Dieter Jacob

Studies on the influence of outboard flaps on the vortex wake of a rectangular wing

Abstract Aircraft trailing vortices constitute a hazard to following aircraft, and are therefore one of the main concerns for airport capacity constraints. At the Institute of Aerospace Engineering (ILR) experiments on wake vortices up to a distance of 60 spans behind the model of a rectangular wing are conducted in a towing tank. The motivation behind the presented experiments is the alleviation of the rolling moment induced on following aircraft by constructive means at wings and flaps of the preceeding aircraft. Following the approach of Ortega et al., the influence of the geometry of outboard flap extensions on the wake of a rectangular wing is investigated. Towing tank experiments using Particle Image Velocimetry are conducted, and the induced rolling moment is calculated from the experimentally determined downwash. In addition a stability analysis is performed for the experimentally investigated four-vortex configurations utilizing the linearized Biot–Savart law as proposed by Crow. It is assumed that the vortex system attunes to the most unstable eigenform. The theoretical results help giving an interpretation of the experimental data. It is shown that a significant reduction of the induced rolling moment by more than 50% can be achieved by the use of outboard flap extensions. This is due to an increase of the effective vortex size resulting from the counter-rotating vortex pairs produced by the flaps.

Behavior of wake vortices near the ground over a large range of Reynolds numbers

Abstract Numerical simulations based on the two-dimensional vorticity-stream function formulation are used to investigate the behavior of wake vortices near the ground over a wide Reynolds number range and to determine the maximum height the primary vortices reach far downstream of the lifting wing. All cases within the studied Reynolds number range (3 · 102 ≤ ReΓ ≤ 3 · 106) show the separation of boundary layer vorticity from the ground, the formation of vortices in the separation region and one or several rebounds of the primary vortex pair. The amount of circulation produced within the boundary layer shows only minor variations, while an increasing Reynolds number results in an increasing number of generated vortices with decreasing circulation. The minimum altitude of the primary vortex pair increases with a decreasing Reynolds number, while the maximum altitude far downstream does not show a regular dependence on the Reynolds number. For all Reynolds numbers the maximum altitude of the primary vortices far downstream is smaller than 3.1 times their initial spacing. This result is confirmed by theoretical deductions yielding an upper limit for the maximum altitude of the primary vortices after several rebounds.

Experimental Analysis and Modulation of Vortices

This paper presents the results of an experimental investigation of the wake vortex structure behind different wings with and without flaps. The wind-tunnel test is part of a project funded to investigate the flow structure in the wake of different wings. The purpose of this investigation is to study the main features of lift generated vortices in order to find ways to alleviate hazardous wake vortex encounters for following airplanes during take-off and approach such that an increase in airport capacity can be achieved. Therefore, the wakes of different wings without flaps are investigated to examine the influence of different wing planforms on the structure of the wing tip vortex and the induced rolling moment. Then, the wake of a swept and tapered wing is investigated for different flap settings. Finally, a wing fin is mounted on the wings with and without flaps to examine its alleviating influence on the wake structure. The results show that the wing planform and the flap deflection have a considerable influence on the wake structure and the induced rolling moment on a following aircraft and that a wing fin can lead to an alleviation of the wake.

Active Control of Aircraft Vortex Wakes by Means of Preset Ailerons

Aircraft trailing vortices constitute a hazard to following aircraft, and are therefore one of the main concerns for airport capacity constraints. At the Institute of Aerospace Engineering (ILR) experiments on wake vortices up to a distance of 50 spans behind the model of a rectangular wing are conducted in a towing tank. The motivation behind the presented experiments is the alleviation of the rolling moment induced on following aircraft by constructive means at wings and flaps of the preceeding aircraft. On the basis of earlier experimental studies on fourvortex-wakes a rectangular wing with preset ailerons is designed. The resulting vortex wake consists of two counter-rotating vortex pairs. These vortex systems show stronger co-operative instabilities than co-rotating vortex pairs emerging from conventional transport aircraft. Cooperative instabilities are instabilities due to the presence of multiple vortices. The vortex system is investigated experimentally with Particle Image Velocimetry and the stability of the identified vortex system is analysed utilising the linearised BiotSavart law as proposed by Crow. It is assumed that the vortex system attunes to the most unstable eigenform. The stability analysis now provides the most unstable movement pattern of the vortices together with the corresponding wavenumber. The co-operative instabilities lead to a disintegration of the vortex system. This disintegration is accelerated by perturbing the vortex system with the wavenumber resulting from the stability analysis. Each half-wing is equipped with two adjacent ailerons that oscillate in counter-phase. This configuration ensures that constant lift is maintained. It is shown that within 50 spans behind the wing a reduction of the induced rolling moment by approximately 50% can be achieved by an appropriate aileron preset alone. This is due to an increase of the effective vortex size resulting from the counter-rotating vortex pairs produced by the ailerons. Oscillating the ailerons induces a sinusoidal perturbation of the vortices and accelerates the onset of rapid decay. The reduction of induced rolling moment is obtained significantly sooner than without oscillation.

Influence of Different Flap Settings on the Hazard Posed to Following Aircraft

In the present paper the influence of different flap settings of the wake generating aircraft on the hazard posed to a following aircraft is investigated. The near wake region of a rectangular wing with flaps is computed by means of a 3D panel method while the far wake region is simulated by a 2D Navier Stokes scheme in vorticity form. The hazard posed to a following aircraft is quantified by the value of the maximum induced rolling moment which is calculated by means of 2D strip theory. A preceding experimental investigation showed that the maximum induced rolling moment on the following aircraft related to the wing lift of the generating aircraft is decreased by deflecting the flaps. This result is confirmed by the numerical simulation for moderate flap deflections whereas higher flap angles lead to an increase of the maximum induced rolling moment related to the wing lift. The computation of the far wake region shows that in these cases the merger of the wing tip and flap vortex leads to a reconcentration of circulation and a reduction of the alleviating effect of the flap deflection.

Simulation of Vortex Sheets at Take-Off and Landing

In this paper a numerical investigation of the wake structure of wings with different planforms and flap settings as well as of wings with wing fins is presented. The near wake region is computed by means of a 3D panel method while the far region is simulated by a 2D Navier-Stokes scheme. The hazard posed to a following aircraft is quantified by the value of the maximum induced rolling moment which is calculated by means of 2D strip theory. The results of the numerical investigation confirm the tendencies that were obtained experimentally in the near field of the wings. Additionally, the development of the wake structure in the far field could be determined. As a result, the alleviating effect of a suitable wing geometry as well as of an additional wing fin that was obtained in the near field is decreased in the far field of the wings.

Analysis of the Oblique Flying Wing Concept’s Aerodynamic Potential by Means of Parameterization and Optimization

A parameterized description of a widely arbitrary Oblique Flying Wing geometry has been developed. In extension to previous studies with more or less symmetric geometries, asymmetric planform (chord distribution and sweep), dihedral bending and profile distribution were allowed. The aerodynamic properties of these geometries have been calculated with a higher order panel method. A parallel genetic algorithm has been developed and applied to determine geometries with optimized lift to drag ratios under realistic constraints concerning the total aircraft size and the available cabin space.