Early in April, Airbus plans to start the second phase of a 150 flight-hour test campaign aimed at confirming technology that is expected to cut aerodynamic drag and fuel consumption significantly. The A340BLADE (for Breakthrough Laminar Aircraft Demonstrator in Europe) project, which uses the original A340-300 prototype modified with new natural laminar-flow (NLF) outer-wings, is part of the region's Clean Sky joint-undertaking research program that aims to reduce aircraft CO₂, greenhouse gas, and noise emissions.

Initial results from the first phase, flown during September to December last year, have been encouraging, says the manufacturer. "So far, with limited analysis, we are very satisfied with the observed behavior, with results meeting—and sometimes even exceeding—expectations," said flight-test engineer Philippe Seve. 

"The aircraft has shown satisfactory handling qualities and flaw-free system behavior. Also, the flight-test instrumentation [FTI], thanks to extensive preparation work and redundancy precautions, was very mature from first flight."

Now, the campaign's second phase, which involves "[aerodynamic] imperfections flights," is intended to "extensively test and characterize 'laminarity' robustness in representative operational conditions." The A340BLADE is expected to fly about every three weeks, governed by changes to aircraft configuration, from the beginning of April to an undetermined date.

Commercial Applications

Choice of the A340-300 prototype (manufacturer's serial number [MSN] 001) was driven by availability and its natural wing "split" at the outboard engines, permitting replacement NLF outer-wings. Airbus hopes wing-friction drag will fall by 50 percent, which would translate into a reduction of "up to 5 percent" in block-fuel and CO₂ emissions if applied to a short-range airliner over a range of up to 800 nm.

Airbus (Chalet CD17, Stand J23) says that this is the first test aircraft "to combine a transonic laminar-wing profile with a true internal primary structure." Laminar—or "streamline"—flow relates to boundary-layer air adhering to the airfoil until interrupted by surface contaminants and other factors that generate turbulence (and consequent higher drag and reduced lift).

The A340BLADE campaign 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," say Clean Sky researchers. It is testing the robustness and sustainability of NLF in operational service to enable commercial-aircraft manufacturers to properly specify production-tolerance requirements and design laminar components, including wings.

The program's primary aim is to accelerate future industrialization (or mass production) of laminar wings, according to Airbus. Earlier technology had not been mature enough for airliners, nor been fully validated in flight. "The last 50 years have seen a more-than-70 percent improvement in relative fuel-burn/seat and 90 percent [reduction] in noise emissions, but a practical application of laminar flow has not been achieved," said Airbus research and technology senior vice president Axel Flaig.

"Rapid and recent development of numerical-flow simulation tools enables us to design, build, demonstrate, and validate an optimized NLF wing," said Airbus. "Aerodynamic flight-control laws related to the specific shape of the NLF wing [have] been defined and validated through ground-test wind-tunnel [and] simulator sessions."

Noting that laminar flow is "much better at lower speed, but requires transonic airflow," BLADE project leader Daniel Kierbel points to many so-called "imperfection" factors that can inhibit NLF generation on an "industrial" wing. External considerations include wing leading-edge and surface erosion or contamination (de-icing fluids and grease, dents, dust, insects, and scratches); atmospheric disturbance; and acoustic disturbance and vibration.

Internal factors include deformation of fastener heads and joints, gap-filler material, local and "global" wing deformation, and system integration.

Flight-test Progress

The first round of flight tests has allowed Airbus to assess aircraft handling, extend the flight envelope, and obtain initial indications of achieved NLF, said Seve. The A340BLADE completed 23 test flights in the 13-week first phase, clocking 65:20 flight hours with up to three flights a week.

"We [were able to] demonstrate our capacity to fly and measure in night conditions," said the engineer. "All this enabled us to move on quite far. In total, we have performed 165 cruise measure points for several Mach, angle-of-attack, Reynolds, sideslip, and aileron settings. Each point provided 'laminarity' extent, pressure distribution, [and] surface-deformation status."

Testing began on September 26 with an initial "shakedown" flight—for both FTI and the airframe—that also saw the machine ferried from a modification center at Tarbes in southwest France to the manufacturer's Flight and Integration Test Centre in Toulouse. Second and third flights with the aircraft confirmed the low- and high-speed flight envelope, including stalls and flutter.

Laminar flow was seen on both new outer wings during the three-hour, 38-minute first flight, which reached a speed of Mach 0^.78 and cruise and maximum altitudes of 32,000 feet, according to Seve. “We achieved our objective to fly at the design Mach number, at a reasonable altitude and check everything was fine. We checked that the FTI was working as expected; to identify further fine-tuning for the next flights.” 

Experiments to better comprehend and control the transition between laminar and turbulent flow had prepared the ground for full-scale testing, "the first time such realistic test data has been available for future aircraft design," according to Clean Sky.

The A340BLADE's new 8m-span outer wings are similar in shape and size to those of a potential future short-range, single-aisle airliner. At first, the project had considered use of a complete NLF wing on a smaller flying testbed, but applying a smaller proportion of a large wing lowers the risk of control loss, while establishing the limit of laminarity, explained Kierbel.

The NLF sections, produced by Sweden's Saab and GKN UK, offer different solutions, while meeting stringent requirements for accuracy, stability, and tolerance to ensure laminarity in all regimes of flight. Saab's left outer wing is comprised of a single-piece carbonfiber-reinforced plastic (CFRP) wing "cover" (or skin) and leading edge (including integrated spar cap and wing-rib feet), while GKN's outer right wing has a mechanically fastened CFRP cover and metal leading edge with a joint at the forward spar.

A new wingbox interface connects the "old" and new sections of each wing, the latter very obviously identified by reduced 20-degree sweepback; specialist FTI is mounted in the cabin.

NLF wings attachment and modification of MSN001's tailfin to contain an infrared-camera pod (to relay wing behavior) represent "a combination of experience and [a] technology bridge," according to Kierbel. The pod also holds a pair of cone-flow cameras and anchors an anenometric airspeed/altitude-calibration "trailing cone."

Developing the A340BLADE demonstrator involved "21 European partners, 500 contributors, [and] 6,500 parts," according to Airbus. It also required production of 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 FT equipment; and digital mock-up of the new wing section outboard of the No 1 (left outer) engine.

The A340BLADE exercise has "no link to any possible future aircraft program," Airbus said.