The Physics Behind The Flight

Ever since humankind first had the capacity to wonder, the sight of a flying animal must have been astounding. It is intuitively strange for an animal to fly almost effortlessly when we cannot (without our technological adaptations for flight). Early humans must have thought: “How the heck do they do that? Why can’t we?”

There are many factors that go into a bird’s ability to fly. Physical characteristics, behavior, and local air conditions all help define how birds fly, including:

  • Wing Shape
    A bird’s wing is thicker at the front than at the back, and more curved across the top than underneath. This causes air to move more quickly over the longer surface of the upper wing than the shorter surface below the wing. This disparity in airspeed above and below the wing causes lower air pressure on top and stronger pressure below, which is the lift that raises the wing and propels the bird higher.
  • Wing Motion
    As a bird flaps, its wing subtly twists to take advantage of wing shape and create more thrust to propel the bird forward and up into the air. This pushes the bird through the air, similar to how a swimmer will push through the water with every stroke as they change the position of their shoulders, hands, and arms.
  • Body Structure
    A bird’s entire body is built to help it fly. Bird skeletons are a lattice-like structure or honeycomb shape filled with air hollows, reducing the overall weight of the bird. Fewer redundant organs, such as a single ovary rather than a pair, also reduce a bird’s weight so it can fly more easily. Larger chest muscles give more power to a bird’s wings for stronger flight.
  • Body Activities
    The internal workings of a bird’s body also help it fly more efficiently. A bird’s body temperature is higher to permit muscles to work more quickly, and both the circulatory and respiratory systems operate more efficiently to support the massive effort required to fly. Birds also have higher metabolic rates so they can digest food more quickly to turn it into energy for flight.
  • Feathers
    A bird’s feathers are more than just a colorful covering for its body. Each feather is aerodynamically shaped and precisely positioned to assist flight by adjusting airflow over and around the bird’s wings and body. Birds can adjust some key feathers to help steer through the air, and tail feathers are often used as a rudder for broad movements.
  • Streamlined Shape
    Birds’ streamlined shapes help make flight easier by reducing drag and friction in the air. The tapered point of a bird’s bill slices through the air, and the smooth curves of the bird’s body guide the air around their bulk with as little resistance as possible. Birds even tuck their legs and feet up while flying to reduce drag even further.
  • Leg Power
    Some birds use their powerful legs to assist their flight by providing the initial thrust needed to get into the air. For many birds, this is from a first leap as the bird jumps into flight. Similarly, many waterfowl use their strong legs and webbed feet to build up speed across the surface of the water as they take off into flight.
  • Air Conditions
    Not only are birds spectacularly equipped to be efficient fliers, but they also take advantage of air conditions for more efficient flight. Air currents, wind, and air temperature differences all contribute to flight dynamics and help birds fly. Birds can sense subtle air changes with their sensitive skin, and will change their flight behavior to fly more easily in different air conditions.

Drag, Lift, and Thrust

To comprehend the biomechanics of flight, a few simple physical principles must be kept in mind. First we have to recognize that air is a fluid, just like water. It is not a liquid, like water, but is a called a fluid because the force needed to deform it depends on how fast it is deformed, not on how much it is deformed (try moving your hand quickly, then slowly through a basin of water for an example). Solids are substances for which the force needed to deform the substance is dependent on the extent of deformation rather than the rate of deformation (so it takes the same amount of force to break a pencil quickly as it does to do it slowly; try this with a pencil that is devoid of sentimental value to you). As is common in nature, there are subtle gradations between the artificial dichotomy of fluids and solids; we have given you a generalized definition for each of the two ends of the continuum. We’ll use “fluid” interchangeably with “air” here, and “object” interchangeably with “animal.”

Drag is a force exerted on an object moving through a fluid; it is always oriented in the direction of relative fluid flow (try running against a high wind and you’ll feel drag pushing you back in the direction of relative fluid flow). Drag occurs because the fluid and the object exchange momentum when impacting, creating a force opposing the motion of the object. Drag is higher when (1) the surface area of the object exposed to the fluid flow is higher, (2) the object is moving faster (or the relative fluid flow is faster), and (3) the fluid has more momentum, or inertia (the viscosity and density of the fluid are high) — this is generally low for air relative to other fluids such as water. Trying to walk in a strong wind will demonstrate drag for you. A dropped weight falls faster through air than through honey largely because of drag forces.

Lift is another force exerted on an object moving through a fluid; it is generally (but not always) directed upwards (perpendicular to the drag force), opposing the weight of the animal that is pulling it down to Earth. In animals that generate significant lift forces (like true flyers), the angle of the wings against the flow of air creates a resistance that has the net effect of moving the wing (and the animal) upward. The majority of lift in gliders and flyers is produced at the proximal part (base) of the wing, where the wing area is largest. Lift is higher when (1) the area of the bottom of the wing is larger, (2) the animal is moving faster, and (3) again, fluid viscosity and density are higher.

Thrust is the third force that we will discuss. It is only present in true fliers; it is produced by powered flight (wing flapping), especially at the distal (end) of the wing. Thrust is a force induced in the direction of the animal’s flight, opposing the drag force. To fly at a steady speed in a completely horizontal direction, an animal must generate enough thrust to equal the drag forces on it. Thrust is produced by flapping the wings (describing the shape of a figure-eight if viewed from the side), which creates a vortex wake that has the net effect of pushing the animal forward. Different kinds of wakes are formed in slow flight, fast flight, and bounding (or intermittent) flight, which you can often see in birds such as goldfinches. If the thrust force is greater than the drag force, the animal will accelerate; likewise the animal will decelerate if the drag is greater than the thrust, and when thrust force equals drag force, the animal moves at a constant speed. Thrust is a force basically dependent on the power output of the flight muscles of the animal.

The Weight of an aircraft is a limiting factor in aircraft design. A heavy plane, or a plane meant to carry heavy payloads, requires more lift than a light plane. It may also require more thrust to accelerate on the ground. On small aircraft the location of weight is also important. A small plane must be appropriately “balanced” for flight, for too much weight in the back or front can render the plane unstable. Weight can be calculated using a form of Newton’s second law:W = mg

where W is weight, m is mass, and g is the acceleration due to gravity on Earth.

Bernoulli’s principle helps explain that an aircraft can achieve lift because of the shape of its wings. They are shaped so that that air flows faster over the top of the wing and slower underneath. Fast moving air equals low air pressure while slow moving air equals high air pressure. The high air pressure underneath the wings will therefore push the aircraft up through the lower air pressure.

Research : Massachusetts Institute of Technology, University of California, Berkeley and Illinois Institute of Technology.

Image Courtesy : The Times Of India, Unsplash.

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