How does wind speed affect helicopter rotor speed?

So on the left it shows wind flowing with the direction of the rotor, yet it represents that it would reduce the rotor speed. I'm wondering how it does this, wouldn't the wind minimally help increase rotor speed by pushing it? Even if the effect was minimal to the point that it was neutral, why would it slow the rotor down?

If anyone could explain that would be great! Thanks.

• Equal and opposite on the other side it would be pushing against the blade and cancel out the effect. Dec 2, 2016 at 17:58
• @Ron Beyer, the relation between drag and speed is not linear so it does not simply cancel out. Dec 2, 2016 at 18:08
• Are you perhaps misreading this? The 90 and 110 refer to the speed of the relative wind (the airflow) not the tutor speed) Dec 2, 2016 at 18:26
• Assume the rotor is moving with a constant speed in relation to the body of the helicopter. If the rotor has a headwind (the left side), it will move at a slower speed through the air. It's sort of like understanding the difference between airspeed and groundspeed. Dec 2, 2016 at 20:10
• As you suspect, the angular speed (speed of the rotor around the rotor axis) doesn't change. What you miss is the graphic is not about angular speed, but about linear velocity of a given point of a blade relative to the airflow.
– mins
Dec 4, 2016 at 2:03

The rotor RPM is fixed all around, but in forward flight—say at 10 knots forward flight like your example—the advancing rotor will have an additional free 10 knots of airspeed (check image below). The other side will be rotating away from that airstream, losing 10 knots.

Red arrows indicate direction of motion, dotted arrows indicate airstream direction, same direction add, different subtract.

The faster side will produce more lift compared to the slower side. This is called dissymmetry of lift.

Solution?

The blades are allowed to flap (change angle) independently from each other, to be able to counter this phenomenon.

If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize lift across the rotor disk. Articulated rotor systems, usually with three or more blades, incorporate a horizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as they rotate. A semi-rigid rotor system (two blades) utilizes a teetering hinge, which allows the blades to flap as a unit. When one blade flaps up, the other blade flaps down.

• Video in French explaining the flapping (good video even if you don't speak French, it's well illustrated).
• Animation for flapping.