Flying is a phenomenon governed by physical forces and principles of physical science. Birds fly by flapping their wings and gliding whereas balloons fly on buoyancy principle. Likewise, man-made aircraft rely on these principles to overcome the force of gravity and achieve flight.
Airfoil & Lift: An airfoil or aerofoil is the shape of an object such as wing or blade or a sail. An airfoil-shaped body moved through a fluid with an angle of attack produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift which is perpendicular to the flow and the component parallel to the direction of motion is called drag.
In order for an aircraft to rise into the air, a force must be created that equals or exceeds the force of gravity. This force is called lift. In heavier-than-air craft, lift is created by the flow of air over an airfoil. The shape of an airfoil causes air to flow faster on top than on bottom. The fast flowing air decreases the surrounding air pressure. Because the air pressure is greater below the airfoil than above, a resulting lift force is created.
Airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with a symmetric curvature of upper and lower surfaces. The lift on an airfoil is primarily the result of its angle of attack and shape. When oriented at a suitable angle, the airfoil deflects the oncoming air, resulting in a force on the airfoil in the direction opposite to the deflection. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. An aircraft's wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections. Airfoils are also found in propellers, fans, compressors and turbines. Airfoils are more efficient lifting shapes, able to generate more lift and to generate lift with less drag. The shape of a typical airfoil is asymmetrical - its surface area is greater on the top than on the bottom.
Airfoil Design: Airfoil design is a major part of aerodynamics. There is no predetermined shape for a wing airfoil, it is designed based on the function of the aircraft it will be used for. Various airfoils serve different flight regimes. Asymmetric airfoils can generate lift at zero angle of attack, while a symmetric airfoil may better suit frequent inverted flight as in an aerobatic airplane. In the region of the ailerons and near a wingtip a symmetric airfoil can be used to increase the range of angles of attack to avoid spin–stall. Modern aircraft wings may have different airfoil sections along the wing span, each one optimized for the conditions in each section of the wing.
Weight: 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.
Drag: Every physical body that is propelled through the air will experience resistance to the air flow. This resistance is called drag. Drag is the result of a number of physical phenomena. Pressure drag is that drag which you feel when running on a windy day. Skin friction, or viscous drag, is that which swimmers may experience. A rough surface will induce more frictional drag than a smooth surface. Likewise, an aircraft's wing is designed to be smooth to reduce drag. Like lift, drag is proportional to dynamic pressure and the area on which it acts.
Thrust: Propulsion involves a number of principles of physical science. Thermodynamics, aerodynamics, fluid mathematics, and physics all play a role. Thrust itself is a force than can best be described by Newton's second law. The basic form of this law is: F = ma which states that force (F) is equal to mass (m) times acceleration (a). Acceleration is the rate of change of velocity over time. Thrust (T) is produced therefore by accelerating a mass of air.
Balancing the Forces: Heavier-than-air flight is made possible by a careful balance of four physical forces: Lift, Drag, Weight, and Thrust. In the flight of an aircraft, lift must balance its weight, and its thrust must exceed its drag. A plane uses its wings for lift and its engines for thrust. Drag is the resistance offered by air to move forward, which is reduced by a plane's smooth shape.
