PRINCIPLES OF FLIGHT
Lift is created by the flow of air over an aerofoil (the cross-sectional shape of an object that, when moved through air, creates an aerodynamic force), in which the greater the curve of the aerofoil and its speed through the air, the more lift is created. The top surface of a wing is shaped such that the air which flows between it, and the undisturbed air closely above it, is, in effect, forced through a constriction. Thus, the air flows over the wing at an increased speed – and therefore a reduced pressure – compared with the surrounding atmosphere. The resulting pressure difference between the air above and below the wing creates lift. The whole surface of a wing, top and bottom, is affected by the airflow. Therefore, there are pressure forces acting all over the wing and it follows that there can be lift forces all over it. However, the top surface generates more lift than the bottom – at some angles of attack (the angle at which the leading edge of the wing comes into contact with undisturbed air in front of it) it is as much as 80% of the total. The greatest amount of lift on the top surface occurs where the aerofoil section is curved the most, normally approximately a third of the way from the front. All lift forces act at 90° to the direction of the airflow. A good example of a bird generating a high amount of lift is an Osprey catching its prey. Often, as it nears the water, it veers out of its dive and places its wings in a high lift position, to slow down and catch its prey with its feet efficiently positioned. The Osprey holds its wing in a highly curved aspect, and the otherwise slower moving air over the wing caused by the reduction in speed is increased as the bird extends its alula (short, very stiff feathers at the ‘wrist’ joint) which cause a ‘slot’ configuration which causes the air to move faster over the more curved form, creating sufficient lift to maintain flight at a slower speed. The distal section of the outer primaries of an Osprey are deeply emarginated, creating a second ‘slot’ effect, and after the Osprey has caught its prey it vigorously flaps its wings and the emarginated outer primaries travel further and faster through the air than the inner wing and provide the power needed to increase the bird’s speed. At the same time the inner wing is creating lift, allowing the bird to leave the water and return to the air.