Lift-induced drag

About: Lift-induced drag is a research topic. Over the lifetime, 2861 publications have been published within this topic receiving 41094 citations.

Aerodynamic effects of local dihedral on a raked wingtip

Abstract: An experimental study was conducted to examine the impact of dihedral in the tip region of a wing on the induced drag as previous theoretical work has indicated a potential for drag reduction. Wake profiles were obtained with a seven hole probe for a systematic variation of the dihedral and sweep of a raked tip on a 30° swept wing. These profiles were analyzed with the Boeing Universal Wake Survey Data Analysis Code to obtain lift and drag distributions. The data indicated that the tips with negative dihedral, and associated smaller span, yielded a lower induced drag than the greater span, zero dihedral configuration. The tips with positive dihedral showed no decrease of induced drag, within experimental error, compared to the zero dihedral case. The raked tip with the lower sweep angle exhibited a lower induced drag then the tip with the higher sweep in all cases. Several unexpected dynamics were observed in the flowfield, possibly as a result of the highly curved tip. A second vortical structure was present inboard of the main trailing vortex and the axial velocity in the trailing vortex displayed a dual "jet like" (high velocity) and "wake like" (low velocity) nature. Lift and drag distributions indicated that this second vortex locally decreased the lift and increased the induced drag. AR b Ab CD CL CP s u,v,w U^ a e A. Nomenclature aspect ratio span of wing tip span drag coefficient lift coefficient pressure coefficient wing area x,y,z velocity components freestream velocity angle of attack dihedral angle taper ratio t Graduate Student, Member AIAA tt Assistant Professor, Senior Member AIAA Copyright © 2002 by K. Visser. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission Background and Motivation Aerodynamic improvement through the modification of flow in the wingtip region of an aircraft has been a focus of many years of research and study. Passive devices, such as tailored wingtips, end plates, winglets and tip sails have been investigated. The fundamental premise of these fixed geometry concepts is to reduce the induced drag and thereby minimize the trailing vortex strength. Increasing the effective aspect ratio with devices such as endplates and winglets does occur, although this is partially offset by the increase in viscous drag from the added wetted area. Well-designed winglets can direct a component of the lift in the thrust direction and tip sails have indicated the potential for reductions in induced drag . Much work has also been performed to study the effects of tailoring the wingtip planform geometry such as by shearing the tip, referred to as a 'raked tip', or through the use of even more exotic geometry such as crescent shaped tips'. Determining the drag savings for a given tip modification is not always an easy task, however. Studies employing active devices have also been undertaken to try to reduce the drag at a given lift by insertion of tip turbines ' , propellers , and jets of air into the vortex region. The tip turbines, however, were found to yield the largest drag reduction while fixed and although the turbines generated power while spinning, this did not offset the increase in drag. The tip propellers were reported to displace the vortices further outboard, increasing the effective aspect ratio. Improvements were also found from the blowing jets, however the weight and complexity of all these devices generally precludes their use on an aircraft Wingspan extensions can reduce the drag of a given airplane, but result in a weight increase due to the weight of the extension itself and strengthening of the original wing to support the increased bending loads. Additional weight penalties can also occur, if the extension aggravates flutter for instance. The potential for increased drag savings has led to several new tip designs appearing on commercial aircraft. 1 American Institute of Aeronautics and Astronautics (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. Wingtip design work at the Boeing Company has led to the development of a novel raked wingtip currently in production on the 767-400ER as shown in Figures 1 and 2 and detailed in a patent granted in July, 2000. The raked tips were designed as in-plane extensions with an unprotected leading edge (no slats). The successful introduction of this large raked wing tip extension has been shown to be a viable alternative to the winglet, resulting in a low cost, low weight, no moving parts, removable tip that performs acceptably at both low and high speeds. Deliveries of the aircraft to airline customers Delta and Continental began at the end of August 2000. Figure 1. 767-400ER Raked Tip Assembly (courtesy of The Boeing Company) The new Envoy 7 corporate/executive jet from Fairchild Dornier is also employing a unique raked winglet concept, denoted Super SharkTM Winglets. Predictions show the winglet will reduce overall aircraft cruise drag by 3.5 percent and decrease fuel burn by more than 5 percent on a 4,000 nautical mile mission. Figure 3. Envoy 7 (courtesy of Fairchild Dornier) Recent work by Eppler has suggested that a local small dihedral in the tip region of a wing may be an effective means to reduce the drag of the wing through the nonlinear effect of induced lift, which has the same effect as decreasing the induced drag for a given lift. Induced lift is caused by the velocity that the lifting vortices induce on themselves and does not develop on a planar wing. Several interesting results were concluded from his study: 1. A wingtip with a dihedral of about 10° has a lower induced drag than a planar wing with the same length', that is a smaller span than the planar wing. 2. Positive dihedral induces a negative lift on the wake and therefore a positive lift on the wing. Negative dihedral indicates the opposite and thus a wing with a downward pointing tip performs poorer. The present investigation examined the impact of dihedral on the performance of a swept wing fitted with a raked tip to determine if the effects described above can be verified experimentally. According to Eppler , although the span is reduced by deflecting the tip, the wing will perform better than a planar wing with the same length, or wetted area. The performance of the wing with a deflected tip was examined against the baseline planar tip of the same length. The wing tip region was given positive and negative dihedral, but the length of the wing was kept constant. Hence the span of the deflected tip geometry was less than the planar wing. Figure 2. 767-400ER Raked Tip in Flight (courtesy of The Boeing Company) American Institute of Aeronautics and Astronautics (c)2002 American Institute of Aeronautics & Astronautics or Published Lift and drag increments were determined using a wake integral method data reduction scheme encoded at Boeing Commercial Airplane Group by Kusunose " . The code uses a method initially derived by Betz and later refined by Maskell which can be used to determine form and induced drag components separately. A brief review is given below and further details can be found in Kusunose Lift and Drag From Wake Measurements The Boeing wake drag reduction code, used to obtain lift and drag from wake measurements, is based on theories 17 1R put forward first by Betz and later refined by Maskell . This method is based on momentum integrals written in terms of flow variables taken from within the wing's vortical wake. As only the vortical part of the wake need be measured, it is possible to use this as a practical wind tunnel testing method. There are a few advantages of using a wake survey method for wind tunnel testing. The wake integral method allows for the determination of lift, induced drag, and profile drag separately. The method can provide sectional distributions of these quantities so that specific parts of wind tunnel models (i.e. slats, flaps and nacelles) can be evaluated for their effect on lift and drag. The code can also be used to reveal complex physics based on vortex interactions such as airframe noise/vortex strength correlations and ground effect in high lift conditions [Kusunose with Permission of Author(s) and/or Author(s)' Sponsoring Organization. • No blowing or suction through the tunnel walls and on model surfaces is allowed ("solid surface assumption") • The tunnel has a uniform effective cross section ("parallel wall assumption"). The effective boundary of the stream is defined as the displacement surface of the boundary layer on the tunnel walls. This assumption excludes the possible effect of buoyancy, which is caused by an axial pressure gradient in the

