PRIN­CI­PLES OF FLIGHT

Bird Watching (UK) - - Id Secrets -

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.

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