When a fighter pilot engages an enemy at near sonic speeds, abrupt wing stall is definitely not part of his flight plan. Yet for the past 50 years, all aircraft that can operate at velocities near the speed of sound, and angles of attack near maximum lift, have experienced some form of uncommanded lateral motion – where the aircraft undergoes a one-sided or side-to-side upset from the intended direction of flight. At the very least, it causes loss of advantage. At its worst, it could result in a loss of the aircraft.
"This is not the first time this aerodynamic behavior has been seen" notes Lawrence Ash, the Office of Naval Research program manager leading a national study. "It can sometimes be fixed by modifications to the aircraft flight control system. But on the F/A-18E/F jet fighter prototypes, the motion happened so quickly and in such a narrow range of the operating envelope that the computer-driven flight control system wasn't able to compensate for it in time.
The question was: why did the F/A-18E/F jet fighters experience abrupt wing stall (AWS) when the F/A-18C/D jet fighters did not?
"It was the same question that a Secretary of Defense-commissioned Blue Ribbon Panel asked.The Blue Ribbon Panel called for a national research effort to: (1) understand the aerodynamic phenomena causing abrupt wing stall, (2) develop test and analysis methods to predict it, (3) characterize and quantify the severity of an AWS event, and (4) develop modeling and simulation procedures to assess the impact of an AWS encounter on an aircraft's flying qualities.
In response, Office of Naval Research, formed a team of scientists and engineers from NASA, the Naval Air Systems Command (NAVAIR), the Air Force Research Laboratory (AFRL), and several universities and government contractors to study the problem. The team made use of all the nation's data on aircraft – both existing and historical – capable of operating at airspeeds near and beyond the speed of sound. With support from the F/A-18, AV-8B, and F-16 program offices, and the Department of Defense High Performance Computing Modernization Office, the team conducted high-speed wind tunnel tests, performed hundreds of computational fluid dynamics calculations, and conducted both piloted and un-piloted simulations of AWS models. An AWS simulation model was developed and flown on a NAVAIR flight simulator by the Navy's F/A-18E/F chief test pilot. Both qualitative and quantitative simulation data were compared with actual flight test results.
The effort required five years. "It was a fairly aggressive research schedule," says Ash, " And ultimately we were successful."
The team successfully developed new tools and procedures for an early assessment of an aircraft's susceptibility to AWS. These include experimental, computational, simulation, and flight test figures of merit that can indicate if a new aircraft design will be vulnerable to AWS anywhere in its flight envelope. The results of their study were presented in 18 papers at the American Institute of Aeronautics and Astronautics 41st Aerospace Sciences Meeting in January 2003, and will be published in two upcoming special editions of the AIAA Journal of Aircraft.
"I'm confident," Ash says, "that future aircraft programs can make use of these modeling and simulation tools to screen their new wing designs for a susceptibility to AWS earlier in their development phase, allowing any required modifications to the aircraft to be much less expensive, and less time consuming to undertake."