The helicopter is moving forward, thus having an incident air flow. The main rotor is rotating, so each blade is experimenting their own incident air flow too. If we now visualize the rotor mounted on the moving helicopter, both situations combines.

Now, the blade is no longer seeing a constant incident flow (constant at a certain radius from the rotor), it will depend on the current location of the blade in a rotation. The velocity will be the same at both the most forward and backward location, however, the flow velocity will increase at the advancing side and decrease at the retreating side (right and left for a counterclockwise rotation). Actually, some area on the left side will be experimenting incident flow while going travelling backwards. There will also be some other area on the retreating side (left) where the blade is stalled. Having learnt that, we already know that there is a variable incident flow seen by the airfoil on the blade, but there is more to it...

If the blades had a fixed angle of attack (and we know lift is directly proportional to the AoA of the airfoil), since the incident air flow velocity is variable, the lift would also be variable. In the described case, there would be an excessive amount of lift on the advancing side and an insufficient lift on the retreating side, which would make the helicopter to bank left.

For a level flight, we want a lower angle of attack on the "faster" advancing blade and a higher angle of attack on the "slower" retreating blade. This happens naturally by letting the blades move up and down (blade flapping). The advancing blade will go up as it has more lift by going faster and, by going up, it will perceive a lower angle of attack of the incident flow. The opposite happens to the retreating blade, going slower, less lift, it will drop increasing its relative angle of attack. Depending on the rotor, the blade may have another degree of freedom twisting around its pitch axis.

Knowing all this phenomena, we tried to work on some unsteady CFD cases and get them working.

We took Wang and Zhao study on helicopter blade aerodynamics to have a reference and check whether our cases were similar in terms of the fluid dynamics properties and the meshes.

For a deeper dive in our CFD experiments, have a look at the document below: