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CFD and Transonic Airfoils
The analysis of transonic flows suffers from the same problems as those for the supersonic blunt body discussed above. Just considering the flow to be inviscid, the governing Euler equations are highly nonlinear for both transonic and hypersonic flows. From the numerical point of view, both flow fields are mixed regions of locally subsonic and supersonic flows. Thus, the numerical solution of transonic flows originally encountered the same problem as that for the supersonic blunt body problem: whatever worked in the subsonic region did not work in the supersonic region, and vice versa. Ultimately, this problem was solved from two points of view. Historically, the first truly successful CFD solution for the inviscid transonic flow over an airfoil was carried out in
1971 by Earll Murman and Julian Cole of Boeing Scientific Research Laboratories, whose collaborative research began at the urging of Arnold "Bud” Goldburg, then Chief Scientist of Boeing.[776] They treated a simplified version of the Euler equations called the small-perturbation velocity potential equation. This limited their solutions to the flows over thin airfoils at small angles of attack. Nevertheless, Murman and Cole introduced the concept of writing the finite differences in the equations such that they reached in both the upstream and downstream directions when in the subsonic region, but they reached in only the upwind direction in the supersonic regions. This is motivated by the physical process that in subsonic flow disturbances propagate in all directions but in a supersonic flow disturbances propagate only in the downstream direction. Thus it is proper to form the finite differences in the supersonic region such that they take only information from the upstream side of the grid point.
Today, this approach in modern CFD is called "upwinding” and is part of many modern algorithms in use for all kinds of flows. In 1971, this idea was groundbreaking, and it allowed Murman and Cole to obtain the first successful numerical solutions of the transonic flow over a body. In addition to the restriction of thin airfoils at small angles of attack, however, their use of the small perturbation velocity potential equation also limited their solutions to isentropic flows. This meant that, although their solution captured the semblance of a shock wave in the flow, the location and flow changes across a shock wave were not accurate. Because many transonic flows involve shock waves embedded in the flow, this was definitely a bit of a problem. The solution to this problem involved the numerical treatment of the Euler equations, which, as we have discussed early in this article, accurately pertain to any inviscid flow, not just one with small perturbations and free of shocks.
The finest in such CFD solutions were developed by Antony Jameson, then a professor at Princeton University (and now at Stanford), whose work was heavily sponsored by the NASA Langley Research Laboratory. Using the concept of time marching in combination with a Runge-Kutta time integration of the unsteady equations, Jameson constructed a series of outstanding transonic airfoil codes under the general code name of the FLO codes. These codes entered standard use in many aircraft companies and laboratories. Once again, NASA had been responsible for a
major advancement in CFD, helping to develop transonic flow codes that advanced the design of many airfoil shapes used today on modern commercial jet transports.[777]
Software for Phase II was also larger and more complex than that used in Phase I because of the need for new pilot interface devices. Flight control modes still included the direct (DIR) mode, the stability augmentation (SAS) mode, and the control-augmentation (CAS) mode. A pitch maneuver-load-control feature was added to the CAS mode, and a digital autopilot was fitted that incorporated Mach hold, altitude-hold,
The backup analog flight control system was also redesigned for Phase II, and the secondary actuators were upgraded. Sperry supplied an updated version of the Phase I Backup Control System using the same technology that had been used in the Air Force’s YF-4E project. Signals from the analog computers were now force-summed when they reached the actuators, resulting in a quicker response. The redesigned secondary actuators provided 20 percent more force, and they were also more reliable. The hydraulic actuators used in Phase I had two sources of hydraulic pressure for the actuators; in those chosen for Phase II, there were three hydraulic sources that corresponded with the three channels in each of the primary and secondary flight control systems. The secondary electronic actuators had three channels, with one dedicated to each computer in the primary system. The actuators were shared by the analog computer bypass system in the event of failure of the primary digital system.
Flying simulated Shuttle landing profiles with the DFBW F-8, the pilots gathered extremely valuable data that supported the Shuttle program in establishing sampling rates and control law execution limits. The DFBW F-8 flight control software had been modified to enable the pilot to vary transport delay times to evaluate their effect on control response. Transport delay is the elapsed time between pilot movement of his cockpit control and the actual movement of the flight control surfaces. It is a function of several factors, including the time needed to do analog-to-digital conversion, the time required to execute the appropriate flight control law, length of the electrical wires to the actuators, and the lag in response of the hydraulic system. If transport delay is too long, the pilot may direct additional control surface movement while his initial commands are in the process of being executed by the flight control system. This can result in overcontrol. Subsequent attempts to correct the overshoot can lead to a series of alternating overshoots or oscillations that are commonly referred to as a PIO. The range of transport delay times within which the Shuttle would be unlikely to encounter a PIO was determined using the DFBW F-8, enabling Dryden to develop a PIO suppression filter for the Shuttle. The PIO suppression filter was
During Phase II, 169 flights were accomplished with several other test pilots joining the program, including Stephen D. Ishmael, Rogers Smith, and Edward Schneider. In addition to its previously noted accomplishments, the DFBW F-8 successfully evaluated adaptive control law approaches that would later become standard in many FBW aircraft. It was used in the Optimum Trajectory Research Experiment (OPTRE). This involved testing data uplink and downlink between the F-8 and a computer in the then-new Remotely Piloted Vehicle Facility. This experiment demonstrated that an aircraft equipped with a digital flight control system could be flown using control laws that were operating in ground-based digital computers. The F-8 conducted the first in-flight evaluations of an automatic angle-of-attack limiter and maneuvering flaps. These features are now commonly used on nearly all military and commercial aircraft with fly-by-wire flight controls. The DFBW F-8 also successfully tested an approach that used a backup software system known as the Resident Backup System (REBUS) to survive potential software faults that could cause all three primary flight control system computers to fail. The REBUS concept was later used in other experimental aircraft, as well as in production fly-by-wire flight control systems. The final flight-test effort of the DFBW program involved the development of a methodology called analytical redundancy management. In this concept, dynamic and kinematic relationships between dissimilar sensors and measurements were used to detect and isolate sensor failures.[1164]
Ensuring proper aircraft propulsion has been a powerful stimulus. In the interwar years, the NACA researched propellers, fuels, engine cooling, supercharging, and nacelle and cowling design. In the postwar years, the Agency refined gas turbine propulsion technology. NASA now leads research in advancing environmentally friendly and fuel-conserving propulsion, thanks to the Agency’s strengths in aerodynamic and thermodynamic analysis, composite structures, and other areas.
