Traditionally, performance drives military-aircraft design decisions and the energy implications of those choices are secondary. But as fuel costs eat into reduced budgets, the balance is shifting. Energy is fast becoming a critical constraint on operations, and the results could reshape aircraft design.

For now, the U.S. Air Force's efforts to cut fuel bills are focused on its transport and tanker fleet, which consumes two-thirds of the aviation fuel the service burns each year. While near-term retrofits—such as formation flying, winglets and other drag-reduction devices—can reduce the fuel consumption of existing aircraft, they will not provide the scale of savings sought in the long term.

The name of the Air Force Research Laboratory's (AFRL) Revolutionary Configurations for Energy Efficiency (RCEE) program says it all: Dramatic changes in aircraft design may be required to achieve significant reductions in fuel consumption.

The goal of RCEE Phase 1, which ran from 2009-11, was to define a next-generation mobility fleet that would use 90% less fuel than today's transports and tankers. Under Phase 2, which began in 2011 and will run until 2015, companies are taking a closer look at specific configurations.

In Phase 1, Boeing defined a mixed fleet that met the 90% savings target: an all-electric truss-braced-wing design with 20-metric-ton payload; a 40-ton-payload distributed-thrust hybrid-electric design; and a 100-ton payload hybrid-electric blended wing-body (BWB). In Phase 2, the company is taking a closer look at the distributed-thrust, hybrid-propulsion design.

Lockheed Martin, meanwhile, studied a wide range of configurations and technologies in Phase 1 in search of the 90% goal, concluding a hybrid wing-body (HWB) offers the most potential. In Phase 2, the company is further refining the concept, which combines a blended wing and forebody for aerodynamic and structural efficiency with a conventional aft fuselage and tail for compatibility with current airlift missions, including airdrop.

The twin-engine HWB is designed to take off in less than 6,500 ft. and fly 3,200 nm carrying 220,000 lb. of payload, including all the outsize cargo now airlifted by the Lockheed C-5. Lockheed calculates the aircraft will burn 70% less fuel than the Boeing C-17 through a combination of better aerodynamics, newer engines and lighter structures. “We use mature technologies to be affordable and could build it today,” says Rick Hooker, an aeronautical engineer at Lockheed Martin Aeronautics

The HWB study is marked by a high degree of aerodynamic optimization using computational fluid dynamics (CFD) tools not available when today's airlift fleet of C-17s and Lockheed C-130s and C-5s was designed. Starting with a cruise Mach number of 0.7 as originally lofted, extensive shape optimization using CFD increased cruise speed to Mach 0.81 and reduced transonic drag by 45%, says Lockheed aeronautical engineer Andrew Wick.

Lockheed estimates the aircraft is 65% more aerodynamically efficient than the C-17, which is penalized by its 1980s design and the requirement for short-takeoff-and-landing (STOL) capability. The HWB is 30% more efficient than a C-5, and Lockheed says it is even able to achieve an aerodynamic efficiency 5% better than the Boeing 787, albeit at a lower Mach number.

That efficiency comes from several sources. To start, the blended forward fuselage carries 25% of the lift and moves the wing roots outboard, extending span and reducing drag without increasing wing weight. The spanwise lift distribution is improved and wing aspect ratio increased to 12 for the weight of a conventional aspect-ratio 9 wing,

The aft fuselage, meanwhile, ensures the aircraft is compatible with current loading and airdrop operations—a challenge for pure flying-wing designs like the BWB, says Hooker. The conventional T tail incurs a 5% drag penalty relative to a pure BWB, but provides robust control and avoids the cost and risk of developing new control effectors and algorithms for a flying wing to enable STOL and manage the abrupt center-of-gravity (CG) shift when airdropping heavy loads.

The aft fuselage is designed to provide a smooth flow field around the aft paratroop doors and cargo ramp, similar to a C-5, says Hooker. The tail is sized to handle a CG range of 20% mean aerodynamic chord, the same as a C-5. And the aircraft is designed so the tail is not needed for trim in the cruise, avoiding a drag penalty.

An unusual aspect of the HWB design is that the blended forebody encloses a circular pressurized fuselage. Some cargo is carried in unpressurized outer bays—pallets are loaded via the rear ramp, moved forward on floor rollers, then sideways through fuselage doors and into the outer bays on ball mats. The result is a pressurized fuselage that is smaller and lighter than the C-5's despite the similar cargo capacity. Lockheed calculates the HWB's structure is 18% lighter than a conventional design.

Another unconventional element of the configuration is the engine location above the wing trailing edge. Over-wing nacelles have long been avoided in aircraft design because of adverse transonic interference with the wing, but careful optimization by Honda of the engine location on the HondaJet has given the configuration new credibility.

Lockheed studied cruise interference drag with engines mounted in several locations—under and over the wing leading edge, over the trailing edge and on the aft fuselage—and generated more than 15,000 Navier-Stokes CFD solutions. The results showed that mounting the nacelles over the inboard trailing edge improved lift-to-drag ratio, regardless of engine type, for an aerodynamic benefit of up to 5% over a conventional under-wing location.

Three potential powerplants have been identified. General Electric's GEnx is available today, providing a 25% reduction in specific fuel consumption (sfc) over the C-17 and C-5M engines. Rolls-Royce's conceptual Ultra Fan has a 30% lower sfc and could be available by 2030. Third is a GE open rotor that could be available after 2025 with a 35% lower sfc. Combined with the improved aerodynamic efficiency and lighter weight, lower sfc results in the HWB burning 70% less fuel than a C-17 with GEnx engines, 75% with Ultra Fans and 80% with open rotors, Lockheed calculates.

Interestingly, despite diameters ranging from the GEnx's 11.8 ft. to an open rotor's 21 ft., “the wing optimized out to the same shape for all three engines,” says Wick. “The same wing for all three allows the engine installation to be modular. We could build it today and it would be designed to be able to be reengined.”

Analysis showed the over-wing installation offers other benefits, he says. The long wing chord ahead of the nacelle acts as a flow straightener to reduce inlet distortion and also shields fan noise from the ground. The overhang from the trailing edge means the engine is still accessible for maintenance and removal. And a smaller tail is possible with over-wing engines, says Hooker.

There is a powered-lift benefit from placing the engine nacelles over the trailing edge of the wing. “The inlet flow provides a large amount of suction lift on the wing,” says Hooker. This has a similar effect to the high-pressure area generated by under-wing engines blowing over deflected flaps, as happens in the C-17, and allows the over-wing engines to achieve a similar 15% increase in maximum lift coefficient.

To provide STOL capability, excess fuel volume could be traded for flap blowing to create a circulation-control wing, as in the STOL airlifter concept developed by Lockheed for AFRL's Speed Agile program. Another possibility is deflecting thrust downward, using flaps aft of the engine, core flow vectoring with an F-35B-style swiveling nozzle, or rotating the engines when the flaps deploy, “so they go along for the ride,” Hooker says.

Although RCEE is just a study effort, the Air Force will have to begin work on its next strategic airlifter in the near future if the C-17 is to be retired as planned starting in 2033. Noting it took 21 years to field the C-17, Hooker says ,“We need to start today to avoid a future gap.”