Aft-swept Wing Research Articles

Aerodynamics, Stability and Performance Characteristics of X-Tandem Aircrafts

This paper details design, analysis and validation of tandem aircraft with one wing swept forward and the other wing swept backwards. The design objective is to investigate the aerodynamic and stability characteristics of this configuration created with the motive of exploiting the manoeuvring and post stall characteristics of a forward swept wing along with the structural robustness and reliable performance of the conventional aft-swept wing. Parameters such as wing sweep, wing position, anhedral and dihedral were varied to develop a range of designs. This gave considerable information regarding the aerodynamic and stability characteristics which enabled a preliminary design of a military combat aircraft exploiting this configuration. All performance characteristics and parameters of the final design compared with current operational military aircraft give a favourable picture regarding the effectiveness of this design.

Demonstration of an in situ morphing hyperelliptical cambered span wing mechanism

Research on efficient shore bird morphology inspired the hyperelliptical cambered span(HECS) wing, a crescent-shaped, aft-swept wing with vertically oriented wingtips. Thewing reduces vorticity-induced circulation loss and outperforms an elliptical baselinewhen planar. Designed initially as a rigid wing, the HECS wing makes use ofmorphing to transition from a planar to a furled configuration, similar to that of acontinuously curved winglet, in flight. A morphing wing concept mechanism ispresented, employing shape memory alloy actuators to create a discretized curvatureapproximation. The aerodynamics for continuous wing shapes is validated quasi-staticallythrough wind tunnel testing, showing enhanced planar HECS wing lift-to-dragperformance over an elliptical wing, with the furled HECS wing showing minimalenhancements beyond this point. Wind tunnel tests of the active morphing wing prove themechanism capable of overcoming realistic loading, while further testing may berequired to establish aerodynamic merits of the HECS wing morphing maneuver.

Aerodynamic Characteristics of Forward and Aft Swept Arrow Wings

D ESPITE aerodynamic advantages, aeroelastic complications have precluded the wide-scale adoption of the forward swept wing planform [1]. Forward swept wings may demonstrate liftdependent drag benefits due to spanwise load distributions that are closer to elliptic then an equivalent aft swept wing [2]. Fuselageinduced spanwise flow is also beneficial for forward swept wings; it reduces the effective sweep, whereas the opposite is true for aft swept wings [3]. The most apparent benefit of the forward sweep configuration is in lateral control; spanwise flow toward the root delays flow separation over the ailerons, whereas the opposite is true for aft swept wings [4]. Little information is available on the effect of highly swept wingswith forward sweepwhere flow separation is enforced. Consequently, an investigation has been undertaken using a 65 deg sweep arrow wing with fore and aft extensions.

Effect of Stator Design on Stator Boundary Layer Flow in a Highly Loaded Single-Stage Axial-Flow Low-Speed Compressor

The paper describes an experimental investigation of the stator hub and blade flow in two different stators of a highly loaded single-stage axial-flow low-speed compressor. The first stator (A) is a conventional design with blades of rectangular planform. The second stator (K) is an unconventional, more advanced design with blades of a special planform, characterized by an aft-swept leading edge with increasing sweep angle toward hub and casing. The experimental results show that stator K exhibits a much better hub performance than stator A, finally leading to a better overall performance of stage K compared to stage A. The better hub performance of stator K is, primarily, the result of a planform effect of the newly introduced blades with an aft-swept leading edge and the aerodynamics of an aft-swept wing.

Hydrodynamic characteristics of a lunate shape oscillating propulsor

Research was conducted to study the hydrodynamic efficiency of a foil with aft-swept wing tips. A potential flow based time domain panel method was formulated to predict the performance of a lunate and rectangular foil in large amplitude, unsteady motion. Skin drag was approximated and boundary layer growth and separation were also estimated. Hydrodynamic efficiency was evaluated in terms of propulsive efficiency and thrust coefficient of the foil. Results are presented for a lunate shaped planform and for a rectangular foil. Predictions show that the lunate shaped planform has a substantially higher propulsive efficiency (13% higher) than the rectangular foil under heavy load conditions when the feathering parameter is zero, throughout a range of reduced frequencies (0.2 to 1.8). Under a medium load condition, however, the rectangular foil gave a higher propulsive efficiency at reduced frequencies less than 0.5 and the same efficiency value at a reduced frequency of 1.8. For a practical range of reduced frequencies between 0.5 and 1.0, the lunate tail gave higher propulsive efficiency. The lunate planform gave a lower thrust coefficient at a heavy load and higher thrust at a medium load condition than the rectangular planform for all reduced frequencies.

Aeroelastic stability of aircraft with circulation control wings

The aeroelastic stability of aircraft with circulation control wings is examined. A beam finite element is used to model a high aspect ratio wing attached to a rigid fuselage. The fuselage is allowed to undergo rigid-body pitch and plunge motions about the aircraft center of gravity. Unsteady aerodynamic loads on the wing are calculated using a linear, time-domain unsteady aerodynamic model based on the indicia1 response method. Steady experimental circulation control airfoil data are used to determine the airfoil lift curve slope and aerodynamic center location as a function of Mach number and blowing level. Several aircraft configurations are examined, and a parametric study of design parameters is performed. The results indicate that when a high level of blowing is applied a instability of the first wing bending mode can occur. If the blowing pressure ratio is held constant, the wing restabilizes at higher speed. With high blowing the aircraft short-period mode also becomes unstable. The inclusion of rigid-body motion is shown to weaken the CC flutter instability due to inertia coupling of the wing elastic modes with the aircraft rigid-body motion. An improvement in stability also results when the fuselage pitch inertia is decreased and when the wing is swept aft. The effect of a horizontal tail and forward movement of the aircraft center-of-gravity was stabilizing for the 45-deg aft-swept wing configuration but is negligible on the 45-deg forward-swept wing configuration.

