Drag-reducing laminar flow is the "new" big thing in research into more environmentally friendly aircraft, and Dassault has completed a test flight to evaluate technology for measuring the laminar flow acheived in flight. The test was conducted as part of Europe's Clean Sky research program, which plans to fly an Airbus A340-300 fitted with laminar-flow wing sections.
Dassault's test evaluated an infrared camera capable of measuring temperature gradients on a wing at high altitudes and low temperatures and pressures. The camera measured differences in surface temperature between laminar and turbulent areas of the horizontal tailplane of a Falcon 7X. (Heat transfer is higher in turbulent flow than in laminar.)Photo: Dassault
The 7X was not designed for laminar flow, Dassault says, but at high altitude laminarity up to 40% of chord was predicted on the upper surface of the tailplane. To measure this, the IR camera (developed by FLIR Systems) was mounted on top of the vertical tail looking down on to the black-painted surface. The same measurement technique will be used when the A340 demonstrator flies in 2014.
The A340 will be modified under Clean Sky's Smart Fixed Wing Aircraft technology demonstration. The outer wing panels will be replaced with new sections (in green, below) with a low-drag aerofoil, smooth surfaces and reduced sweep to promote natural laminar flow. Graphic: Clean Sky
NASA, meanwhile, plans flight tests to determine whether tiny bumps on the wing surface - called distributed or discrete roughness elements (DRE) - can increase the extent of natural laminar flow. DREs modify the instabilities that cause laminar-to-turbulent transition over a swept wing, and NASA believes they can sustain natural laminar flow to 60% chord at 737-scale flight conditions. The flight tests will involve a glove (below) fitted over the left wing of a Gulfstream III.Graphic: Aviation Week
None of this work is really "new", but the focus on finding ways to sustain laminar flow in everyday operations is. Clean Sky is working to demonstrate that a tight-tolerance laminar flow wing can be manufactured and will operate as predicated. NASA is hoping DREs will allow requirements for surface smoothness and cleanliness to be relaxed, making laminar-flow wings more practical and usable.