being equal, reducing the pressure across the propulsor reduces the exhaust velocity, which therefore increases propulsive efficiency. The relevant design parameter is the fan (or propulsor) pressure ratio, Figure 2.5. At constant thrust, as fan pressure ratio is reduced, more airflow and thus a larger propulsor area are needed. This requires either a larger fan (or propeller) diameter or more fans (or propellers). Either approach will have important design implications for an airplane, ranging from landing gear height to overall airframe configuration.
Turbofan engines mount the fan in a duct to serve several purposes: Doing so isolates the fan aerodynamics from aircraft speed, thereby facilitating higher airspeeds; it attenuates fan noise; and, in the case of a catastrophic engine failure, it improves safety by containing debris from the fan blades exiting radially and potentially striking the aircraft. The penalty paid for the duct is an increase in weight and drag. As the duct diameter grows, the propulsor efficiency increases, but at some point the increased weight and drag of the fan, duct, and nacelle cancel the efficiency gain, so that further increasing the size of the fan and its duct would increase the fuel burn of the aircraft. Also, larger diameter fans and nacelles may require longer and thus heavier landing gear. Therefore, at a given level of engine, nacelle, and aircraft technology, there is an optimum fan diameter for minimum fuel burn (see Figure 2.6). Moving the optimum fan pressure to lower values requires improving some combination of engine weight, engine inlet and duct length, nacelle weight and drag, landing gear weight, and nacelle–wing interface technologies.
Propellers and other unducted propulsors avoid the weight and drag penalties of a fan duct but also miss the advantages of ducted fans in terms of noise attenuation, safety, and high speed. While current turbofan aircraft mount engines on pylons standing off from the wing or fuselage, most commercial propeller aircraft in service today use a wing-mounted, tractor configuration in which the engines are cantilevered forward of the wing. Currently, large commercial aircraft, which are powered exclusively by turbofans, are designed for cruise speeds of Mach 0.78-0.86. Commercial propeller aircraft can cruise at speeds up to about Mach 0.68. Advanced propellers have been designed and demonstrated capable of flying in the Mach 0.7-0.78 range, although efficiency decreases as design airspeed is increased. At lower speeds, aircraft optimized for propellers can offer better efficiency than ducted approaches. Major impediments to the fielding of such aircraft at larger sizes include those associated with airframe integration, especially noise reduction and safety, and longer flight times.
The engineering metrics relevant to propulsors are propulsive efficiency, weight, and drag. Isolated (from the airframe) propulsor efficiencies are shown in Figure 2.5. State-of-the-art commercial turbofan aircraft in service today have propulsive efficiencies of 70-80 percent. Overall fuel burn must consider weight and installation aero-
dynamic effects, so that direct comparisons are best done at the aircraft level. It is the installed performance of the propulsion system that is important, not that of the engine or propulsor alone.
Distributed propulsion, which is discussed below in the section on advanced aircraft–propulsion integration concepts, uses multiple, relatively small motors and fans. Distributed propulsion may overcome the integration challenges of finding room for larger diameter fans by instead increasing the number of fans, though this has its own integration challenges.
Safety is essential for aeronautical systems, especially those used in commercial aircraft. One key to achieving the current extraordinary low accident rate of airliners is an empirically based, stringent set of design requirements for aircraft and their propulsion systems. The requirements for specific design functionalities, material properties, and redundancy are significant factors in the cost and complexity of current propulsion systems. In addition, if existing certification standards cannot assure the safety of new technologies, then new standards must be developed and validated. Any consideration of changes or innovations proposed to reduce energy consumption need to be mindful of such requirements since aircraft safety cannot be compromised. For example, the high volatility and low temperatures of cryogenic fuels introduce significant safety challenges, especially in the area of crashworthiness, though much can be learned from the utilization of cryogenics in space platforms and their associated design requirements.
