Aerodynamics:
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Reaction Force - Applied to Drag
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We might visualize Drag as being like a rope tied to the back of the airplane
and being pulled on to hold us back.
But just as when we talked about swinging something on a string
we can easily see that if there was a rope tied to our airplane
that while we would see the force as holding us back, someone at
the other end of that rope would see the airplane as pulling them
forward.
So, we should realize that if there is drag - meaning that the
air pulls "backwards" on the airplane. Then by Newton's
third law the airplane must pull forward on the air. |
In a wind tunnel the model does not travel through the air, the air travels
past the model. Did you know that in wind tunnels, drag is measured NOT
by the force on the model but by the amount the air is slowed after passing
over the model?
In the real world the air is stationary (prior to the airplane flying
through it.) Once the airplane flies through a parcel of air that air
has a forward motion, which is the result of the reaction to drag (that
tries to slow the airplane.)
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The picture to the left shows this idea. The upper part of the
picture shows undisturbed air well ahead of an approaching airplane.
After the airplane passes, the lower part of the picture shows
that the air is moving downward (reaction to lift) and forward (reaction
to drag.)
The angle of the vector for the air in motion is very significant.
I drew it more down than forward to indicate that lift is greater
than drag. The exact balance between lift and drag will be an important
point of our consideration in this text. For now just realize that
the proportion of downward and forward deflection is proportional
to lift and drag. |
Next Lesson: Conservation of Rotation Rate
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