Airbus's (Outdoor exhibit 13, 25, K*/Hall 1, Stand 1105) flight-testing of a converted A340 quadjet confirms that "the door is wide open" to apply laminar-flow technology to future airliners, says research and technology senior vice president Axel Flaig. Ten months ago, the manufacturer embarked on a two-year project dubbed Breakthrough Laminar Aircraft Demonstrator in Europe (BLADE).

Conducted under the auspices of Europe's Clean Sky 1 joint-undertaking environmental research program, BLADE aims to achieve a reduction of 10 percent in aircraft drag and up to 5 percent in CO 2 emissions. After a successful early round of test flights in late 2017, Airbus has resumed operations with the "Flight Lab" test aircraft—the original prototype A340-300 (manufacturer's serial number 001)—with plans to analyze laminar flow over a contaminated wing and after modification with a fixed Krüger flap.

Initial flight tests permitted Airbus to assess aircraft-handling, extend the flight envelope, and record early indications of achieved natural laminar-flow (NLF). Flaig says that investment in the project is "really paying back."

The €1.6 billion ($1.9 billion) Clean Sky 1 program has been running for 10 years, and the BLADE exercise is assessing the feasibility of introducing NLF wing technology on a large airliner. Airbus believes it could be applied to a next-generation narrowbody aircraft by 2030.

The company claims its demonstrator aircraft, which has been modified with two slightly different NLF outer-wings, is the first in the world "to combine a transonic laminar wing profile with a true internal primary structure." It is testing the robustness and sustainability of NLF during flight operations to enable commercial-aircraft manufacturers to properly design laminar components (including wings) and to specify production-tolerance requirements.

The theoretical benefits of laminar (or non-turbulent) boundary-layer airflow are well known, challenging engineers to produce smooth and aerodynamically stable wing surfaces on what Flaig calls "an industrial scale." Now, the BLADE project is intended "to validate the area of 'laminarity' that can be achieved for a large variety of cruise-flight conditions with respect to altitude, Mach number, and wing loading," according to Clean Sky researchers.

Understanding that laminarity, particularly the factors influencing the phenomenon, is at the heart of continuing flight-testing by Airbus. By early May, the A340 Flight Lab—"the largest flight-test demonstrator ever launched in Europe"—had logged some 70 flight-hours and as of last month was almost midway through an expected 150 hours of testing that will continue until next year.

Flight-test Findings

The manufacturer notes a number of flight-testing firsts arising from the project. These include using infrared cameras to monitor laminar-flow "transition points," an acoustic generator to measure the influence of sound on laminarity, and a real-time "reflectometry" system to gauge overall flow deformation in flight.

Before the second phase of investigation began in April, the demonstrator aircraft had completed 23 test flights in the initial 13-week trials campaign last year, accruing 65 flight hours with up to three flights a week. Flaig reports that laminar flow—more stable than had been expected—was seen from the A340's very first flight.

Airbus has now concluded that the modified outer wings have reduced aircraft drag by more than was initially believed. This has encouraged the manufacturer to suggest that applying the technology to a future generation is more feasible than previously thought.

Flight-testing has demonstrated that the aerodynamic benefits of laminar-flow can be sustained at speeds of up to Mach 0.78, compared with a predicted requirement of Mach 0.75 to achieve targeted fuel savings. Despite slight differences in the increased aerodynamic efficiency of the two slightly different modified outer wings, Airbus confirms that the effects can be sustained.

The second flight-test phase now underway involves what Airbus terms aerodynamic "imperfections," involving laminar flow over a contaminated wing surface and after installation of a fixed Krüger leading-edge flap. This is intended to "extensively test and characterize" the robustness of laminarity in "representative operational conditions." The test aircraft is expected to fly at intervals of about three weeks, governed by changes to aircraft configuration.

Development of modifications for the test aircraft involved 21 European partners, 500 contributors, and some 6,500 components to produce the required parts:

wing-joint "aero-fairings" that separate the "turbulent" inboard wing from the NLF sections;

NLF-section wingtip pods that provide a defined flow pattern and accommodate flight-test equipment; and

a digital mock-up of the new wing section outboard of the number-one (outer left) engine.

In April, having previously confirmed that the A340-based BLADE exercise has "no link to any possible future aircraft program," Airbus signed an "accord" with numerous stakeholders "to build on this program in the European framework of Clean Sky."