Why Are Airplane Wings Angled Backwards?
Watch the video and answer the following questions:
What types of aircraft wings do you know?
What is the name of the pilot who makes a test flight?
What aircraft tests do you know of?
AIRPLANE STABILITY
Stability is the inherent ability of a body, after its equilibrium is disturbed, to develop forces or moments that tend to return the body to its original position. In other words, a stable airplane tends to return to the original condition of flight if disturbed by a force such as turbulent air. This means that a stable airplane is easy to fly; however, this does not mean that a pilot can depend entirely on stability to return the airplane to the original condition. Even in the most stable airplanes, there are conditions that will require the use of airplane controls to return the airplane to the desired attitude.
Stability is classified into three types: (1) positive, (2) neutral, and (3) negative.
Positive stability can be illustrated by a ball inside of a bowl. If the ball is displaced from its normal resting place at the bottom of the bowl, it will eventually return to its original position at the bottom of the bowl.
Neutral stability can be illustrated by a ball on a flat plane. If the ball is displaced, it will come to rest at some new, neutral position and show no tendency to return to its original position.
Negative stability is in fact instability and can be illustrated by a ball on the top of an inverted bowl. Even the slightest displacement of the ball will activate greater forces which will cause the ball to continue moving in the direction of the applied force.
It should be obvious that airplanes should display positive stability, or perhaps neutral stability, but never negative stability.
Stability may be further classified as static and/or dynamic. Static stability means that if the airplane's equilibrium is disturbed, forces will be activated which will initially tend to return the airplane to its original position. However, these restoring forces may be so great that they will force the airplane beyond the original position and continue in that direction.
On the other hand, dynamic stability is a property which dampers the oscillations set up by a statically stable airplane, enabling the oscillations to become smaller and smaller in magnitude until the airplane eventually settles down to its original condition of flight. Therefore an airplane should possess positive stability which is both static and dynamic in nature.
Axes of Rotation
The airplane has three axes of rotation around which movement takes place. These are (1) lateral axis — an imaginary line from wingtip to wingtip, (2) longitudinal axis — an imaginary line from the nose to the tail, and (3) vertical axis — an imaginary line extending vertically through the intersection of the lateral and longitudinal axes. The airplane can rotate around all three axes simultaneously or it can rotate around just one axis. These axes are imaginary axes around which the airplane turns, much as a wheel would turn around axes positioned in these same three planes.
The three axes intersect at the center of gravity and each one is perpendicular to the other two.
Rotation about the lateral axis is called pitch, and is controlled by the elevators. This rotation is referred to as longitudinal control or longitudinal stability.
Rotation about the longitudinal axis is called roll, and is controlled by the ailerons. This rotation is referred to as lateral control or lateral stability.
Rotation about the vertical axis is called yaw and is controlled by the rudder. This rotation is referred to as directional control or directional stability.
Stability of the airplane then, is the combination of forces that act around these three axes to keep the pitch attitude of the airplane in a normal level flight attitude with respect to the horizon, the wings level, and the nose of the airplane directionally straight along the desired path of flight.
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