Gottfried Sachs

Technische Universität München, München, Bavaria, Germany

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Publications (96)30.55 Total impact

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    ABSTRACT: Dynamic soaring is a small-scale flight manoeuvre which is the basis for the extreme flight performance of albatrosses and other large seabirds to travel huge distances in sustained non-flapping flight. As experimental data with sufficient resolution of th ese small-scale movements are not available, knowledge is lacking about dynamic soaring and the physical mechanism of the energy gain of the bird from the wind. With new in-house developments of GPS logging units for recording raw phase observations and of a dedicated mathematical method for postprocessing these measurements, it was possible to determine the small-scale flight manoeuvre with the required high precision. Experimental results from tracking 16 wandering albatrosses (Diomedea exulans) in the southern Indian Ocean show the characteristic pattern of dynamic soaring. This pattern consists of four flight phases comprising a windward climb, an upper curve, a leeward descent and a lower curve, which are continually repeated. It is shown that the primary energy gain from the shear wind is attained in the upper curve where the bird changes the flight direction from windward to leeward. As a result, the upper curve is the characteristic flight phase of dynamic soaring for achieving the energy gain necessary for sustained non-flapping flight.
    Journal of Experimental Biology 11/2013; 216(Pt 22):4222-32. · 3.00 Impact Factor
  • Gottfried P. Sachs
    AIAA Atmospheric Flight Mechanics (AFM) Conference; 08/2013
  • Gottfried P. Sachs, Jakob Lenz, Florian Holzapfel
    AIAA Atmospheric Flight Mechanics (AFM) Conference; 08/2013
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    ABSTRACT: THE MALE ALBATROSS IS FINALLY BACK FROM his foraging, and now there is no time to lose. His mate has been patiently sitting on their nest awaiting his return, without food, for nearly a month, and we have to get to her before she flies off to forage for herself. · Our colleague, biologist Anna Nesterova, crawls slowly toward the bird. All of a sudden, she lunges: With her left hand she expertly grabs the 10-kilogram albatross by the beak, and with her right arm she hugs its body and lifts it off the nest and its precious cargo, a single egg. Together we then fix a GPS logger onto the feathers of the bird's back with adhesive tape and glue. · Soon after we release her, the mother albatross takes two or three steps into the furious wind, opens her 3-meter-wide wings, and takes flight. Four weeks from now, she'll return to this island in the southern Indian Ocean bearing a data log of where and how she flew-data that at last will put to the test our theories of how she stays aloft so long, almost never touching down, barely even flapping her long, elegant wings. If we could get our aerial robots to emulate that feat, they might someday orbit Earth for months, surfers of the winds of the uttermost sky.
    IEEE Spectrum 07/2013; 50(7):46-54. · 0.94 Impact Factor
  • G. Sachs, J. Lenz, F. Holzapfel
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    ABSTRACT: The maximum-range flight of a powered sailplane with a retractable electric motor is treated as a periodic optimal control problem. The periodic nature of the maximum-range flight, known as saw-tooth mode, is due to the high drag when the motor is extended and the low drag when the motor is retracted. An optimization treatment based on energy considerations is performed to develop analytical solutions and to deepen the insight into the physical mechanism underlying the superiority of the saw-tooth mode. This is complemented by a treatment using a modeling based on point mass dynamics and an efficient optimization method to construct solutions for maximum-range saw-tooth flight. As a main result concerning the range performance, it is shown that the maximum range achievable with optimal saw-tooth flight is considerably larger than the greatest range possible with the best steady-state cruise.
    11/2012;
  • Gottfried Sachs
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    ABSTRACT: The effects of the wind on the energy expenditure of bounding flight and on the travelling speed are dealt with. For this purpose, a mathematical model of bounding flight in moving air is developed. Introducing an appropriate non-dimensionalization, results and findings of generally valid nature are derived. It is shown that bounding flight yields a flight mechanical advantage in headwinds when compared with continuous flapping flight. This is because the minimum energy expenditure is lower and the associated travelling speed is higher. The body lift in the bound phase has an advantageous influence. The effects of tailwinds yield less differences between bounding flight and continuous flapping flight.
    Journal of Theoretical Biology 09/2012; 316C:35-41. · 2.35 Impact Factor