Since commercial aviation is a business, the cost of the aircraft propulsion system is always an important design consideration and directly relevant to reducing energy consumption. Roughly speaking, current engines account for about 20 percent of the purchase cost of a commercial airplane. Propulsion system concepts that are significantly more costly will not be implemented unless they offer clear value in the eyes of the airline customer or are necessary to satisfy a regulatory requirement. The designs of current engines often reflect this as a constraint on the economics of propulsion. At a given level of technology, gas turbine performance can be traded for cost as represented by such factors as material choice, design complexity, and maintenance intervals. In other words, turbofan engines could be designed to consume less energy if engine purchase price and maintenance cost constraints were relaxed. Thus, it would be appropriate for comparative analyses of alternative approaches that raise the overall purchase and operating cost of the propulsion system and airplane to consider the performance of a gas turbine solution at the same overall purchase and operating costs.
The value of reducing carbon emissions (i.e., increasing aircraft efficiency) is strongly dependent on the price of energy/fuel/carbon. At the historical average price of jet fuel of less than $1/gal (2012 U.S. dollars), fuel represented about 20 percent of the direct operating cost of a twin-aisle airliner, not including aircraft depreciation. At that price, fuel is less than an airplane’s depreciation, so even a 5 percent fuel burn deficit did not render a product uncompetitive or eliminate it from the market. The historic high fuel price in 2008 was close to $5/gal, at which point fuel represented 60 percent of the aircraft’s operating cost. At that level, a 5 percent reduction in fuel burn had immense economic impact. As of April 2016, with the spot price of crude oil at $36, the spot price of jet fuel was about $1 per gallon. The U.S. Energy Information Administration projects that the spot price of crude oil in 2040 will be 2 to 7 times as much as current prices (in constant dollars), and it also projects that the cost of jet fuel will track the cost of crude oil.4,5 The economic viability of technologies that significantly increase the cost of propulsion systems and aircraft but reduce energy cost is very strongly dependent on the value placed on carbon emissions relative to other aircraft properties such as economy, speed, noise, and so on.
4 Energy Information Agency, 2016, Petroleum and Other Liquids: Spot Prices, Washington, D.C. https://www.eia.gov/dnav/pet/xls/PET_PRI_SPT_S1_D.xls.
5 Energy Information Agency, 2015, Annual Energy Outlook 2015 with Projections to 2040, DOE/EIA-0383(2015), Washington, D.C., http://www.eia.gov/forecasts/aeo/pdf/0383(2015).pdf.
In all cases, the propulsor and motor must be mounted to the aircraft in order to react against the aircraft and transmit the thrust forces and torque produced. The manner of this mounting, which is one aspect of propulsion integration, has many important aerodynamic, structural, and safety implications for the aircraft. Aircraft–propulsion integration considerations are a function of many details of both the aircraft configuration and the propulsion system. Such considerations are extremely important in selecting the best propulsion system and in optimizing an aircraft for low energy consumption.
Aircraft–propulsion integration refers to the aerodynamic, structural, and subsystem (fuel, pneumatic, hydraulic, electrical, control, etc.) interfaces between engines and airframe. These subsystems are important to the operation of the airplane, are in many cases safety critical, and consume on the order of 5-8 percent or more of the energy required for flight. Safety-critical examples include wing and inlet anti-icing and cabin environmental control. These functions are all required independent of the energy or propulsion system powering the aircraft. To a large degree, they are also independent of aircraft efficiency in that their power and energy requirements do not necessarily decrease as aircraft and propulsion systems improve. In fact, the electrical power demand of aircraft subsystems will likely increase as more utility functions are powered by electricity. Thus, one would expect that subsystem power and energy needs may grow as a fraction of total aircraft load unless technology is advanced in this area.
Today, all commercial aircraft share a configuration known as tube and wing. This configuration is characterized by thin wings, swept for efficiency at high subsonic speeds, which are mounted to a roughly circular cross-section tubular body. Engines on current turbofan-powered commercial aircraft are mounted on pylons, which stand the engines off the wings or fuselage in order to isolate the engine and airframe aerodynamic characteristics. The pylons must transmit the thrust loads from the engine to the airframe as well as convey all fluid and electrical interconnections. The engines are enclosed by fairings known as nacelles, which contain many of the subsystems important to the operation of the aircraft such as the electrical generators. The nacelles also serve other purposes, including aerodynamic fairing of the engine, conditioning of airflow into the engine, thrust reversing, and noise attenuation. The aerodynamic, structural, and subsystem integration of the engine and nacelle with the airframe is important to determining aircraft performance and optimum engine characteristics such as propulsor diameter and fan pressure ratio. Whatever the propulsor, either a fan or a propeller, virtually all engines today require nacelles.