As the jet engine made its way from Europe to the United States and designs for the basic turboprop, turbojet, and turbofan were refined, the NACA during the early 1950s began one of the earliest noise-reduction programs, installing multitube nozzles of increasing complexity at the back of the engines to, in effect, act as mufflers. These engines were tested in a wind tunnel at Langley Research Center in Hampton, VA. But the effort was not effective enough to prevent a growing public sentiment that commercial jet airliners should be seen and not heard.
NASA started a project in the mid-1990s known as the Advanced Subsonic Technology program. Like HSR before it, the AST focused heavily on reducing emissions through new combustor technology. The overall objective of the AST was to spur technology innovation to ensure U. S. leadership in developing civil transport aircraft. That meant lowering NOx emissions, which not only raised concern in local airport communities but also by this time had become a global concern because of potential damage to the ozone layer. The AST sought to spur the development of new low-emissions combustors that could achieve at least a 50-percent reduction in NOx from 1996 International Civil Aviation Organization standards. The AST program also sought to develop techniques that would better measure how NOx impacts the environment.[1417]
Despite their relative success, however, NASA’s efforts to improve engine efficiency and reduce emissions began to face budget cuts in 2000. Funding for NASA’s Atmospheric Effects of Aviation project, which was the only Government program to assess the effects of aircraft emissions at cruise altitudes on climate change, was canceled in 2000.[1420] Investments in the AST and the HSR also came to an end. However, NASA did manage to salvage parts of the AST aimed at reducing emissions by rolling those projects into the new Ultra Efficient Engine Technology program in 2000.[1421]
Under UEET, NASA worked on ceramic matrix composite (CMC) combustor liners and other engine parts that can withstand the high temperatures required to maximize energy efficiency and reduce carbon emissions while also lowering NOx emissions. These engine parts, particularly combustor liners, would need to endure the high temperatures at which engines operate most efficiently without the benefit of cooling air. Cooling air, which is normally used to cool the hottest parts of an engine, is unacceptable in an engine designed to minimize NOx, because it would create stoichiometric fuel-air mixtures—meaning the number of fuel and air molecules would be optimized so the gases would be at their hottest point—thereby producing high levels of NOx in regions close to the combustor liner.[1427]
Another area of NASA involvement in the development and use of alternative energy was work on solar propulsion for High-Altitude Long – Endurance (HALE) unmanned aerial vehicles (remotely piloted vehicles). Work in this area evolved out of the Agency’s Environmental Research Aircraft and Sensor Technology (ERAST) program that started in 1994. This program, which was a joint NASA/industry effort through a Joint Sponsored Research Agreement (JSRA), was under the direction of NASA’s Dryden Flight Research Center. The primary objectives of the ERAST program were to develop and transfer advanced technology to an emerging American unmanned aerial vehicle industry, and to conduct flight demonstrations of the new technologies in controlled environments to validate the capability of UAVs to undertake operational science missions. A related and important aspect of this mission was the development, miniaturization, and integration of special purpose sensors and imaging equipment for the solar-powered aircraft. These goals were in line with both the revolutionary vehicles development aspect of NASA’s Office of Aerospace Technology aeronautics blueprint and with NASA’s Earth Science Enterprise efforts to expand scientific knowledge of the Earth system using NASA’s unique capabilities from the standpoint of space, aircraft, and onsite platforms.[1519]
The ERAST program involved the development and testing of four generations of solar-powered UAVs, including the Pathfinder, the Pathfinder Plus, the Centurion, and the Helios Prototype. Because of budget limitations, the Helios Prototype was reconfigured in what could be considered a fifth-generation test vehicle for long-endurance flying (see below). Earlier UAVs, such as the Perseus, Theseus, and Proteus, relied on gasoline-powered engines. The first solar-powered UAV was the RAPTOR/Pathfinder, also known as the High-Altitude Solar (HALSOL) aircraft, which that was originally developed by the U. S. Ballistic Missile Defense Organization (BMDO—now the Missile Defense Agency) as part of a classified Government project and subsequently turned over to NASA for the ERAST program. In addition to BMDO’s interest in having NASA take over solar vehicle development, a workshop held in Truckee, CA, in 1989 played an important role in the origin of the ERAST program.