Showing papers in "Journal of Aircraft in 1976"

Lifespan of trailing vortices in a turbulent atmosphere

Abstract: The lifespan of aircraft trailing vortices is controlled by a mutual induction instability excited by atmospheric turbulence. The instability itself is well understood. The purpose here is to incorporate the effects of turbulence and thereby predict wake lifespan as a function of meteorologica l conditions. Eddies of the relevant size are assumed to lie in the Kolmogorov inertial subrange, characterized by an energy dissipation rate e. The appropriate dimensionless measure of turbulence intensity proves to be T/ = (€^) 1/3/(T/27r&), where T is the circulation around the vortices, and b is their separation. Similarity considerations imply a mean wake lifespan of the form (2-Kb2/r)r(ij), where T is a universal function of rj. A statistical definition of lifespan is proposed, and T is computed in the limit of large 17, when the vortices are too weak to influence their own deformation. Vortex induction then is included, and T is computed in the opposite limit of small i? by the method of stationary phase. In that limit, vertical currents are the chief excitors of instability. The two asymptotic forms for 7(17) join in a smooth curve in reasonable agreement with the few published data. The paper concludes with a review of a practical method for actively exciting the mutual induction instability. The method would shorten the typical lifespan of a 747 wake by a factor of 3.

Correlation of turbulent trailing vortex decay data

Abstract: A correlation function, derived on the basis of self similar variable eddy viscosity decay, is introduced and utilized to correlate aircraft trailing vortex velocity data from ground and flight experiments. The correlation function collapses maximum tangential velocity data from scale model and flight tests to a single curve. The resulting curve clearly shows both the inviscid plateau and the downstream decay regions. A comparison between experimental data and numerical solution shows closer agreement with the variable eddy viscosity solution than the constant viscosity analytical solution.

Nonlinear prediction of the aerodynamic loads on lifting surfaces

Abstract: A numerical procedure is used to predict the nonlinear aerodynamic characteristics of lifting surfaces of low aspect ratio at high angles of attack for low subsonic Mach numbers. The procedure utilizes a vortex-lattice method and accounts for separation at sharp tips and leading edges. The shapes of the wakes emanating from the edges are predicted, and hence the nonlinear characteristics are calculated. Parallelogram and delta wings are presented as numerical examples. The numerical results are in good agreement with the experimental data.

Fixed-range optimum trajectories for short-haul aircraft

Abstract: An algorithm, based on the energy-state method, is derived for calculating optimum trajectories with a range constraint. The basis of the algorithm is the assumption that optimum trajectories consist of, at most, three segments: an increasing energy segment (climb); a constant energy segment (cruise); and a decreasing energy segment (descent). This assumption allows energy to be used as the independent variable in the increasing and decreasing energy segments, thereby eliminating the integration of a separate adjoint differential equation and simplifying the calculus of variations problem to one requiring only pointwise extremization of algebraic functions. The algorithm is used to compute minimum fuel, minimum time, and minimum direct-operating-cost trajectories, with range as a parameter, for an in-service CTOL aircraft and for an advanced STOL aircraft. For the CTOL aircraft and the minimum-fuel performance function, the optimum controls, consisting of air-speed and engine power setting, are continuous functions of the energy in both climb and descent as well as near the maximum or cruise energy. This is also true for the STOL aircraft except in the descent where at one energy level a nearly constant energy dive segment occurs, yielding a discontinuity in the airspeed at that energy. The reason for this segment appears to be the relatively high fuel flow at idle power of the engines used by this STOL aircraft. Use of a simplified trajectory which eliminates the dive increases the fuel consumption of the total descent trajectory by about 10 percent and the time to fly the descent by about 19 percent compared to the optimum.

Dynamic Stall Analysis in Light of Recent Numerical and Experimental Results

Abstract: An earlier developed engineering analysis of dynamic stall is reviewed in light of recent numerical and experimental results. It is found that the concept of equivalence between boundary-layer improvement due to piich-rate-induced effects and increasing Reynolds number is supported by the available numerical and experimental results. The existence of the postulated plunging-induced improvement of the boundary layer and associated delay of stall, the controversial "leading-edge jet'* effect, is indicated by oscillatory stall data for different oscillation centers and by the measured negative aerodynamic damping for plunging oscillations in the stall region. More work is needed before the dynamic stall characteristics can be predicted for high frequencies. Until then, the present technique offers a reliable means for prediction of low-frequency (d)<0.5) dynamic stall characteristics from static experimental data.