It is also possible to embed engines within the wings as was done on the Comet airliner in the 1950s, or within the airframe, as was done on the B-2 bomber, which has an unusual blended wing/body configuration. To date, embedded configurations have not proven advantageous compared to the pylon mount common to airliners today. One major challenge is that these installations required relatively long ducts to move air to and from the engines. Such duct lengths, with relatively large viscous losses due to large surface area, are not compatible with the low fan pressure ratios used to improve propulsive efficiency on modern commercial airliners.
There has been much study of various commercial blended wing/body configurations over the past several decades, including configurations with engines on pylons and ones with embedded engines. To date no such commercial aircraft has been developed. Indeed, refinements of pylon-mounted engine, tube-and-wing configurations have made a contribution to the 70 percent fuel efficiency gain illustrated in Figure 2.3.
The National Aeronautics and Space Administration (NASA) has promulgated a set of goals known as N + 3 for aircraft entering service 20 or 30 years or so from now aimed at reducing noise, fuel burn, and emissions.6 Depending on the reference, the energy reduction goal is 60-70 percent better than the reference 1990s design aircraft for similar missions. Several organizations and teams have published designs that they claim have the potential to meet these goals given sufficient investment. All require innovation in aircraft design (which is beyond the scope of this committee), in propulsion, and in aircraft–propulsion integration. The propulsion approaches under consideration include ultrahigh bypass ratio turbofans, turboprops, distributed propulsion, and hybrid-electric schemes.
Several proposed advanced aircraft designs include configurations in which the boundary layer developing along the aircraft is ingested into the propulsor to reduce the velocity defect in the aircraft wake (also known as
6 NASA, “ARMD NRA: Advanced Concept Studies Awards,” last update October 6, 2008, http://www.aeronautics.nasa.gov/nra_awardees_10_06_08.htm.
wake cancellation), thus reducing the thrust needed to propel the aircraft and so reducing energy consumption. While it can be debated whether the effect of boundary layer ingestion (BLI) should be considered one of increased propulsive efficiency or decreased aircraft drag, it is clear that it requires a much higher level of propulsion–aircraft integration than is common today and imposes new constraints and requirements on both the airframe and the propulsion system. BLI configurations have been proposed using a variety of propulsor drive systems, including direct-drive turbofan engines, geared mechanical drives, and electrical drives. In some approaches, distributed propulsion may be synergistic with BLI, but this is not necessary.
BLI can be used in designs where a propulsor is integrated into the aft fuselage to capture the fuselage boundary level or on an upper surface to capture part of the fuselage or wing boundary layer. Theoretical designs of several configurations have been documented in the literature, including three that pass the boundary layer of the top surface of the flattened fuselage through propulsors (e.g., the MIT/NASA D8, NASA N3-X, and Cambridge/ MIT SAX-40 aircraft concepts) and two with circumferential ingestion at the rear of a circular fuselage (i.e., the Bauhaus Luftfahrt Ce-Liner7 and Boeing SUGAR Freeze 8 aircraft concepts). Figure 2.7 illustrates four concept aircraft proposed by different organizations. The D8 (A) and the Propulsive Fuselage (B) ingest the boundary layer directly into the fans of turbofan engines.9 The SAX-40 (C) uses a gear drive system to power three fans from each of its three gas turbines,10 while the N3-X (D) uses a turboelectric configuration in which two gas turbines drive electric generators that in turn power 13 electric motor–driven fans.11 Not shown is the Boeing SUGAR Freeze, which has an aft fuselage BLI configuration for an aircraft powered by cryogenic fuel (liquefied natural gas).12 BLI configurations also can benefit from using the larger number of fans with low fan pressure ratio to produce a higher propulsive efficiency (same trend as in Figure 2.5). Alternatively, a fan pressure ratio benefit can also be obtained with distributed propulsion that does not use BLI. Some boundary layer ingestion configurations use a small number of relative large fans (1, 2, or 3); others use more (10-15) smaller ones. In either case the fans can conceptually be driven by electric or gas turbine motors, either directly, through shafts and gear trains,13 or electrically through power extracted from the gas turbines and distributed via electrical cables.
