Study of the Reverse Delta Wing Afaq Altaf, * Ashraf A. Omar, † Waqar Asrar, ‡ and Hani Bin Ludin Jamaluddin * International Islamic University Malaysia, Kuala Lumpur 50728, Malaysia DOI: 10.2514/1.C031101 Particle image velocimetry was used in a low-speed wind tunnel to investigate the vortex structures of a slender reverse delta wing at various angles of attack and roll. This work investigates the characteristics of the vortices generated downstream in planes perpendicular to the freestream direction and their dependence on angles of attack and roll at a chord-based Reynolds number of Re c  3:82  10 5 . The peak tangential velocities at > 5  show a trend similar to a delta wing. A six-component balance was used to obtain the aerodynamic coefficients for a reverse delta wing as well as a delta wing for comparison. A simulation of the streamlines, velocity vectors, and surface pressure contours was carried out using computational fluid dynamics software to show the characteristics of the flow over a reverse delta wing. Nomenclature C D = drag coefficient C L = lift coefficient C m = moment coefficient c = chord length, m k = number of points L=D = lift-to-drag ratio Re c = Reynolds number based on chord r = radius, m r c = core radius, m V  = tangential velocity, m=s V 1 = freestream velocity, m=s x = streamwise coordinate, m y = spanwise coordinate, m z = transverse coordinate, m  = angle of attack, deg  = circulation, m 2 =s  = roll angle, deg I. Introduction T HE purpose of this paper is to investigate the flow over a reverse delta wing. Reverse delta wings may be used in vortex alleviation [1]. They may also be used in forward-swept-winged aircraft [2]. Vortex flows play a vital role in modern aerodynamic applications such as in the control of wingtip vortices of large aircraft so as to minimize the hazard posed by trailing aircraft from such wake-vortex encounters [1,2]. Vortices created by aircraft are an inevitable consequence of the creation of lift. Vortices persist for many miles, and wake-vortex encounters pose a grave hazard to trailing aircraft that fly in close proximity near the airport runway, especially during takeoff and landing [3], because the tip vortex circulation is at a maximum. This limits the spacing between aircraft within the takeoff and landing corridors at busy airports and hence increases the time intervals between consecutive landings and takeoffs [4]. Worldwide research has been focused on increasing airport capacity by minimizing the wake-vortex hazard. The studies of forward-swept wings, which resemble a reverse delta wing, have shown promising results. Interest in forward-swept-wing aircraft is growing. The aerodynamics of a forward-swept wing shows that air moving over it tends to flow inward toward the root of the wing instead of outward toward the wingtip, as occurs on sweptback wings. This reverse airflow does not allow the wingtips and their ailerons to stall at high angles of attack. Both X-29 and Sukhoi Su-47 supersonic aircraft make use of forward-swept wings for superb maneuverability and operation at angles of attack up to 45 deg or more [5]. This type of configuration is made possible by lightweight nonmetallic composite materials that can withstand the increased amounts of aerodynamic forces. This configuration would provide a number of advantages, such as higher lift-to-drag ratio, higher capability in dogfight maneuvers, higher range at subsonic speed, improved stall resistance and antispin characteristics, improved stability at high angles of attack, a lower minimum flight speed, and shorter takeoff and landing distances [6]. A reverse delta wing has certain favorable aerodynamic char- acteristics that can be exploited for efficient supersonic flight. Early investigations into the aerodynamics of reverse delta wings were carried out by NACA in 1947 [7]. Gerhardt [7] has also studied the reverse delta wing at high Mach numbers and has observed significant differences between the regular delta wing and a reverse delta wing. According to his study, surface pressure contours of a reverse delta wing are expected to exhibit a more regular change in pressure than a delta wing and strong pressure gradients are expected to be confined to the trailing-edge regions. In 1999, a group of Northrop Grumman designers came up with an innovative supersonic transport design incorporating an un- usual reverse delta wing. Designers from Northrop Grumman claimed that, “The reverse delta wing design allows additional lift to be created at low speeds, reducing power requirements and therefore noise, during the environmentally crucial takeoff and landing phases” [8]. Recent investigations into the aerodynamics of a reverse delta wing were carried out by Elsayed et al. [9]. The vortex characteristics of the reverse delta wing showed promising results. Their inves- tigation suggests that a reverse delta wing, as a wake alleviation add- on device, may excite some instability through stable laminar or unstable wave phase or through modifying the vortex roll-up process as a result of interaction with the turbulent phase. These can lead to rapid diffusion of vorticity, which can enhance wake-vortex decay and thus lead to wake-vortex alleviation. A lot of research has been done on delta wings, especially on leading-edge vortex breakdown control [10], vortex bursting [11], simulation [12], and experimental and numerical investigation of delta wings [13]. Most of the research on delta wings has been per- formed using flow visualization and simulation to better understand Received 7 May 2010; revision received 12 August 2010; accepted for publication 20 August 2010. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 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 0021-8669/11 and $10.00 in correspondence with the CCC. * Research Assistant, Department of Mechanical Engineering, Gombak, P.O. Box 10. † Professor, Department of Mechanical Engineering, Gombak, P.O. Box 10. Member AIAA. ‡ Professor, Department of Mechanical Engineering, Gombak, P.O. Box 10. JOURNAL OF AIRCRAFT Vol. 48, No. 1, January–February 2011 277