MICHAEL PHELAN / LONDON
A US Air Force Research Laboratory (AFRL) project to study the next generation of unmanned military surveillance aircraft is examining the suitability of high-aspect ratio (AR) joined-wing designs for long-endurance loiter missions. Participants in the Hilda project hope that the novel shape of the aircraft could result in 20-30% weight savings over conventional designs.
The requirements of a potential Northrop Grumman RQ-4A Global Hawk replacement unmanned air vehicle would include a 5,500km (3,000nm) transit range to and from the surveillance area at Mach 0.6 and a 40-60h loiter above the target area at M0.25 at 60,000-70,000ft (18,300-21,350m), requiring high performance across a wide coefficient of lift (CL)/altitude range.
The AFRL study includes the provision of large antenna apertures for lower frequency radar capability for foliage penetration, and the joined-wing configuration would enable the incorporation of such devices. The diamond shape also offers good stealth performance.
Bristol, UK-based Nangia Aero Research (NAR) is helping to evaluate the best wing shape designs to ensure laminar flow and efficient tip effects. The configuration being investigated by NAR is a 61m-span diamond planform aircraft, with either one or two fuselages and high AR laminar flow wings. The company is focusing on evaluating wingtip designs and sustaining laminar flow at high CL.
"Laminar flow delivers 25% more loiter time. With forward-swept tips we can preserve laminar flow up to a CL of 1.6, compared with 1.4/1.5 for unswept tips," NAR consulting engineer Raj Nangia told the Royal Aeronautical Society's Aerodynamics Research Conference last week. "Forward-swept tips require three or four times less aileron deflection to control sideslip," he added.
The Hilda project aims to establish limits of aerodynamic performance and structural weight-saving opportunities before recommending the best configuration. Shape-changing materials and flow control techniques are also being evaluated.
Source: Flight International
