Wolf-Ernst Reif, a German researcher researching sharkskin fossils in the early 1970s, discovered microscopic longitudinal grooves using newly accessible scanning electron microscopes. He later discovered identical microstructures in the skins of modern fast-swimming sharks. When Reif spoke with Dietrich Wolfgang Bechert, a turbulence scientist at the German aerospace center DLR, they understood the purpose of the microstructures, now known as riblets: they reduce friction drag by limiting contact between vortices in the turbulent boundary layer to the sharp tips of the micron-high grooves.
Fast forward to November 2019. After testing it for three years, Lufthansa Technik (LHT) received the first supplemental type certificate (STC) to install an adhesive film that mimicked Sharkskin Fossils on the Boeing 747 400. MicroTau, an Australian firm, began testing a competing riblet film two years later.
As more aircraft types receive STCs, they may appreciate a 1% or higher increase in fuel efficiency. Engine upgrades are typically used to provide such advances during the service life of an aircraft. Many operators must retain older aircraft in operation longer due to new-build delivery delays, and they may welcome a new option to reduce their fuel costs.
LHT’s AeroShark is now in service on 27 aircraft, the majority of which are Boeing 777-300ERs and 777Fs, with four 777-200ERs and one 747-400. The film is made up of prism-shaped 50-micron riblets. Place the riblets at different angles to follow the airflow to install the adhesive film. AeroShark, which is developed for the cruising phase, is more suited to long-haul flights than short hops, according to LHT engineers.
The clip begins at Door 1 and spans the entire length of the airplane. The film covers three-quarters of the fuselage and the nacelles from the front, leaving the crown region unobstructed. According to Jens-Uwe Mueller, Aeroshark’s product sales lead, the film on a 777-300ER covers more than 900 m2 (9,700 ft.2), with 50 m2 for each nacelle.
Mueller explains that the crown is tougher. “Because of the aircraft’s slightly nose-up attitude in cruise flight, the film would have less influence in the area,” he elaborates. Furthermore, we would have to guarantee that the coating does not harm the stabilizer’s airflow. And it is significantly more challenging to place on the crown.” Mueller intends to expand the film’s coverage from 40% to 80% of the 777’s surface.
AeroShark adds around 150 kilograms (330 lb) to the 777. LHT estimates it saves 1% of gasoline on a 10-hour flight. According to Mueller, the return on investment takes 2-3 years, depending on aircraft utilization, fuel prices, and installation costs, which vary with labor expenses at the modification facility.
LHT is currently designing a variant for the Airbus A330. According to Mueller, the A321XLR could potentially be an interesting target for future development, as an AeroShark alteration could provide more range.
Since the product’s introduction, the company has received over 50 orders, with the 27th and 28th being modified. According to an LHT spokeswoman, the first installation has been in operation for nearly six years and has shown little to no wear and tear.
Meanwhile, MicroTau is attempting to get STCs for its Riblet Modification Package for commercial aircraft, starting with the Airbus A320 and Boeing 767. The business hopes to conduct flight testing and certification on the A320 in collaboration with Jetstar Airways within the next year, according to founder and CEO Henry Bilinsky. The 767 will fly with riblets next year, and MicroTau has worked with Delta Air Lines to get FAA approval.
MicroTau tested its package on a US Air Force Lockheed Martin C-130J airlifter in December, thanks to support from the Pentagon’s Defense Innovation Unit. In 2021, the first trials with a riblet film applied to the wing of a Zivko Edge 540 aerobatic aircraft revealed a speed increase equivalent to a 1-2% reduction in drag across the flight profile. MicroTau plans to reduce fuel consumption by up to 4% on commercial airplanes.
According to Bilinsky, MicroTau’s manufacturing technology enables manufacturing more complicated riblet designs. The company can create 3D riblets with constantly changing cross-sections or print numerous concurrent riblet geometries on a single film substrate, allowing it to conform to local airflow streamlines.
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