Induced Drag

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Induced drag is generated by the airflow circulation around the rotor blade as it creates lift. The high-pres-sure area beneath the blade joins the low-pressure air above the blade at the trailing edge and at the rotor tips. This causes a spiral, or vortex, which trails behind each blade whenever lift is being produced. These vortices deflect the airstream downward in the vicinity of the blade, creating an increase in downwash. Therefore, the blade operates in an average relative wind that is inclined downward and rearward near the blade. Because the lift produced by the blade is perpendicular to the relative wind, the lift is inclined aft by the same amount. The component of lift that is acting in a rear-ward direction is induced drag. [Figure 2-13]

Figure 2-13. The formation of induced drag is associated with the downward deflection of the airstream near the rotor blade.

As the air pressure differential increases with an increase in angle of attack, stronger vortices form, and induced drag increases. Since the blade’s angle of attack is usually lower at higher airspeeds, and higher at low speeds, induced drag decreases as airspeed increases and increases as airspeed decreases. Induced drag is the major cause of drag at lower airspeeds.