Airlines and airplane manufactures have been pulling out all the stops searching for ways to increase the efficiency of commercial airliners and reduce their environmental footprint. I have posted about Alaska Airlines commitment to sustainable fuels and KLM, Airbus and Boeing blended-wing airplane deigns. Now, in an effort to maximize fuel savings, aeronautical engineers are thinking about taking a tip from birds.
Birds are the masters of flight. They have been flying for millions of years. Over eons, migratory birds have learned the best ways to save energy on their intercontinental journeys. Migratory birds flying in the V formation is a sight everyone is familiar with.
Birds take advantage of the wingtip vortices generated from lift created by the wings of the bird ahead. Birds and airplanes generate lift by creating a pressure differential over the wing surfaces. Airflow over the curved surface on top creates an area with lower pressure compared to the air pressure from the airflow over the flat surface under the wing. This pressure differential triggers a rollup of the airflow from the wingtip resulting in swirling air masses trailing downstream of the wing. When an airplane generates lift, it leaves a wake consisting of two counter-rotating cylindrical vortices.
Wingtip vortices linger in the air, sinking at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft. Wingtip vortices, also known as “wake turbulence,” are hazards near the ground.
Although wake turbulence is invisible, every pilot learns ways to anticipate and avoid it when taking off and landing. Wake turbulence from a large plane can easily lead to induced rolling moments that exceed the roll-control authority of a small airplane, i.e. an uncontrollable situation.
For saving energy, the trick for commercial airliners is to copy birds and take advantage of the updraft from the smooth current of rotating air which is next to the vortex. To perfect this, Airbus is conducting a series of flight tests called “fello’ fly.”
Taking advantage of the free lift in this updraft of air is referred to as “wake-energy retrieval.” Determining the proper formation is the first step. Airbus flight tests will use two A350s with the trailing aircraft flying 1.5 to 2 nautical miles behind, below, and slightly offset from the leader. For trailing aircraft, fuel savings on long-haul flights could amount to 5% to 10%.
Operational and financial considerations will also need to be worked out. Systems must be developed to maintain the correct position, reducing pilot workload and ensuring a smooth ride for passengers by avoiding the more turbulent components of the wake. Government aviation agencies will need to be convinced to change regulations to allow for much closer aircraft separation standards than are currently in place.
In addition, flight planning procedures will have to be arranged for planes to match routes, positions and altitudes with other aircraft. And a process to share fuel savings among airlines will be required.
In the future, flights from San Francisco, Los Angeles, and Las Vegas may takeoff, rendezvous over Utah, and fly in an energy-saving, wake-energy retrieval formation to England for example.
It is in the interest of commercial aviation and the environment to make flying as energy efficient as possible. Doing that requires exploring a variety of options, even ones that come from the birds.