Distributed Propulsion Systems to Maximize the Benefits of Boundary Layer Ingestion

ISABE-2015-20288

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.

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 enabling technologies will be as an example. This four-engined aircraft design was needed prior to launching a DP aircraft design, in intended for short takeoff and landing from small order to justify the investment. A problem with all regional and city centre airports. Such an aircraft is

compromised by its need for a low approach speed the Boeing 7J7, a typical open rotor aircraft design. and a low wing loading, so it should not be expected Counter-rotating open rotors offer significant fuel to match the operating economics of an aircraft burn reductions through greatly increased propulsive designed to operate from regular length runways. efficiency and reduced nacelle drag.

Example: BAe146 / Avro Regional Jet

Spec: • Four low specific thrust geared fan engines • Fan diameter not limited by ground clearance • Low cruise speed, short takeoff and landing • No thrust reverser, low interference drag nacelle

Pros: Cons: • Lightweight powerplants • Niche short-field market • Good propulsive efficiency even • Poor thermal efficiency from small at a relatively low cruise Mach core engines number • Maintenance costs associated • Quiet with having four engines • No benefit from BLI Figure 3: Typical Open Rotor Aircraft Figure 1: High Wing Aircraft Pros and Cons Benefiting from BLI Nevertheless having a high wing and/or four-engined The above examples have shown different ways in arrangement would enable lower specific thrust which aircraft could evolve without exploiting BLI. engines to be used without running into problems with These alternative aircraft and propulsion options must ground clearance or interference drag between the not be dismissed when considering the potential nacelle and the wing. advantages of more novel airframe designs and BLI propulsion systems. Open Rotor Aircraft The reasons why ingesting boundary layer air into a Open rotor powerplants can be single rotation or propulsion system can be beneficial, and whether this double rotation, tractor or pusher designs. Having a should be accounted as an improvement in propulsive second contra-rotating blade row straightens out the efficiency, or as a reduction in aircraft drag, are swirl from the first rotor, increasing overall efficiency. discussed in an accompanying paper [1]. It should be Figure 2 shows a typical open rotor powerplant appreciated that it takes less power to generate thrust proposal. by reaccelerating slow moving boundary layer, than to take the same amount of free-stream air to generate the same amount of thrust by accelerating it to a higher velocity. Another way of looking at this is to consider that the propulsion system will be more efficient if it minimizes the residual disturbance and kinetic energy left in the atmosphere after the aircraft has passed. This is illustrated in figure 4. The top half of this diagram shows an aircraft with a BLI propulsor integrated with the aft fuselage, and the bottom half shows an aircraft without a BLI propulsor. The red line indicates power exported from a wing mounted Figure 2: Open Rotor Powerplant (Tractor) engine, which enables its jet velocity to be reduced.

The rotor exit swirl is not so significant for a lightly Free-stream velocity (wing-tip vortex loaded propeller, and when it is mounted ahead of a Aircraft with BLI effects omitted) wing, the wing will help to straighten the flow. But Turbofan Thrust with power higher speed aircraft need higher disc loadings and offtake to aft BLI fan BLI intake Reduced because rotor tip speeds are limited to reduce noise, residual KE single rotors would have very high exit swirl angles that would significantly increase SFC. Consequently Drag counter-rotating designs where the second blade row Regular High turbofan shares the work and cancels out the swirl are preferred residual KE

for high speed applications. To minimize cabin noise, Aircraft without BLI open rotors in pusher configuration can be mounted Air velocity re. Kinetic energy in aircraft wakes on pylons either side of the rear fuselage rather than on the wings. Figure 3 shows this type of engine on Figure 4: Reducing atmospheric disturbance

