A common myth about ornithopters and birds is that they flap their wings to produce lift. Actually, they produce lift in the same way as an airplane, simply by their forward motion through the air. What the flapping does is continue that forward motion. To understand how a wing produces lift, let's start with the simple case of gliding flight.

Gliding Flight

When a bird is just gliding, it moves forward through the air, with its wings held in a fixed position. The wings are at a slight angle, so they deflect the air gently downward. Pushing the air downward causes a reaction force in the opposite direction. You will notice a reaction force, any time you push against anything! The reaction force is called lift. Lift is a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.

In gliding flight, a bird's wings deflect air downward, causing a lift force that holds the bird up in the air.

There is also air resistance or drag on the body and wings of the bird. This force would eventually cause the bird to slow down, and then it wouldn't have enough speed to fly. To make up for this, the bird can lean forward a little and go into a shallow dive. That way, the lift force produced by the wings is angled forward slightly and helps the bird speed up. Really what the bird is doing here is giving up some height in exchange for increased speed.The bird must always lose altitude, relative to the surrounding air, if it is to maintain the forward speed that it needs to keep flying.

By tilting forward and going into a slight dive, the bird can maintain forward speed.

Angle of Attack

Recall that the wings are angled slightly, which allows them to deflect the air downward and produce lift. The slight angle of the wings is called the angle of attack. If the angle of attack is too great, the wing will produce a lot of drag. If the angle is too small, the wing won't produce enough lift. The best angle depends on the shape of the wing, but it's usually just a few degrees! Notice that what matters is the angle relative to the direction of travel, not relative to the horizon.

The angle of attack determines the amount of lift and drag made by the wing.		The bird wing has a cambered, or curved, cross-section.

The ornithopter wing usually consists of a thin fabric membrane, which takes on a curved or cambered shape, when it pushes against the air. Birds have more of a rounded leading edge to help reduce air resistance. The inner part of the wing, near the bird's body, is more curved than the outer part. As you read on, see if you can figure out why!

Flapping-Wing Flight

The wings flap with an up-and-down motion. This may change in special situations, but we aren't going to talk about those until later. When the wings move up and down, they are also moving forward through the air along with the rest of the bird. Close to the body, there is very little up and down movement. Farther out toward the wingtips, there is much more vertical motion.

As the bird is flapping along, it needs to make sure it has the correct angle of attack all along its wingspan. Since the outer part of the wing moves up and down more steeply than the inner part, the wing has to twist, so that each part of the wing can maintain just the right angle of attack. The wings are flexible, so they twist automatically. This picture shows how the wing must twist in the downstroke, to keep each part of the wing aligned with the local direction of travel.

As the wing moves downward and twists, the lift force in the outer part of the wing is angled forward. This is what would happen if the whole bird went into a steep dive. However, only the wing is moving downward, not the whole bird. Therefore the bird can generate a large amount of forward propulsive force or thrust, without any loss of altitude.

The wing twists as shown to maintain the correct angle of attack for the downstroke.

The bird's wing produces lift and thrust during the downstroke.

The air is not only deflected downward, but also to the rear. The air is forced backward just as it would be by the propeller of an airplane. You can feel this blast of air when a bird takes off from your hand.

If thrust is produced in the downstroke, you might be wondering what happens in the upstroke. Often people have the wrong-headed notion that the upstroke will somehow cancel out the downstroke. But remember, the force produced depends on the angle of attack. It can be controlled. And here is what birds do to make the upstroke more efficient:

• The outer part of the wing points straight along its line of travel so it can pass through the air with the least possible resistance. In other words, the angle of attack is reduced. Ornithopters do this too.
• The bird partially folds its wings. This reduces the wingspan and eliminates the draggy outer part of the wing. This is not strictly necessary though. Most insects and most ornithopters lack the capability.

I think the idea people have, about the upstroke cancelling out the downstroke, results from thinking of the wing as a moving mass instead of an aerodynamic surface. If you swing a baseball bat up and down, the upstroke and downstroke will cancel out, and there won't be any net lift.

The inner part of the wing is different from the outer part. There is little up-and-down movement there, so that part of the wing continues to provide lift just as a result of its forward motion. Only the inner part of the wing produces lift in the upstroke, so the upstroke as a whole offers less lift than the downstroke. As a result, the bird's body will bob up and down slightly as the bird flies.

The inner part of the wing produces lift, even during the upstroke.

The outer part of the wing is angled to pass through the air with little resistance.

So, like an airplane, lift and thrust functions are separated. The outer part of the wing provides thrust. The inner part of the wing produces lift.

What you've read so far is a basic description of how birds fly, when they are already up to speed and just cruising along. Birds also have other flying techniques, which they use when taking off or landing, or for other special maneuvers. Books on bird flight will tell you more about these techniques as well as the special adaptations birds have for flight.

Bird Flight Myths

Q: Doesn't a bird push its wings backward against the air?
A: No. Some insects fly this way, but it doesn't work well at larger scales.

Q: Doesn't a bird separate its feathers during the upstroke?
A: Yes, but only in special maneuvers like takeoff or landing.

Q: Do birds move their wings in a figure-8 motion?
A: Hummingbirds do this when they hover.