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Name: AG W.
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 2/26/2003


Question:
How does an aeroplane manage to fly upside-down, if it normally gets its lift from the differential airflow above and below its wings?


Replies:
AG -

You are half right. What produces lift is a combination of Bernoulli's principal - causing a low pressure area above the upper surface of the wing - and Newton's Third Law - a reaction to the air particles being pushed down by the bottom of the wing cause the wing to move up. We generally consider these two laws to contribute to lift.

The amount that each of these contributes to lift is a function of the cross section of the airfoil and angle of attach - the angle between the wing (actually the chord line of the wing) and the relative wing (the wind that is made by the forward motion of the airfoil). Some airfoils have a greater curvature (camber) on the upper surface of the wing making the Bernoulli effect greater at lower angles of attack.

Getting to your question... an airfoil that is made to fly with a greater curvature on the top can fly - though not as efficiently - inverted as long as the angle of attack is established in the upward (opposite of gravity) direction. Some airfoils are made symmetrical to allow them to fly equally well right-side-up and up-side-down. Airliners are unlike candidates for symmetrical airfoils, but nimble aerobatic aircraft often feature these.

The short answer... angle of attack controls lift.

Larry Krengel
Aviation Instructor, retired


Several factors affect the lift obtained from an airfoil. Primarily they are related to the differential speed of the air flowing over the top of the wing vs the air flowing underneath the wing. The air on top has to flow farther, thus it must flow faster and the air pressure on top is lower, resulting in lift.

The difference in distance the air has to flow is controlled by the shape of the airfoil and the angle of attack (i.e., the angle with respect to the airflow. Airfoils are designed for a relatively shallow angle of attack under normal flight conditions. When flying upside down a high angle of attack will allow sufficient lift to fly.

They probably adjust the ailerons and flaps to increase lift as well.

Greg Bradburn



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