Wall oscillation induced drag reduction zone in a turbulent boundary layer

Abstract: Spanwise oscillation applied on the wall under a turbulent boundary layer flow is investigated using direct numerical simulation The temporal wall-forcing produces a considerable drag reduction (DR) over the region where oscillation occurs Three simulations with identical oscillation parameters have been performed at different Reynolds numbers with one of them replicating the experiment by Ricco and Wu (Exp Therm Fluid Sci 29, 41–52, 2004) The downstream development of DR in the numerical simulation and experiment is nearly identical The velocity profiles and the indicator function are investigated with respect to the variation in DR and Reynolds number The DR affects the slope of the logarithmic part of the velocity profile in accordance with previous theoretical findings Low speed streaks are visualized and the bending of longitudinal vortices related to the drag reduction phenomenon is discussed In addition, the visualization is compared with the corresponding results from the experiments The spatial transient of the DR before reaching its maximum value is analyzed and is found to vary linearly with the oscillation period An analysis of the energy budget is presented and the fundamental differences compared to the streamwise homogeneous channel flow are elucidated While the power budget improves with increasing Reynolds number, it is shown that the net power remains negative for the wall forcing parameters considered here, even under ideal conditions On the other hand, the analysis together with channel and boundary layer flow data in the literature provides an estimation of net energy saving for boundary layer flows which depends on the streamwise extent of the oscillating zone

Effect of Rear-End Extensions on the Aerodynamic Forces of an SUV

Abstract: Under a global impulse for less man-made emissions, the automotive manufacturers search for innovative methods to reduce the fuel consumption and hence the CO2-emissions. Aerodynamics has great potential to aid the emission reduction since aerodynamic drag is an important parameter in the overall driving resistance force. As vehicles are considered bluff bodies, the main drag source is pressure drag, caused by the difference between front and rear pressure. Therefore increasing the base pressure is a key parameter to reduce the aerodynamic drag. From previous research on small-scale and full-scale vehicles, rear-end extensions are known to have a positive effect on the base pressure, enhancing pressure recovery and reducing the wake area. This paper investigates the effect of several parameters of these extensions on the forces, on the surface pressures of an SUV in the Volvo Cars Aerodynamic Wind Tunnel and compares them with numerical results. To decrease the dependency of other effects within the engine bay and underbody, the SUV has been investigated in a closed-cooling configuration with upper and lower grille closed and with a smoothened underbody. These results might change if the study would be conducted with a less smooth underbody and in an open-cooling configuration. Extensions with different shapes and dimensions have been placed around the perimeter of the base exterior. The chosen design philosophy of the extensions allowed for different combinations with variable inclination angles depending on their position along the base perimeter, multiple extension lengths and shapes to be investigated. The results show that the extension shape is an important factor in reducing the aerodynamic drag. Significant drag reductions could be obtained while maintaining the vehicle's rear lift within acceptable levels for stability with a kicker attached to the extension. The investigation shows the reduction with a kicker holds for up to 7.5° yaw angles. With a beneficial shape, the extension length can be significantly reduced. The reduced drag is visible in the wake by a more concentric wake.