Scientists have always peaked into nature in order to mimic its most interesting features into industrial processes. This adaptation process is called biomimicry, and the most recurring example is the history of how the Swiss engineer George de Mestral invented fabric hook-and-loop fasteners (Velcro) in the 1950s, after he saw how burdock seeds got attached to his dog’s fur. Modern scientists use biomimicry very intensively, e.g. trying to create miniature flying robots resembling winged bugs, or build solar cell systems resembling tree branches in order to improve its efficiency; the example list is virtually endless.
From a mechanical point of view, a very interesting animal is the albatross (Diomeidae). These birds can travel huge distances with almost no energy consumption during their foraging trips, and they do not even need to flap their wings to achieve it. Trips of 15,000km and maximum speeds of 127km/h have been reported for wandering albatrosses (Diomedea exulans). An estimation of the energy they require to counter the drag forces that appear while flying determines that it is equivalent to 0.9l of fuel per day. Some other bird species have been reported to lose 0.34g/km of weight during flying. Extrapolating this rate to the albatrosses, after 15,000km they would have lost half their weight, which implies that they must eat enough preys during this foraging trip in order to replace the lost weight and subsist for further 10-20 days while keeping their eggs warm.
The actual foraging strategies of the albatrosses would not provide enough energy for this behaviour, so there should be some adaptations that reduce these energy expenditures. The first one is an anatomic adaptation, as they have an elbow-lock system that allows them to keep their wings wide open without using any muscle. The second strategy is their flight mode: dynamic soaring. Albatrosses have a flying pattern that allows them to gain energy from the strong winds that occur in the open seas. However, the exact mechanism of how albatrosses obtain this energy is still unknown.
There are many theories about how the albatrosses use this energy for propulsion, such as i) surface level gust-wind propelling, where the energy is obtained by the fairly regular wind discontinuities at low altitudes; or ii) wind speed gradient theory, where the albatrosses obtain the energy from the high variations of wind speed when they modify their flying altitude. These theories have never been validated, as the available tracking devices did not have the necessary resolution in positioning and sampling rate. This study1 presents experimental data of the small scale movements of albatrosses in order to end the controversy between the different theories.
Untangling the energy transformation mechanism
Albatrosses, and birds in general, do not fly in a straight line. In Figure 1b it is depicted the flight path for c.a. 13.5 km projected to the sea surface, and there is not a single straight section, but a cycle-like behaviour. If we zoom into one of these cycles and plot the position in three dimensions (Figure 1c) a height gradient of c.a. 15m can be noticed. The bird is constantly repeating this kind of cycle, which consists of 1) climbing perpendicular to the wind (windward), 2) curving at highest altitude point to face the wind direction (leeward), 3) leeward descending and 4) low-altitude curve from leeward to windward.
The total energy of the albatross is calculated as the sum of potential and kinetic energy, this is, the energy due to the altitude above the sea plus the energy due to its speed. Figure 2a shows its vertical position and speed for the analysed cycle, while 2b shows the potential and the total energy along the same cycle. The red lines mark the minimum and maximum energy values. The minimum energy occurs after the albatross starts to rise, while the maximum occurs after descending at maximum speed, before it reaches the sea level. The energy gain is huge, c.a. 360% of the minimum value. It can be noticed that the energy gain is mostly due to the kinetic energy, as the potential energy levels are quite low through all the soaring cycle.
To fully understand the energy and trajectory of the dynamic soaring, Figure 3 depicts them all together. The global picture allows to better understand the trajectory and energy interaction: the energy gain starts when the trajectory changes form windward to leeward and is increased until it reaches sea level. Then the bird starts to slowly steer back to windward while soaring at sea level. But the important question is: what mechanism drives the energy gain? Figure 4 depicts the mechanical power balance during dynamic soaring cycle, relative to the dissipation effect of drag forces. The mechanical power available in a flapping bird is also depicted, and it is c.a. 10 times lower than the maximum power, so it is not the flapping the driver of the energy changes. There is a much more powerful mechanism, the interaction between the wind speed and the trajectory of the bird.
Solution to the theoretical controversy
The wind gusts theory claims that birds obtain the energy from surface-level wind gusts. The total energy plot in figure 2b shows that the energy variation is smooth, without any discontinuities or pulses. The bird extracts the energy from the wind in a continuous manner, and moreover, the energy extraction occurs during the time that the bird is at high altitude, not close to the surface.
For the validation of the wind-gradient soaring, the wind gradient vs. the altitude is plotted in Figure 5. In the section where the energy gain is obtained, this wind speed gradient is very low and thus cannot be the cause for the increase in energy.
This means that low altitude wind gusts or high altitude wind speed gradient have a negligible influence on the energy gain. Instead, the flight at no energy cost is achieved by strategically changing the flight direction in the maximum altitude curve.
This mechanism used for flying without spending any energy would be extremely useful for aeronautical engineers, as aeroplanes are the most energy consuming mean of transport, both per person and per load unit.
The dynamic soaring flying technique that albatrosses use will not be directly implemented for improving modern aircrafts, and will never allow flying at no energy cost. However, it could be a good inspiration for obtaining more energy efficient aircrafts.