1 ISABE-2015-20288 Distributed Propulsion Systems to Maximize the Benefits of Boundary Layer Ingestion Andrew Rolt Propulsion Engineering Centre, School of Aerospace, Transport & Manufacturing, Cranfield University Cranfield, Bedfordshire, MK43 0AL, United Kingdom John Whurr Rolls-Royce plc Derby, Derbyshire DE24 8BJ UK Abstract Introduction This paper reviews the merits of alternative aircraft Propulsion systems for advanced aircraft designs configurations and propulsion systems including featuring BLI and distributed propulsion (DP) will distributed propulsion (DP) and potential synergies need to address some major technical and economic with boundary layer ingestion (BLI). It compares the challenges to realize significant benefits relative to performance benefits of alternative propulsion more conventional evolutionary designs. If a third systems for tube and wing aircraft featuring rear main engine is added to a twin-engined aircraft and fuselage BLI and in particular considers adding a third the size of each engine is reduced, then only a small propulsion system to reenergize the rear fuselage reduction in thermal efficiency and a modest increase boundary layer and to improve the propulsive in maintenance costs would be acceptable, otherwise efficiency of an otherwise more conventional twin- the propulsive efficiency improvements are likely to engined aircraft. The BLI propulsor could be powered be outweighed. Alternatively, if power is transferred by a third engine, or by power transmitted just from to a centreline propulsor from just two main engines, the two other engines. Stored energy could also be then the transmission system will need to be highly integrated into a hybrid electric propulsion system to efficient and lightweight. boost the takeoff and climb performance. Improved If it is also desired to use stored electrical energy to propulsive efficiency, reduced drag and reduced noise boost power at critical conditions and for the engines are the major drivers for DP and BLI in civil aviation. to be optimally sized for cruise, then the batteries and Glossary their associated systems must have high power density and high energy density to show a benefit. ACARE Advisory Council for Aerospace Research in Europe Current Aircraft and Engine Configurations APU Auxiliary Power Unit BLI Boundary Layer Ingestion Aircraft performance is highly dependent on having BWB Blended Wing-Body (aircraft) efficient and right-sized propulsion systems that can COC Cash Operating Cost provide sufficient thrust for all phases of a design mission and in anticipated failure cases. The levels of core The power generating part of an engine thrust demanded depend on the weight of the aircraft, DEAP Distributed Electrical Aerospace Propulsion its lift/drag ratio and its required rates of takeoff DP Distributed Propulsion acceleration, climb and descent, throughout the flight Extended-range Twin-engine ETOPS envelope, but the maximum takeoff weight also Operational Performance Standards depends on the design payload-range mission and the FAA Federal Aviation Administration performance, weight, nacelle drag and overall Leading Edge Asynchronous Propeller LEAPTech efficiency of the propulsion systems. Technology For a new aircraft design with new engines, both the LEBU Large-Eddy Break-Up (device) airframe and the propulsion system should be scaled LP Low Pressure to find the optimised combination. This process is A high speed propulsor or propulsion system open rotor complicated if it is desired to develop a family of with one or two rows of propeller blades aircraft having different payload-range combinations propulsion All of the components needed to provide while maintaining greater or lesser commonality in the system thrust, integrated with the aircraft airframe, engines and systems, and meeting noise A fan, propeller, open rotor or any other propulsor targets etc. The aircraft designers would also want to thrust producing device be able to take advantage of potential improvements SFC Specific Fuel Consumption to the propulsion systems when considering future TeDP Turbo-electric Distributed Propulsion developments of their new aircraft. Copyright 2015 by Rolls-Royce plc. Published by the University of Cincinnati, with permission. 