Aeroservoelastic tailoring for lateral control enhancement

The need for effective aileron power for aircraft lateral control and turning maneuvers dates back to the Wright Brothers and their wing warping concept for active stabilization of their aircraft. Early researchers in Great Britain, Japan, Germany and the United States explored ways to increase the effectiveness of control aileron to generate a roll moment. The basic problem of aileron effectiveness and the interrelationship between structural distortion and the loads applied by the control surface is illustrated. A rigid wing/aileron surface will develop the capability to generate increased roll rates as airspeed increases. A flexible surface will become less effective as airspeed increases because of the twisting distortion created by the aft-mounted control surface. This tendency is further worsened by bending distortion of an aft swept wing. This study focuses its attention on the ability of a combined effort between structural redesign of a wing and sizing and placement of a control surface to create specified roll performance with a minimum hinge moment. This design optimization problem indicates the advantages of simultaneous consideration of structural design and control design.

Reductions in induced drag by the use of aft swept wing tips

SummaryRecent experimental and computational studies have indicated that rearward curvature of a wing can reduce the induced drag factor to values less than that obtained from the unswept elliptical wing considered optimal in classical wing theory. Wake non-planarity associated with a wing with aft swept tips has been suggested as a reason for this behaviour. This paper provides a systematic examination of the wake non-planarities induced by a variety of wing planforms and quantifies the induced drag reductions which may be expected. Results suggest a dependence of induced drag factor upon angle of attack, and therefore upon lift coefficient. The crescent wing is identified as the planform to derive the most benefit from its non-planar wake shape, typically a 4% reduction in induced drag factor at moderate angle of attack.

Aeroelastic deformation effects on the stopped-rotor dynamics of an X-wing aircraft

A new design concept in the development of vertical takeoff and landing aircraft with high forward flight speed capability is that of the X-wing. The X-wing is a stiff, bearingless helicopter rotor system that can be stopped in flight and the blades used as two forward-swept wings and two aft-swept wings. Because of the unusual configuration in the fixed-wing mode, there is a high potential for aeroelastic divergence or flutter and coupling of blade vibration modes with rigid-body modes. An aeroelastic stability analysis of an X-wing configuration aircraft was undertaken to determine whether these problems were likely. This paper reports on the results of dynamic stability analysis in the lateral and longitudinal directions, including the vehicle rigid-body and flexible modes. A static aeroelastic analysis using the normal vibration mode equations of motion was performed to determine the cause of a loss of longitudinal static margin with increasing airspeed. This loss of static margin was found to be due to aeroelastic wash in of the forward-swept blades and wash out of the aft-swept blades moving the aircraft aerodynamic center forward of the center of gravity. This phenomenon is likely to be generic to Xwing aircraft.

Aeroelastic tailoring of aft-swept high-aspect-ratio composite wings

The integrating matrix method is used to study the aeroelastic performance of aft-swept high-aspect-ratio wings. Aeroelastic stability boundaries are shown as a function of fiber angle and dimensionless modulus parameters for straight wings of aspect ratio 14. Certain laminates show divergence for wings with low sweep angles, but for most laminates and wing sweeps, flutter was the instability. The bending-torsion coupling that is beneficial for forward-swept wings is shown to be of no advantage for aft-swept wings, for which torsional stiffness is much more significant. Both symmetric and nonsymmetric ply orientations were studied, and it is demonstrated that there need not be a severe penalty for using general laminates.

Assessment of certain merits of FSW and effect of the sign of the sweep angle on the stressing of a wing structure

Forward Swept Wings have a number of advantages over Aft Swept Wings which have been appreciated for a number of years. In the past, however, their use has been restricted to limited degrees of sweep because of the severe weight penalty necessary to prevent aerolastic divergence. Recently, it has been shown that the outstanding directional properties of fibrous composites could help overcome this problem by adequate selection of ply orientations and laminate thickness distribution of the wing skin.

Wind tunnel investigation of the transonic aerodynamic characteristics of forward swept wings

This paper discusses some of the results of a series of wind tunnel investigations conducted on forward and aft swept wings in order to compare the relative drag performance of both wings at a transonic maneuver condition and to determine the associated drag penalty of the forward swept wing for a high supersonic cruise condition. At the transonic maneuver design point, the results indicate a significant reduction in the profile drag of the forward swept wing relative to the aft swept wing. The forward swept wing exhibits extreme sensitivity to wing root height and incidence variations. A drag penalty was recorded at M— 2.0 for the forward swept cruise wing. The cruise wings have the same sweep and box geometries as the transonic maneuver wings. The difference in supersonic cruise drag is attributed to the difference in the leading-edge sweeps (ALE = +48.7 and -28 deg for the aft and forward swept wings, respectively). This drag penalty decreases at lower supersonic Mach numbers. The results of the test indicate that the aft swept wing aerodynamic design and analysis methods used in the study can be used on forward swept wings with only minor modifications.

Spanwise lift distribution of forward-and aft-swept wings in comparison to the optimum distribution form

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