FAAV

AURA Lab

Flapping-wing Aerial Aquatic Vehicles

Aerial-aquatic robots have the potential to perform remote, autonomous sensing missions for environmental and biological tasks thanks to their hybrid operation capability. They can cross large distances, fly over obstacles and even change from one body of water to another. Flapping Wing Aerial Aquatic Vehicles (FAAVs), are inherently safe, quiet and robust devices due to the low velocities of the wings and the lack of propellers. This project studies such robots to deepen our understanding of their locomotion in air, in water, and in-between.

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Transitions

The water-air transition is the most demanding maneuver the FAAV performs. Water is 800 times denser than air, so the same wing stroke that cruises a bird through the sky meets enormous resistance underwater. Leaving the surface involves the FAAV outputing a short energetic burst, accelerating a wet body through the interface against gravity in under one second using 8-10 wing strokes. Our research shows that three conditions must all be met for a successful transition: intermediate wing stiffness, a short tail (a long taildrags and pitches the robot back), and a ~70° exit angle — too shallow and the tail drags it back, too steep and it tumbles.

Transition merge
Transition sequence 1
Transition sequence 2

Aerial

In flight the FAAV uses biologically-inspired flapping wings to generate lift and thrust simultaneously — no propellers, no exposed blades. The wings are flexible membranes that bend passively in dense water and snap back to full span in air, adapting to both media without any folding mechanism. Flight has been validated indoors and in the WindShape wind tunnel at EPFL.

  • Cruise speed: 6.3 m/s  (range 4.6–7.4 m/s)
  • Estimated range: ~6 km per charge
  • Flapping frequency: 5–11 Hz
  • Wingspan: 88 cm tip-to-tip
FAAV in WindShape wind tunnel FAAV flight merge

Underwater

The FAAV uses the same flapping wings to propel the robot underwater as in air. Water's high density causes the flexible wings to bend up to 90%, passively reducing the effective stroke and protecting the motor from burning out. Unlike propellors, flexible flapping wings shrug off bumps and don't foul on weeds or debris the way a rigid propeller does - useful in cluttered places like coral reefs, riverbeds and kelp forests.

  • Max swim speed: 0.95 m/s
  • Estimated range: ~2 km per charge
  • Flapping frequency: 0.1–6 Hz
  • Buoyancy: Neutrally buoyant
FAAV swimming

Quick Facts

250 g
Total mass
88 cm
Wingspan
6.3 m/s
Cruise speed in air
0.95 m/s
Swim speed
<1 s
Water exit duration
~$300
Component cost
6 km
Range in air
2 km
Range in water

Why such robots?

More than ever before we are realizing the importance of our lakes, rivers and oceans. Conservation of our water ecosystems as well as climate models and flood management all depend on the availability of local, rapid and extensive environmental data. There are important open questions regarding the impacts of climate change on water temperature and water flow alteration, sedimentation in glacial flows, algal blooms, coral reef degradation, fishery exploitation, pressures due to invasive species, emissions of pesticides, pollutants and microplastics. To this day, all these issues cannot be well quantified due to a lack of available data. The importance of advancing our knowledge of water systems advocates for the development of aerial-aquatic robots as a transformative solution for effective monitoring. Data collection has improved (for example from remote satellite imagery or autonomous sinking-rising buoys e.g. ARGO floats), but coverage in space and time is still insufficient to provide a consistent picture globally, and the existing solutions are expensive.

Objectives

FAAV figure 1 FAAV figure 2

This project aims to push our knowledge in mobile robotics, focusing on locomotion capabilities in air and in water. Biology provides a strong source of inspiration for solving the extremely challenging constraints that aerial-aquatic operation poses. Indeed, over 20 species of diving birds routinely fly and swim, some even reaching depths of over 100 m. These examples serve as design starting points, which need to carefully integrate engineering limitations. The specific objectives of this project is to understand how flapping flight can adapt to two vastly different media in a small, mobile system. Through the use of robotics, flight, swimming and transitions locomotion are explored. In conclusion, hybrid locomotion with flapping propulsion has been successfully achieved in a limited set of conditions. This is clearly just the beginning towards these robots seamlessly moving in and out of our aquatic environments, but the application potential is vast and the current results demonstrate the validity of the approach. Further efforts are required to improve control methods, efficiencies and propulsive power, as well as continued research regarding the impact of wind, wave and current natural elements, which are ubiquitous outdoors.