On repetitive flutter calculations in structural design

Abstract: The use of continually updated natural modes, called changing modes, is discussed together with the employment of fixed modes in the design process. 'No-derivatives' methods are considered. The flutter solution process is separated into parts which are mode dependent and parts which are mode independent. On the basis of this separation an attempt is made to identify the type of design problems which are suitable for the use of changing modes, and the type of problems which are more suitable for fixed modes. The use of derivative methods is also examined.

Transonic airfoil analysis and design using Cartesian coordinates

Abstract: An inverse numerical technique for designing transonic airfoils having a prescribed pressure distribution is presented. The method uses the full potential equation, inverse boundary conditions, and Cartesian coordinates. It includes simultaneous airfoil update and utilizes a direct-inverse approach that permits a logical method for controlling trailing edge closure. The method can also be used for the analysis of flowfields about specified airfoils. Comparison with previous results shows that accurate results can be obtained with a Cartesian grid. Examples show the application of the method to design aft-cambered and other airfoils specifically for transonic flight.

On modeling ATC work load and sector capacity

Abstract: This paper describes a semi-empirical, deterministic work load model and an evaluation procedure intended to aid in the design and evaluation of those units of airspace (sectors) under the jurisdiction of a team of air traffic controllers. The technique relates the traffic variables, route and sector geometry, and control procedures to an index that quantifies the work load required on the part of the air traffic control (ATC) team. Work load is considered to constitute the required sector evaluation criterion when maximum overall ATC facility capacity and manning efficiency are desired. With proper calibration, the model may be used to assess the impact on work load and sector capacity of future automation features. An example evaluation of an actual high altitude, enroute sector is included.

Experimental Study of the Effect of Span Loading on Aircraft Wakes

Abstract: Measurements were made in the NASA-Ames 40- by 80-Foot Wind Tunnel of the rolling moment induced on a following model in the wake 13.6 spans behind a subsonic transport model for a variety of trailing edge flap settings of the generator. It was found that the rolling moment on the following model was reduced substantially, compared to the conventional landing configuration, by reshaping the span loading on the generating model to approximate a span loading, found in earlier studies, which resulted in reduced wake velocities. This was accomplished by retracting the outboard trailing edge flaps. It was concluded, based on flow visualization conducted in the wind tunnel as well as in a water tow facility, that this flap arrangement redistributes the vorticity shed by the wing along the span to form three vortex pairs that interact to disperse the wake.

Failure Accommodation in Digital Flight Control Systems by Bayesian Decision Theory

Abstract: A design method for digital control systems which is optimally tolerant of failures in aircraft sensors is presented. The functions of this system are accomplished with software instead of the popular and costly technique of hardware duplication. The approach taken, based on M-ary hypothesis testing, results in a bank of Kalman filters operating in parallel. A moving window of the innovations of each Kalman filter drives a detector that decides the failure state of the system. The detector calculates the likelihood ratio for each hypothesis corresponding to a specific failure state of the system. It also selects the most likely state estimate in the Bayesian sense from the bank of Kalman filters. The system can compensate for hardover as well as increased noise-type failures by computing the likelihood ratios as generalized likelihood ratios. The design method is applied to the design of a fault tolerant control system for a current configuration of the space shuttle orbiter at Mach 5 and 120,000 ft. The failure detection capabilities of the system are demonstrated using a real-time simulation of the system with noisy sensors.

Ejector performance at high temperatures and pressures

Abstract: Attention has recently been given to the use of thrust augmenting ejectors in the wings of V/STOL aircraft. Laboratory experiments using low temperature and pressure primary air have measured high-performance levels with well-designed ejectors. The present experiments were motivated by aircraft designers' questions regarding the effects of realistic temperatures and pressures on ejector performance. The simplest geometry was used: a convergent nozzle issuing into an axisymmetric duct that entrained from and exhausted to ambient conditions. The length of the ejector was varied from 12 to 0.75 diam. Primary temperatures and pressures spanned the intervals 60 to 1000°F and 10 to 80 psig. In support of existing theory, the mass entrainment performance usually decreased with increasing primary pressure although an aeroacoustic interaction reversed the trend over small intervals. Increasing the primary temperature decreased the performance of long ejectors but had little effect on the performance of short ejectors. The results are interpreted in terms of measurements of the pressure along the wall of the mixing duct and total pressure and temperature profiles acquired at the exhaust plane of the ejector.

