Laminar Flow – Final Frontier of Aerodynamics

Developments into achieving laminar flow in the past have mostly suffered from a lack of knowledge of physics and difficulties involving wing smoothness. Particularly in the United States where extensive and exhaustive research has been carried out by individual manufacturers, NASA and numerous research bodies and institutions. Boeing has been working intensively on laminar flow for the last ten years and even has a 757 demonstrator flying using the laminar flow concept, yet it has been in Europe that some of the latest advances have taken place.

The European Laminar Flow Investigation, or ELFIN is one of the largest and most comprehensive research projects undertaken under the auspices of the Brite/Euram Aeronautics Programme, supported by the Commission of the European Communities. The participants in the project read like a who’s who of European aircraft manufacturers with Deutsche Airbus, Aerospatiale, Alenia, British Aerospace, CASA, Dassault Aviation, Dornier, Fokker and Saab all playing major roles. ELFIN was initiated in 1989 to improve the physical understanding of laminar flow and to develop the necessary research and experimental tools. The aim is to make laminar flow technology available for application to future commercial aircraft. The Europeans have identified that to remain at the leading edge of technology, incorporating laminar flow into future aircraft design will become extremely important because of the massive fuel savings available.

So how does it work? Air flowing across the top of a conventional aircraft wing becomes turbulent, thus creating drag once it has passed the leading edge. Reshaping the wing and increasing the smoothness of its surface allows the air to flow across the wing uniformly without turbulence. This is called laminar flow and when implemented into aircraft construction it can potentially reduce an aircraft’s total drag by as much as 15%. Such an enormous reduction in drag, which translates into a two to three percent saving in direct operating costs, is sure to bring a smile to many an airline executives face. Advances in laminar flow are not just confined to the wing either, empennage, engine nacelles and even the fuselage can all in the longer term have laminar flow technology applied to them. Also on the plus side, the dramatic reduction in fuel burn equates to a reduction in pollutive emissions released into the environment.

Two types of laminar flow have currently been developed, depending on aircraft size, speed and wing sweep angle. Natural laminar flow is designed to be incorporated into small to medium sized aircraft. It involves reducing drag by increasing the smoothness and changing the profile of the wing. Hybrid laminar flow, the second form of laminar flow involves mechanically inducing laminar flow by sucking air through perforations in the wing leading edge so that air flow over the wing can be stabilised. Hybrid laminar flow is suitable for large aircraft such as Boeing and Airbus airliners, which have enormous wing areas.

The ELFIN program focuses on four main areas. The first task involves flight testing of ahybrid laminar flow model in a wind tunnel, while the second task looks at the development of an air suction device and of testing techniques for wind tunnel and flight experiments. The third area looks at improving computer methods for laminar flow predictions. Stage four, which is currently underway involves flight testing of natural laminar flow on a modified Fokker 100 testbed.

The Fokker 100 testbed has been fitted out with a full chord, part span glove fixed on top of the Fokker’s existing wing. Extending between the two starboard flap rail fairings, the glove is made from a type of foam and has a fiberglass skin, which is fixed onto the wing structure. Precisely made and extremely smooth, the contour was produced from a highly accurate mould.

Incorporated into the testbed are a number of highly sensitive measuring devices designed to measure airflow and air pressure across the wing. Tiny holes in the surface of the wing are designed to measure surface pressure. Wake rakes, consisting of a number of small tubes fixed to a pylon running between the two flap fairings behind the wing measure the pressure of the air once it has passed over the wing. The device used is hot film, a fine layer of metal embedded in the composite substructure. Through a process of applying heat to the piece of metal, drag can be measured with a very high degree of precision. The hot film device has been flush fitted into the wing leading edge. Finally infrared cameras record the airflow over the upper wing surface while in flight.

Flight testing of the testbed began in November 1991. Initial test flights were flown to measure handling and performance of the aircraft with the glove fitted. These test flights have reportedly been very successful with laminar flow being achieved in a number of flight conditions.

However, much flight testing and laboratory research remains to be in done into laminar flow before it can be adapted to be suitable for production aircraft. Once laminar flow does reach maturation though, the perceived reduction in costs for airlines and the world aviation community will be fantastic.