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have you ever wondered what force holds
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a helicopter back as it moves through
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the air That force is called drag and
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it's one of the four fundamental
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aerodynamic forces that act on any
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in helicopters drag is the force that
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resists movement through the air it's
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produced when lift is developed by the
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rotor blades and it's something the
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engine must constantly fight against to
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keep the rotor turning by nature drag
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always acts parallel to the relative
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wind that's the airflow experienced by
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the moving blades total drag on a
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helicopter is the combination of three
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types profile drag induced drag and
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profile drag develops from the
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frictional resistance of the blades
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passing through the air it does not
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change significantly with the air foil's
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angle of attack but increases moderately
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when air speed increases profile drag is
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composed of form drag and skin friction
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form drag results from the turbulent
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wake caused by the separation of air
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flow from the surface of a structure the
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amount of drag is related to both the
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size and shape of the structure that
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protrudes into the relative wind skin
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friction is caused by surface roughness
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even though the surface appears smooth
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it may be quite rough when viewed under
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a microscope a thin layer of air clings
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to the rough surface and creates small
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eddies that contribute to drag induced
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drag is created as a direct result of
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the lift being generated by the rotor
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blades when lift is produced high
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pressure air beneath the blade moves
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toward the low pressure area above it
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especially at the blades trailing edge
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this interaction forms spiraling air
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patterns known as vortices which trail
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behind each blade these vortices push
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the airream downward increasing what's
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called down wash as a result the rotor
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blade experiences a relative wind that
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angles slightly downward and rearward
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since lift always acts perpendicular to
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the relative wind this tilted flow
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causes the lift vector to tilt backward
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and that rearward component of lift is
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what we call induced drag as the angle
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of attack increases the pressure
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difference around the blade grows
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creating stronger vortices and more
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induced drag at lower air speeds angle
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of attack is higher so induced drag
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increases at higher air speeds angle of
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attack drops and induced drag decreases
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this makes induced drag the dominant
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form of drag at low speeds especially
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during hovering or slow flight parasite
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drag is the drag a helicopter
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experiences anytime it's moving through
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the air and it comes from all the parts
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that don't produce lift components like
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the fuseloff rotor mast landing gear and
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tail all contribute to this drag even
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small openings like those for engine
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cooling disrupt the air flow and
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increase parasite drag the key thing to
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remember is parasite drag increases
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rapidly with air speed in fact it
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increases with the square of velocity so
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if you double the helicopter's speed the
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parasite drag becomes four times greater
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that's why parasite drag becomes the
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dominant form of drag at high speeds
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especially during cruise or fast forward
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flight total drag in a helicopter is the
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combined effect of all three drag types
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profile induced and parasite drag as the
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helicopter picks up speed induced drag
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begins to decrease parasite drag rises
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sharply and profile drag stays mostly
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constant with only a slight increase at
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higher speeds when we combine all these
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forces we get a total drag curve the
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lowest point on that curve is where drag
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is at its minimum and this is known as
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the point of maximum lift to drag ratio
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at this speed the helicopter is
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performing at its most efficient
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generating the most lift for the least
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amount of drag this point is crucial for
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optimal flight performance and fuel