Aircraft Designs Incorporating BLI Electrically driven fans have several potential advantages. In the event that one core engine fails, BLI is applicable to a wide range of airframes, and the remaining thrust can still be evenly distributed several recent studies have applied BLI to BWB across the span of the aircraft, eliminating the airframes. Figure 5 considers the potential benefits of asymmetric thrust moment that normally accompanies a BWB airframe with BLI propulsors mounted above an engine failure. This greatly reduces the size of the the centre-body near the trailing edge. There is a vertical tail fins that were needed to counteract the potential synergy between the BWB airframe and BLI asymmetric thrust and reduces the skin friction drag if the propulsors on the upper surface can ingest a associated with the fins. In the N3-X design concept larger proportion of the overall airframe drag. This is the fins are eliminated altogether and yaw control is because the velocity of the passing air is higher on the maintained using differential thrust between the port upper “suction” surface of the aircraft than it is on the and starboard fans [2]. lower “pressure” surface, so more drag is created by the former. In principle it should also be possible to Despite a potential synergy between BWB airframes position some additional BLI propulsors along the and BLI, the overall advantage of the BWB lower surface to capture even more of the aircraft’s architecture is unproven for commercial passenger drag, or one row of ducted fans could be fed from aircraft. Conventional tube and wing designs are intakes on both the upper and lower surfaces. probably a more realistic prospect for the early application of BLI propulsion technologies. Table 3 Example: Silent Aircraft SAX-40 design compares tube and wing designs with BWB airframes. • Three engines ingest much of the BWB airframe upper surface boundary layer • Each engine has three mechanically Table 3: Pros and cons of different airframes driven fans arranged side-by-side • The fans ingest boundary layer and Benefits of a tube and wing arrangement: free-stream air • Fuselage length is easily varied in a family of aircraft Pros: Cons: • Unobstructed cabin space with simple egress • High propulsive efficiency from • Fans see high inlet distortion • Cylindrical pressure cabin is structurally efficient high bypass ratio fans and BLI • Engine cores are also subject • • Passengers are insensitive to typical rates of roll For a large aircraft three engines to some inlet flow distortion provides a good compromise and pressure loss • The fineness ratio of the tubular fuselage gives low between weight and thermal • Intake and exhaust losses are efficiency higher than for conventional form drag and permits high cruise Mach numbers • BWB airframe provides useful podded installations shielding for fan noise Benefits of a BWB arrangement: • Increased wingspan for given wing structural weight Figure 5: BWB aircraft with BLI turbofans • Higher potential internal volume to surface area ratio The Silent Aircraft project study SAX-40 aircraft as • Potential to ingest more of the airframe boundary layer shown in figure 5 is a design with three turbofan • Improved lift/drag at modest Mach numbers engines, each engine having three parallel flow fans ______arranged side by side. The central fan in each engine is driven directly from the LP turbine while the two For future advanced aircraft that are tube and wing outer fans are driven via bevel gears and cross- designs, several potential performance improvement shafting – well established transmission technologies. options are compared in table 4. These technologies are not mutually exclusive, so a wide range of Figure 6 shows the NASA N3-X BWB aircraft combinations may be considered. proposal. This is a more ambitious design that uses a superconducting electrical transmission to take power Table 4: Some candidate technologies for advanced from two wing-tip mounted turboshaft engines to a tube and wing aircraft bank of sixteen BLI fans arranged along the upper • Higher aspect ratio strut-braced wings trailing edge of the aircraft’s centre-body. • Natural or hybrid laminar flow wings • Continuous curvature and/or area ruling of fuselage

Example: NASA N3-X study aircraft • Riblets or LEBU devices on fuselage to reduce drag • Two turbo-generators drive multiple • Wing-tip rotors to reduce induced drag BLI fans • Power electronics for flight control • Blown wings or flaps to reduce wing area and drag • Fans ingest boundary layer air and free-stream air • Aft fuselage BLI propulsors, ducted or open rotor • Pros: Cons: A wider double-bubble fuselage • Relatively powerful core engines give • Fans see high inlet distortion • Integrated or semi-embedded propulsion systems high thermal efficiency • Reliant on a very efficient, • Multiple small propulsors along wing • High propulsive efficiency from very lightweight and flight critical low pressure ratio fans and BLI superconducting electrical ______• Airframe provides shielding for noise transmission system from fans • Cryo-cooling system needed • Eliminates drag from tail fins (or liquid hydrogen coolant) The most appropriate technologies will depend on the size and range of the aircraft and the market drivers in Figure 6: N3-X turbo-electric DP BWB aircraft each sector. Different configurations should emerge