Low-speed wind tunnel tests of the D8 configuration documented a 6 to 8 percent reduction in cruise power requirement with BLI.14 Other studies have claimed a potential reduction of more than 20 percent if the entire vehicle boundary layer is cancelled. BLI configurations bring a very high distortion level into the fan, which impacts efficiency, fatigue life, and noise—at least partially offsetting potential gains from wake cancellation.
It should be emphasized that all of the configurations shown in Figure 2.7 are advanced concepts only, with significant uncertainty surrounding the performance of an operational aircraft based on these concepts. Considerable detailed study is required to assess their viability and
7 M. Bradley, 2012, “NASA N+3 Subsonic Ultra Green Aircraft Research SUGAR Final Review,” Boeing, http://aviationweek.typepad.com/files/boeing_sugar_phase_i_final_review_v5.pdf.
8 Boeing’s Subsonic Ultra Green Aircraft Research (SUGAR) Program had multiple elements, including SUGAR Freeze, which focused on cryogenic fuels and superconducting electrical components, and SUGAR Volt, which focused on nonsuperconducting electric propulsion.
9 S.A. Pandya, 2012, “External Aerodynamics Simulations for the MIT D8 ‘Double-Bubble’ Aircraft Design,” presented at the Seventh International Conference on Computational Fluid Dynamics, https://www.nas.nasa.gov/assets/pdf/papers/ICCFD7-4304_paper.pdf.
11 J.L. Felder, 2014, “NASA N3-X with Turboelectric Distributed Propulsion,” NASA Glenn Research Center, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150002081.pdf.
12 M. Bradley, 2012, “NASA N+3 Subsonic Ultra Green Aircraft Research SUGAR Final Review,” Boeing, http://aviationweek.typepad.com/files/boeing_sugar_phase_i_final_review_v5.pdf.
14 A. Uranga, M. Drela, E.M. Greitzer, N.A. Titchener, M.K. Lieu, N.M. Siu, A.C. Huangk, G.M. Gatlin, and J.A. Hannon, 2014, “Preliminary Experimental Assessment of the Boundary Layer Ingestion Benefit for the D8 Aircraft,” presented at the 52nd Aerospace Sciences Meeting, AIAA SciTech, AIAA Paper No. 2014-0906, http://web.mit.edu/drela/Public/N+3/Uranga2014_compressed.pdf.
their value relative to more conventional approaches at the same level of technology. The studies reviewed by the committee focused on the propulsion and some aerodynamic considerations of wake cancellation; many more disciplines will need to be explored to turn these ideas from advanced concepts into promising design approaches. Questions of aerodynamic performance, weight, subsystem requirements, and safety all need to be addressed to arrive at an overall integrated airplane assessment.
Considerable research will be required to establish the net energy reduction benefit of practical BLI configurations and to identify any impacts on other aircraft requirements such as noise, safety, and reliability. All things considered, boundary layer ingestion configurations may be a productive research path for reducing aircraft fuel burn.
