Refer to figure.
Profile Drag or Rotor Profile Drag is the drag created when the rotor is moved through the air, even when no lift is being generated and it is made up of the blades' form (pressure) and skin friction drag.
Rotor Profile Drag varies as the square of speed (V2). Rotor RPM remains relatively constant, so it is expected that the profile drag will be the same at all speeds. But, this would ignore the fact that the forward speed of the helicopter also contributes to the speed of flow over the rotor blades.
When the helicopter has forward speed, the speed of the flow over the rotating blades will be:
- The free stream flow plus the flow caused by the rotor's rotational speed, when the rotor is advancing - more drag.
- The free stream flow minus the rotor's rotational speed, when the blade is retreating - less drag.
When forward airspeed is low, the two effects nearly cancel each other out. But, when forward airspeed is high, the fact that profile drag is proportional to V2 means that the extra drag produced by the advancing blade is much greater than the drag reduction associated with the retreating blade.
Thus, although, it is almost constant at very low airspeeds, rotor profile drag slightly increases at high airspeeds.
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