as favourites for regional, medium range, long range maximum efficiency, it seems reasonable that if a rear commercial and business aircraft. fuselage BLI propulsor is to be included in the design, then this should also be an open rotor. For shorter range regional routes, slower turboprop aircraft are very cost competitive with small turbofan Another propulsion system configuration potentially aircraft, though the latter may still be preferred by applicable to regional and business aircraft has a passengers. For example, ATR claims its 70-seat single propulsor mounted on the centreline of the aft ATR72-600 turboprop has around 45% lower fuel cost fuselage. In principle this could be a ducted fan, a and 30% lower Cash Operating Cost (COC) than an pusher propeller, or an open rotor. Many small single- existing 70-seat regional jet aircraft flying 250 nm engined aircraft have been designed with single- routes in Europe. The comparison assumes broadly rotation pusher propellers. A few recent examples comparable maintenance costs [3]. While this may include the Cirrus VK-30, the OMAC Laser 300 and overstate the benefit of a state-of-the-art turboprop the General Atomics MQ-9 Reaper Uninhabited aircraft relative to an equivalent state-of-the-art Aerial Vehicle, but these aircraft have single engines. turbofan aircraft, the former clearly derives a Commercial and business aircraft would need to have substantial benefit from its lower cruise speed and at least two core engines driving the propeller. engines that have higher propulsive efficiency. The LearAvia Lear Fan 2100 business aircraft, shown In this regional market sector it seems that future more in figure 8, had two PT6B turboshaft engines driving a fuel-efficient aircraft should start with turboprop or four-bladed single-rotation pusher propeller through a open rotor propulsion systems before considering the combining gearbox [6], [7], but the project collapsed additional benefits that could be obtained from BLI. in 1985 when funding was withdrawn after failure to obtain FAA certification. The objections related to Some interesting regional aircraft concepts with unproven gearbox reliability and the potential after multiple smaller diameter propellers have been effects of total oil loss. Provided such concerns can proposed that would use wing leading edge and wing- be addressed, aircraft certification with a propulsion tip mounted rotors to increase wing loading and system having a single propeller should be possible reduce the induced drag from wing-tip vortices. under the existing rules for propeller driven aircraft. Figure 7 illustrates a proposed NASA Armstrong “LEAPTech” distributed electric propulsion aircraft concept researched with Joby Aviation [4], [5]. The principle behind this concept is distinct from BLI, as the benefit derives from improving the low speed performance of the aircraft. To improve the high speed performance one might have arranged BLI pusher propellers along the wing trailing edge, but such rotors would suffer an efficiency loss from the severely distorted inlet flow field just behind the wing. Figure 8: LearAvia Lear Fan 2100 Aircraft

Possible alternative drive arrangements for the aft fuselage propulsor would be for each core engine to have its own reduction gear train with a separate oil system and clutch, or to have a pair of contra-rotating propellers, each rotor driven by its own core engine, a drive configuration used, for example, in the 1950s Fairey Gannet naval aircraft, though this had tractor propellers [8]. It is far less obvious that a single or contra-rotating

ducted fan propulsor could be certificated as the sole Figure 7: NASA Armstrong Blown Wing Concept, propulsor for a commercial aircraft, because fan blade Proposed Technam P2006T Based Demonstrator failures normally have to be considered, and any such failure is likely to result in a total loss of thrust. A However, a future regional aircraft might incorporate further potential problem is the risk of cross-engine a rear fuselage BLI propulsion system as well and use debris, whereby an uncontained failure in one engine an electrical transmission system to enable power to could incapacitate the other, again resulting in a total be optimally distributed between multiple propulsors loss of thrust. This concern is likely to preclude under all flight conditions. Since a regional aircraft mounting two engines side by side inside the fuselage should have turboprop or open rotor propulsors for of a commercial aircraft.

