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The effect of helicopter rotor blade planform sweep on rotor vibratory hub, blade, and control system loads has been analytically investigated. The importance of sweep angle, sweep initiation radius, flap bending stiffness and torsion bending stiffness is discussed. The mechanism by which sweep influences the vibratory hub loads is investigated.
The effects of passive leading-edge devices on the boundary layer of a swept cylinder in supersonic flow with very low free-stream disturbance levels are investigated. Tests are conducted at Mach 3.5 over a free-stream Reynolds number based on model diameter of 15,000 to 150,000, and sweep angles of 60 deg and 76 deg are used to evaluate possible sweep-angle effects. The devices tested include a sawtoothed leading edge, square device, and a fence. It is observed that at a sweep angle of 76 deg, relaminarization of the supersonic attachment-line flow is achieved by several of the devices, while no devices are successful at a sweep angle of 60 deg.
Because the effective 'area' of the Neptunian rings is larger than that of the inner moons, the sweeping of energetic particles by the rings is perhaps the dominant process for particle loss in the magnetosphere within 5 R(N). In this paper, a theory for calculating the absorption probability of energetic charged particles by the rings is described. The effects of a large tilt and an offset between the planet and dipole centers are included. It is found that the probability of absorption for protons is so high that the sweeping lifetime is only a few times the gradient-curvature drift period. For electrons, the sweeping lifetime is even less. The pitch angle dependence for sweeping manifests itself strongly only at large equatorial pitch angles. Lower-energy particles have higher absorption rates by the rings.
A computational and experimental study of generic 3D sidewall compression inlets is conducted to examine the effects of fore and aft leading edge sweep on the internal shock structure. Inlets with leading edge sweeps of +30 deg and -30 deg with sidewall compression angles of 6 deg were tested in the NASA Langley Mach 4 air tunnel at a geometric contraction ratio of 1.87. The principal difference in performance was determined to be in the mass capture. Spillage was identified as having two components: a pressure induced component and a sweep induced component. It was found that while the direction of the leading edge sweep had a large influence on the spillage, the pressure effects were more important.
Studies of the stalling characteristics show that the stall begins at the tip and moves inboard with increasing angle of attack at positive sweep; the sta11 begins at root and moves outboard at negative sweep (sweepforward). At +/-45 deg sweep the stall was less sharply defined than at the lower angles of sweep. No effect of Mach number on the.flow patterns as indicated by tufts was found in the speed range of these tests which extended to a Mach number of 0.55.
Detailed flow behavior in a modern transonic fan with a composite sweep is investigated in this paper. Both unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) methods are applied to investigate the flow field over a wide operating range. The calculated flow fields are compared with the data from an array of high-frequency response pressure transducers embedded in the fan casing. The current study shows that a relatively fine computational grid is required to resolve the flow field adequately and to calculate the pressure rise across the fan correctly. The calculated flow field shows detailed flow structure near the fan rotor tip region. Due to the introduction of composite sweep toward the rotor tip, the flow structure at the rotor tip is much more stable compared to that of the conventional blade design. The passage shock stays very close to the leading edge at the rotor tip even at the throttle limit. On the other hand, the passage shock becomes stronger and detaches earlier from the blade passage at the radius where the blade sweep is in the opposite direction. The interaction between the tip clearance vortex and the passage shock becomes intense as the fan operates toward the stall limit, and tip clearance vortex breakdown occurs at near-stall operation. URANS calculates the time-averaged flow field fairly well. Details of measured RMS static pressure are not calculated with sufficient accuracy with URANS. On the other hand, LES calculates details of the measured unsteady flow features in the current transonic fan with composite sweep fairly well and reveals the flow mechanism behind the measured unsteady flow field.
A parametric experimental study was performed with sweeping jet actuators (fluidic oscillators) to determine their effectiveness in controlling flow separation on an adverse pressure gradient ramp. Actuator parameters that were investigated include blowing coefficients, operation mode, pitch and spreading angles, streamwise location, aspect ratio, and scale. Surface pressure measurements and surface oil flow visualization were used to characterize the effects of these parameters on the actuator performance. 2D Particle Image Velocimetry measurements of the flow field over the ramp and hot-wire measurements of the actuator's jet flow were also obtained for selective cases. In addition, the sweeping jet actuators were compared to other well-known flow control techniques such as micro-vortex generators, steady blowing, and steady vortex-generating jets. The results confirm that the sweeping jet actuators are more effective than steady blowing and steady vortex-generating jets. The results also suggest that an actuator with a larger spreading angle placed closer to the location where the flow separates provides better performance. For the cases tested, an actuator with an aspect ratio, which is the width/depth of the actuator throat, of 2 was found to be optimal. For a fixed momentum coefficient, decreasing the aspect ratio to 1 produced weaker vortices while increasing the aspect ratio to 4 reduced coverage area. Although scaling down the actuator (based on the throat dimensions) from 0.25 inch x 0.125 inch to 0.15 inch x 0.075 inch resulted in similar flow control performance, scaling down the actuator further to 0.075 inch x 0.0375 inch reduced the actuator efficiency by reducing the coverage area and the amount of mixing in the near-wall region. The results of this study provide insight that can be used to design and select the optimal sweeping jet actuator configuration for flow control applications.
