We have developed four-winged bird-shaped robots, called ornithopters, that can take off and fly with the agility of the speedy, hummingbirds, and insects. We did this by reverse engineering the aerodynamics and biomechanics of these creatures.
Our ornithopters have the potential to outperform and exceed existing drone configurations with static wings or propellers.
Read more: How modern technology is inspired by the natural world
What are ornithopters?
Ornithopters are flying machines based on the design of birds. Existing drone configurations are based on propellers and static wings. The ornithopters flap their wings to generate forward thrust. The complex relationship between aerodynamics and wing movements allows birds and insects to fly in ways impossible for conventional drones.
Why do we want ornithopters?
Ornithopters fly differently than conventional drones. They can glide, float, and perform aerobatics. In different situations, they can save energy by flying like a normal airplane or choosing to float. They can slowly take off and land in tight spaces, but they can quickly fly upward to perch like a bird.
Today’s multi-rotor drones fly very well, but use even more energy in forward flight than in hover, so they really can’t travel far. Fixed-wing drones can travel efficiently at high speeds, but it’s typically not possible to move around without compromising the entire design. There are hybrid concepts, usually with wings and rotors. Hybrid aircraft perform poorly when moving and sailing compared to other designs due to the added weight and drag due to having more parts.
Flapping wings are nature’s original solution to the need to fly both quickly and slowly, as well as landing and taking off from anywhere. For a bird or insect, each part of the system is used to fly and fly, without carrying redundant thrusters or additional wings.
Existing fixed-wing and rotary-wing drones are so well understood that designs are now close to the limits of how efficient they can be. Adding something new comes at a cost to other aspects of performance.
In principle, ornithopters are capable of more complex missions than conventional aircraft, such as flying long distances, occasionally flying, and maneuvering in tight spaces. Ornithopters are less noisy and safer to use around humans, due to their large wing area and slow heartbeat.
How do we make a working ornithopter?
Chin et al., Science Robotics, Author provided
An ornithopter is a highly complex system. Until now, flapping drones have flown slowly and have not been able to achieve the speed and power required for vertical aerobatics or sustained hover.
The few commercially available ornithopters are designed for forward flight. They climb slowly like an underpowered aircraft, and cannot fly or climb vertically.
Our design is different in several ways.
One difference is that our ornithopters use the clap and throw effect. The two pairs of wings flap in such a way that they meet, like clapping hands. This does enough extra thrust to lift your body weight when hovering.
We improved efficiency by adjusting the wing / body hinge to store and recover energy from the moving wing when the wings change direction, like a spring. We also discovered that most of the energy loss occurred because the gears bent under the load of driving the wing. We solved this with tiny bearings and rearranging the shafts in the transmission to keep the gears properly spaced.
The large tail, comprising a rudder and lift, creates a lot of turning force. This allows aggressive acrobatic maneuvers and rapid changes from horizontal to vertical flight.
The system was designed to be able to lift the nose, rapidly increasing its angle of attack to the point where the wing does not generate lift, a phenomenon called “dynamic blocking”. The dynamic stop creates a lot of resistance, turning the wing into a parachute to slow down the plane. This would be undesirable on many drones, but the ability to enter this state and recover quickly adds to the maneuverability. This is useful when operating in messy environments or landing on a hanger.
Catching up with evolution
One of the main conclusions of our work was that a practical ornithopter could achieve similar efficiency to a propeller-powered airplane. Various behaviors became possible for the ornithopter once some additional power was released.
This really showed that optimizing the flight apparatus is key to making these new aircraft designs viable. We are now working to use wing designs copied from nature. We look forward to equally great improvements.
In some ways, the huge efficiency gains from design changes in these new systems shouldn’t be surprising. Winged organisms have been optimized by evolution for hundreds of millions of years. Humans have been at it for less than 200 years.
