Robert J. Englar

Development of advanced circulation control wing high-lift airfoils

Abstract : Recent experimental and flight test programs have developed and confirmed the high lift capability of the Circulation Control Wing (CCW) concept. These CCW airfoils employ tangential blowing of engine bleed air over circular or near circular trailing edges, and are capable of usable lift coefficients triple those of simple mechanical flaps. Earlier versions of these blown airfoils made use of relatively complex leading and trailing edge devices which would have to be retracted mechanically for cruise flight. In a continuing program to reduce the complexity, size and weight of the CCW system, several series of advanced CCW airfoils have been developed which can provide STOL capability for both military and commercial aircraft using much smaller, less complex high lift systems. This paper describes these configurations and presents the experimental results confirming their aerodynamic characteristics. Comparisons to previous CCW and more conventional high lift systems are provided.

Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part I: Airfoil Development

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) blown high-lift airfoils, and to address specific issues related to their application to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic airfoils to increase high-lift system performance in the terminal area while reducing system complexity. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations, 2) development of effective pneumatic leading-edge devices, 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities, and 4) investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases of this program are described in Part I of this article. Applications to the high-lift and control systems of advanced subsonic transport aircraft and resulting performance are discussed in the continuation of this article, Part II. Experimental lift coefficient values approaching 8.0 at zero incidence and low blowing rates were demonstrated by two-dimensional CCW configurations that promised minimal degradation of the airfoils performance during cruise. These results and experimental/CFD methods will be presented in greater detail in the following discussions.

Design of the Circulation Control Wing STOL Demonstrator Aircraft

Research and development have been conducted at the David W. Taylor Naval Ship Research and Development Center to develop the STOL capability of the circulation control wing concept. This simple high lift system employs tangential blowing over the wings rounded training .edge, and can more than double the lifting capability of conventional high performance aircraft. Based on the associated STOL benefits, design and flight testing of the concept on a full-scale A-6A flight demonstrator have been completed by Grumman Aerospace Corporation. The present paper addresses experimental development of the vehicle, details of the full-scale aircraft design, predicted STOL performance benefits, and some flight test results.

Computational Evaluation of the Steady and Pulsed Jet Effects on the Performance of a Circulation Control Wing Section

Circulation Control Wing (CCW) technology is a very effective way of achieving very high lift coefficients needed by aircraft during take-off and landing. This technology can also be used to directly control the flow field over the wing. Compared to a conventional high-lift system, a Circulation Control Wing (CCW) can generate the required values of lift coefficient C(sub L,max) during take-off/landing with fewer or no moving parts and much less complexity. Earlier designs of CCW configurations used airfoils with a large radius rounded trailing edge to maximize the lift benefit. However, these designs also produced very high drag. These high drag levels associated with the blunt, large radius trailing edge can be prohibitive under cruise conditions when Circulation Control is no longer necessary. To overcome this difficulty, an advanced CCW section, i.e., a circulation hinged flap was developed to replace the original rounded trailing edge CC airfoil. This concept developed by Englar is shown. The upper surface of the CCW flap is a large-radius arc surface, but the lower surface of the flap is flat. The flap could be deflected from 0 degrees to 90 degrees. When an aircraft takes-off or lands, the flap is deflected as in a conventional high lift system. Then this large radius on the upper surface produces a large jet turning angle, leading to high lift. When the aircraft is in cruise, the flap is retracted and a conventional sharp trailing edge shape results, greatly reducing the drag. This kind of flap does have some moving elements that increase the weight and complexity over an earlier CCW design. But overall, the hinged flap design still maintains most of the Circulation Control high lift advantages, while greatly reducing the drag in cruising condition associated with the rounded trailing edge CCW design. In the present work, an unsteady three-dimensional Navier-Stokes analysis procedure has been developed and applied to this advanced CCW configuration. The solver can be used in both a 2-D and a 3-D mode, and can thus model airfoils as well as finite wings. The jet slot location, slot height, and the flap angle can all be varied easily and individually in the grid generator and the flow solver. Steady jets, pulsed jets, the leading edge and trailing edge blowing can all be studied with this solver.

Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part II: Transport Application

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) airfoils and to address specific issues related to the application of these blown high-lift devices to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic configurations to increase high-lift system performance in the terminal area while reducing system complexity and aircraft noise. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations; 2) development of effective pneumatic leading-edge devices; 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities; and 4) the investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases were presented in Part I of this article. This segment, Part II, describes the fourth phase of the program. Experimental lift coefficient values approaching 8.0 at zero incidence were demonstrated by two-dimension al CCW configurations and were reported in Part I. These were used to predict 70-80% reductions in takeoff and landing distances for a three-dimensional advanced subsonic transport configuration employing a simplified pneumatic high-lift system. These results and the methodology used to obtain them will be presented in greater detail in the following discussions.