Flight Performance of a Circulation Controlled STOL Aircraft

Abstract: Theoretical and wind tunnel studies have been performed on various high-lift airfoils using circulation control by blowing over a circular trailing edge. On the basis of these studies, a full scale Technology Demonstrator STOL aircraft was designed, constructed, and flight tested/Circulation control blowing air was provided by bleed air from a gas turbine. The first series of flight tests have recently been completed. Satisfactory STOL performance and handling characteristics were obtained. Advantages of this system are high lift to power ratio, and near level aircraft attitude at all speeds.

Prediction of Vortex Flow Characteristics of Wings at Subsonic and Supersonic Speeds

Abstract: The leading-edge suction analogy of Polhamus, which has been successful in the prediction of vortex lift characteristics on wings with pointed tips at subsonic and supersonic speeds, has recently been extended to account for the vortex flow characteristics for wings with side edges. Comparisons of experimental data and other currently used methods with the extended method are made for wings having side edges at subsonic and supersonic speeds. Recent data obtained for a low-aspect-rat io cropped-delta wing with various amounts of asymmetrical tip rake, simulating a roll control device, are also presented.

Periodic control and the optimality of aircraft cruise

Abstract: ENGINEERING NOTES are short manuscripts describing new developments or important results of a preliminary nature. These Notes cannot exceed 6 manuscript pages and 3 figures; a page of text may be substituted for a figure and vice versa. After informal review by the editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).

Thermal-Structural Design/Analysis of an Airframe-Integrated Hydrogen-Cooled Scramjet

Abstract: This paper presents the salient features of a preliminary thermal-structural design and analysis study of a hydrogen-fueled, regeneratively cooled, airframe-integrated scramjet. This three-dimensional fixed geometry scramjet concept, which was developed at NASA Langley Research Center, is designed to operate over a flight Mach number range from 4 to 10. The thermal-structural study was focused on a scramjet application to one concept for a hypersonic research vehicle and was based on technology developed under the NASA Hypersonic Research Engine Project. State-of-the-art analytical methods consisting of lumped system and finite-difference steady-state thermal analyses and a finite-element structural analysis were used. The results of the study indicated that this scramjet concept is viable from both a structural mass and cooling requirement standpoint. However, advances in material and fabrication technology for hydrogen-cooled structures appear necessary for acceptable engine life for commercial application.

Propulsive efficiency from an energy utilization standpoint

Abstract: The standard but ill-defined concept of propulsive efficiency, classically employed in the analysis of turbojet and turbofan engines, can be approached from the point of view of energy utilization by applying basic thermodynamic principles. First and second law analysis of propulsive powerplants as general thermodynamic systems leads to a universal definition for propulsive efficiency. This definition clearly accounts for the energy unavailability production separated into two distinct parts: 1) the unavailable energy associated with the thermodynamic cycle's rejected heat; and 2) the wasted energy produced by inefficiencies inherent to the conversion of available cycle energy to propulsive power. As a result, conversion or transfer inefficiencies are consistently reflected in the propulsive efficiency, whereas cycle inefficiencies are most properly identified with cycle, or thermal, efficiency for a powerplant.

Development of Theoretical Models for Jet-Induced Effects on V/STOL Aircraft

Abstract: Successful design and development of V/STOL aircraft requires an understanding and accurate prediction of aircraft forces and moments caused by jet-induced phenomena. Several computerized analytic procedures have been developed using potential flow theoretical solutions to model the entrainment and displacement effects of a propulsion system efflux. The various flow regimes and theoretical models will be described for out-of-ground effect and in-ground effect, where multijet interactions with the ground plane cause significantly increased entrainment and fountain forces. These jet models are coupled with a panel method for calculating the aerodynamic forces and moments. Excellent agreement between calculated and model test data is displayed.

