Understanding Autorotation

The first step in learning about autorotation is to understand that rotor blades are rotary wings. We can all imagine an airplane flying without a motor (glider), but it's a bit more difficult to visualize a helicopter doing the same. By recognizing that each blade is a wing that acts like the wing of a glider, autorotation is more comprehensible. The blade element diagram can be used to understand the forces acting on the blade during an autorotation.

Blade Element Diagram
Normal Powered Flight

In May's edition of RC Heli, we defined the components of the blade element diagram and how they work during normal flight: The blade sees a combination of rotational flow (1) and downward induced flow (2) called relative wind (3). The angle of attack (4) is the angle formed between the relative wind and the chord line, and the pitch angle (5) is formed between the rotor plane and the chord line. Lift (6) is the total aerodynamic force perpendicular to the relative wind. For a helicopter in hovering flight, the lift vector is tilted aft. The lift vector can be broken into two components: a vertical component (7), which is the total force that generates vertical lift, and the rearward component called the induced drag (8), formed by the acceleration of air mass (downwash) and the energy spent in the creation of trailing vortices. Induced drag must be overcome to develop lift, and power is required to the rotor system to overcome this drag. The remaining vector on the blade element diagram is profile drag (9), a result of air friction acting on the blade element.

Blade Element Diagram

To keep the blades turning during normal flight induced drag must be overcome by engine power. During an autorotation, the power that turns the rotors comes from another source-potential energy (gravity), as the helicopter loses altitude. The rotor head will initially slow down, feeding on its own rotor inertia. Lowering collective will stop the decay. The increasing up flow of air through the rotor system reverses airflow. With
the lift vector always perpendicular to the relative wind, induced drag reverses and the lift vector is tilted forward providing a Pro-Autorotative force that turns the rotor head. A component of profile drag (In-Plane Drag) acts in opposite direction to the Pro-Autorotative force.

The Autorotative Regions of the Blade

As the rotor blades travel around the arc, each part of the blade sees a different relative wind--from lowest velocity at the hub, to highest velocity at the rotor tip. This is because the rotor tip has to travel farther in the same period of time as the part of the blade near the hub. During an autorotation the rotor blade sees three different regions: prop, auto, and stall region.

Auto Region • Rotational speed combines with induced flow, shifting the relative wind below the rotor plane. Notice that the lift vector is tilted forward, providing a pro-autorotative force. This region is increased and shifted toward the blade tip at higher pitch settings, decreasing rotor rpm and slowing the sink rate.

Prop Region • Relatively high rotational speed at the outer portion of the rotor disk combines with induced flow, shifting the relative wind towards the horizontal. Notice the lift vector is tilted more vertical than forward, providing less pro-autorotative driving force. In this region, the profile drag is the largest and causes greater anti-autorotative force. This region is increased with lower pitch settings and higher rpms, thus reducing the auto region, resulting in a faster sink rate.

Stall Region • The stall region is at the blade root, where rotational flow is reduced to the point where lift is not generated and profile drag dominates. As pitch angle is increased, rotor rpms are reduced and the stall region increases across the blade, reducing the auto region and prop region.

Putting it
all together
As blade pitch and rpm changes, the three regions change across the blade. It is very important that during an autorotation, pitch angle and rpm are controlled for the most practical use of the blade regions.

Autorotation Entry
When the motor decides to quit at the most inconvenient time, the lift vector is pointed aft, quickly slowing the rotor head without the motor to overcome the induced drag. The pilot quickly lowers the collective to decrease the stall region on the blade.

During the descent, the pilot starts to control the pitch of the blades in order to adjust the amount of prop and auto region. If the pilot wants more lift, he adds collective, slowing the rotor head and increasing lift. For a higher sink rate, the pilot decreases pitch, increasing rotor rpm. This balancing act is optimized in order to find a suitable rate of descent that will get the pilot to the desired landing point.

Getting close to the ground, the pilot trades airspeed for rotor head power by flaring, increasing the reversed induced flow through the rotor head, increasing the lift vector, and tilting it more forward, causing a higher rotor rpm.

As airspeed is traded, the helicopter starts to settle. At this point the pilot trades energy from rotor inertia into greater lift. Hopefully the pilot didn't go too deeply into the bank account of rotor energy. Before the blades stall, the helicopter should settle safely on the ground.

I hope this took some of the mystery out of autorotation. By adjusting pitch you control the direction and magnitude of the relative wind, lift vector, and pro-autorotative force. Proper collective management through the decent, flare, and touch down will save you from buying a new set of skids and will result in a magnificent crowd-pleasing auto.