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44-45     ARMY AIR FORCES

(3) Wind direction and speed.—Wind is designated by the direction from which it blows. Its speed is expressed in mph or knots.
(4) Ground speed.—The actual speed of an aircraft relative to the earth's surface in mph or knots.
(5) True course.—The direction over the surface of the earth, expressed as an angle, with respect to true north, that an aircraft is intended to be flown. It is the course laid out on the chart or map. Strictly, track is the actual path of an aircraft over the surface of the earth that has been flown. In practice, "track" and "course" are freely used interchangeably.
c. Some general rules for setting up vector diagrams follow:
(1) The reference system for direction of vectors is always the same. The true north line is the reference line and all angles are measured from it clockwise, through 360°.
(2) Choose a scale which will give lines large enough for accurate measurement.
(3) Label all vectors as they are drawn.
(4)"Heading" and "air speed" determine one vector.
(5)"Course" and "ground speed" determine one vector.
(6) Wind always blows the aircraft from the heading onto the course.
45. Type I.—The simplest triangle of velocity problem involves finding the true course and ground speed when wind speed, wind direction, air speed, and heading are known. 
a. Example: The pilot has been told to fly a heading of 56° in an airplane the air speed of which is 100 mph. The wind is blowing 25 mph from 110°. Find the true course and ground speed.
NOTE.—See back of manual for figure 54.

(1) Parallelogram method.—To find the true course and ground speed, add the wind velocity vector and the air speed heading vector by the parallelogram method. The length of the resultant determines the ground speed and the direction of the resultant is the true course direction. The length of the resultant is 8.7 cm, thus the ground speed is 87 mph. The direction is 42°; thus, true course equals 42°.
(2) Triangle method.—To find the true course and ground speed, add the wind velocity vector and the air speed heading vector by the triangle method.
(a) Draw the north line.
(b) Draw the air speed heading vector at an angle of 56° from the north, making it 10 cm long (1 cm=10 mph; hence 10 cm=100 mph).
(c) Draw the wind vector away from 110°, making it 2 1/2 cm long (2 1/2 cm=25 mph) and from the head of the air speed heading vector.

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