Airborne Windmills and Communication Aerostats

Abstract: to the onset of static stall, as = 11°. It is interesting to note that the unslotted airfoil had a more severe decrease in Vf with increasing a0 than the slotted airfoil, for ot0

Near-hover control of a helicopter with a hanging load

Abstract: Piloting a helicopter with a hanging load is a difficult task, especially when the mass of the load is a significant fraction of the mass of the vehicle and there are gusty winds An autopilot logic is proposed here for controlling the helicopter in this configuration and for precision hover It is proposed that the vehicle position be measured using a lightweight cable from the helicopter to a point on the ground near the desired hover point Simulation with one version of S-61 Sikorsky helicopter shows satisfactory controller performance under both design conditions and for parameter changes from one mission to another Assuming noise-free measurements for feedback is found to be far too optimistic in predicting performance, the sensor/estimator design is a key element in the controller

Aerodynamic characteristics of slender wings with sharp leading edges - A review

Abstract: This paper presents an overview of the current state-of-the-art regarding slender wings with sharp leading edges; i.e., wings characterized by the presence of leading edge separation at most angles of attack. Several theoretical methods are discussed in detail and their results are compared with experimental data. Both steady and some unsteady flows are considered. No one theory adequately predicts all aspects of the flow process, and more work is needed, particularly in the fields of vortex control and unsteady flow.

Practical Design of Minimum-Weight Aircraft Structures for Strength and Flutter Requirements

Abstract: Several methods for sizing the finite elements of an aircraft structural idealization to achieve minimum-weight design under combined strength and flutter-speed requirements are developed and evaluated. Two basic categories are considered: methods based on a combination of energy principles and optimality criteria; and procedures employing numerical-search techniques. Drawing upon the experience gained from studies of both of these basic methods, a resizing algorithm is developed that employs a uniform-flutter-velocity-derivative optimality criterion for flutter-critical elements and the fully-stressed-design criterion for strength-critical elements. The final result is a practical, automated approach for dealing with large-scale idealizations having both structural and mass-balance design variables.

New Air Force Requirements for Structural Safety, Durability, and Life Management

Abstract: The past five years have seen significant changes in the Air Force philosophy and approach in achieving structural safety and durability in military aircraft. These changes have been motivated by problems of high cost, late system development programs, with a high level of in-service structural maintenance and modification costs (poor durability) and, in some cases, less than desired fracture resistance (poor damage tolerance/safety). The problem area has been attacked along a number of avenues; one major thrust has been a thorough examination and revision of the structural design and test specifications, the MIL-A-8860 series. In this paper, certain aspects of the overall problem are discussed; an overview of the pre-1969/70 Air Force approach is presented, along with its short-comings; and, finally, significant aspects of current policy are listed, giving comparisons with the old requirements.

Approximate Prediction of Airframe Noise

Abstract: Theme IRFRAME noise from aerodynamically clean airframes A is assumed to be trailing-edge noise caused by convection of the wing turbulent boundary layer past the wing trailing edge. Resulting maximum flyover noise is predicted from an existing solution for trailing-edge noise plus estimates of turbulent boundary-layer integral scale length and turbulence intensity. Typical airplanes with retracted or well-faired landing gear but with wing-mounted engine nacelles and extensive trailing-edge flap-track shields generate about 8 dB more than this minimum trailing-edge noise. Resulting predicted acoustic intensity varies with airspeed t o the fifth power, in agreement with a linear regression analysis of flight data. ' Trailing-edge noise has been previously inferred' to be the mechanism for airframe noise because of the observed fifthpower velocity dependence. In contrast, other analyses 3.4 have assumed a lift dipole directivity associated with fluctuating-drag noise mechanisms and have predicted a sixthpower velocity dependence.

Leading-Edge-Vortex Augmentation in Compressible Flow

Abstract: Leading-edge-vortex enhancement by blowing has been explored experimentally. Conceptual half-span windtunnel tests were conducted on a wing-body-tail configuration with a cambered and twisted wing with leadingedge flaps. Blowing vortex augmentation is shown to be effective in improving both lift and drag due to lift at high angle of attack for the Mach numbers tested, M=0.3 and 0.75. Drag improvement results, in part, from an apparent vortex-suction effect on the cambered wing.