Two aspects of the use of full-and partial-span Gurney flaps

A GURNEY flap is a long and narrow plate attached to a wing’s trailing edge on the pressure side and perpendicular to it. It is quite simple, yet very effective in bringing about significant changes in the flow around the wing. The primary effect is the added downward flow deflection at the trailing edge (thus, increased circulation). The wing CLmax increases: the larger the height of the flap, the more the lift improves. The wing drag, however, is also affected, almost always in the adverse way, and the overall effect on the wing L=D is detrimental. The flap was first proposed and used successfully for race-car applications in the 1970s. In the same time frame, Liebeck [1] did an analysis of the device. Since then, a number of other researchers have studied various aspects of this modification. The following is a brief discussion of some of the most recent efforts. Zhan and Wang [2] studied the effects ofGurney flaps and apex flaps on the aerodynamic performance of a delta wing. They found that the Gurney flap produced a significant increase of CL before stall accompanied by a drag penalty. Traub et al. [3] studied the effects of Gurney-flap porosity, inclination, and spacing on the wing’s lift, drag, and moment characteristics. The same three authors conducted a preliminary study to examine the possibility of using a jet flap for hingeless control and compared the approach with a Gurney flap at the same location [4]. The tests showed noticeable lift augmentation, as well asCD increase, throughout the range of tested. Meyer, et al. [5] conducted investigations with the aim to stabilize the wake flow to achieve drag reduction. They used Gurney flaps with slits and holes and vortex generators. Their results showed the usual increases in both CL and CD. Although modifying Gurney flaps by adding holes had no effect onCL,CD was somewhat lowered, but still higher than that of the baseline wing. They supplemented their experiments by numerical simulation. The reader is referred to [6] for a short discussion of the most important work on the topic. Reference [7] gives a rather extensive review of the Gurney-flap research. The present author has been studying Gurney flaps since 2004, primarily for their potential as a trailing-vortex-reduction device. This supposition is based on the following twopremises [6]: First, the flap’s ability to significantly alter the wing’s lift, connected with the fact that a wing’s lift and its trailing vortex are intimately related. Second, due to the spanwise components of the velocities of the airflows leaving awing’s trailing edge, which give rise to the sheet of vorticity that evolves into distinct trailing vortices downstream of the generating wing, it appears reasonable to wonder: What effect might a Gurney flap have on the spanwise flow direction as the flow leaves the wing’s pressure side at the trailing edge? Would the presence of the flap impede the flow? A single Gurney flap was tested in combination with a rectangular wing. Using oil-smoke flow visualization and a newly proposedmodel of a vortex rollup tightness factor (RTF), it was shown that the flap did affect the trailing vortex rollup in the near field [6]. To bring closer Gurney-flap and wing wake vortices a study was conducted to address the supposition that, according to the Helmholtz vortex laws, the presence of a Gurney flap (as with any flap, for that matter) must lead to the creation of additional trailing vortices [7]. The flow hypothesis of [1] makes no reference to these vortices. The study clearly confirmed the existence of such vortices [7]. With these vortices confirmed, the reasoning that Gurney flaps may affect the wake rollup appeared even more justified. Subsequently, two additional full-span Gurney flaps of varying height were investigated [8]. As might be suspected, the results showed that the higher the Gurney flap, the more pronounced its effect on thewing aerodynamics and also on trailing vortex reduction in the near field. Following the findings of these three studies, while considering the mechanism through which a Gurney flap increases a wing’s CL, the following appeared a reasonable question:Would a part-spanflap installed at various spanwise locations be able to produce the desirable vortex reduction with a lower increase of the wing’s drag and weight? The plausibility of this supposition in the case of a rectangular wing seemed to be enhanced by the well-known separation pattern on this planform. It seemed justifiable to assume that a Gurney flap positioned around the wing’s centerline would help most by energizing the suction-side airflow as is increased. How effective would a flap positioned outboard be in energizing the still attachedflow?Also, can aflap installed in thisway create trailing vortices capable of interfering with themain wingtip vortices? Based on these thoughts, it seemedwarranted to commence a studywith the objective to examine these flow phenomena. Apparently, the next question involved the effects of the trailing vortices originating at the flap’s ends. The study presented in this paper was undertaken with the goal to address these questions. Received 4 December 2006; revision received 14 June 2007; accepted for publication 15 June 2007. Copyright © 2007 by Vojin R. Nikolic. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/07 $10.00 in correspondence with the CCC. ∗Professor. Associate Fellow AIAA. JOURNAL OF AIRCRAFT Vol. 44, No. 5, September–October 2007