These objections can be overcome if the rear fuselage will be less attractive on account of the sensitivity of propulsor produces no more than half of the total their efficiency to flight speed and their size, weight thrust in an aircraft having two or more propulsors. and noise impact, but for shorter-range middle of the market aircraft with aft fuselage propulsion systems Several twin-engined aircraft designs have had both exploiting BLI, a wide variety of potential design tractor and pusher propellers in the so-called push-pull configurations seems possible, both for the propulsors arrangement. Probably the most successful was the and the means of powering them in a DP aircraft. Cessna Skymaster, almost 3000 of which were built [9]. The Gulfstream American Hustler aircraft had a Propulsor Selection turboprop in its nose and a turbofan in its tail, though the turbofan intake was not configured for BLI and its Several factors influence the choice of propulsor, not tractor propeller would not have helped to reduce drag least: propulsive efficiency, nacelle drag, ease of on the fuselage [10]. installation and maintenance, cost, weight, noise, general appearance and provision of reverse thrust. From the aerodynamic point of view it seems better to have at least three propulsors: a BLI propulsor at the Reverse thrust is widely considered a requirement for aft end of the fuselage and two further units mounted commercial applications to provide an effective way in, on, or below the wings, or on either side of the aft of stopping an aircraft on an icy runway. Most fuselage. Each of these three propulsors could be an commercial aircraft have reverse thrust, though the open rotor or a ducted fan, depending on the cruise A380 has reversers only on its inboard engines and the speed of the aircraft and the severity of the noise Bae146 does not have any. targets it has to meet. Figure 9 compares some of Thrust reversal for large turbofan engines comes with these options. significant penalties because cascade and pivot-door type thrust reversers increase SFC and add cost, weight and nacelle drag, whereas turboprops and open rotors can have variable pitch blades that can be set to

provide reverse thrust with relatively little penalty. The need to provide reverse thrust is a disincentive to the development of turbofan engines with larger diameter fans that would give higher propulsive efficiency, but a possible solution is to delete the usual thrust reverser components and use variable pitch fan blades instead. This would enable a shorter, slimmer, Figure 9: Alternative Configurations for BLI lighter and lower drag nacelle, but the variable pitch fan itself adds weight and complexity, and auxiliary Each propulsor could have a dedicated power source, intakes may be required to achieve reliable flow or it could have power transmitted to it from two or reversal at high forward speeds. There is also a risk more independent power systems. These may include that the disturbed airflow entering the core engine in fuel cells, batteries, gas turbines, reciprocating engines reverse thrust mode could trigger rotating stall or or compound cycle engines. compressor surge. It is worth noting that the modern semi-rigid Zeppelin One way of reducing the penalties from thrust NT airships have three engines driving propeller, one reversers would be to have two under-wing turbofans on each side and a pusher unit at the back that gives without them and a single centreline BLI propulsor to efficient cruise propulsion, benefiting from BLI [11]. provide some reverse thrust. This should shift the balance in favour of lower specific thrust turbofans. For a short to medium term “middle of the market” 100-200 passenger short to medium range aircraft, a Adding a third propulsor offers further useful benefits low wing design with a near circular fuselage cross- in comparison with more conventional twin turbofan section is likely to remain the preferred configuration aircraft. These include its effects on airframe weight, and the targeted cruise speed could be a little higher or drag, and external and internal noise, and avoiding lower than the 0.78-0.79 Mach number typically problems with larger diameter fans and open rotors. quoted for Boeing 737 and Airbus A320 aircraft. The choice depends on the trade-off between lower fuel Table 5 considers the pros and cons of the different burn and higher productivity. A lower cruise speed arrangements shown in figure 9. Table 6 considers should favour open rotors and laminar flow wings, the potential benefits of the arrangement with two whereas a higher cruise speed may favour ducted fans. under-wing turbofans and a centre-line pusher open rotor for rear fuselage BLI while table 7 notes the For larger longer range aircraft, higher cruise speeds potential disadvantages, though these do not seem are likely to be considered essential and open rotors sufficiently penalising to invalidate the concept.

