One big advantage to a helicopter’s rotor system is the vertical thrust that allows the aircraft to hover. However, when this same rotor system is flown edge wise through the air it creates an aerodynamic problem that limits the helicopter’s forward speed. The term used to describe this is dissymmetry of lift.
To generate lift a helicopter’s rotor blades spin to create airflow. A rotor system’s RPM is fixed at a certain value for all phases of normal flight and increasing the blade’s angle of attack (the angle between the relative wind and the blade’s cord line.) with the collective control generates lift. In a hover with no wind, lift is essentially equal across the entire rotor disc. However, as the helicopter begins to move forward it creates a relative wind. One side of the rotor disc has a blade that is advancing into the relative wind (think headwind) and the other side has a blade that is retreating (think tailwind). The difference in airspeed each blade encounters between the two sides grows as the helicopter gains forward speed. This creates an imbalance of lift problem that early helicopter engineers had to solve to maintain controllability.
To help understand how they did it, consider the equation for lift.
Lift = CL ½ p S V2
CL = Coefficient of Lift, which is a function of angle of attack and blade shape
p = air density
S = total blade area
V = airspeed
At a given moment in time, air density, total blade area and blade shape are all fixed values, so as each blade’s airspeed changes the rotor system must respond by changing the blade’s angle of attack to keep total lift constant. This is done primarily by allowing the blades to move up or down in a process known as flapping. Two bladed rotor systems (known as semi-rigid) use a single teetering hinge that allows the blades to flap as a unit (one go up, the other goes down). Rotor systems with more than two blades (typically known as fully articulated) use a flapping hinge on each blade allowing the blades to move up or down independently of each other.
How flapping works is by changing the angle of attack in response to the varying airspeeds the blade encounters as it moves around the rotor disc. When the advancing blade experiences a higher airspeed, the lift on that blade increases forcing it to move up. This upward movement changes the direction of the blade’s relative wind reducing its angle of attack. On the retreating side just the opposite happens. The reduced airspeed causes a decrease in lift causing the blade to move down, increasing its angle of attack. Each blade’s angle of attack changes in direct relation to its relative airspeed. As each blade’s relative airspeed increases, angle of attack decreases and vice versa to maintain equal lift across the disc as the helicopter’s airspeed increases.
As you might have guessed, this creates a problem on the retreating side. You can only increase an airfoil’s angle of attack so much before it stalls. As the helicopter continues to fly faster the retreating side must continue to increase its angle of attack to compensate. At some airspeed the retreating blade stalls and this is what limits the helicopter’s forward airspeed. This is referred to as retreating blade stall.
There is more to discuss on this subject so part 2 is coming up next time.