Enhancement of Roll Maneuverability Using Post-Reversal Design. Part I: Nonlinear Analysis

Abstract

A nonlinear analysis and associated parametric study of the post-reversal behavior of both a cantilevered wing and a rolling aircraft are presented. For the simpler case, a highaspect-ratio cantilevered wing is modeled with geometrically-exact composite beam theory and a static aerodynamic model. The objective is to determine the effectiveness of the ailerons according to the wing’s contribution to rolling moment within the post-reversal regime. A second more involved model uses the same nonlinear aerodynamic and structural models, except they are applied to a rolling aircraft, the wings of which are of rectangular planform and have trailing-edge control surfaces attached to them. The objective is to determine the resulting steady roll rate that can be achieved within the post-reversal regime as a measure of the roll effectiveness of the ailerons. Aerodynamic coefficients for the wing-aileron system are computed by XFOIL and validated using experiment and advanced CFD codes. The results are used to build up a full range “air table” for a table-lookup within the multi-flexiblebody computer program DYMORE. Structural nonlinearities are included in DYMORE based on geometrically-exact composite beam theory, and static stall type aerodynamic nonlinearities are included through the table-lookup. Results demonstrate that there are configurations that reverse at a relatively low dynamic pressure, thus enabling the aircraft to operate at a higher level of aileron efficiency. In particular, one can get considerably more (negative) lift for a positive flap angle in this unusual regime than positive lift for a positive flap angle in the conventional setting. These findings may have important implications for development of highly maneuverable aircraft. I. Introduction HE notion of getting higher lift from a reversed control surface is not new. In the helicopter industry, the Kaman servo-flap rotor is designed to operate in the post-reversal regime in order to greatly increase the effectiveness of the trailing-edge flap. Of course, for the helicopter application there is no problem with having to control the blade at the point of reversal since that occurs at a lower rotor angular speed when the aircraft is still on the ground. In this paper the concept is revisited with a focus on fixed-wing aircraft configurations. The only fixed-wing application known to date is the Active Aeroelastic Wing (AAW) program, which indeed intended to address this design concept. The idea of AAW technology initially came from Rockwell International Corporation’s Active Flexible Wing (AFW) program. The concept exploited wing flexibility to provide weight savings and improved aerodynamic performance. Weight savings were realized via increased wing flexibility and removal of the horizontal tail. In the AFW wing design, large amounts of aeroelastic twist provide improved maneuver aerodynamics at several design points. However, degraded roll performance (in the form of aileron reversal) over a significant portion of the flight envelope is a direct result of wing twist. In a typical aircraft design, a differential horizontal tail control would be added to provide acceptable roll performance. Instead, in the AFW design, multiple leading- and trailing-edge wing control surfaces were used in various combinations