Moving in Air
Perhaps one of the most transformative feats humans have accomplished in locomotion is learning to fly. It was hundreds of years between Leonardo da Vinci's drawings of a flying machine to the Wright brother's first successful airplane flight. And yet, animals have been successfully flying for hundreds of thousands of years. What is it that makes flying so difficult? How have animals overcome those difficulties?
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Just like with moving on land, the biggest obstacle to moving through the air is gravity. This force is constantly pulling you towards the Earth. To counteract this force, you need to generate a force in the opposite direction called lift. This can be accomplished in a variety of ways that we will consider. But when flying, you don't just want to go up, you want to go forward too. To do this, you need to generate thrust, enough thrust to overcome the drag created by friction with the air. Air is a fluid, like water, and a lot of the same principles apply. In fact, the free-body diagram, shown left, looks very similar to the free-body diagram for water with the exception that we have traded in buoyancy for lift.
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If lift in air is similar to buoyancy in water, can we generate lift the same way fish generate buoyancy? Recall that fish generate buoyancy by inflating a bladder of air inside them. Because the air is lighter than the water, it generates an upward force, buoyancy. In order to do that in air, we would need to inflate a container with something lighter than air. It turns out that's how blimps work. We fill blimps with a gas that's lighter than the gases that make up our atmosphere and that generates lift. What is lighter than air? Helium. We used to use hydrogen, which is also lighter than air. But it turned out that hydrogen was flammable, and so it was dangerous. Helium is not flammable but it is still lighter than the air around it. Although humans use this method to generate lift in blimps, animals and airplanes use a slightly more complicated method.
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In many ways, the challenges of moving through water and moving on land are combined in moving through the air. Air is a fluid, just like water, and behaves in many of the same ways as water. But air is not as viscous as water, and therefore does not help much with evading gravity. Not only must flying animals contend with gravity, they also must contend with air moving in different ways (currents, vortices, eddies, and turbulence). Differences in temperature can create air currents (hot air rises and colder air moves to fill in where the hot air used to be). The presence of large objects, like mountains and skyscrapers can cause air to move around them in odd ways. Aircraft and animals flying through the air also create movement that must be contended with by other flying animals.
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As in water, the shape of animals in air has a huge impact on how they move through the air. Consider the shapes of the plane and animals to the left. What do you notice about their overall body shapes? They have the same fusiform (cigar-shaped) body shapes found in fish, but with wings attached. This fusiform body shape helps to reduce drag in air, just as it does in water. Whereas in water, we would call this shape hydrodynamic, in air we call it aerodynamic. While birds may be more aerodynamic if they didn't have wings, they would not be able to produce thrust or lift if they didn't have wings, and would therefore not be able to fly. That doesn't mean that wings have to create a ton of drag though. Many animals have fine-tuned their wings and feathers to be very aerodynamic.
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Reducing drag alone, is not enough to produce flight. You also need to generate lift. Birds, bats, and insects produce lift primarily with their wings. As moving air hits a wing, the air on top of the wing moves faster than the air below the wing. Because the air above the wing is moving faster than the air below the wing, the pressure on top of the wing is less than the pressure below. The pressure pushing on the bottom of the wing creates a net force upwards, called lift.
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The amount of this lift changes with different angles of attack (the angle the wing makes with the oncoming wind). Generally, with an increasing angle of attack, lift increases, but only up to a point. Typically, with an angle of attack of greater than 20° the wing will stall, and the bird will fall out of the sky. Different animals have different ways of dealing with this stall angle. Most avoid it and keep a shallow angle of attack, which is why you don't often see birds falling out of the sky. But the hummingbird can have higher angles of attack by creating lift on both the downstroke and the upstroke. Check out the video to the left to find out how they pull that off!
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This property of wings to create lift doesn't require flapping. It only requires the air to be moving over the wing. That's why airplanes don't need to flap their wings to stay aloft. So why do animals flap their wings? Because lift will only allow you to go up. In order to get anywhere, you need thrust as well, to move you forward. Flapping the wings generates thrust. Take a look at the cockatiel flying to the right. Notice that it makes its wing very large on the downstroke (to generate a lot of thrust and a little lift as well) and folds its wings back to make them small on the upstroke (to limit drag undoing the effects of the downstroke). If a wing just went straight up and down without this change in size and shape, the upstroke and downstroke would cancel each other out, and the bird would go nowhere. This subtle change in shape is what allows birds to fly.
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So how do airplanes generate thrust? They can generate lift using their wings as airfoils, like birds do. But birds need to flap their wings to generate thrust. If airplanes had to flap their wings, flying across the country would be decidedly less pleasant for the passengers. Instead, planes use propellers (sometimes propellers inside of jet tubes) to generate thrust in the same way that motor boats use propellers in the water. Can you guess why birds don't use propellers?
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Biomechanics is at the cross-section of two major branches of science: biology and physics. This tutorial has barely touched the surface of this vast field. Check out the Get More Information page to learn more about this subject or to find other cool tutorials.