Aircraft require power for a variety of functions such as electric power for avionics, cabin air supply and conditioning, deicing, and actuators for flight controls and landing gear. On most aircraft these functions are supplied by a variety of propulsion engine–derived sources including electrical generators, pneumatic bleeds, and hydraulic pumps. Together they constitute the engine offtake. It is also possible to use electrical power only, in
which case the aircraft is known as a more- or all-electric aircraft. Aircraft variation in power demand does not match the variation of power output of the engines. At takeoff the offtake needed represents less than 1 percent of engine power. During descent, it represents effectively 100 percent. In other words, during descent the engine power setting is determined by the need to provide offtake power rather than to propel the aircraft. Currently, about 6 to 8 percent of the fuel consumed during cruise is to supply engine offtake functions. As aircraft have become more efficient, engine offtake requirements as a fraction of propulsion system output have grown because the propulsive power needed drops but the power needed to support passengers and crew or to deice the aircraft have not changed dramatically. At the same time as fan pressure ratios have dropped and bypass ratios have increased to improve propulsive efficiency, there is even less power available in the engine to supply offtake requirements. This mismatch between needs and capabilities now results in designs that are not optimized for overall fuel burn. There are significant opportunities to improve aircraft fuel burn by more tightly integrating these now distinct aircraft systems. New architectures for these future integrated systems could consider energy storage, load leveling, auxiliary power unit (APU) utilization or elimination, higher distribution voltage, thermal management, safety, and redundancy.
Finding. Rationale for Aircraft–Propulsion Integration. Advances in integrating aircraft and propulsion are needed to enable many aspects of low-carbon aviation that are not achievable by incorporating discrete improvements in individual component technologies. Areas of interest include both evolutionary configurations such as lower fan pressure ratio engines in nacelles on standard tube-and-wing aircraft and significant departures from standard configurations, including modified aircraft platforms, distributed propulsion concepts, and boundary layer ingestion configurations.
Low fan pressure ratios are needed to reduce exhaust velocities and thereby improve propulsive efficiency, regardless of whether the fan is driven by a gas turbine or an electrical motor. For a constant level of thrust, this requires that the effective fan area increase so as to avoid commensurate increases in weight, drag, and integration losses.
At constant thrust, lowering the fan pressure ratio requires increasing the fan area. The typical approach is to increase fan and nacelle diameter, which increases aircraft weight and drag. At the same time, lower pressure rise across the fan means that internal flow wall drag and nozzle losses become more important, driving designs toward shorter ducts, in turn reducing the duct area available for noise attenuation. Because slower fan speed also lowers fan noise frequencies, noise attenuation requires duct liners of greater volume. Rather than making a single fan larger, multiple fans (distributed propulsion) can also be used to increase total fan area, but the power must be distributed to the multiple fans by an electrical system or mechanical shafting, which adds weight and reduces transfer efficiency. Overall, the low pressure ratio fans needed to improve propulsive efficiency present installation challenges so that the performance penalties caused by higher noise, weight, and drag and lower transfer efficiencies do not cancel out the gain of lower exhaust velocity.
To use boundary layer ingestion and wake cancellation to reduce aircraft cruise energy requirements, an aircraft configuration integrated with a propulsor design is needed in which the overall benefits of wake cancellation outweigh the costs in terms of propulsor efficiency, noise, and weight.
The potential for wake cancellation to reduce aircraft cruise energy requirements, which could be considered as either drag reduction or improved propulsive efficiency, has long been recognized. Many implementations have been proposed, but none have been realized; all represent significant departures from legacy conventional aircraft configurations. All also result in the propulsor—fan or propeller—operating in significantly distorted inflow, which is known to reduce propulsor efficiency, threaten rotor mechanical integrity, and generate significant noise.
Relative Economic Value
A balanced technology investment portfolio in aircraft–propulsion integration research is needed to ensure that economic uncertainties arising from changes in net fuel price do not slow continued reductions in emissions by commercial aircraft.
The relative economic value of different aircraft configurations and propulsion systems can change dramatically as fuel prices change, so the relative value of technology approaches can change as well. Currently, the price of jet fuel is well within the range of historic norms, so aircraft depreciation is a larger cost than fuel. Thus technical approaches to reducing fuel burn that significantly increase the purchase price of an airplane are not favored commercially. The future net cost of jet fuel is highly uncertain and could rise dramatically due to economic factors and policy responses to climate change. In this case the relative balance could change between, on the one hand, operators’ cost and, on the other, the value society places on the importance of carbon emissions and other aircraft environmental effects such as oxides of nitrogen (NOx), particulates, and noise. The challenge is to construct a balanced technology investment portfolio that is robust to a key economic uncertainty of commercial aviation—namely, net fuel price.
