Interested in aerodynamics? Make Natural Laminar Flow your weekend reading - Tim Robinson

TIM ROBINSON reports from Tarbes, France, as Europe's most ambitious aerospace demonstrator gets set to take on the final frontier of aerodynamics – natural laminar flow.

Almost ten years after the start of the project, a European flying lab will take-off later this month to explore and research the Holy Grail of aerodynamics – natural laminar flow (NLF).

The Clean Sky 2 BLADE (Breakthrough Laminar Aircraft Demonstrator in Europe) demonstrator, was rolled out of its hangar in Tarbes in the south of France at the start of September. BLADE is a highly-modified Airbus A340 airliner and the culmination of nearly ten years of research, planning and ground tests by Airbus and its industrial and academic partners on this pan-European aerospace project. With its outer wings removed and replaced by two new panels packed with sensors, BLADE is set to study laminar flow in flight to a level of detail never seen before. 

If successful, its flight tests could lead to the way to a step-change in NLF aerodynamics for civil airliners with up to 8% drag reduction for a short-range airliner. This would translate into 5% block fuel burn saving on a typical 800nm single-aisle mission. Given that millions are invested each year to improve engine fuel efficiency by an average of 1%, exploiting NLF represents a tantalising goal for the industry in helping to meeting the challeng environmental targets set by Europe’s ACARE 2020.      

Go with the flow

Airbus A340 MSN001 has had major surgery to turn it into an laminar flow demonstrator. (Airbus)

Advances by civil aircraft manufacturers since the dawn of the jet age are sometimes taken for granted. Notes Axel Flaig, Head of R&T, Airbus, which is the leader on this project: “Aviation has achieved lot in past 50 years with more than a 70% reduction in fuel flow, and noise down by 90%.” These advances have, however, been mostly driven by engine manufacturers, developing, cleaner, quieter, fuel-sipping turbofans. With aero engine development (at least for conventional kerosene-powered turbofans) now plateauing in terms of efficiency: “The next big step is to look at friction" says Flaig. The ball is now in the airframers’ court. Flaig says that BLADE’s goal is to reduce wing friction drag by a whopping 50%. 

Harnessing natural laminar flow, of course, is not a new idea and has been around for decades, with perhaps one of the first attempts being the North American P-51 Mustang. Simply put, a laminar flow wing, with a different side-on profile and extremely smooth surface, aims to delay the separation of the air as it races across the top of the wing - greatly reducing drag. The longer the flow over the wing can be smoothly laminar, 'sticking' to the wings surface, as opposed to turbulent, the better.

”It is the most advanced flight test installation we have ever done at Airbus"

Though the basics are well understood, converting NLF from laboratory or wind-tunnel tests into a practical wing able to be built and survive the rigours of everyday airline operations has proved frustratingly elusive. NLF over a wing in the air exists in a very fickle state and can separate into turbulent flow by a whole range of microscopic flaws or tiny movements, including wing contamination (ice, grease, insects, dents) deformation (wing flexing, joints), loose fastener heads and even acoustic disturbances and vibrations.

These demanding requirements for an ultra-smooth surface and stiffer wing also mean that new design and manufacturing techniques are needed to first, create a wing that is super slippery, and second, build one that can be mass-produced, can be easily maintained and retains its NLF qualities in regular airline operations.

These challenging requirements for an ‘industrial wing’ to be used in everyday service (as opposed to a highly prepared wing in wind-tunnel testing in controlled conditions) explains why it is only now, with today’s sophisticated CFD flow simulation and advances in precision manufacturing, an optimised industrial-quality natural laminar flow wing is now almost within aerospace’s grasp. With BLADE, Airbus is aiming for laminar flow from the leading edge to 50% chord.   

Enter BLADE

BLADE is a Clean Sky project, with the A340 part of Airbus's new in-house FlightLab technology demonstrator arm. (Airbus)

To test this, Airbus and its partners will use a heavily modified four-engine A340 airliner (in reality MSN001, the first A340 prototype). This has had its outer wings replaced by two reduced sweep (20 deg) laminar flow outer wings, that are designed to be two-thirds the size of a single-aisle airliner wings. Each of the outer NLF wing sections are supplied by two different manufacturers (GKN Aerospace and Saab) with each featuring a different construction and assembly. GKN's wing, on the starboard, features a more conventional architecture with a metal leading edge and composite cover. Saab’s wing on the port wing, on the other hand, features a one-piece composite surface, with integrated rib feets and spar cap.

In addition, the wing sections are separated and bookended by aerodynamic pods that both keep the NLF airflow separate from the inner wing section and are also packed with advanced sensors (see below).