Numerical Simulations of the Steady and Unsteady Aerodynamic Characteristics of a Circulation Control Wing Airfoil

The aerodynamic characteristics of a Circulation Control Wing (CCW) airfoil have been numerically investigated, and comparisons with experimental data have been made. The configuration chosen was a supercritical airfoil with a 30 degree dual-radius CCW flap. Steady and pulsed jet calculations were performed. It was found that the use of steady jets, even at very small mass flow rates, yielded a lift coefficient that is comparable or superior to conventional high-lift systems. The attached flow over the flap also gave rise to lower drag coefficients, and high L/D ratios. Pulsed jets with a 50% duty cycle were also studied. It was found that they were effective in generating lift at lower reduced mass flow rates compared to a steady jet, provided the pulse frequency was sufficiently high. This benefit was attributable to the fact that the momentum coefficient of the pulsed jet, during the portions of the cycle when the jet was on, was typically twice as much as that of a steady jet. NOMENCLATURE

DOE's Effort to Reduce Truck Aerodynamic Drag Through Joint Experiments and Computations

Class 8 tractor-trailers are responsible for 11-12% of the total US consumption of petroleum. Overcoming aero drag represents 65% of energy expenditure at highway speeds. Most of the drag results from pressure differences and reducing highway speeds is very effective. The goal is to reduce aerodynamic drag by 25% which would translate to 12% improved fuel economy or 4,200 million gal/year. Objectives are: (1) In support of DOEs mission, provide guidance to industry in the reduction of aerodynamic drag; (2) To shorten and improve design process, establish a database of experimental, computational, and conceptual design information; (3) Demonstrate new drag-reduction techniques; and (4) Get devices on the road. Some accomplishments are: (1) Concepts developed/tested that exceeded 25% drag reduction goal; (2) Insight and guidelines for drag reduction provided to industry through computations and experiments; (3) Joined with industry in getting devices on the road and providing design concepts through virtual modeling and testing; and (4) International recognition achieved through open documentation and database.

Numerical Investigation of Circulation Control Airfoils

Reynolds-averaged Navier-Stokes simulations are presented for a circulation control airfoil. Comparisons with measured load characteristics are presented. The effects of turbulence models and the spatial accuracy of the simulations on the predictions are examined. It is observed that increasing the spatial accuracy from third-order to seventh-order had only a minor effect on the solution accuracy on the fine grid simulations reported here. Two-equation turbulence models, and particular the κ-ω /κ-e blended baseline model, tended to perform better than the one-equation model. The predictions were found to be sensitive to the turbulent kinetic energy level of the jet. Inclusion of the plenum chamber and the jet nozzle allowed the physics of the flow to be captured in greater detail than modeling the jet only at the jet slot, but had only a minor influence on the overall loads.

Noise Reduction Through Circulation Control

Circulation control technology uses tangential blowing around a rounded trailing edge or a leading edge to change the force and moment characteristics of an aerodynamic body. This technology has been applied to circular cylinders, wings, helicopter rotors, and even to automobiles for improved aerodynamic performance. Only limited research has been conducted on the acoustic of this technology. Since wing flaps contribute to the environmental noise of an aircraft, an alternate blown high lift system without complex mechanical flaps could prove beneficial in reducing the noise of an approaching aircraft. Thus, in this study, a direct comparison of the acoustic characteristics of high lift systems employing a circulation control wing configuration and a conventional wing flapped configuration has been made. These results indicate that acoustically, a circulation control wing high lift system could be considerably more acceptable than a wing with conventional mechanical flaps.

Drag reduction, safety enhancement, and performance improvement for Heavy Vehicles and suvs using advanced pneumatic aerodynamic technology

Blown aircraft aerodynamic technology has been developed and applied to entrain separated flow fields, significantly reduce drag, and increase the fuel economy of Heavy Vehicles and SUVs. These aerodynamic improvements also lead to increases in stability, control, braking, and traction, thus enhancing safety of operation. Wind-tunnel results demonstrating model Heavy Vehicle drag coefficient reductions of up to 84% due to blowing and related configuration improvement are reviewed herein. Data confirming the elimination of directional instability due to side-winds plus generation of aerodynamic forces which are not currently used for control of large vehicles are also shown. These data have guided the design and modification of a full-scale road-test vehicle. Initial confirmation road test results of this patented concept on the modified blown HV rig are presented. An SAE Type-II Fuel Economy test was also conducted. Here, various blowing configurations were tested, and results compared to a baseline reference tractor-trailer to confirm the improved fuel economy due to blowing. Full-scale wind-tunnel tests of this pneumatic technology applied to a GM Suburban SUV were also conducted, and the positive effects of blowing on drag reduction, vehicle aerodynamic stability, and operational safety are shown. Comparative results presented include wind-tunnel data for both unblown and blown configurations, full-scale blowing and fuel-economy data, and comparisons to smaller-scale blown Pneumatic Heavy Vehicle experimental results.