Aircraft Propulsion Systems Technology And Design
Glenn Propulsion Program Opens the Door to a New Era in General Aviation NASA's General Aviation Propulsion (GAP) program has turned vision into reality. At the beginning of the GAP program, NASA promised to transform small aircraft by developing revolutionary new engines and demonstrating them in the year 2000. These radically advanced engines will enable the general aviation industry to produce innovative, affordable engines for the commercial market. Although current general aviation engines are good and have served their purpose well, they require considerable pilot attention, intrude on passenger comfort with noise and vibration, and are costly to buy, operate, and maintain.
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Image right: The Eclipse 500 TM, a six-passenger jet made possible by revolutionary EJ22 turbofans, which are commercial derivatives of the GAP FJX-2 turbofan. Credit: NASA The new GAP engines will change all of that, along with our ideas about what general aviation propulsion systems can be. With their smooth, quiet operation, they provide comfort never before enjoyed in general aviation light aircraft. New engines are crucial to truly new airplane designs. The GAP engines are bringing about a revolution in light aircraft affordability, ease of use, and performance. These new engines are key to creating a small aircraft transportation system in the United States. The potential is especially strong when the benefits of the new propulsion systems are coupled with those of cockpit and airframe technologies developed by the NASA-FAA-industry Advanced General Aviation Transport Experiment (AGATE) consortium.
The GAP Diesel Engine The jet-fueled GAP diesel engine will make possible outstanding new propulsion systems for entry-level aircraft. Such aircraft are usually characterized by a single engine, up to four seats, cruise speeds of 200 knots or less, and easy, well-mannered handling. The GAP program goal for piston engines was to reduce engine prices by half while eliminating the need for leaded gasoline and substantially improving reliability, maintainability, ease of use, and passenger comfort. To achieve this goal, Teledyne Continental Motors and it's industry team (Aerosance, Cirrus Design, Hartzell Propeller, Lancair, and Mod Works) partnered with NASA Glenn to develop a highly advanced piston engine, the GAP diesel engine.
Diesel engines are known for being reliable but have been considered too heavy for use in general aviation. Combining the two-stroke operating cycle with innovative, lightweight construction makes the GAP diesel engine competitive with current piston aircraft engines. The GAP diesel engine, combined with advanced-design low-speed propellers (from related NASA-industry research), offers very quiet operation for both passengers and airport neighbors. Additionally, the new engine is very economical to operate.
It has been designed to burn readily available jet fuel at a low fuel consumption rate of about 25 percent less than current engines. Image left: Revolutionary diesel engine developed in the GAP program. Credit: NASA This engine provides pilots and passengers with the same kind of quiet, easy-to-use power that we have come to expect in our automobiles.
There is no fuel-air mixture or propeller pitch control for pilots to contend with. A single power lever controls the engine and propeller automatically, much like the gas pedal of a car with an automatic transmission. Other special design features ensure extremely smooth, vibration-free operation. And because the GAP diesel's unique design allows use of low-cost mass production manufacturing methods, engine cost could be half that of current aircraft piston engines. The FJX-2 Turbofan Engine Modern turbine engines are highly desirable aircraft propulsion systems because they are user-friendly and environmentally compliant. They are characterized by very high reliability, smooth operation, use of readily available jet fuel, and low noise and emissions. Their reliability and smoothness contribute greatly to aircraft safety and comfort.
But, until now, the use of turbine engines in the light aircraft market has been limited by high cost. Image right: The revolutionary FJX-2 turbofan engine developed in the GAP program. Credit: NASA With the development of the FJX-2 Turbofan engine, the GAP program is helping to reduce the cost of small turbine engines by a factor of ten and revolutionize the concept of personal air transportation. The FJX-2 has enabled a whole new class of aircraft: safe, affordable, fast, efficient small jets in which the family can travel in comfort. At a fraction of the cost, this new turbine engine has made the performance of a commercial jet available to the general aviation community, including cruise speeds of 380 knots (440 mph) and the ability to fly over or around inclement weather. The FJX-2 is a high-bypass-ratio turbofan engine that produces 700 pounds of thrust, yet weighs only 85-100 pounds, about one-fourth the weight of piston engine propulsion systems with similar capabilities. To keep costs low, the FJX-2 team applied many lessons learned from research of automotive gas turbine engines.