The A340 BLADE also features a pod on the vertical tail that houses thermal sensors looking down onto the wings. This pod is a more elegant and easier location than an earlier concept which would have seen a sensor pod mounted on struts above the centre fuselage.       

But if the wings being tested are for a single-aisle airliner, why use an A340? Using a large four-engine design like this offers some advantages says Airbus. For one, the A340 has a natural junction outboard of the outer engines where a new wing can be installed. Secondly, replacing part of a larger aircraft's wing in an inflight flying lab, will give more predictive and better safety margins than if, for example, a smaller entire wing was swapped. Should the results prove unexpected in some way, they aircraft will still have most of its wing producing lift. It is worth noting that a laminar flow wing changes the handling qualities and performance (stall speeds etc) and thus adding a wing to a larger airliner provides an additional safety margin. Finally, replacing the outer wings on an existing larger airliner, is a cheaper solution than building entirely new wings for a smaller airliner. An A320 NLF demonstrator, for example, would also need new nacelles and pylons integrated – a far more complex and expensive project.  

A flood of data

No wool tufts measuring airflow here - the end pod will use HD cameras to detect laminar flow.

While Airbus test aircraft are no stranger to advanced sensors  - the measurements being taken by the A340 BLADE test aircraft will break new ground in accuracy with 87,000 parameters measured by some 2,700 sensors. Indeed, some measure of how precise measurements of NLF and laminar/turbulent separation need to be can be gained in that lasers were rejected as not being accurate enough. “It is the most advanced flight test installation we have ever done at Airbus” says Daniel Keirbel, the BLADE Project Leader since 2015.    

To study NLF and measure laminar/turbulent transition on the new wing test sections, thermal cameras in a pod installed on the vertical tail will measure tiny differences in temperature that show where the cooler laminar-flow air breaks down into a hotter turbulent flow.

But this is only half the story. To cross check this laminar flow, HD cameras in the wing pods will use reflectology to watch a zebra-like test pattern reflected in the mirror-like wings surfaces looking to see if separation is being caused by tiny flexes or deformation in the wing's surface. Eight cameras will be able to see disturbances on the skin as small as 20 microns, or almost a quarter of the width of a human hair. Meanwhile, spread out underneath the wings skin are waviness sensors which, (like the inside of Yale locks) can measure whether the wing itself is distorting in tiny imperceptible ways. 

Other sensors include 1,200 pressure sensors, accelerometers and the aircraft itself is expected to generate a staggering 4Tb of data each flight, the majority (75%) of this from the reflectology HD cameras that will scan for tiny changes in the aircraft’s skin.

Microphones are also fitted to measure acoustic disturbances and the A340 BLADE is also fitted with an extremely unusual piece of equipment for a test aircraft – a loudspeaker. Such is the sensitivity of NLR that turbulent separation can be triggered by acoustics. Therefore Airbus will test to see what the effect of noisy engines are on laminar flow with a series of pure tones. (Although one Flight Test Engineer did jokingly express to aviation reporters a preference to blast out AC/DC at the wings).

The road to Cleaner Skies

BLADE A340 inside low-speed wind-tunnel (Clean Sky)

It has been a long and winding road to get here. The BLADE technology project, as part of the EU’s Clean Sky's Smart Fixed-Wing Aircraft Integrated Technology Demonstrator (SFW-ITD) has involved a large number of industrial and academic partners from around Europe, from Spain to Sweden, and from Romania to the UK. All told, BLADE features 21 partners from across Europe including Airbus, Dassault, Saab, Safran, Aernnova, GKN Aerospace, Romaero, EURECAT, as well as SMEs such as Sertc, Asco, Aritx, FTI-Engineering and research bodies like DLR, NLR and ONERA. 

Launched in 2008, BLADE saw a concept freeze review in 2010, maturity reviews of the wings between 2010 and 2014. Manufacturing of the upper covers began in 2015, while the existing wings were removed and new ones joined in 2016. This year saw aircraft power on in February and flight test instrumentation integration completed in August.

The extensive preparation has also included CFD testing, low-speed wind-tunnel testing as well developing the flight test anti-contamination device.

Flight test

Flight Test Engineers station. BLADE will generate up to 4TB of data each flight.

With BLADE now handed over to the flight test team, the aircraft is set to fly for the first time in the second week of September. The flight test campaign will consist of two phases, in 2017 and in 2018, with the goal of flying around 120-150 hours. The first phase in Q4 of 2017 will see aircraft handling qualities assessed, the flight envelope opened out to obtain initial results from the NLF wing sections. 

