Nature’s innovations can transform aviation

In the 1970s, NASA engineers discovered that microstructures they had developed, ‘riblets,’ were similar to structures found on the skin of fast-swimming sharks. Paradoxically, shark skin is rough to the touch, but it is this structured roughness that imparts its superpower: drag reduction. Sharks use these drag-reducing microstructures – in their case called ‘dermal denticles’ – to swim more efficiently, conserving their energy.

While usually a smoother surface is more aerodynamically efficient, the rough riblet surfaces work by organising turbulent flow. When a fluid is turbulent, microscopic hairpin vortices form that produce additional drag on the surface. The riblets are narrower than the diameter of the vortices, so they cannot fit between the riblets and only have contact with the tips, reducing drag as a result. Riblets have been employed successfully to reduce hydrodynamic drag of water vessels, most notably on the 1987 America’s Cup winner, Stars & Stripes. The use of riblet surfaces was then banned in the series.

But the use of drag-reducing riblets is not limited only to the water. The implication is obvious: if riblets allow sharks to swim more efficiently, then they should allow planes to fly more efficiently, too. These gains in efficiency could theoretically be ‘cashed in’ for a variety of enhancements to the capabilities of aircraft: greater speeds, increased range, longer endurance, and larger payload capacity. But there are larger consequences for the airline industry between the rock and the hard place of volatile fuel costs and the pressures of societal attitudes towards climate change: reductions in fuel cost and reductions in CO₂ emissions.

The race, then, was on. In 1989, 3M completed a full-scale flight test on an Airbus A320 by covering 80 per cent of it in riblets resulting in a 1 per cent fuel reduction. With more modern riblet designs, Lufthansa airlines did a flight test on a Boeing 747 by covering the underside of the fuselage (about 20 per cent of the surface area) and showed a 0.8 per cent fuel reduction. Modest efficiency improvements, to be sure, but applied to the global fleet this would mean billions of dollars worth of fuel saved and millions of tonnes of CO2 emissions abated. And yet, riblets are not yet seen on our interstate business jaunts nor our international long-haul flights. Why not?

Despite the compelling benefit offered by this technology, challenges inherent in manufacturing and product durability have been the downfall of previous attempts at implementation. It has been reported that even the most promising did not survive the harsh UV radiation from the sun, and other attempts at manufacturing these micro-scale features at the macro-scale required for commercial transport aircraft have proved beyond the wit of the most advanced materials science organisations. Now, an Australian start-up has developed a practical method of manufacture that can unlock the significant commercial potential of these surfaces: MicroTau.

MicroTau

Airlift aircraft account for most of the fuel burned by the US Air Force, and the Air Force is the largest consumer of fuels in the US Government. Aware of their outsized fuel consumption, in 2015 the service held a global competition for solutions to improve fuel efficiency of these aircraft. MicroTau’s founder Henry Bilinsky, then a physics and law graduate, proposed not only that the drag reduction delivered by riblets could provide the efficiency required by the Air Force, but also a novel method of producing riblets that showed promise to solve the issues plaguing previous attempts.

His patented production method, Direct Contactless Microfabrication (DCM), was selected out of 296 global applicants by a committee including NASA, the US Federal Aviation Administration, Massachusetts Institute of Technology, Boeing, Lockheed Martin, General Atomics, and Northrop Grumman. As a result, Mr. Bilinsky founded MicroTau to take this solution to market.

Today, MicroTau manufactures riblet films that are applied to the outer-mold-line of aircraft similarly to graphic decals, integrating into existing maintenance procedures. For airlines, this technology provides myriad ways to positively impact revenue; the extended range afforded by riblets can open up new routes and services not previously possible, the fuel cost savings can impact competitiveness by lowering ticket prices, or they can benefit from the positive consumer sentiment associated with real and substantial impacts on emissions reductions targets beyond simple carbon offsets. Ultimately, for airlines operating aircraft like the 787, application of MicroTau’s riblets means fuel cost savings of over $1 million annually.

Aerospace and Sustainability

The aviation industry accounts for 2 per cent of global CO₂ emissions – a paltry 915 million tons annually. Every year, the fuel that leads to these emissions costs around US$198 billion. As a result, the industry operates on razor-thin margins of 1 to 2 per cent. Airlines and operators are screaming out for greater fuel efficiencies, feeling the squeeze from both the bottom line and the environmental impact. Despite this, we don’t yet have the technology to effectively decarbonise air travel, and the industry is locked into using trillions of dollars in fuel-burning assets. Solutions are needed to both reduce their fuel burn and help accelerate the transition to a fleet powered by non-fossil fuel alternatives.

Around half of an aircraft’s fuel consumption is spent on overcoming drag–burning fuel to push an aircraft through the air. This is what makes MicroTau’s solution compelling – a riblet product applied to aircraft can make a meaningful impact on the emissions of fossil fuel-burning aircraft immediately, and continue to provide benefits for those aircraft with novel propulsion systems. Considering only the existing commercial aviation fleet, MicroTau’s riblet product could save US$20 billion of fuel and 92 million tonnes of CO2 annually.

Defence and Capability

Depending on its implementation, drag reduction technology can be employed beyond simply as a cash-saving measure. In the same way that solar powered race cars utilise aerodynamic shapes to maximise range, riblets can achieve this for existing aircraft. Reductions in drag correlate with a reduction in the thrust required to propel an aircraft, and the fuel saved can be redirected to increase range and endurance: characteristics of significant interest to air forces.  Similarly, reduced drag and a lower fuel requirement allows for improved payload. Importantly for Australian sovereign capability, airborne intel, surveillance and reconnaissance (ISR) defence capabilities have been identified as a critical area for improvement, and the ability to extend range and time-on-station of aircraft may be a critical advantage in the current and future warfighting environments. By way of example, MicroTau estimate that their shark-skin riblet film could increase the P-8A Poseidon’s loiter time on station by up to an hour within a 2,500 km radius of action, or increase the MQ-28 Ghost Bat’s ferry range by up to 260 km, potentially bringing southern Taiwan within range if operating out of Darwin.

What Next?

The opportunity extends beyond aviation – anything that interacts with a fluid, be it air, water, or other gases and fluids, can be impacted by the application of riblets. Riblets can improve the efficiency of turbines and gas pipelines and are especially impactful as a drag-reduction measure on the hulls of large ships. In commercial shipping alone MicroTau estimates US$14 billion in fuel savings every year and an additional 133 million tonnes of CO₂ reduction.

The International Air Transport Association (IATA) has set the ambitious target of reaching net zero emissions by 2050. Emissions abatement enabled by this technology is real and actual and presents a significant stepping stone to achieving this goal. With other aviation emissions innovations in development like SAFs, novel airframes, and electric and hydrogen propulsion, this goal now sits firmly within the realms of possibility. MicroTau’s headquarters sits directly under the flight path leading to Sydney International Airport’s main runway, 16 Right. While occasionally challenging during video conferences, this location does always serve to provide a reminder to the engineers and scientists working there of what they’re looking to improve and of just how impactful their work is.