Emphasis was placed on simplifying design and reducing the number of parts. Low-cost design techniques and advanced automated manufacturing methods have led to the first turbine engine that is cost competitive with piston engines. While not as fuel efficient as today's comparable piston-powered aircraft, new turbofan jets will have equivalent or lower takeoff-to-landing fuel consumption. The FJX-2 was developed in partnership with NASA Glenn by Williams International and its industry team (California Drop Forge, Cessna Aircraft, Chichester-Miles Consultants, Cirrus Design, Forged Metals, New Piper Aircraft, VisionAire, Producto Machine, Scaled Composites, and Unison Industries). The Future Is Here The General Aviation Propulsion Program has delivered on its promise to propel the general aviation industry to new heights.
Commercial derivatives of the GAP diesel engine and the FJX-2 turbofan engine will provide a previously unheard-of level of comfort and convenience, and the performance-to-price ratio will soar. The Eclipse 500 TM aircraft, made possible by the revolutionary EJ22 turbofan (a commercial derivative of the GAP FJX-2), is just the first example of a new generation of aircraft. Image left: The FJX-2 turbofan engine in Glenn's Propulsion Systems Lab (PSL) altitude test chamber. PSL is NASA's only ground-based test facility capable of true flight simulation for experimental research on air-breathing propulsion systems.
Credit: NASA The Glenn Research Center is continuing its contributions to NASA's new Small Aircraft Transportation System (SATS) initiative. Innovations such as oil-free engines and affordable high-performance lightweight materials will continue the revolution in propulsion technologies for general aviation. NASA, with its government and industry partners, is putting America on wings!
FS-2000-04-001-GRC For more information contact: Information and Publications Office NASA Glenn Research Center 21000 Brookpark Road Cleveland, Ohio 44135 (216) 433-5573.
5 Propulsion Technologies If the performance required of a UAV is similar to the performance of conventional aircraft, the propulsion system may also be similar. Many UAVs will weigh more than 1,000 pounds, fly at subsonic and supersonic velocities at altitudes below 60,000 feet, maneuver at 9 g’s or less, and will be maintained in ways similar to current military or commercial aircraft. These UAVs will not require unique propulsion technology.
Indeed, many new aircraft of all types are designed to use existing engines to avoid the time and expense of developing new engines. This chapter discusses UAV concepts that require new propulsion technology. Some classes of UAV require new engine technology, new designs, or even new fundamental research and propulsion concepts. For example, a UCAV may require a gas turbine engine that can operate at much more than the 9 g forces that limit manned vehicles. For high g loadings, the entire engine structure, especially the rotor support, will have to be reevaluated. An engine capable of maneuvering at 30 g, for example, would require new design concepts that could require considerable engineering development but not new basic research. Nevertheless, for some UAVs, the propulsion system is a critical limiting technology.
These include subsonic HALE aircraft that must operate above the altitude limits of current engine technologies; MAVs; and very low-cost, high-performance vehicles. (while meeting aircraft performance requirements) and reliability levels commensurate with permissible aircraft loss rate (1 per 10 8 departures for commercial aircraft). Many military missions also require stealth, which greatly affects engine design and installation. For all types of aircraft (including UAVs), engines and fuel typically account for 40 percent to 60 percent of gross takeoff weight, and the performance of the propulsion system has an enormous effect on air vehicle performance. The gas turbine engine is vastly superior to alternative engines in all propulsion metrics. This high level of performance reflects the intrinsic merits of the concept and the $50 billion to $100 billion invested in gas turbine research and development over the past 50 years. The power-to-weight ratio of gas turbines is three to six times that of aircraft piston engines.
The difference in reliability is even greater. The in-flight shutdown (IFSD) rate, a measure of reliability, for gas turbine engines in large commercial aircraft is 0.5 shutdowns for every 10 5 hours of flight. For single-engine military jet aircraft, the IFSD rate is 2 for every 10 5 hours.
The IFSD rate for light aircraft piston engines is considerably worse, about 5 to 10 for every 10 5 hours. Although the IFSD statistics are not available for small piston engines in current UAVs, anecdotally, they are even higher. Gas turbines can also operate for long periods of times (4,000 to 8,000 hours) between overhauls, compared to 1,200 to 1,700 hours for aircraft piston engines. The small piston engines in current UAVs are replaced every 100 hours or less of service. The attractiveness of small piston engines is their low cost and the lack of availability of high-performance gas turbines in very small sizes. Alternative propulsion concepts may only be desirable when suitable gas turbines are not available. Both energy density and power density are important factors for propulsion systems.
