The aviation industry is often credited with making great strides in its effort to improve fuel efficiency and reduce emissions. Progress has been made, driven by executives hunting for margin, and to a lesser degree by the tightening of regulations on pollution. While such advances might feel like a revolution to those on the inside, on the outside the typical airliner looks decidedly unreformed.
The question is, how much more can be done through weight savings and new engine technologies to improve the efficiency of the standard tube with wings? How long will it be before the industry has to employ new aircraft shapes, such as the blended wing body (BWB), in order to continue to reduce fuel burn?
No doubt the BWB is still a long way off, but there are other modifications to the standard airliner configuration that might make a stepping-stone along the way. One such proposal, which is in development at NASA, is for a new type of engine, or ‘propulsor’, that lies partially embedded in the surface of the fuselage itself.
Typically, an airliner’s engines are located in pods away from the body of the aircraft where they are free to ingest a relatively clean and uniform flow of air. Instead, the new engine experiment, which has just undergone a first round of research testing at the NASA Glenn Research Center in Cleveland, ingests the slower-moving and distorted stream of air that develops close to the skin of the aircraft, known as the boundary layer. It is a major cause of drag.
The propulsor combines an engine inlet designed to manage the distorted flow, and an especially resilient fan stage. The team at Glenn has just completed the first ever wind tunnel tests of the inlet and fan combination.
By accelerating part of the boundary layer through an engine in this way, NASA says it is possible to reduce the drag on an aircraft and thus increase fuel efficiency by 4-8% more than the advanced engine designs that airlines are beginning to introduce.
The challenge is to build an engine that is capable of withstanding the effects of the boundary layer flow without resorting to technologies that might compromise efficiency, for example by adding weight to the aircraft.
Speaking to Aerospace Testing International, David Arend, a boundary layer ingestion (BLI) expert at NASA, says the propulsor has the potential to considerably improve aircraft efficiency.
“The team has successfully completed tests of the first-ever boundary layer ingesting propulsor for subsonic cruise aircraft and will begin reporting its results this summer,” he says. “This was the first such test. Much research remains to be done over perhaps the next five years. Beyond that, it would be great to see this technology earn its way into mainstream aircraft design and go on to meaningfully reduce the impact of aviation on the environment.”
The highly experimental engine is known as the Boundary Layer Ingesting Inlet/Distortion Tolerant Fan (BLI2DTF) and was designed with the assistance of NASA’s industry and academic partners: the United Technologies Research Center and Virginia Polytechnic and State University.
The research, which began in 2009, is part of NASA’s Advanced Air Transport Technologies Project. Arend says early studies indicated that approximately 3-5% fuel burn reduction could be achieved with five engines embedded into the upper aft surface of a hybrid wing body aircraft. The study also identified that up to 10% reduction was possible if additional propulsors were added to ingest more of the aircraft’s boundary layer.
“The simplified objective of this research was to generate and evaluate new technologies through design, analysis and test of a multi-use single inlet-fan propulsor experiment,” continues Arend. “Its ultimate goal was – if possible – to achieve significant fuel burn reduction for subsonic cruise aircraft relative to an advanced conventional baseline ultra-high bypass propulsor.”
The first challenge was the requirement for a multidisciplinary approach to the project in order to secure the best chance of making real and credible progress. The design of the combined inlet and fan stage had to be approached holistically, as Arend explains:
“From the outset, the BLI2DTF Task pursued concurrent research in the areas of inlets, turbo-machinery, aeromechanics, acoustics, propulsion airframe integration and nozzles. Included therein is the requirement for a coupled, designed and integrated BLI inlet-fan stage.”
The embedded BLI propulsor testbed for the transonic test section of the Supersonic Wind Tunnel and other experiment hardware was installed last year and research tests of the main wind tunnel experiment were conducted during November and December.
A jet engine creates most of its thrust by accelerating the airflow from the inlet to the nozzle at the rear. The thrust is proportional to the difference between the incoming and outgoing velocities, not the actual values of those velocities. A BLI propulsor achieves this acceleration within a lower range of inlet and outlet velocities than a conventional jet engine, and thus requires less propulsive power input (i.e. fuel burn) to produce the required amount of aircraft thrust.
Commenting on the aerodynamic advantages of the propulsor, Arend says, “Thrust generation with reduced average inflow and outflow velocities requires less propulsive power input, yielding a considerable reduction in the amount of fuel that must be consumed. This is more than enough to offset the efficiency losses incurred due to boundary layer distortion.”
The next challenge for the team was to find a way to manage the impact of the distorted airflow on the performance and operability of the engine, by designing a fan that would be aerodynamically successful.
In the tests, the BLI inlet ingested a considerable amount of simulated aircraft boundary layer airflow, as well as undistorted free stream airflow. Arend says the inlet was designed for that combination to minimize the impact of distortion on the performance of the fan stage. He adds that the fan was designed to structurally withstand and achieve aerodynamically robust performance despite the presence of the managed distortion flow provided by the inlet. “The experiment was designed to ingest the naturally occurring wind tunnel boundary layer – augmented by aerodynamic roughness, which was employed to ensure it was thick enough. Combined with a boundary layer bleed system, we were able to achieve the desired amount of boundary layer ingestion for both on- and off-design operating conditions.”
The experiment included rotating instrumentation arrays at the ‘aerodynamic interface plane’ between the inlet and the fan, as well as downstream of the fan stage to measure its performance.
“It also featured a fast-acting variable area fan nozzle that was used to explore the inlet-fan map, and determine the propulsor’s stability limits when needed,” continues Arend. “The 22in diameter fan experiment was powered by NASA’s ultra-high bypass drive rig. All the instrumentation typically employed for inlet and fan tests was employed.”
The tests have achieved considerable progress: “We successfully completed our test program in December 2016 – including acquisition of the defined minimum success data and much more,” says Arend. “The operating map of our BLI propulsor has been explored and its aeromechanical robustness established over a little more than 104 hours of operation. Data was recorded across its cruise operating map, consisting of BLI inlet static pressure distributions, total pressure recovery and airflow distortion measurements.
“Measurements were also obtained of the fan’s aeromechanical response to BLI distortion and the fan stage’s efficiency and stability margin, as well as other flow physics measurements. We have acquired and are now post-processing the data needed to meet our research objectives.”
The next stage of the project is to employ the multi-use experiment to conduct certain specific research into the fan’s response to simulated BLI inflow conditions through a NASA fan rig test.
As Arend notes, there are many years of work ahead, but he is hoping that the new focus on the potential to exploit the boundary layer will make a major contribution to lowering the impact of aviation on the environment. “In this way, I hope BLI propulsion becomes a positive game-changer for the aerospace and aviation industries.”
George Coupe is a journalist and editor with many years of experience writing for science, engineering and technology publications, as well as the national press.