What is a turn?
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This page was last modified on August 2, 2006
A turn is a curvature in the trajectory of a moving body. In the case of an aircraft, a turn is a curvature in the flight path.
An aircraft's "heading" is the direction that the nose is pointing. Fundamentally, a change in heading is not a turn, though there are several different aerodynamic effects by which a change in heading can cause a turn. Fundamentally, a turn is not a change in heading, though if a turn is sustained for more than a few seconds, it will inevitably lead to a change a heading.
The idea that an aircraft's nose can point in a slightly different direction than the aircraft is actually travelling through the airmass at any given moment is one of the keys to understanding the dynamics of turning flight.
The fundamental cause of a turn is not a yaw torque. In a steady, constant-bank, constant-rate turn the net yaw torque generated by an aircraft is actually zero.
The fundamental cause of a turn is a "centripetal force", acting at the CG of the aircraft, and acting at a right angle to the actual direction of the flight path at any given moment. This steady "centripetal force" makes the flight path curve, just as the steady pull of the sun's gravity makes the earth follow a curving path through space. (Diagram to be inserted.)
A wing is a very efficient generator of aerodynamic force. Therefore, the most efficient way to generate the "centripetal force" that drives a turn is by simply banking the wing to one side. This causes the wing's lift vector to have a horizontal component as well as a vertical component. (Diagram to be inserted.) This horizontal lift component causes the aircraft to follow a curving path through the air. The horizontal lift component from the banked wing is the "centripetal force" that is the fundamental cause of the turn.
Sometimes as a pilot begins to roll an aircraft into a turn, "adverse yaw" causes the nose to swing in the opposite direction as the intended direction of turn. (Diagram to be inserted.) Generally speaking, this "adverse yaw" motion does not cause the aircraft to actually turn in the "wrong" direction, even momentarily. In other words, this "wrong-way" swing of the nose generally does not cause the flight path to curve in the "wrong" direction. With many aircraft, this temporary "wrong-way" swing of the nose will cause a noticeable temporary reduction in the turn rate, i.e. in the rate of curvature of the flight path, in the intended direction of turn. However, for a given amount of "adverse yaw", the resulting decrease in the turn rate is highly dependent on the shape of the aircraft, and will be minimal with certain types of aircraft such as pure "flying wing" designs.
(Diagram to be inserted--tangible forces in a normal turn.)
(Diagram to be inserted--all forces in a normal turn.)
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