Systems Technology Group
Energy density is a measure of the energy in the fuel and the conversion efficiency of the power converter (engine). Power density is a measure of the power converter. For example, the propulsion system weight of a long-range transport aircraft is dominated by the energy density of the fuel consumed (which may be 10 times the weight of the engines). In contrast, a solar-powered vehicle has zero fuel weight and, thus, very high energy density but low power density (the solar cells and power storage system are heavy). Illustrates the range of power and energy densities for current UAVs. Most air vehicles require about twice as much power for takeoff and climbing than for cruising.
Therefore, the design of the propulsion system is a compromise between the weight of the engine (power-to-weight ratio) required for takeoff and the fuel weight required for cruising range (e.g., engine efficiency). The interactions between these factors for particular power system technologies will be discussed below.
Development cost has been a major factor for UAV propulsion systems in the past. The development of an all-new gas turbine engine for a tactical military aircraft can cost more than $1 billion, an inconceivable expense for the UAVs developed to date.
Thus, the practice has been to adapt existing devices in a very budget-constrained, suboptimal manner, usually by sacrificing both performance and reliability. The cost of new technology, especially new concepts, will be as high for UAVs as it has been for conventional aircraft unless new ways for developing propulsion systems can be perfected. High-Altitude, Long-Endurance UAVs Substantial efforts are under way to develop propulsion technologies for HALE surveillance and communications-relay missions. The mission objectives for HALE UAVs are to operate at as high an altitude as possible to maximize the geographic coverage of sensors and communications.
High altitude can also be an important contributor to survivability because high altitude reduces the aircraft’s vulnerability to ground-to-air and air-to-air missiles. However, to be entirely safe from many widely deployed threats, operating altitudes must be above 75,000 or even 85,000 feet. These altitudes cannot be routinely reached with current propulsion technology. At an altitude above 75,000 feet, there is very little air (the air density at 80,000 feet is only 3 percent of the density at sea level), which affects air-breathing fueled propulsion systems in two fundamental ways. First, engine weight is inherently higher. The fuel required to produce a unit of thrust per time is the same at high altitudes as it is at low altitudes, but the fuel-to-air ratio is fixed by the chemistry of combustion. As a result, the required mass flow rate of air is set by the power required.
Second, the large compression ratios required for gas turbines (additional compressor stages must be added), piston engines, and fuel cells (which require several stages of turbocharging) result in weight and drag penalties. The additional compression requirement significantly increases the weight of high-altitude propulsion systems. Because the compression process increases the temperature as well as the air pressure, the required pressure ratios result in temperatures that are too high for current technology. Thus, coolers (heat exchangers) must be added to the compression system. The weight and drag penalties of these heat exchangers are exacerbated by the very low ambient air density.
High-altitude aircraft under development for NASA, which use piston engines, have more area and drag associated with heat exchangers than for the wings. The increased weight and drag of heat exchangers with altitude limit the operating altitude of these designs (Drela, 1996). Areas for research include technology leading to low-weight, low-drag heat exchangers and low-weight, low Reynolds number, high-efficiency compression systems. These technologies will be important for both gas turbine and internal combustion engines, as well as for fuel cell systems (described below). Propulsion approaches other than combustion engines have been proposed, notably fuel cells (Stedman, 1997) and solar power.
Fuel-cell systems have the potential advantage of high energy densities but have relatively low power densities. Turbochargers and heat exchangers similar to those for piston engines would be required at high altitude. Unless fuel cells can operate on hydrogen (whose low density makes it difficult to integrate into an air vehicle), their complexity and weight quickly dominate the design.
No liquid fuel systems are in routine operation today, and none has been designed for use in air vehicles. Fuel cells might be useful for very long-endurance missions for which fuel consumption is the dominant factor. Because of the relatively low energy density of solar radiation, solar-powered aircraft must be extremely light and efficient, and they require exceptionally careful operation. Thus, they are probably only viable for niche military applications. The principal technology requirements for solar-powered aircraft are lighter, more efficient solar cell designs and compact, lightweight energy storage systems (for night operation). Micro Air Vehicles MAVs are currently defined by DARPA as having characteristic dimensions of less than 15 cm. This makes propulsion and power for MAVs very challenging indeed.