2 Performance of turbofan aero-engines has improved highly innovative aircraft designs is that they are significantly since the first high bypass ratio designs likely to take longer to bring to market, by which time were introduced around 1970 and there is every evolutionary developments of more conventional air- indication that it will continue for many years to frames and propulsion systems could catch them up, come, but the rate of improvement is not matching the or market conditions and drivers could have changed. ambitious long-term targets set for the industry by The industry has a long track record of more advanced ACARE and NASA. Therefore more revolutionary aircraft designs being cancelled or taking far longer step changes in powerplant and aircraft design should than anticipated to enter service. be considered. Over the life of a commercial aircraft its operating The general configuration of most commercial and costs exceed its ownership costs, so it will always be business aircraft has changed little since the advent of possible to develop more efficient and sophisticated twin-turbofan low-wing aircraft. Small business jet designs and to sell them at a premium, but this aircraft have retained rear-fuselage mounted turbofan strategy has its risks. For example, the cost of fuel installations, but larger commercial aircraft generally may not continue to increase as anticipated. now have under-wing mounted engines. Only a few Future Airframes regional and commuter aircraft have retained high wings and/or turboprop engines. Higher cruise speed Though many aircraft designers since the 1930s have open rotor powerplants have yet to be adopted. considered flying wing or blended wing body (BWB) designs, all in-service commercial and business The most obvious changes to twin-turbofan aircraft aircraft are “tube and wing” designs. However, over the years have been increasing fan diameters. several of the more recently proposed DP aircraft Table 1 lists some good reasons why the industry has designs have been BWBs or intermediate designs with standardised on these designs. Only the very largest flattened or “double bubble” fuselage cross-sections. aircraft now need to have more than two engines, and Table 2 lists some different cabin arrangements used with the advent of 330 and 370 minute ETOPS in tube and wing aircraft. Most aircraft have a mostly approvals, there are no long-range commercial routes constant fuselage cross-section, though a continuous that cannot be flown by a twin-engined aircraft. curvature design is more aerodynamically efficient Table 1: Twin-Engined Aircraft Pros and Cons and is for example used in the Piaggio Avanti aircraft. Pros: Table 2: Alternative Cabin Arrangements A second engine enables safe operation with one engine inoperative Low wing with cabin floor above wing centre section: High reliability in comparison with a three or four engined aircraft • Turbofans under wings Higher rate of climb and climb ceiling with both engines operating • Turboprops or turbofans above wings High core mass flow gives high component and thermal efficiencies • Turbofans or open rotors aside the rear fuselage Minimum number of engines reduces servicing and maintenance High wing with centre section above cabin and a T-tail: Cons: • Turbofans under wing Two large engines will be heavier than three or more smaller ones • Tractor turboprops or open rotors ahead of wing Very large new or spare engines will be more difficult to transport • Pusher turboprops or open rotors aft of the wing Larger diameter fans and nacelles present installation challenges Mid-height wing with passenger cabin ahead of wing: Cores may be oversized for cruise, reducing thermal efficiency, in • A canard with pusher turboprops or open rotors order to meet one engine inoperative takeoff thrust requirements ________________________________________ High thrust and drag asymmetry with one engine inoperative leads to larger control forces and bigger tail surfaces with increased drag A key design choice is whether to keep the low wing __________________________________________________ configuration. The Piaggio Avanti is a rare example Nevertheless, having more than two propulsors does of a passenger aircraft with a wing at mid-height have benefits and it can be argued that in the near within the fuselage, made possible because all of the future some more complex DP arrangements could passenger cabin space is ahead of the wing spars. challenge the predominance of conventional twin- engined aircraft. A few turboprop aircraft have had low wings with the engines mounted above them, but high wings provide For DP aircraft and propulsion systems to win sales increased ground clearance and better protection of relative to conventional twin-engined aircraft they will the engines from the ingestion of sand and other need to offer their buyers affordability, high dispatch debris that may be encountered particularly if the reliability and reduced operating costs, as well as aircraft is operated from less well prepared runways. reductions in noise, CO2 and other emissions. Figure 1 considers the potential benefits of a high Significant performance improvement and thorough wing configuration, and takes the Avro Regional Jet demonstration of the