Any vehicle traveling on the ground, such as an automobile, moves in the direction in which it is steered or headed and is affected very little by wind. However, an aircraft seldom travels in exactly the direction in which it is headed because of the wind effect.
Any free object in the air moves downwind with the speed of the wind. This is just as true for an aircraft as it is for a balloon. If an aircraft is flying in a 20-knot wind, the body of air in which it is flying moves 20 NM in 1 hour. Therefore, the aircraft also moves 20 NM downwind in 1 hour. This movement is in addition to the forward movement of the aircraft through the body of air.
The path of an aircraft over the earth is determined by the two unrelated factors shown in Figure 4-23:
The motion of the aircraft through the airmass, and
The motion of the airmass across the earth's surface.
Figure 4-23. Two factors determine the aircraft's path. [click image to enlarge]
The motion of the aircraft through the airmass is directly forward in response to the pull of the propellers or thrust of the jet engines, and its rate of movement through the airmass is TAS. This motion takes place in the direction of true heading (TH). This motion of the airmass across the earth's surface may be from any direction and at any speed. The measurement of its movement is called wind and is expressed in direction and speed (wind vector (W/V)).
Drift Caused by Wind
The effect of wind on the aircraft is to cause it to follow a different path over the ground than it does through the airmass. The path over the ground is its track. The terms true course (TC) and track are often considered synonymous. TC represents the intended path of the aircraft over the earth's surface. Track is the actual path that the aircraft has flown over the earth's surface. TC is considered to be future, while track is considered to be past.
The lateral displacement of the aircraft caused by the wind is called drift. Drift is the angle between the TH and the track. As shown in Figure 4-24, the aircraft has drifted to the right; this is known as right drift.
Figure 4-24. In 1 hour, aircraft drifts downwind an amount equal to windspeed.
With a given wind, the drift changes on each heading. A change of heading also affects the distance flown over the earth's surface in a given time. This rate traveled relative to the earth's surface is known as GS. Therefore, with a given wind, the GS varies on different headings.
Figure 4-25 shows the effect of a 270°/20 knots wind on the GS and track of an aircraft flying on headings of 000°, 090°, 180°, and 270°. The aircraft flies on each heading from point X for 1 hour at a constant TAS.
Figure 4-25. Effects of wind on aircraft flying in opposite directions. [click image to enlarge]
Note that on a TH of 000°, the wind causes right drift; whereas on a TH of 180°, the same wind causes left drift. On the headings of 090° and 270°, there is no drift at all. Note further that on a heading of 090°, the aircraft is aided by a tailwind and travels farther in 1 hour than it would without a wind; thus, its GS is increased by the wind. On the heading of 270°, the headwind cuts down on the GS and also cuts down the distance traveled. On the headings of 000° and 180°, the GS is unchanged.
Drift Correction Compensates for Wind
In Figure 4-26, suppose the navigator wants to fly from point A to point B, on a TC of 000°, when the wind is 270°/20 knots. If the navigator flew a TH of 000°, the aircraft would not end up at point B but at some point downwind from B.
Figure 4-26. Aircraft heads upwind to correct for drift.
By heading the aircraft upwind to maintain the TC, drift is compensated for. The angle BAC is called the drift correction angle or, more simply, the drift correction. Drift correction is the correction that is applied to a TC to find the TH.
Figure 4-27 shows the drift correction necessary in a 270°/20- knot wind if the aircraft is to make a good TC of 000°, 090°, 180°, or 270°. When drift is right, correct to the left, and the sign of the correction is minus. When the drift is left, correct to the right, and the sign of the correction is plus.
Figure 4-27. Maintaining course in wind. [click image to enlarge]
Vectors and Vector Diagrams
In aerial navigation, there are many problems to solve involving speeds and directions. These speeds and directions fit together in pairs: one speed with one direction.
By using vector solution methods, unknown quantities can be found. For example, TH, TAS, and W/V may be known, and track and GS unknown. To solve such problems, the relationships of these quantities must be understood.
The vector can be represented on paper by a straight line. The direction of this line would be its angle measured clockwise from true north (TN), while the magnitude or speed is the length of the line compared to some arbitrary scale. An arrowhead is drawn on the line representing a vector to avoid any misunderstanding of its direction. This line drawn on paper to represent a vector is known as a vector diagram, or often it is referred to simply as a vector. [Figure 4-28] Future references to the word vector means its graphic representation. Two or more vectors can be added together simply by placing the tail of each succeeding vector at the head of the previous vector. These vectors added together are known as component vectors.
Figure 4-28. Vector has both magnitude and direction.
The sum of the component vectors can be determined by connecting, with a straight line, the tail of one vector to the head of the other. This sum is known as the resultant vector. By its construction, the resultant vector forms a closed figure. [Figure 4-29] Notice the resultant is the same, regardless of the order, as long as the tail of one vector is connected to the head of another.
Figure 4-29. Resultant vector is sum of component vectors. [click image to enlarge]
The points to remember about vectors are as follows:
A vector possesses both direction and magnitude. In aerial navigation, these are referred to as direction and speed.
When the components are represented tail to head in any order, a line connecting the tail of the first and the head of the last represents the resultant.
All component vectors must be drawn to the same scale.
Effect of Wind
Drift and Ground Track Control (Part One)
Drift and Ground Track Control (Part Two)
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