Using the globe is the only accurate means of representing the spherical surface of the earth, but a globe is impractical to use in the cockpit of an aircraft. It is therefore necessary to use charts to represent the earth surface. The method of representing the earth surface on a chart is called projection.
How to measure distances on the globe. If a string is stretched between two points on the globe, the length will represent the shortest distance between the points.
If the string is extended around the globe then the circle is divided into two halves, the line is called the great circle: The shortest distance between any two points will be a portion of the great circle.
Locating a point on the earth’s surface.
To form a system of reference, meridians running in straight lines are draw from the pole meridian drawn in degrees from east to west of Greenich (England). The prime meridian is called longitude zero.
Lines of latitude are drawn at 90 Degree angles to the meridians starting at the equator, these lines are called latitude lines.
To locate a point the coordinates of the point is given usually latitude followed by the longitude.
How a pilot plans a trip.
(a) draw a straight line between the two points
(b) measure the course in degrees from true north from a point about midway on the course. This is called True Course (T.C).
The effect of winds. When an airplane moves through the air mass at a given airspeed, the airspeed remains constant, but the aircraft’s speed measured by the distance it travels over ground is affected by the movement of the air.
(figure available in print form)
The effect of the wind speed causes the aircraft to drift.
The wind triangle.
A course is the direction towards it’s destination. If there is no wind the aircraft will fly towards it’s direction. If the wind is blowing it will affect the aircraft’s tract. In order to establish a heading direction in which the plane is going a parallelogram of forces is constructed.
(figure available in print form)
The wind triangle is one half of the parallelogram of forces representing the interaction between the airplane’s airspeed and wind velocity.
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problem.
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T.C = A to ,90 degrees
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Distance A to = 150 miles
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Wind Velocity = 35 knots (40 m.p.h.)
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Airspeed = 120 m.p.h
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Variation = 7 degrees East.
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Constructing a wind triangle.
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1) draw a north-south line, locate T.C at the 90 degree point, and the direction from which the air is blowing at 45 degree point.
(figure available in print form)
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2) Draw the wind vector in the direction of its force.
(figure available in print form)
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3) Using the same scale draw a line from the end of the wind arrow.
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4.) The true heading () can be measured directly with the protractor.
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5) The wind correction angle can also be measured with a protractor
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6) ground speed can be measured along T.C. Iines from point E to the vector at point P.
Completed Diagram.
(figure available in print form)
Calculating Time of a Trip and Fuel Consumption.
After the pilot has determined his path, he needs to calculate the time and fuel required for the trip. He needs to have fuel for the trip and enough reserve for 45 minutes of extra flying time.
Calculations Required.
Calculating minutes to hours.
To find the time in flight when groundspeed is known
Time = distance/groundspeed.
To find the distance flown for a given time
groundspeed x time = distance.
The groundspeed in flight is often not the same as calculated, this variation is due to wind velocity. To calculate groundspeed accurately use
groundspeed = distance between check points/time.
To calculate fuel consumption:
Fuel = fuel consumed/time.
