The investigation deals with the dynamic behavior of an airplane encountering aircraft wake turbulence. A digital computer simulation was developed to study the response of an aircraft flying into a trailing vortex wake. The simulation includes the complete six degree of freedom equations of motion, a description of the vortex velocity field, unsteady aerodynamics, and pilot control input. The parameters included the penetration angle, separation distance, aircraft size, and pilot control input. Predicted vortex induced motions are presented for general aviation, business, and light jet transport type aircraft.
Without major changes, the current air transportation system will be unable to accommodate the expected increase in demand by 2025. One proposal to address this problem is to use the Global Positioning System to enable aircraft to fly more closely spaced. This approach, however, might be limited by the wake turbulence problem, which can be a safety hazard when smaller aircraft follow relatively larger aircraft too closely. To examine how this potential hazard might be reduced, Congress in 2005 directed NASA to request a study from the NRC to assess the federal wake turbulence R&D program. This book provides a description of the problem, an assessment of the organizational challenges to addressing wake turbulence, an analysis of the technical challenges in wake turbulence, and a proposal for a wake turbulence program plan. A series of recommendations for addressing the wake turbulence challenge are also given.
The combination of increasing airport congestion and the ad vent of large transports has caused increased interest in aircraft wake turbulence. A quantitative understanding of the interaction between an aircraft and the vortex wake of a preceding aircraft is necessary for planning future high density air traffic patterns and control systems. The nature of the interaction depends on both the characteristics of the following aircraft and the characteristics of the wake. Some of the questions to be answered are: What deter mines the full characteristics of the vortex wake? What properties of the following aircraft are important? What is the role of pilot response? How are the wake characteristics related to the genera ting aircraft parameters? How does the wake disintegrate and where? Many of these questions were addressed at this first Aircraft Wake Turbulence Symposium sponsored by the Air Force Office of Sci entific Research and The Boeing Company. Workers engaged in aero dynamic research, airport operations, and instrument development came from several count ries to present their results and exchange information. The new results from the meeting provide a current picture of the state of the knowledge on vortex wakes and their interactions with other aircraft. Phenomena previously regarded as mere curiosities have emerged as important tools for understanding or controlling vortex wakes. The new types of instability occurring within the wake may one day be used for promoting early dis integration of the hazardous twin vortex structure.
The study illustrated in these pages was developed in the Structural Dynamics and Aeroelasticity Department of the Military Aircraft division of Airbus Defence and Space in Getafe, Madrid (Spain). That department is a multidisciplinary one involving several categories. Some of its competences are the analysis of impacts, acoustics and vibrations for the aircraft and all their systems. Also, the dynamic response of the aircraft to different events is part of the tasks for that department. It is in that field where this project is located. Wake vortex encounters are a dynamic phenomenon similar to other excitations faced by aircraft in its usual operation. That excitation will induce dynamic loads in the aircraft structures as it passes through the wake. Obtaining and analyzing those loads are the main aims of this study. First, it is important to introduce the concept of aircraft wake. In order to achieve the goals of the study, it is necessary to start with the basics of the problem. This is the acknowledgement of the physics of aircraft wakes and how they are generated. Then, how those wakes act as an excitation for other aircraft as they perturb the velocity field of the air. Once the aircraft wakes are introduced, the next step consists of explaining the corresponding aircraft that will face those encounters. In this case, the particular aircraft is an Airbus A400M because that was the one used by Airbus Defence and Space in the campaign of the wake vortex encounters. The following considerations involve the procedures of the Structural Dynamics department in the computation of the numerical simulation for that kind of event. Therefore, it is necessary to know what the tools available for the resolution of this problem are and which aspects face each one. Furthermore, the software and models used in this study have been validated with flight tests and they are an excellent representation of the aircraft and problem considered. After the presentation of the problem, the aircraft considered and tools used, the wake model is the next aspect of study. This is one of the widest fields of aircraft wakes. Having a good wake model is necessary for the accuracy of the results and the main tasks are focused on the wake aging model. That is how the wake evolves and changes as times goes by. With all that, the last aspect of this study is the computation of the dynamic loads induced in wake vortex encounters. As it is an event that is not considered in aircraft regulations, it was difficult to define a particular scenario that is representative of the problem. For that reason, a stochastic analysis of the problem has been chosen to obtain the aircraft loads generated in this kind of event. This analysis consists of scattering the input variables affecting the problem and executing a high number of simulations. In that way, it is possible to derive a statistical behavior of the load response and establish a probability of occurrence for the results obtained. Finally, comparing those results with the reference loads of the aircraft, it is feasible to predict whether wake vortex encounters could be a significant event for the aircraft structure during its operational life.
Investigation of vortex wakes behind various aircraft, especially behind wide bodied and heavy cargo ones, is of both scientific and practical in terest. The vortex wakes shed from the wing’s trailing edge are long lived and attenuate only atdistances of10–12kmbehindthe wake generating aircraft. The encounter of other aircraft with the vortex wake of a heavy aircraft is open to catastrophic hazards. For example, air refueling is adangerous operationpartly due to thepossibility of the receiver aircraft’s encountering the trailing wake of the tanker aircraft. It is very important to know the behavior of vortex wakes of aircraft during theirtakeoff andlanding operations whenthe wakes canpropagate over the airport’s ground surface and be a serious hazard to other depart ing or arriving aircraft. This knowledge can help in enhancing safety of aircraft’s movements in the terminal areas of congested airports where the threat of vortex encounters limits passenger throughput. Theoreticalinvestigations of aircraft vortex wakes arebeingintensively performedinthe major aviationnations.Usedforthispurpose are various methods for mathematical modeling of turbulent flows: direct numerical simulation based on the Navier–Stokes equations, large eddy simulation using the Navier–Stokes equations in combination with subrigid scale modeling, simulation based on the Reynolds equations closed with a differential turbulence model. These approaches are widely used in works of Russian and other countries’ scientists. It should be emphasized that the experiments in wind tunnels and studies of natural vortex wakes behind heavy and light aircraft in flight experiments are equally important.