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    ABSTRACT: ALBATROSSES DO SOMETHING THAT NO OTHER BIRDS ARE ABLE TO DO: fly thousands of kilometres at no mechanical cost. This is possible because they use dynamic soaring, a flight mode that enables them to gain the energy required for flying from wind. Until now, the physical mechanisms of the energy gain in terms of the energy transfer from the wind to the bird were mostly unknown. Here we show that the energy gain is achieved by a dynamic flight manoeuvre consisting of a continually repeated up-down curve with optimal adjustment to the wind. We determined the energy obtained from the wind by analysing the measured trajectories of free flying birds using a new GPS-signal tracking method yielding a high precision. Our results reveal an evolutionary adaptation to an extreme environment, and may support recent biologically inspired research on robotic aircraft that might utilize albatrosses' flight technique for engineless propulsion.
    PLoS ONE 09/2012; 7(9):e41449. · 3.53 Impact Factor
  • G. Sachs, J. Lenz, F. Holzapfel
    AIAA Atmospheric Flight Mechanics Conference; 08/2012
  • G. Sachs, J. Lenz, F. Holzapfel
    AIAA Guidance, Navigation, and Control Conference; 08/2012
  • Gottfried Sachs, Jakob Lenz
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    ABSTRACT: A new modeling approach is presented which accounts for the unsteady motion features and dynamics characteristics of bounding flight. For this purpose, a realistic mathematical model is developed to describe the flight dynamics of a bird with regard to a motion which comprises flapping and bound phases involving acceleration and deceleration as well as, simultaneously, pull-up and push-down maneuvers. Furthermore, a mathematical optimization method is used for determining that bounding flight mode which yields the minimum energy expenditure per range. Thus, it can be shown to what extent bounding flight is aerodynamically superior to continuous flapping flight, yielding a reduction in the energy expenditure in the speed range practically above the maximum range speed. Moreover, the role of the body lift for the efficiency of bounding flight is identified and quantified. Introducing an appropriate non-dimensionalization of the relations describing the bird's flight dynamics, results of generally valid nature are derived for the addressed items.
    Mathematical biosciences 08/2011; 234(2):75-83. · 1.30 Impact Factor
  • Gottfried Sachs, Jakob Lenz, Florian Holzapfel
    AIAA Atmospheric Flight Mechanics Conference; 08/2011
  • Gottfried Sachs, Jakob Lenz, Florian Holzapfel
    AIAA Guidance, Navigation, and Control Conference; 08/2011
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    ABSTRACT: Results from in-flight measurements of dynamic soaring in wandering albatrosses are presented. The experimental results were obtained using a new computational method based on L1 phase measurement as well as miniaturized and low cost GPS logging devices featur-ing a sufficiently high data sampling rate. Thus, it was possible to determine precisely the flight path and the motion quantities of the birds. Wind information was obtained using SeaWinds on QuikSCAT Level 3 Daily, Gridded Ocean Wind Vectors (JPL SeaWinds Pro-ject). The presented results show how albatrosses manage to make progress against the wind in non-flapping flight by means of dynamic soaring.
    AIAA Guidance, Navigation, and Control Conference and Exhibit; 01/2011
  • AIAA Atmospheric Flight Mechanics Conference; 08/2010
  • AIAA Guidance, Navigation, and Control Conference; 08/2010
  • Gottfried Sachs, Jalkob Lenz, Florian Holzapfel
    AIAA Guidance, Navigation, and Control Conference; 08/2010
  • Gottfried Sachs, Mochammad Agoes Moelyadi
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    ABSTRACT: Using a sophisticated aerodynamic method, the effects of extremely large dihedral on the aerodynamic characteristics of birds are determined. With this method, it is possible to generate solutions for the addressed aerodynamic problem, which shows a high complexity due to interference effects caused by dihedral and pronounced 3-dimensional flow properties as well as due to complex wing geometries. From the obtained results it follows that extremely large dihedral has substantial effects on the aerodynamic force characteristics. There are significant changes in the lift, the drag and the side force, thus affecting the flight performance. Furthermore, the obtained results show that the aerodynamic rolling and yawing moment characteristics are influenced by extremely large dihedral to a high degree. This is a significant outcome for lateral-directional stability because the aerodynamic rolling and yawing moment characteristics have a determinative influence here.
    Journal of Bionic Engineering 03/2010; 7(1):95-101. · 1.33 Impact Factor
  • Gottfried Sachs, Jakob Lenz, Florian Holzapfel
    AIAA Guidance, Navigation, and Control Conference; 08/2009
  • Gottfried Sachs, Jakob Lenz, Florian Holzapfel
    AIAA Guidance, Navigation, and Control Conference; 08/2009
  • AIAA Atmospheric Flight Mechanics Conference; 08/2009