Table 5: Alternative BLI Propulsor Arrangements Table 7: Potential problems with adding the open rotor to a twin-turbofan aircraft Concept Pros Cons Open rotors and Maximizes the Large open rotors • Open rotors must be protected from ground-strikes by aft fuselage BLI total propulsive may be too noisy a strake or tail wheel which could limit the angle for open rotor with efficiency aircraft rotation on takeoff, raising the minimum either two or Aft C-of-G and takeoff speed or needing an extending undercarriage three core engines Option to have a no wing bending • Open rotors also need to be protected from any debris mechanical power moment relief thrown up be the wheels, perhaps by deflector plates transmission to • The ingestion of wakes from the wing roots and the centreline BLI Passengers may empennage into an open rotor may generate high noise propulsor prefer aircraft with ducted fans • There is a hazard from a turning rotor to personnel on the ground. Even with propeller rotor brakes applied, Turbofans under Suitable for larger Needs electrical keep-out zones may need to be set up around them if the wings and aft and longer range transmission if there is any risk that they could start to turn. (This fuselage ducted aircraft flying at powered by just already applies for example to the starboard engines of BLI fan or fans higher Mach nos. two core engines. ATR aircraft when their cores are kept running to provide ground air and power in lieu of an APU.) Probably the Installation losses quietest solution associated with • Tail rotors may add to the overall length of an aircraft, BLI ducted fans or require its empennage to be moved forwards, reducing the effectiveness of the control surfaces Turbofans under Possibility of just Needs electrical • Any mass moved from the wings to the tail reduces the wings and aft using the open transmission if wing bending moment relief and also reduces pitch fuselage open rotor for reverse powered by just and yaw stability by shifting the centre of gravity rotor for BLI thrust – deleting two core engines. further aft, increasing aircraft structural weight. reverse thrust • Additional costs associated with adding/maintaining a

from the wing Noise unless open third engine, or extra transmission systems mounted engines rotor can be cut- ______back at fly-over The open rotor will not need a large diameter if it is Table 6: Potential benefits of adding a centreline just acting on the fuselage boundary layer and only open rotor to a twin-turbofan aircraft providing 30-40% of net thrust. To avoid needing to • An open rotor is more efficient for re-energizing the run it in proximity to ground crew, and to reduce boundary layer, as it has no intake or ducting losses airport emissions, a motorized undercarriage could be • The open rotor can generate high levels of reverse used for taxiing. The other concerns would need to be thrust on landing, so the wing mounted turbofans traded-off against the propulsive efficiency or drag probably won’t need to have thrust reversers reduction benefits in more detailed design studies. • The open rotor could be used for taxiing and push- back, without needing to start the turbofans engines* For an aircraft with three propulsors, each propulsor could be a self contained “engine”, or it could have • The turbofans can produce less thrust than for an equivalent twin engined aircraft, so for given ground power transmitted to it from elsewhere on the aircraft. clearance they can have lower fan pressure ratio and In the latter case it still makes sense that the port and specific thrust to improve propulsive efficiency, or starboard propulsors are integral with the core engines smaller fan diameters giving lower drag to minimize the power to be transmitted over longer • Cabin noise is minimized by the ducted fans and by distances. They will also have sufficient separation to positioning the open rotor as far aft as possible reduce the risk of cross-engine debris impact in • External noise is minimized if most thrust comes the event of an uncontained failure, but the BLI from the ducted fans and the open rotor power is propulsor in the tail could be provided with power cut-back after takeoff and during approach exported from the other engines. Relative to having • In common with all aircraft having three propulsors: three core engines, this gives reduced parts count and greatly reduced thrust asymmetry in the event of loss maintenance costs and gives larger core components of thrust from one propulsor at takeoff enables much that will be more aerodynamically efficient. It also smaller tail surfaces with reduced weight and drag avoids positioning the third engine core where its ______performance will be compromised either by ingesting * Note taxiing on the asymmetric thrust from just one disturbed, lower pressure air, or by needing to have an wing mounted engine makes opposing turns difficult. extended offset S-duct intake, as in the Bauhaus Luftfahrt “Propulsive Fuselage” concept [12]. The concept seems to be a good compromise for a middle of the market aircraft with a fairly high cruise Power Transmission speed. The open rotor would not need a very large diameter if it just acts on the fuselage boundary layer There are several ways in which power may be and only provides 30-40% of net thrust. transmitted from the port and starboard engines to the

