Airfoils ( Penampang Sayap Pesawat )

Cross-Section of a Typical Airfoil
• Mean line: A line equidistant between the upper and lower surfaces.
• Chord line: A straight line joining the intersections of the mean line with the leading and trailing edges of the airfoil.
• Camber: The degree of curvature of the mean line.
• Upper and lower ordinates: The distance of the upper and lower surfaces from the chord line, usually expressed in in percent of chord length.

Air Flow Over a 2-Dimensional Airfoil
Consider an airfoil in a fluid stream (air), with the airfoil placed at an angle relative to the direction of the flow. The “angle of attack,” denoted by the Greek letter a (alpha) is the term used to designate the angle of the chord line of the airfoil relative to the remote direction of the air flow (the freestream velocity vector).

• Note that angle of attack is not defined relative to a local or earth-based axis system.
Observe the stream lines in the photo on the following page. Notice that the airflow over the top of the airfoil is deflected more than the flow over the bottom, hence the velocity over the top will be greater than over the bottom.
Notice also that the farther away from the airfoil, the less the deflection of the streamlines. Moving far enough away the streamlines would be parallel to the free-stream airflow.
This situation is similar to (but not exactly the same) as a stream-tube having a reduction of cross section where the airfoil is located. Flow velocity above and below the airfoil is greater than the velocity ahead or behind the airfoil. This results in a reduction of the static pressures above and below the airfoil.

(Actual photo from wind tunnel test. The lines are produced by injecting smoke into the tunnel. Smoke trails follow the streamlines of the air flow over the airfoil.)

• Similar to the symmetric airfoil seen earlier at no angle to the flow, the flow velocities and resulting pressure distribution may be shown for the airfoil at a positive angle of attack.

• The length of the arrows indicates the amount of pressure reduction or increase from the free-stream ambient pressure.
• Arrows are drawn perpendicular to the airfoil because pressure always acts at a right angle to the surface.
• The pressure forces may be resolved into a single equivalent
force. The location of that force on the wing is called the center
of pressure.

• There is no moment about the center of pressure, since the force vector passes right through it and there is thus no moment arm about the center of pressure.

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Reff : Flight Operations Engineering - Boeing

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