Stalls will be tested as the modification to a new aerofoil section will change the aircraft's handling necessitating a careful approach. To incorporate the NLR sections, the existing wings have also had their slats disabled, making for a higher landing speed. As one Airbus source noted, it will be a 'bizarre aircraft to fly' with wings that are two-thirds 1980's technology and one-third, the latest cutting-edge. optimised laminar flow design.

Interestingly, the reflectology measuring sensors means that there are limitations due to the angle of the Sun, with a four-hour sweet spot either side of the zenith where the miniscule deformations in the skin can be observed by the cameras. For this reason, from around late October onward, the aircraft will have to curtail flight testing (or move south) during winter as the Sun will not be at the correct angle.

Project engineers will start slowly, with the goal of flying at the sweet spot for NLF – Mach 0.75. Then the speed will be increased to 0.79 and above, with the goal of measuring in detail never-before recorded when separation happens on a full-size wing in flight.

Sharp-eyed observers will notice that the wing sweep of the outer test NLF wings are around 20degs, compared with 30degs for the rest of the swept wing. The reason highlights a tricky compromise for future airliner designers – optimise a wing with (shallower sweep) that maintains laminar flow at around Mach 0.75 and save fuel, or fly faster at the usual cruising speed of Mach 0.82-0.85 and see boundary layer separation as the aircraft goes faster. One solution might be hybrid laminar-flow, either passive or active, to 'suck' the airflow to the wing and keep the drag down at higher speeds. A slower, but more fuel efficient, straighter wing thus means that the first applications could be in narrowbody airliners on short-haul routes, where slower cruise speeds may not make too much of a difference compared to long-haul.

A second phase of flight testing, in 2018, will test the robustness of laminarity in representative operational conditions.  This will include artificially introducing contaminants such as grease, or finding concentrations of insects to dirty the wing, with the goal of seeing when NLF breaks down A 2m fixed Krueger flap section will also be tested to act as a possible anti-contamination device that could be a solution for protecting the wing from insects on take-off and landing in regular operations. A peel-off paper strip, activated by pilots using a cable after take-off will protect the leading edge from dead bugs during flight tests. 

BLADE also includes aspects of maintainability – with Airbus drawing on its in-house maintenance experts to judge whether a contaminated wing can be recognised by eyesight alone and to help define what airline maintenance requirements might be.

Ease of maintainability, will thus be a crucial factor in selling an airliner with NLF to airline customers in the future. Oddly, one unexpected spin-off of Lockheed Martin F-35 in the future will be a global pool of military maintenance technicians with experience of looking after an aircraft with unique coatings and a need to keep the surface extremely smooth. Some of these will naturally migrate to the civil sector and therefore there may be a body of experience well versed already in some of the requirements for supporting NLF-winged airliners  – when it finally enters the market. 

Clipped wings?

The NLF wing is optimised to be extremely smooth for laminar flow, but has also been designed to be industrially feasible.  

Readers from the UK may note the irony that just as wing design, testing and development is set to enter a new era of aerodynamics, Britain is set to exit the EU in two years time, potentially hampering a field in which otherwise it would be considered a natural leader in commercialising and productionising this technology. Airbus in the UK, with its Wing of the Future project, is still set to play a central role and, indeed, should BLADE prove successful, then it is highly likely this technology could find its way onto Airbus' next generation single-aisle product. However, as an EU project, it is unclear what access UK companies and academic institutions may have in the follow-on Clean Sky 3. Some companies have already begun highlighting their European facilities and factories and stressing that they should be considered European, rather than UK entities. 

Summary

BLADE is Europe's most advanced aerodynamics demonstrator. (Airbus)

The A340 BLADE flight tests, on Europe’s largest ever flight test demonstrator are thus set to be a landmark in aerodynamics research – as the first industrial transonic laminar flow flight test. The sheer amount and granularity of data expected from this testbed should prove invaluable in helping engineers design ultra-efficient wings and airframes for the next generation of airliners. This sort of research, with a large flying demonstrator or 'X-plane', will also help keep Airbus and the European aerospace sector ahead of rivals, just as the duopoly faces new entrants keen to muscle in on Toulouse and Seattle's turf.

Another spin-off benefit is that having developed this flying, instrumented testbed, it could form the basis to explore laminar flow even further, for example, with active or passive hybrid flow solutions, or perhaps even self-cleaning nanotechnology coatings in follow-on trials.   

For this BLADE runner – it is wake-up, time to fly.

Tim Robinson
8 September 2017