Tunnel/Predictor Display for Trajectory Control in Hypersonic Flight

A tunnel/predictor display which presents guidance information in a 3-dimensional format is considered for improving trajectory control in hypersonic flight. The displayed 3-dimensional information comprises a tunnel image and a predictor for indicating the aircraft position at a specified time ahead. The 3-dimensional guidance information is introduced to support the pilot in controlling the flight path. It is considered that piloting problems can be avoided which exist with conventional trajectory control techniques due to path-attitude decoupling. A predictor control law is constructed which yields controlled element properties (predictor-aircraft system) requiring minimum pilot compensation. This predictor control law forms the basis of the trajectory control improvement goal. Results from hypersonic flight simulation tests at the NASA Dryden Flight Research Center are presented for experimental verification. This paper is an outcome of a joint research effort of the NASA Dryden Flight Research Center, and the Institute of Flight Mechanics and Flight Control of the Technische Universität München and the Department of Aeronautics, Naval Architecture and Railway Vehicles (former Department of Aircraft and Ships at the Budapest University of Technology and Economics

Unsteady Aerodynamics of Flapping Wing of a Bird

The unsteady flow behavior and time-dependent aerodynamic characteristics of the flapping motion of a bird's wing were investigated using a computational method. During flapping, aerodynamic interactions between bird wing surfaces and surrounding flow may occur, generating local time-dependent flow changes in the flow field and aerodynamic load of birds. To study the effect of flapping speed on unsteady aerodynamic load, two kinds of computational simulations were carried out, namely a quasi-steady and an unsteady simulation. To mimic the movement of the down-stroke and the upstroke of a bird, the flapping path accorded to a sinus function, with the wing attitude changing in dihedral angle and time. The computations of time-dependent viscous flow were based on the solution of the Reynolds Averaged Navier-Stokes equations by applying the k-e turbulence model. In addition, the discretization for the computational domain around the model used multi-block structured grid to provide more accuracy in capturing viscous flow, especially in the vicinity of the wing and body surfaces, to obtain a proper wing-body geometry model. For this research, the seagull bird was chosen, which has high aspect ratio wings with pointed wing-tips and a high camber wing section. The results include mesh movement, velocity contours as well as aerodynamic coefficients of the flapping motion of the bird at various flapping frequencies

Frigate birds track atmospheric conditions over months-long transoceanic flights

International audienceUnderstanding how animals respond to atmospheric conditions across space is critical for understanding the evolution of flight strategies and long-distance migrations. We studied the three-dimensional movements and energetics of great frigate birds (Fregata minor) and showed that they can stay aloft for months during transoceanic flights. To do this, birds track the edge of the doldrums to take advantage of favorable winds and strong convection. Locally, they use a roller-coaster flight, relying on thermals and wind to soar within a 50- to 600-meter altitude band under cumulus clouds and then glide over kilometers at low energy costs. To deal with the local scarcity of clouds and gain longer gliding distances, birds regularly soar inside cumulus clouds to use their strong updraft, and they can reach altitudes of 4000 meters, where freezing conditions occur

Philosophy Enters the Optics Laboratory: Bell's Theorem and its First Experimental Tests (1965-1982)

This paper deals with the ways that the issue of completing quantum mechanics was brought into laboratories and became a topic in mainstream quantum optics. It focuses on the period between 1965, when Bell published what now we call Bell's theorem, and 1982, when Aspect published the results of his experiments. I argue that what was considered good physics after Aspect's experiments was once considered by many a philosophical matter instead of a scientific one, and that the path from philosophy to physics required a change in the physics community's attitude about the status of the foundations of quantum mechanics.Comment: 57 pages, accepted by Studies in History and Philosophy of Modern Physic

Performance Enhancement by Wing Sweep for High-Speed Dynamic Soaring

Dynamic soaring is a flight mode that uniquely enables high speeds without an engine. This is possible in a horizontal shear wind that comprises a thin layer and a large wind speed. It is shown that the speeds reachable by modern gliders approach the upper subsonic Mach number region where compressibility effects become significant, with the result that the compressibility-related drag rise yields a limitation for the achievable maximum speed. To overcome this limitation, wing sweep is considered an appropriate means. The effect of wing sweep on the relevant aerodynamic characteristics for glider type wings is addressed. A 3-degrees-of-freedom dynamics model and an energy-based model of the vehicle are developed in order to solve the maximum-speed problem with regard to the effect of the compressibility-related drag rise. Analytic solutions are derived so that generally valid results are achieved concerning the effects of wing sweep on the speed performance. Thus, it is shown that the maximum speed achievable with swept wing configurations can be increased. The improvement is small for sweep angles up to around 15 deg and shows a progressive increase thereafter. As a result, wing sweep has potential for enhancing the maximum-speed performance in high-speed dynamic soaring

Comparison of Power Requirements: Flapping vs. Fixed Wing Vehicles

The power required by flapping and fixed wing vehicles in level flight is determined and compared. Based on a new modelling approach, the effects of flapping on the induced drag in flapping wing vehicles are mathematically described. It is shown that flapping causes a significant increase in the induced drag when compared with a non-flapping, fixed wing vehicle. There are two effects for that induced drag increase; one is due to tilting of the lift vector caused by flapping the wings and the other results from changes in the amount of the lift vector during flapping. The induced drag increase yields a significant contribution to the power required by flapping wing vehicles. Furthermore, the power characteristics of fixed wing vehicles are dealt with. It is shown that, for this vehicle type, the propeller efficiency plays a major role. This is because there are considerable differences in the propeller efficiency when taking the size of vehicles into account. Comparing flapping and fixed wing vehicles, the conditions are shown where flapping wing vehicles have a lower power demand and where fixed wing vehicles are superior regarding the required power. There is a tendency such that fixed wing vehicles have an advantage in the case of larger size vehicles and flapping wing vehicles have an advantage in the case of smaller size ones

Flight of frigatebirds inside clouds – energy gain, stability and control

International audienceInvestigating the unique ability of frigatebirds of flying inside clouds, it is shown that they achieve a large energy gain by ascents to high altitudes in strong updrafts of trade cumulus clouds. Frigatebirds often perform that kind of flight, at daytime as well as in the night. This suggests that they are capable of flying inside clouds in a controlled and stabilized manner. The control requirements for ascents in terms of a circling flight in updrafts of trade cumulus clouds are analyzed, and the necessary aerodynamic control moments are determined. Based on a stability investigation, it is shown that there are restoring effects which act against disturbances causing possible deviations from the circling flight condition. The aerodynamic moments which effectuate that stabilization are identified. Furthermore, the problem of neutral azimuth stability which generally exists in the flight of birds and which is the reason for continually increasing deviations from the course is dealt with. It is shown for the circling flight mode of frigatebirds inside clouds that, here, deviations are small and remain constant, suggesting that a corrective control action is not required. This is particularly important for circling flight in conditions without a visual reference, like inside clouds