A study was conducted by the Massachusetts Institute of Technology’s Lincoln Laboratory on both the propulsion requirements and the technology options available to meet these requirements (Davis et al., 1996). Illustrates how the amount of power required varies as a function of vehicle size for a class of conventional airplane configurations. In the figure, the flight power curve refers to the power (thrust times flight velocity) the vehicle requires for level flight. (Climbing and maneuvering may require 50 percent to 100 percent more power than level flight.) The flight power requirement is independent of the type of propulsion system. The shaft power curve in the figure refers to the mechanical power a motor must provide with a propeller propulsion system, regardless of the type of motor (e.g., electric, internal combustion, gas turbine).
Assuming that the motor is electric, the electric power curve then represents the power that must be supplied by the source of electricity. Thus, vehicles of this type need on the order of 3 to 5 watts for cruising and 6 to 10 watts for climbing. Conceptually, different propulsion systems have different relationships between motor weight and fuel weight, so the relative, overall mass of the propulsion system is a function of flight duration requirements.
Shows the trade-offs at the 50-watt level that would be required for some of the less power-efficient UAV concepts (e.g., hovering vehicles) (NRC, 1997b). Illustrates the propulsion system mass (including fuel where appropriate) to propel a vehicle with a takeoff weight of 50 grams for various flight times with different power systems (the only option that has been demonstrated is electrically driven. FIGURE 5-3 Typical power requirements for propeller-powered MAVs. Source: Massachusetts Institute of Technology, Lincoln Laboratory. The nominal weight allowance for propulsion in the design is 36 grams; thus weights of more than 36 grams do not meet the specified flight times. The most attractive (lowest total weight) propulsion systems are airbreathing systems.
The current DARPA MAV program is investigating four propulsion options: batteries, microdiesels, fuel cells, and micro gas turbines. The last three are projected to have about the same fuel consumption per unit power, but the micro gas turbine is considerably smaller and lighter. Low-Cost, High-Performance UAVs Reliable aircraft propulsion systems are expensive to develop, manufacture, and operate. Typical list prices range from $130 to $200 per pound of thrust for civilian engines and $200 to $400 per pound for military engines (civilian engine prices generally include amortization of the development costs; military engine prices do not).
The price per pound increases as size is reduced because of relatively higher development costs and engine accessory costs (e.g., fuel pumps, controls, and electrical generators). Even “low-cost,” short-lived (10-hour) cruise missile engines cost about $150 per pound.
With current technology, an engine designer can trade off lower cost for lower performance by selecting less expensive materials and manufacturing approaches and reducing the number of parts. The most important question for many UAVs will be how to realize high performance while dramatically reducing costs, especially in the smaller engine sizes.
FIGURE 5-4 System mass vs. Energy for several advanced, small energy systems. Source: Massachusetts Institute of Technology, Lincoln Laboratory. Significant cost reduction over the lowest cost with current technology will require advances in fluid mechanics, heat transfer, and materials technologies that emphasize cost instead of performance, which is traditionally emphasized. For example, increases in airfoil and end-wall boundary-layer loading can reduce the number of compressor and turbine stages, as well as the number of airfoils per stage. These increases might be realized through progress in passive (e.g., suction or casing treatment) or active (e.g., involving feedback) boundary-layer control. Another example would be reducing the cost of hot sections (combustors and turbines) through the development of low-cost, high-temperature materials and coatings.
An alternative approach would be to develop new cooling schemes that would reduce the cost of producing air-cooled parts. (A typical small engine may require drilling more than 100,000 cooling holes). Also, cooling is often less efficient in small engines because of limitations in manufacturing technology. Many fundamental problems with using vapor and liquid cooling approaches in engine environments will require basic research to be resolved.
TABLE 5-1 Total Propulsion System Mass for 50-Gram MAV Mass for 30-minute flight (in grams) Mass for 60-minute flight (in grams) Rocket (hydrogen-oxygen) 83 140 Pulse jet 45 80 Electric motor (0.38 W/gram, 60% efficient) Batteries 55 79 Solar 35 35 Thermal photovoltaic 25 26 Microturbine generator 20 24 Advanced fuel cell 25 31 Microfan jet 8 12 Internal combustion engine (5% efficient) Otto cycle 13 22 Diesel cycle 9 13 Note: Propulsion system design mass is 36 grams aSolar panel size may exceed the available surface area. BExcludes cooling drag. Another major issue for engines of all sizes, but increasingly important as engine size is reduced, is leakage flows through the clearances between stationary and rotating parts. These leakages have a first-order impact on engine efficiency and operability. Engine complexity and costs are increased significantly by design features to reduce leakage. New technology and approaches for airfoils, end-wall flows, seals, and thermostructural interaction could reduce the impact of leakage.
