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This Symposium is intended to bring together the often distinct cultures of the Stability and Control (S&C) community and the Computational Fluid Dynamics (CFD) community. The COMSAC program is itself a new effort by NASA Langley to accelerate the application of high- end CFD methodologies to the demanding job of predicting stability and control characteristics of aircraft. This talk is intended to set the stage for needing a program like COMSAC. It is not intended to give details of the program itself.
While there are many reasons to have this Symposium, a direct motivation for this event was the Flight Prediction Workshop.
This chart, by Doug Ball of Boeing Commercial, highlights the large amount of wind tunnel resources that are dedicated to determining stability and control characteristics, certification requirements, and low-speed lines. CFD has not generally penetrated these needs areas.
Impacts occur across of vehicle classes--767,F/A-18E, C130J, T-45,X-43 Stack, 777, Lear 23, AV-8B, and 737NG.
• 767--Stall for 767-400 model with raked tips more rapid than expected--vortilon pattern had to be developed • F/A-18E--wing drop at transonic speeds. Impact: program almost canceled.
• C-130J--wing drop due to propeller induced effects. Impact: delayed deliveries, increased development costs • T-45--low speed approach wing drop. Impact: redesigned wing • X-43 Stack--inaccuracies of S&C aero data base. Impact: lost research vehicle • 777--missed horizontal tail effectiveness. Impact: larger than needed horizontal • Lear 23--Laminar separation bubble breakdown leading to wing drop on approach.
Impact: safety of flight, development costs • AV8B--wing drop and wing rock. Impact on operational envelopes (considered minimal) • 737--737NG (400 to 800) sensitivity to wing rigging with unacceptable number of aircraft not passing acceptance flights. Impact: production expenses and development costs Existing tools and methods for predicting characteristics when flow is primarily attached are adequate. However, when separation becomes significant, analytical tools are inadequate and CFD methods have not been calibrated, in general.
While wind tunnel availability is decreasing, needs for aero data bases are increasing.
Computational tools will be needed to complement wind tunnel data to an increasing extent in the future.
As will be reported in this Symposium, current and emerging CFD methods offer the exciting promise of new approaches to address the S&C needs. This will be even more important as emerging flow-control concepts are brought on line.
The pyramid shows the general evolution of algorithms and computer power as a function of decade. The level V is labeled RANS+ because of the addition of methodologies such as either Large Eddy Simulation (LES) or Detached Eddy Simulation (DES). The bottom line is that there are new developments in algorithms which, when combined with increasing availability of computer resources, will enable the community to address problems that previously were untenable. The challenge now facing the CFD community is to take the latest levels of technology and begin making the sort of impacts in the stability and control arena that it has already made in the performance arena.
This list shows just a few of the many applications that have been addressed by the authors reporting during this Symposium. This is merely to communicate that a lot of work has already been done by a lot of organizations.
I would like to show one example with which I am familiar that comes from the Abrupt Wing Stall (AWS) program. This work was by Jim Forsythe and utilized a Detached Eddy Simulation (DES) implementation. The insight into the flow physics of this example changed the thinking of the S&C folks.
While there are examples of successes in applying CFD to S&C problems, it is still unclear within and outside of the CFD community that the current state-of-the-art is up to the task of predicting the very complicated, sometimes time dependent, flows associated with massively separated flows. What is clear, however, that it was appear that if separation is a large player in the flow field, it will be necessary to bring to the problem RANS or RANS+ levels of technology. This means that large resources will be required to address these problems. So ways will have to be found apply these codes with as much automation and robustness as possible. Of course, CFD credibility must be established in the S&C community by demonstrating that the codes can predict the answer before knowing it. Finally, while cultural differences are a challenge in bringing together the two disciplines, some of the reduced accuracy requirements associated with S&C may reduce some of the resource requirements.
This chart contrasts the differences between the two communities.
NASA has been involved with trips to different organizations to make sure we understood the level of technology and the needs of the communities.