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.
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