Factors of flight
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 is 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), air moves more quickly over the top of the animal's wings than the bottom of the wings, creating a low pressure area over the wings, following Bernoulli's Principle. This low pressure area "sucks" the wing (and its attached body) upwards, creating lift; which is how hard the object is sucked upwards. 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. Another important concept is the fact that lift can exist only in the presence of a moving fluid. This is also true for drag. It doesn't matter if the object is stationary and the fluid is moving, or if the fluid is still and the object is moving through it. What really matters is the relative difference in speeds between the object and the fluid.
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, pushing the air contacted by the wings downwards and forwards (describing the shape of a figure-eight if viewed from the side), which creates a pressure difference around the airfoil wing, and hence thrust. 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.
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