Progress on experimental efforts to optimize sweeping jet actuators for active flow control (AFC) applications with large adverse pressure gradients is reported. Three sweeping jet actuator configurations, with the same orifice size but di↵erent internal geometries, were installed on the flap shoulder of an unswept, NACA 0015 semi-span wing to investigate how the output produced by a sweeping jet interacts with the separated flow and the mechanisms by which the flow separation is controlled. For this experiment, the flow separation was generated by deflecting the wing's 30% chord trailing edge flap to produce an adverse pressure gradient. Steady and unsteady pressure data, Particle Image Velocimetry data, and force and moment data were acquired to assess the performance of the three actuator configurations. The actuator with the largest jet deflection angle, at the pressure ratios investigated, was the most efficient at controlling flow separation on the flap of the model. Oil flow visualization studies revealed that the flow field controlled by the sweeping jets was more three-dimensional than expected. The results presented also show that the actuator spacing was appropriate for the pressure ratios examined.
The development of a multimission military aircraft would be highly desirable both from the standpoint of easing the strain on the national budget by reducing the number of aircraft types and of providing versatility which would increase the effectiveness of the aircraft in the performance of a given mission. Some of the capabilities which might be required of such an aircraft are shown in figure 1 along with their respective aerodynamic and configuration requirements. The first three capabilities are grouped together since they all require good subsonic characteristics. The first, a long loiter capability for combat air patrol and the second, a long ferry range for efficient aircraft deployment both require a high subsonic lift-drag ratio. The third capability, STOL, is desirable for carrier and short-field operation and requires the development of high lift. All three of these capabilities can best be obtained with a high-aspect-ratio wing having a large span and a low-sweep angle. The fourth capability is that of a high-altitude supersonic attack or intercept and requires a high lift-drag ratio at supersonic speeds which dictates a rather slender configuration with a moderate-span wing which is either very thin or highly swept. The fifth capability listed in figure 1 is that of a low-altitude high-speed attack that would increase the probability of long-range penetration of antiaircraft defenses. The high dynamic pressures encountered on the deck at high speeds require a low-lift-curve slope to reduce the gust-induced normal accelerations, and low friction and wave drag (drag due to lift is insignificant at high dynamic pressures) to assure sufficient speed and range. In order to best satisfy these requirements, a slender aircraft having little or no wing is required. It is apparent from figure 1 that these five capabilities are highly incompatible and that an efficient multimission aircraft will require a means of varying its aerodynamic characteristics. This can be best accomplished with some type of variable-wing geometry. There are, of course, several types of variable wing geometry. However, in view of the extremely large variations in wing span desired, variable wing sweep, as indicated In the lower right sketch of figure 1, appears to provide the best method. The Langley Research Center of the National Aeronautics and Space Administration has therefore initiated a research program to provide the aerodynamic information needed for the development of a variable-sweep multimission military aircraft, and it is the purpose of this paper to briefly describe some of the results of this program.
Measured values of lift, drag, and pitching moment at a Mach number of 1.53 and Reynolds numbers of 0.31, 0.62, and 0.84 million are presented for a wing-fuselage combination having a wing leading-edge sweep angle of 63 degrees, an aspect ratio of 3.42, a taper ratio of 0.25, and an NACA 64A006 section in the stream direction. Data are also presented for sweep angles of 57.0 degrees, 60.4 degrees, 67.0 degrees, and 69.9 degrees. The experimentally determined characteristics were less favorable than indicated by the linear theory but the experimental and theoretical trends with sweep were in good agreement. Boundary-layer-flow tests showed that laminar boundary-layer separation was the primary cause of the differences between experiment and theory.
Sweeping jet actuators are currently being studied as a method of active flow control to increase the performance of wings and tails. Computational Fluid Dynamics simulations show that the jet does not sweep at a constant rate, instead spending more time at the minimum and maximum of its extent. As part of a test done to provide dynamic flow measurements for Computation Fluid Dynamics validation in NASA Ames Fluid Mechanics Lab, Particle Image Velocimetry (PIV) was performed on a single sweeping jet actuator designed by NASA and Boeing that was inset in a small airfoil. However, even at low jet pressures, the PIV could not be acquired at high enough frequencies to both spatially and temporally resolve the stickiness of the jet motion. Therefore, a small portion of the PIV laser was picked off and used to acquire flow visualization at higher frequencies through a new technique, laser speckle Retroreflective Background Oriented Schlieren (RBOS).
