Dynamic stall control for advanced rotorcraft application

Abstract

Advanced concepts designed to improve the lift, drag, and pitching moment characteristics of rotor blades have been investigated for the purpose of enhancing rotor maneuver capability. The advantages and disadvantages of these concepts have been evaluated using both computational and experimental means. The concepts that were considered in this study included a leading-edge slat, a deformable leading-edge, and upper-surface blowing. The results show the potential of these concepts for substantially improving the performance of a rotor. HE next generation of rotorcraft will be required to operate at much higher performance levels than in the past, particularly in the areas of nap-of-the-ea rth (NOE), deep-penetration operations, and air-to-air combat. These new requirements will require highly maneuverable, agile, and survivable rotorcraft, far exceeding the capabilities of those in the current inventory. The objectives of this project include an increase in the maneuverability/agility capability of the helicopter and a reduction in the acoustic detection range. The single most important element of the rotorcraft for meeting these requirements is the rotor itself, since it is the primary source of lift, control, and speed. At the same time, the rotor is also a major source of acoustically detectable radiation. Among the many factors affecting rotorcraft performance, the aerodynamic characteristics of the rotor system are the most important and are the main subject of this paper. The maneuvering capability of a rotorcraft can be improved by re- ducing or suppressing the vibratory loads on the rotor blades caused by aerodynamic separation and stall. This would have the effect of expanding the stall-limiting boundary of the rotor and thereby increase the available load factor in all flight regimes. The con- ventional way to obtain higher lift is to increase the blade area, however, this usually results in a heavier rotor that is also less ef- ficient. With regard to compressibili ty effects and acoustic radia- tion, improvements have been obtained by sweeping, tapering, and thinning the tip region of the rotor blade. As a result, numerous families of airfoils and planform shapes have evolved that offer bet- ter advancing-blade characteristics. However, improvements on the retreating-blade side have not been as impressive. One reason for this imbalance may be that design codes are available for treating blades at low angles of attack and high Mach number (characteris- tic of the advancing side), whereas the design strategy has had to depend heavily on costly empirical studies for blades at high angles of attack and having some amount of separation (characteristic of the retreating side). Increasing the tip speed of the rotor to achieve a maneuvering ad- vantage may produce a dangerous condition with regard to acoustic detection. Rapid advancements in passive acoustic sensor arrays and advanced signal processing technologies pose a serious threat to the mission effectiveness of Army helicopters. Since the rotor blade generates acoustic radiations that can be easily detected and identified, airfoil and planform shapes must be carefully optimized to reduce the detection range of the rotorcraft. The requirements for improved maneuverability and reduced sus- ceptibility will clearly demand a substantial growth in the technolo- gies for addressing rotor aerodynamics. New control techniques must be considered, both passive and active, and these must be ac- companied by a more thorough physical understanding of these flow phenomena along with substantially improved prediction capabili- ties. To meet these requirements, computational and experimental efforts have been initiated to evaluate the effectiveness of various concepts. At present these concepts include airfoils with slats and slots, airfoils that deform, and airfoils with flow energizers. Description of Experiment and Computational Fluid Dynamics Code