In support of the Aeronautical Systems Center's (ASC) Global Hawk Systems Group, AFRL has undertaken a program to study manufacturing tolerances for laminar flow on aircraft wings. On the drawing board, air vehicle designs have perfectly smooth aerodynamic surfaces, yet it is nearly impossible for manufacturers to fabricate those surfaces without some imperfections. Any surface imperfection, no matter how slight, can affect the properties of the boundary layer— the air flowing nearest an air vehicle's body during flight. In turn, this airflow dramatically impacts the amount of drag an air vehicle experiences. When the boundary layer is smooth, or laminar, drag is minimal; as the boundary layer becomes more turbulent, drag increases. Nonetheless, decreasing the amount of surface imperfection is not always a practical solution, because as the manufacturing processes become more stringent, they also become increasingly expensive and time-consuming endeavors. It is therefore vitally important to determine the relationship between the height, shape, and location of surface imperfections and the resulting performance degradation.
Studying the effects of surface imperfections on aerodynamic performance is not new. During the 1940s and 1950s, Mr. A. Fage and Mr. A. Smith, among other notable researchers, conducted extensive research into the effect of surface imperfections (e.g., bumps, steps, and waves) on a boundary layer's transition from laminar to turbulent flow. Although this early work was very general in its scope, its results continue to set the standard for determining today's airframe manufacturing tolerances. However, this initial research did not accurately determine the effect of an aerodynamic surface's pressure gradient on the occurrence of turbulent flow. The pressure gradient constitutes the change in air pressure from a surface's leading edge to its trailing edge. In their studies, Mssrs. Fage and Smith incorrectly concluded that pressure gradients did not affect the corresponding tolerances.
The aerodynamic surfaces of today's air vehicles, such as the Global Hawk, incorporate quite a bit of curvature. This curvature results in a favorable pressure gradient, wherein upstream air pressure is lower than downstream air pressure. AFRL and ASC engineers suspected the Global Hawk's favorable pressure gradient would offset a certain amount of performance loss and delay the onset of turbulence caused by manufacturing defects. If the engineers could prove their supposition correct, the manufacturer would essentially be able to relax surface defect tolerances without adversely impacting Global Hawk performance. To investigate this question, AFRL partnered with Northrop Grumman and Washington State University (WSU) to conduct a series of wind tunnel tests designed to determine the effect of various surface imperfections on the airflow over moderate gradient aerodynamic surfaces. They conducted their tests at WSU's Contractionless Boundary Layer Wind Tunnel. This wind tunnel is one of only a few US test facilities capable of producing airflow that is almost completely absent of free-stream turbulence—a necessity for this type of testing.
To conduct the study, the 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, incorporating skin friction measurement instruments to indicate laminar flow (see figure). They used a second plate mounted above the test specimen to adjust the pressure gradient on the specimen to positive, neutral, or negative. The entire study required more than 400 test runs to adequately examine the many variables.
After analyzing the test data, the engineers were able to quantify the effects of surface imperfection height, shape, and location, as well as the impact of pressure gradient on the airflow's transition from laminar to turbulent. Results indicate that a favorable pressure gradient does indeed mitigate some of the negative effects of surface imperfections. For example, the requirements derived from the earlier studies allow manufacturing imperfections with a height of .008 in. within the first 10% of a wing's chord length. However, as a result of the studies conducted at WSU, AFRL's new recommendations permit imperfections of heights up to .018 in. within the first 10% of the wing's chord length, provided the wing configuration generates a favorable pressure gradient. Although both the earlier and the more recent measurements represent very small increments, it is significant that the newer data allows surface imperfections over twice the size formerly permitted, with no expected decrease in aerodynamic performance. The widespread adoption of these new standards could thus provide substantial manufacturing savings.
AFRL engineers would now like to expand these test results by conducting a more extensive examination of a favorable pressure gradient's influence on flow parameters. The tests performed at WSU verified engineering predictions for a limited range of conditions; the wind tunnel was too small to examine the entire flight envelope experienced by today's air vehicles. Typically, aircraft such as the Global Hawk and the SensorCraft concept vehicle have Reynolds numbers in the range of 4-10 million. Engineers use the dimensionless Reynolds number to determine whether a flow will be laminar or turbulent, because this number indicates the length scales involved in the flow's perturbations. In the tests conducted to date, engineers have reached Reynolds numbers up to 700,000, but to fully understand the relationship between pressure gradient and resulting turbulent flow characteristics, they must collect data over a larger Reynolds number range. Nevertheless, these recent tests successfully demonstrate the potential for dramatically relaxing manufacturing tolerances with no loss in performance. This information may prove beneficial to current air vehicles, and it will definitely impact the design and manufacture of future air vehicles such as the SensorCraft.
Ms. Melissa Withrow (Azimuth Corporation), formerly of the Air Force Research Laboratory's Air Vehicles Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document VA-H-06-03.