Determining Direction for Optimization of Movable Wing Tip Strake

S TRAKES are lifting surfaces featuring low aspect ratios and highly swept sharp leading edges conventionally located at the fuselage–wing junction. The benefits of using strakes on various airplane configurations have been known since the early 1970s. By imposing controlled flow separation and creating powerful leadingedge vortices that then subject both the strake and the main wing to high rotational velocities, strakes generate significant amounts of additional lift. Movable tip strake is an extension of the strake concept first proposed by the author [1]. Wing-fuselage strakes have been the subject of numerous studies; see, for example, [2–12]. The reader is referred to [1] for an extensive review of strake research. Movable wing tip strake is a novel design idea. Other researchers have studiedfixedwing tip strakes in the past, notablyMa [13], Traub et al. [14], and Staufenbiel and Vitting [15]. The author proposed making them movable in flight. Then the advantageous effect of the strakes on the aircraft performance is achieved without having to set the aircraft to a high , accompanied by a high CD. Instead, the optimal angle of operation of the strakes is attained by simply setting independently the strakes relative to thewing. This setting angle then represents an additional variable for controlling the airplane configuration. First, a simple semidelta movable tip strake (MTS) shown in Fig. 1 was tested in combination with a rectangular baseline wing [1]. The results showed that this configuration outperformed by a factor of 2.24 a modified wing obtained by extending the span of the baseline wing and maintaining the same airfoil and having the same aspect ratio ( 2 S ) as the wing with movable tip-mounted strakes. By deflecting the strakes up or down, the L=D benefit shifted to a lower, or higher, respectively, of the main wing, as it would be expected, because the flow conditions on the strake depend on both the of the wing and ds. In subsequent studies, the author studied various planform shapes of MTSs with the objective of maximizing their positive effect as measured by theL=D of thewing-strake configuration for a givenCL [16–21]. Ninevarious strake planforms,MTS1–9,were tested. These included strakes with straight leading edges swept at angles from 60 to 80 deg, a strake for which the leading edgewas a parabolic arc, and a series of strakes featuring two straight-line portions of the leading edge swept at different angles, thus resulting in a leading-edge break point. It has been found that the best performance is achievedwith the MTS4, which had a cropped double-delta strake planform featuring an inboard portion swept at 80 deg and an outboard portion swept at 45 deg with the transition, or the leading-edge break point, located at 57.5% of the strake root chord. This strake is shown in Fig. 2. When used at moderate-to-high of the wing and set to the neutral setting with respect to the wing chord plane, that is, at ds 0 deg, this strake improved the L=D by approximately 26%. This beneficial effect of the strake persisted at both positive and negative settings of the strake (ds > 0 deg, the strake leading edge up, and ds < 0 deg, the strake leading edge down) and the optimal point corresponding to the L=D max shifted to a lower or higher of the main wing, respectively, as it would be expected. This confirmed the supposition that ds would represent a new and useful variable for wing configuration control. To continue to optimize the movable strake design, it appeared of interest to examine the effect of the leading-edge break point position, xBP, while maintaining the inboard and outboard sweep angles of MTS4. Apparently, by moving the break point forward (and, consequently, slightly inboard), the area of the inboard, highly swept portion of the strake decreases and that of the outboard, moderately swept portion increases. Moving the break point backward (and somewhat outboard) accomplishes the opposite effect. The relative sizes of the inboard and outboard segments of the strake presumably have an effect on the formation and disposition of the generated leading-edge vortices and the resulting wing-strake aerodynamics. This study has been commenced with the goal to investigate those effects.