We develop a simple, high-order, conservative and robust positivity-preserving sweeping procedure for the density and the nonlinear pressure function in the compressible Euler equations. Using the scaling limiter in Zhang and Shu (J Comput Phys 229:3091–3120, 2010), we obtain a non-trivial extension of the scalar sweeping technique in Liu et al. (J Sci Comput 73:1028–1071, 2017) for the positivity of pressure. The sweeping procedure developed in this paper is a post-processing technique, which can be applied to any concave functions of the conserved variables in hyperbolic conservation law systems. Thus, it has applications beyond the Euler equations. This procedure preserves positivity and conservation of physical quantities without destroying the accuracy of the underlying scheme. The algorithm works for general schemes including finite difference, finite volume and discontinuous Galerkin methods; however, in this paper we focus on finite difference weighted essentially non-oscillatory (WENO) methods. As a result, we provide numerical tests of the fifth-order finite difference WENO scheme to demonstrate the accuracy and robustness of the technique.
A second-order method for numerically solving control optimization problems has been developed. The method, referred to as the modified sweep method (MSM), differs from the successive sweep method (SSM) proposed by McReynolds and Bryson (1965) in that the conditions for local control optimality are used to determine the control as an explicit function of the state variables and time. The control is eliminated from the problem and the solution to the resulting two-point boundary value problem can be obtained by linear perturbation methods. The modified sweep method proposed here uncouples the perturbation equations for the state variables and the Lagrange multipliers by using a generalized matrix-Riccati transformation of variables. The resulting algorithm for the numerical iteration process is concerned with determining the initial values of a set of Lagrange multipliers rather than correcting a numerical control programme over the entire time interval of interest.
An investigation has been made to determine the effects of the location of the wing pivot and geometry of the forewing on the static longitudinal aerodynamic characteristics at subsonic speeds of a model representing a variable-sweep supersonic fighter airplane. Results indicate that as the wing-pivot location moves aft and outboard, the change in static margin due to wing sweep is reduced. Increasing the forewing area resulted in a forward shift of the aerodynamic center as well as a slight reduction in the aerodynamic-center variation due to wing sweep.
A hybrid computer program is presented which can process as many as 10 channels of sweep frequency data simultaneously. The program needs only the sine sweep signal used to drive the system, and its correponding quadrature component, to process the data. It can handle a maximum frequency range of 0.5 to 500 hertz. Magnitude and phase are calculated at logarithmically spaced points covering the frequency range of interest. When the sweep is completed, these results are stored in digital form. Thus, a tabular listing and/or a plot of any processed data channel or the transfer function relating any two of them is immediately available.
Low-speed wind tunnel tests were conducted to study the influence of sweep angle on the pressure distributions of an ogee-tip configuration with relation to the effectiveness of the ogee tip in diffusing a line vortex. In addition to the pressure data, performance and flow-visualization data were obtained in the wind tunnel tests to evaluate the application of the ogee tip to aircraft configurations. The effect of sweep angle on the performance characteristics of a conventional-tip model, having equivalent planform area, was also investigated for comparison with the ogee-tip configuration. Results of the investigation generally indicate that sweep angle has little effect on the characteristics of the ogee in diffusing a line vortex.
A generalized wind-tunnel model, with canard and wing planforms typical of highly maneuverable aircraft, was tested in the Langley high-speed 7- by 10-foot tunnel at a Mach number of 0.30. The test was conducted in order to determine the effects of canard sweep and canard dihedral on canard-wing interference at high angles of attack. In general, the effect of canard sweep on lift is small up to an angle of attack of 16 deg. However, for angles of attack greater than 16 deg, an increase in the canard sweep results in an increase in lift developed by the canard when the canard is above or in the wing chord plane. This increased lift results in a lift increase for the total configuration for the canard above the wing chord plane. For the canard in the wing chord plane, the increased canard lift is partially lost by increased interference on the wing.
A detailed investigation of flight buffeting response of an F-111A aircraft was performed. AIAA Paper No. 74-358 presented results of an initial study of wing and fuselage responses measured at subsonic speeds and wing leading-edge sweep of 26 degrees. The present paper gives additional results for wing sweeps of 26, 50 and 72.5 degrees at Mach numbers up to 1.2 including horizontal tail responses. Power spectra, response time histories, variations of rms response with angle of attack, and effects of Mach number and wing sweep angle are discussed.