centreline BLI propulsor. If the engines are mounted Barring some unanticipated developments in more either side of the aft fuselage, a direct mechanical conventional electrical transmission systems, a highly drive might be used, or the partly expanded exhaust efficient superconducting electrical power trans- gasses could be ducted to power turbines used to drive mission system seems to be essential where a high the BLI propulsor. A mechanical drive from a wing proportion of the power generated by the core engines mounted turboprop or open rotor powerplant in a high is transmitted to the BLI propulsors. The systems wing aircraft might also be feasible, but if the main need superconducting power cables and at least partly engines are mounted under low wings, then an superconducting electrical machines to match the electrical power transmission system is probably more performance and weight of mechanical transmissions practical, given its path length, as shown in figure 10. and to meet the required efficiency and weight targets. For a simple electrical transmission, the generators on This is because the power/weight requirements for the the main engines could be synchronous with the technologies used on the aircraft must increase in motors driving the BLI propulsors. proportion to the linear scale of the aircraft and with its airspeed in order to avoid a disproportionate increase in the mass of the total propulsion system. For example, electric cable mass is proportional to the length of the cables as well as to the power transmitted, and the volumes and masses of electrical machines tend to scale with torque, so for a given mechanical limit on rim speed, the masses will scale as power4/3. It is relatively easy to make an electrically powered model aircraft using current technology, but it is much more challenging to engineer a high speed

and full sized electrically powered passenger aircraft. Figure 10: Sending Power to a BLI Propulsor For example, a single generator on the LP spool of each engine could power one half of a pair of contra- rotating tail rotors. The pitch of each variable-pitch rotor would determine the work-split between that open rotor and the main engine’s fan. Alternatively, if the BLI propulsor uses multiple small ducted fans, then each generator could power fans on the opposite side of the aircraft, minimizing thrust asymmetry in the event of an engine failure. Figure 11: Airbus E-thrust TeDP Aircraft Concept Hybrid Electric Propulsion Even fully superconducting transmission systems will Total flexibility may be achieved through the use of not be 100% efficient because they still suffer from power electronics to reconfigure a transmission magnetic flux leakage and alternating current losses, system as and when required, and to run each so some heat is still generated and continuous cooling propulsor at its optimum speed and power level is needed. While superconductivity is possible at the independently of the frequency of the power source. boiling point of liquid nitrogen, practical machines Using power electronics would also permit integration need to operate at much lower temperatures today and of energy storage systems. A downside is that the need cooling with a cryo-cooler or liquid hydrogen. power electronic units would add extra weight and Because of its high cost, the widespread use of liquid complexity to the aircraft, and because they are not hydrogen as an aviation fuel can only be a very long- 100% efficient they would add to transmission losses term prospect, but with the right infrastructure small and need a dedicated cooling system. amounts might be loaded onto aircraft to provide the The NASA N3-X shown in figure 6 is an example cooling for superconducting electrical systems. The of a study aircraft with distributed hybrid electric gassified hydrogen could then be fed to a fuel cell or propulsion and potential for integrating energy storage APU to produce additional power. Liquid methane is systems. Rolls-Royce has investigated the electrical a possible alternative cryo-fuel that could be used as a equipment that this aircraft would need [13], [14]. heat sink for a cryo-cooler [16]. Another concept is the Airbus E-Thrust aircraft shown Conclusions in figure 11 and further studies have been made in the Distributed Electric Aerospace Propulsion (DEAP) BLI offers the potential for significant improvements programme by Airbus, Rolls-Royce and Cranfield and in propulsive efficiency and drag reduction, given Cambridge Universities [15]. closer integration of aircraft and propulsion systems.

In the longer term, novel DP airframes with hybrid References electric and superconducting transmissions may prove their worth, but in the more immediate future BLI [1] Rolt, A. and Whurr, J., Optimizing Propulsive might be applied to more conventional twin-engined Efficiency in Aircraft with Boundary Layer Ingesting “tube and wing” aircraft, simply by adding a third Distributed Propulsion, ISABE-2015, Phoenix, USA, propulsor to the tail to re-energize the aft fuselage October 2015. boundary layer air. The most efficient propulsors are open rotors and contra-rotating open rotors would be [2] Felder, J., Brown, G., Kim, HD. and Chu, J., preferred for aircraft with higher cruise speeds, though Turboelectric Distributed Propulsion in a Hybrid the largest and fastest aircraft might prefer ducted fans Wing Body Aircraft. ISABE-2011-1340, Goteborg, to give the smallest possible noise footprint. Sweden.

