Definition of the relative wind
This page was last modified on August 29, 2006
Throughout these tutorial pages we'll make many references to the "relative wind". The relative wind is simply the airflow created by an aircraft's movement through the airmass. In wings-level, straight-line flight where nothing is making the aircraft yaw to one side, the aircraft will travel in the direction the nose is pointing, and the relative wind will blow from that direction. For example if an aircraft's nose points east toward a heading of 090 degrees, then the relative wind will blow from 090 degrees. The external, meteorological wind has no effect on the relative wind, at least when we aren't dealing with gusts or gradients.
During turning flight, since the flight path is curving, the direction of the flight path and relative wind are constantly changing, so we'll often speak of "the direction of the flight path and relative wind at any given moment".
During turning flight, an aircraft's nose is often not exactly aligned with the actual direction of the flight path and relative wind at any given moment. In other words, during turning flight, an aircraft is slightly travelling through the air in a slightly different direction than the nose of the aircraft is actually pointing. For example, in "What is a turn?", we noted that adverse yaw often swings an aircraft's nose out of alignment with the actual direction of the flight path and relative wind as the aircraft enters a turn.
When we speak of the "relative wind", we aren't taking into consideration the localized changes in the direction of the airflow caused by the physical presence of the aircraft. We'll generally make the simplifying assumption that the physical presence of the aircraft is not affecting the direction of the airflow. In other words we'll assume that the physical presence of the aircraft is not causing any change in the position of the molecules of the airmass, which obviously is not entirely true! If we want to take into account localized changes in the direction of the airflow caused by the physical presence of the aircraft, we'll avoid using the term "relative wind" in these instances.
At various points in the tutorial pages we'll make use of the concept of the "curvature in the relative wind". The "curvature in the relative wind" during a turn, or during any yawing, rolling, or pitching motion, is a reflection of the fact that different parts of the aircraft are actually travelling at slightly different linear speeds and in slightly different linear directions at any given moment. This is due to the fact that the body as a whole has a linear motion plus a rotational motion. For example, during a turn the outboard wingtip moves faster (and experiences a faster relative wind) than the inboard wingtip. We'll explore other aspects of the "curvature in the relative wind" in more detail elsewhere in the tutorial pages.
Throughout these tutorial pages, when we speak of an aircraft's "flight path", we always mean the flight path through the airmass, never the ground track. When there is a wind, the "flight path" or flight path through the airmass will be different from the ground track. We'll rarely be concerned with the ground track in these tutorial pages, except when discussing crosswind landings. From time to time we'll point out that an aircraft flies "within" the airmass, and does not "feel" the external, meteorological wind. An external, meteorological wind is simply a horizontal motion of the airmass (or from the aircraft's point of view, a horizontal motion of the ground!).
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