Drag is the force that opposes the motion of an aircraft through the air.
Total drag produced by an aircraft is the sum of the profile
drag, induced drag, and
parasite drag. Total drag is primarily a function of airspeed. The
airspeed that produces the lowest total drag normally determines the
aircraft best-rate-of-climb speed, minimum rate-of-descent speed for
autorotation, and maximum endurance speed.
The following picture
illustrates the different forms of drag versus airspeed:
is the drag incurred from frictional resistance of the blades passing through
the air. It does not change significantly with angle of attack of the airfoil
section, but increases moderately as airspeed increases.
is the drag incurred as a result of production of lift. Higher angles of
attack which produce more lift also produce increased induced drag. In
rotary-wing aircraft, induced drag decreases with increased aircraft airspeed.
The induced drag is the portion of the total aerodynamic force which is
oriented in the direction opposing the movement of the airfoil. Think of it as
lift which is in the wrong direction.
is the drag incurred from the nonlifting portions of the aircraft. It includes
the form drag and skin friction associated with the fuselage, cockpit, engine
cowlings, rotor hub, landing gear, and tail boom to mention a few. Parasite
drag increases with airspeed.
Curve "A" shows that parasite
drag is very low at slow airspeeds and increases with higher airspeeds. Parasite
drag goes up at an increasing rate at airspeeds above the midrange.
Curve "B" shows how induced drag
decreases as aircraft airspeed increases. At a hover, or at lower airspeeds,
induced drag is highest. It decreases as airspeed increases and the helicopter
moves into undisturbed air.
Curve "C" shows the profile drag
curve. Profile drag remains relatively constant throughout the speed range with
some increase at the higher airspeeds.
Curve "D" shows total drag and
represents the sum of the other three curves. It identifies the airspeed range,
line "E", at which total drag is lowest. That airspeed is the best airspeed for
maximum endurance, best rate of climb, and minimum rate of descent in