Technical and policy issues surrounding certification of aircraft and propulsion concepts and technologies not covered by current certification procedures are important for guiding technology development. In addition, manufacturers must have confidence that these issues will be resolved in a timely fashion before they are likely to begin advanced development of the relevant aircraft and propulsion systems.
Rules for certification of new aircraft and engines are intended to establish that aircraft are in compliance with standards for safety and environmental impact. Existing certification rules and procedures have evolved over many decades, benefiting from aviation’s vast experience. These rules are a major factor in the extraordinary safety record of commercial aviation. This experience, however, is based on a narrow range of possible commercial aircraft and propulsion configurations, namely tube-and-wing aircraft powered by pylon-mounted jet engines. Novel aircraft and propulsion proposals often contain features that are not consistent with current regulations or are not addressed by them. Inappropriate or inadequate certification regulations and procedures can impede the development of new approaches. Important questions include these: Is a new aircraft propulsion system approach sufficiently safe? What design approaches and technologies are needed to generate the required safety level? Which procedures are needed to demonstrate safety? Answers to these questions are important for guiding basic
technology and for maturing aircraft designs. They need to be addressed on both a technical and policy basis well before aircraft development is started.
The two most promising technical directions in gas turbine research are (1) nacelle and integration technologies to enable ultrahigh bypass ratio propulsors so as to realize high propulsive efficiencies and (2) technologies that enable boundary layer ingestion. Power systems that are highly integrated on the aircraft level may reduce fuel burn, but the possible gain is estimated to be less than items (1) and (2), so a power system research project is not recommended as a high priority. While not called out explicitly, simulation and modeling improvement are important to all three of these projects.
Develop nacelle and integration technologies to enable ultrahigh bypass ratio propulsors.
Improving propulsive efficiency requires reducing fan pressure ratios. Fan pressure ratios at least as low as 1.25 are of interest over the 30-year timeframe addressed by this report. Realizing such low pressure ratios means that technologies and design approaches will need to be developed to address challenges in all the areas relevant to installation of such fans. This includes internal and external aerodynamics, acoustics, thrust reversing, operability, manufacturing, and, of course, overall weight. The goal is to enable compact nacelles, including their installation, that are lighter and have less drag than today’s nacelles in order to optimize propulsive efficiency. Many of the technologies for reducing internal propulsor flow path losses are relevant to conventional configurations with engines podded in nacelles and to configurations in which the propulsion system is imbedded within the airframe. This research project is closely related to the gas turbine research project on low pressure ratio fan propulsors, and work on the two should be closely coordinated.
Pursue technologies that can enable boundary layer ingestion to reduce the velocity defect in the aircraft wake (also known as wake cancellation) and thus reduce cruise energy consumption.
BLI promises at least theoretically to significantly reduce aircraft fuel consumption, all else being equal. The propulsor–aircraft integration of most proposed BLI configurations, however, is significantly different than conventional designs, so all else is not equal. The benefits and costs of BLI are confounded by many other significant aircraft and propulsion changes in these advanced designs, necessitating careful, detailed aircraft design studies to guide investment in component and subsystem technologies. The highest priorities are as follows:
- Design approaches. Explore the aerodynamic, structural, subsystem, control, and safety implications of BLI configurations, including detailed systems analyses of alternative approaches such as various advanced propulsion system options. The value of these studies would be greatly enhanced if they include careful, apples-to-apples comparisons with more conventional configurations using similar levels of technology.
- Fans in distorted flow fields. Develop technologies for propulsion fans and their installation consistent with operating in the highly distorted flow fields that are characteristic of BLI configurations. One central technical challenge to the realization of BLI is overcoming the penalties associated with fan efficiency, noise, operability, and life that would accrue if BLI were implemented at the current state-of-the art fan and fan installation. Overcoming this challenge will require detailed assessment of penalties inherent in current technology, as well as pursuing design and technology approaches that mitigate such penalties. Progress in this area requires both analysis and testing at representative Mach numbers.