history of flight..
http://www.ueet.nasa.gov/StudentSite/historyofflight.html
An aircraft flies due to aerodynamic reactions that happen when there is relative motion between air and a wing.
If a cross-section of a typical aircraft wing is viewed, the top of the wing can be seen to be curved downwards, while the bottom of the wing is less curved or straight, but angled to the airflow. This shape, called an airfoil or aerofoil, creates lift when it travels through the air.
Lift is created as an airstream passes by something which deflects it. The force created by this deflection of the air creates an equal and opposite force on the wing according to Newton's third law of motion. The deflection of airflow downward during the creation of lift is known as downwash.
Nearly any shape will produce lift if curved or tilted with respect to the air flow direction. However, most shapes will be very inefficient and create too much drag. One of the primary goals of wing design is to devise a shape that produces the most lift while producing the least lift-induced drag.
A false explanation for lift has been put forward in mainstream books, and even in scientific exhibitions. Known as the equal transit-time fallacy, it states that the parcels of air which are divided by an airfoil must rejoin again; because of the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom. Therefore, because of its higher speed the pressure of the air above the airfoil must be lower. Despite the fact that this "explanation" is probably the most common of all, it is false in that there is no requirement that divided parcels of air rejoin again, and in fact they do not do so.
Nevertherless, the deflection of the air does cause changes in the pressure above and below the wing; generally a higher pressure area forms below and a lower pressure forms above; these changes in pressure are necessary for the deflection to occur, and it is these pressure differences over the area of the wing that provides the force that holds the aircraft in the air.
2006-09-10 02:41:02
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answer #1
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answered by Anonymous
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