The third propulsor could be a self contained third [3] ATR-600 series Brochure, downloaded 14 May engine, or it could take power delivered from the two 2015 from ATR Aircraft: main engines via mechanical or electrical transmission systems. Hybrid electric arrangements are possible http://www.atraircraft.com/products/list.html and would enable power from an APU or an energy store to be integrated into the aircraft power system to [4] Stoll, A. M., Bevirt, A., Moore, M. D., Fredericks, facilitate reduced emissions around airports. W. J., and Borer, N. K., Drag Reduction Through Distributed Electric Propulsion, ATIO Conference The twin engine configuration with power transmitted Atlanta Georgia, June 2014. to a tail mounted open rotor merits further study. It

gives a useful propulsive efficiency benefit at cruise [5] Fredericks, W. J., Impact of Distributed Electric without the cost and complexity of a third engine or Propulsion (DEP) on Aircraft Design. IMechE the loss of thermal efficiency associated with down- Special Conference on Disruptive Green Propulsion sizing the engine cores in three-engined aircraft. Technologies, London, November 2014. Initial application of these concepts could be to a regional or middle of the market aircraft and perhaps [6] LearAvia Lear Fan. Retrieved 17 May 2015 from also to a business jet, but development of more Wikipedia: efficient superconducting electrical transmissions will http://en.wikipedia.org/wiki/LearAvia_Lear_Fan open up the design space envelope, allowing more power to be transmitted over longer distances, so that [7] Learavia LearFan 2100 – executive transport. even the largest commercial aircraft could benefit Retrieved 17 May 2015 from Aviastar: from DP and BLI. http://www.aviastar.org/air/usa/learavia_learfan.php It is often argued that longer range aircraft have more to gain from technologies like BLI that reduce fuel [8] Fairey Gannet Anti-Submarine Warfare Aircraft consumption, because the benefit is compounded by (1953), retrieved 17 May 2015: not needing to carry the weight of the fuel saved http://www.militaryfactory.com/aircraft/detail.asp?air during the flight. However, as aircraft and propulsion craft_id=709 systems get to be more efficient, this compounding of the benefits is becoming less significant. [9] Cessna Skymaster. Retrieved 16 May 2015 from The synergy between BLI, DP and energy storage Wikipedia: may be more pertinent. Energy storage in a TeDP http://en.wikipedia.org/wiki/Cessna_337_Skymaster aircraft is more likely to buy its way on to a shorter range aircraft that can recharge its batteries more [10] Gulfstream American Hustler. Retrieved 22 June often and use their power to help reduce emissions 2015 from Wikipedia: around sensitive airports. Given that such aircraft https://en.wikipedia.org/wiki/Gulfstream_American_ would already need to have electrical transmission Hustler systems, adding a BLI propulsor would give an additional benefit at relatively low cost. [11] Brandt, T., Zeppelin NT - The Utility Airship Acknowledgements Zeppelin NT as a Platform for Remote Sensing for Environmental and Industrial Applications. AIAA The authors are grateful to Cranfield University and 2007-7879, Seventh AIAA ATIO Conference, Belfast, Rolls-Royce plc for permission to publish this paper. September 2007.

[12] Seitz, A., Bijewitz, J., Kaiser, S., Wortmann, G., (2014) "Conceptual investigation of a propulsive fuselage aircraft layout", Aircraft Engineering and Aerospace Technology: Vol. 86 Iss: 6, pp.464 – 472. AEAT-2014-079.

[13] Kim, H. D., Felder J. L., Tong, M.T. and Armstrong, M., “Revolutionary Aeropropulsion Concept for Sustainable Aviation: Turboelectric Distributed Propulsion.” ISABE-2013-1719, ISABE Conference, Busan, South Korea, September 2013.

[14] Armstrong, M., “System Level Design Con- siderations of a Superconducting TeDP Microgrid.”, IMechE Disruptive Green Propulsion Technologies Conference presentation, London, 18 November 2014.

[15] Parker, R., Challenges of Advanced Propulsion Systems Development for Future Civil Air Transport, General Lecture 1, ICAS-2014-0.2 ICAS Conference 2014, St. Petersburg, Russia, 9 September 2014.

[16] Palmer, J., Pagonis, M., Malkin, P., "Configuration Options for High-Power, Low Weight Aerospace Superconducting Distributed Propulsion Cryocoolers", ISABE 2015, Phoenix, USA, October 2015.