One example that has been tried is shape-memory alloys to control compressor blade clearances (Schetky et al., 1998). Gas bearings are feasible in small sizes and are used in small turbomachinery, such as APUs. If gas bearings were used in small aircraft engines, they could reduce the complexity and cost of the bearing and lubrication systems. Currently, most military engines are designed for specific applications; thus development costs for each new aircraft are substantial. One radical approach to reducing these costs would be to develop a miniature, high-performance, low-cost engine that could be grouped to provide greater thrust. This “one-size-fitsall” approach, however, is well beyond the state of the art and would require basic research.
Existing technology can produce only miniature, low-performance, high-cost (per unit thrust) engines. In addition to the advances discussed above, the technologies for this new approach would include very small, low-cost accessories. MEMS could be an important element in miniature engines.
Low-Cost, Storable, Limited-Life Propulsion Systems As currently envisioned, propulsion systems for UAVs can be divided into two broad categories: (1) vehicles operated routinely in peacetime (e.g., highaltitude reconnaissance UAVs), and (2) vehicles used only in wartime, for which most, or even all, training will be done by simulation. Engines for the first category of UAVs will have conventional operations and maintenance requirements. But the requirements of store-in-peace/use-in-war vehicles will be closer to those of cruise missiles. These vehicles will require engineering solutions for subsystems, such as fuel and lubrication systems, that must be capable of unattended storage for years and very fast start-up.
Traditionally, much of the profit for manufacturers of gas turbines has come from the sale of spare parts to replace parts consumed during military training. If vehicles are used only in wartime, manufacturers will have little or no opportunity to sell spare parts in peacetime (and thus no industry geared up to produce them), necessitating a different pricing structure for these engines. Therefore, although overall engine-related program costs might be reduced, costs would be shifted from the operations and maintenance budget to the procurement budget (i.e., the purchase price of engines would increase).
Engines are now nominally optimized for minimum life-cycle costs under the current market structure. A different life cycle can have different optimal conditions. For a given thrust, the optimum design for a 500-cycle engine life in a UCAV will be different than for a 4,000-cycle life (typical for a modern fighter) or for a 20,000-cycle life (for commercial aircraft). These differences will be apparent, for example, in the lower requirements for material creep life, maintenance, and survivability. The lower requirements might also be reflected in the selection of materials (for lower cost and weight), lighter weight structures (especially rotating parts), and less emphasis on aging and maintainability characteristics (e.g., thinner airfoils, more welds, and fewer bolted joints). Technology for storable engines already exists for cruise missiles and smaller engine sizes (700-lb.
Thrust and below) with very limited lives (tens of hours). However, this technology has not been used for larger engines (more than 1,000-lb.
Thrust) with longer lives (500 hours), which are contemplated for UCAVs. TABLE 5-2 UAV Propulsion Technologies Type of UAV HALE HSM Very Low-Cost General Topics High-altitude propulsion E VTOL propulsion E Modeling I I I Cost reduction I I Specific Topics Low Reynolds number turbomachinery E E Low Reynolds number heat rejection E Turbomachinery tip-clearance tolerance I E E Leakage desensitization I I Thrust vectoring I I Magnetic bearings I I Air bearings I I Solid lubricated bearings I I Low-cost accessories E I Low-cost vapor and liquid cooling schemes I Affordable high-temperature materials I I I Cooling for small engines I E I = important E = enabling. Air Force should include research on propulsion systems for UAV applications in its long-term research program. Air Force (USAF) planners have envisioned that uninhabited air vehicles (UAVs), working in concert with inhabited vehicles, will become an integral part of the future force structure. Current plans are based on the premise that UAVs have the potential to augment, or even replace, inhabited aircraft in a variety of missions. However, UAV technologies must be better understood before they will be accepted as an alternative to inhabited aircraft on the battlefield. Air Force Office of Scientific Research (AFOSR) requested that the National Research Council, through the National Materials Advisory Board and the Aeronautics and Space Engineering Board, identify long-term research opportunities for supporting the development of technologies for UAVs.
The objectives of the study were to identify technological developments that would improve the performance and reliability of “generation-after-next” UAVs at lower cost and to recommend areas of fundamental research in materials, structures, and aeronautical technologies. The study focused on innovations in technology that would “leapfrog” current technology development and would be ready for scaling-up in the post-2010 time frame (i.e., ready for use on aircraft by 2025). Contents.
i–xvi. 1–6. 7–19. 20–36.
37–38. 39–49. 50–58.
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82–91. 92–96. 97–100.
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