As with the flapping mechanism, there are many ways to build an effective ornithopter wing. But there are even more ideas that don't work. Here, I'll present some of the best designs that have been proven to work. First, let's review some basic flapping-wing aerodynamics.

airplane wing

To see how an ornithopter or bird can fly, first you must understand how a simple airplane wing operates. As the wing moves forward through the air, it is held at a slight angle, called the "angle of attack". This causes the wing to deflect the air gently downward. Like when you push down on anything, there is an equal and opposite reaction force. This is the "lift" force that keeps the airplane up in the air.

There is some drag, or air resistance, whenever any object moves through the air. This would tend to make the airplane slow down, reducing the lift, and the airplane wouldn't be able to stay aloft. That is why an airplane needs a propeller. The propeller overcomes the air resistance and keeps the plane moving.

glider

There is another way to solve this problem. An unpowered, glider type aircraft can maintain its speed by going into a shallow dive. The wing is angled forward so some of the lift of the wing counteracts the drag on the aircraft body. To maintain its speed, a glider must keep moving downward, relative to the surrounding air.

bird downstroke

The bird or ornithopter applies power in the downstroke of the wings. The wing in downstroke works something like a glider when it goes into a dive. The downward motion and angle of the wing cause a strong lift force with a forward thrust component. Unlike a glider, only the wings are going down. The body stays up!

bird upstroke

People often ask why the upstroke doesn't cancel out the downstroke. There are two reasons why. First, the part of the wing near the body has little upward motion. It acts like an airplane wing and produces lift at all times. Birds line up the outer part of the wing with the wing's direction of travel. This allows the wing to get back in position for the next downstroke without causing too much air resistance.

In most cases, the angle of attack is regulated through "aeroelasticity". This means that the wing structure twists in response to the aerodynamic forces acting on it. Ornithopters, like birds or insects, have a stiff spar at the leading edge of the wing. The rest of the wing is flexible. The right amount of flexibility allows each part of the wing to stay correctly aligned with the local airflow at various stages of the wingbeat cycle. There is no need to provide a greater flexibility on the upstroke, compared with the downstroke. The slight positive angle at the wing root causes the downstroke to have a large positive angle of attack and the upstroke to have a very low angle of attack in the outer part of the wing.

It is possible to actively drive the twisting of the wing, instead of relying on aeroelasticity. In principle, this would allow more control over the angles of attack, perhaps resulting in more efficient flight. However, active wing twisting requires a more complex wing design and a more complex mechanism to drive the twisting as well as flapping of the wings.

Construction of Membrane Wings

The most consistently successful wing design is the simple membrane type. These wings are very simple to construct, consisting of a single leading edge spar, bearing a thin sheet of wing material which may be plastic film, fabric, or paper. Additional battens or bracing may be added, but they are not strictly necessary. If you are going to use any additional structure, it is important to understand how to do this correctly.

The wing surface should conform to the shape of a cone.

Radial battens (red) help support the outer part of the wing membrane. They are not always necessary, because the membrane under load takes on a cambered shape that provides some support. The battens allow the membrane to extend farther out, which may be helpful in micro air vehicle competitions or in other cases where you want to increase the wing area without increasing the wingspan. The battens should always radiate outward from the front, inside corner of the wing. This allows the membrane to take on a cambered shape under load, which approximates the surface of a cone.

Diagonal bracing (green) was introduced by Percival Spencer in order to regulate the aeroelastic twisting of the wing. If the main wing spars are too flexible, they will bend under load, causing the wing surface to deform excessively. Since the aftward bending of the spar is a major component of its deflection under load, the wing can be stiffened by adding a diagonal brace within the wing surface. However, the brace must be flexible enough to conform to the conic shape that the wing has under load. If the bracing rods are too stiff, they will cause a discontinuity in the cambered cross-section of the wing, making the airfoil less efficient.

The Ornithopter Design Manual provides detailed instruction on how to make these wings from a variety of different materials and at various scales. Some designs having a more airplane-like, yet torsionally flexible, wing structure are also described.