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Drag is defined as the force that acts parallel to the relative wind. Relative WindRelative wind is the "wind" that acts exactly opposite to the direction of flight. In the picture to the left the relative wind is horizontal so this airplane is in level flight.
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In the picture to the left the relative wind is from the upper left. That means that the airplane is in a climb. Notice that when the airplane climbs the drag vector is still parallel to the relative wind - as shown. In this case the drag is no longer horizontal. The picture also shows that lift is defined as being at right angles to the relative wind. It is therefore correct to say that drag is always at right angles to lift. |
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In the picture to the left the airplane is descending. Once again you can see that the drag vector acts parallel to the relative wind.
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There are two types of drag known as parasite drag and induced drag. We will examine each separately.
Parasite drag is drag caused by friction and pressure differences ahead of and behind the airplane. It is the easiest type of drag to understand.
Induced drag is drag due to the production of lift. Why should producing lift cause any drag at all is the obvious question. We will tackle that question after we have explored parasite drag thoroughly.