Aileron buzz investigated on several generic NASP wing configurations

Abstract

Transonic aileron buzz characteristics are presented for several generic National Aerospace Plane (NASP) wing models with full-span ailerons. Flutter characteristics, in addition to buzz characteristics, are also presented for one of these models. The objective of this study was to observe how variations in wing geometry and aileron hinge stiffness would affect the occurrence or behavior of buzz. The effects of minimizing airflow through the gap at the aileron hinge was also investigated on two of these models. All tests were conducted in NASA Langley's Transonic Dynamics Tunnel (TDT) in air. The Mach number, dynamic pressure and aileron oscillation frequency when buzz (and/or flutter) was observed are provided for these wing configurations. This work was conducted in support of the NASP Technology Maturation Plan. Aileron buzz is a phenomenon that can occur with trailing edge control surfaces on an aircraft that is operating at transonic speeds and is characterized by singledegree-offreedom oscillations of the control surface. Buzz oscillations are normally high frequency and low amplitude although explosive instabilities resulting in catastrophic failure have also occurred on control surfaces. The occurrence of buzz on modern aircraft has been greatly minimized by the use of hydraulically actuated control systems. However, because the National Aerospace Plane (NASP) is weight critical, it is not known if current design practices implementing such a control system will be acceptable. Because the NASP wings may have full-span ailerons, the immediate objective of this study was to determine buzz characteristics for this type of control surface on generic NASP wing models. This study is also a part of an ongoing investigation to better understand the physical mechanisms associated with buzz. This understanding would aid in the development of future analytical methods and vehicle design guidelines. Early investigations into aileron buzz suggest that the occurrence of buzz is shock related.lS2 In these studies, buzz occurred in the presence of an oscillating shock which was visible on the upper and lower surface of the wing. It was also found that the observed shock oscillations were synchronized, but not in phase, with the motion of the * Member AIAA control surface. This phase lag between the shock wave motion and the motion of the control surface is believed to play a significant role in at least one of the mechanisms producing control surface buzz. In a report3 published by Lambourne in 1960, the shock wave activity associated with aileron buzz was captured on high speed film using the schlieren method of flow visualization. Several alternate frames from this film sequence show the shock waves, boundary layers, and corresponding aileron deflections during a buzz cycle (figure 1). The photographs indicate that during the buzz oscillation, the upper and lower shocks executed a fore and aft motion in opposite directions of each other. According to Lambourne, each shock also had a phase displacement with respect to the flap. Prior to buzz, the shock waves were arranged symmetrically on the upper and lower surfaces of the airfoil. Lambourne suggests that the dynamic instability arises from a fluctuation in boundary layer strength (alternations between attached and separated flow) induced by the shock. Experimental evidence supporting this can be found in reports summarizing the results of wind tunnel tests conducted on YP-80A ailerons.4~5 Figure 1. Aileron buzz cycle for a symmemc airfoil Thls paper is declared a work of the U. S. Government and is not subject ot copyright protection in the United States. 1702 Another article by Lambourne worth noting is found in the AGARD Manual on ~ e r o e l a s t i c i t ~ . ~ The author suggests that control surface buzz can be associated with three different types of flow. The first involves a shockboundary layer interaction ahead of the control surface hin e which results in subsonic flow aft of the control hinpe. The second involves shock waves on the control surface between the hinge line and trailing edge. This results in a mixed supersonic and subsonic flow over the control surface. The third involves supersonic flow over the entire control surface with the shock waves attached to the trailing edge. The purpose of this paper is to present experimental buzz results obtained by testing several highly swept deltawing models (representative of NASP) equipped with fullspan, trailing edge control surfaces. Also the effects of variations in wing geometry and aileron hinge stiffness on buzz characteristics are discussed. Finally, the effects of minimizing airflow through the gap at the aileron hinge are presented. The Transonic Dynamics Tunnel (TDT) is a continuous flow tunnel with a slotted test section. The tunnel test section is 16 feet square with cropped comers. Either air or a heavy gas (R-12) can be used as a test medium. The present tests were conducted in air. The TDT operates to a maximum Mach number (M) of approximately 1.2 and at pressures ranging from a near vacuum to atmospheric. The tunnel operating envelope for air is presented in figure 2. The tunnel is equipped with quick-opening bypass valves which can be activated to reduce test section dynamic pressure (q) and speed rapidly when an instability occurs. These capabilities make the TDT ideally suited for buzz or flutter testing.