AFRL Continues to Find Ways to Decrease Air Vehicle Costs
By Plans and Programs Directorate, AFRL/XP
/ Published December 12, 2006
WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- An AFRL-led team determined that an air vehicle's pressure gradient can mitigate the detrimental effect that small surface defects have on air vehicle performance. This finding may result in less restrictive manufacturing tolerances, reducing air vehicle manufacturing and maintenance expenses.
During the 1940s and 1950s, scientists conducted extensive research on the effect of surface imperfections (e.g., bumps, steps, and waves) on the air flowing nearest the air vehicle's body during flight (i.e., in the boundary layer). As the boundary layer transitions from laminar (i.e., smooth) airflow to turbulence, the drag increases, which reduces the aircraft's performance. The research results of this broad concept continue to set the standard for today's manufacturing tolerances. However, these past studies incorrectly concluded that pressure gradients did not affect manufacturing tolerances.
In support of the Global Hawk Systems Group, AFRL teamed with the Aeronautical Systems Center, Northrop Grumman, and Washington State University to study the effects of surface imperfections on the laminar airflow over an air vehicle's wing. Throughout 400+ test runs at the Washington State University Contractionless Boundary Layer Wind Tunnel, engineers fit various surface imperfections, such as forward- and rearward-facing steps, bumps, and surface waves, into a slotted, flat plate with a rounded leading edge, which represented an air vehicle's wing.
Engineers adjusted the pressure gradient (i.e., the change in air pressure from the plate's leading edge through the trailing edge) from positive to neutral and then negative. They used instruments that measured skin friction (an indicator of laminar flow) in order to gather statistics. Data analysis showed that a favorable pressure gradient has a very significant benefit that cancels some of the negative effects of surface imperfections. The new measurements allow over twice the manufacturing errors with no loss in performance characteristics. Following the new standards would offer substantial manufacturing savings without sacrificing capability.