Cyclorotor

Cyclorotor before installation on small-scale cyclogyro

A cyclorotor, cycloidal rotor or propeller, is a fluid propulsion device that converts shaft power into the acceleration of a fluid using a rotating axis perpendicular to the direction of fluid motion. It uses several blades with a spanwise axis parallel to the axis of rotation and perpendicular to the direction of fluid motion. These blades are cyclically pitched twice per revolution to produce force (thrust or lift) in any direction normal to the axis of rotation. Cyclorotors are used for propulsion, lift, and control on air and water vehicles. An aircraft using cyclorotors as the primary source of lift, propulsion, and control is known as a cyclogyro. When implemented on ships, cyclorotors are sometimes referred to as Voith–Schneider propellers.

Operating Principle

Cyclorotos produce thrust by varying the blade angle-of-attack while following an orbital motion. In hover, the blades are actuated to a positive pitch (outward from the center of the rotor) on the upper half of their revolution and a negative pitch (inward towards the axis of rotation) over the lower half inducing a net upward aerodynamic force and opposite fluid downwash. By varying the phase of this pitch motion the force can be shifted sidewise or downward. Before blade stall, increasing the amplitude of the pitching kinematics will magnify thrust.

A cyclorotor generates thrust by altering the pitch of the blade as it transits around the rotor.

Design Advantages and Challenges

Rapid Thrust Vectoring

Cyclorotors provide a high degree of control. Traditional propellers, rotors, and jet engines produce thrust only along their axis of rotation and require rotation of the entire device to alter the thrust direction. This rotation requires large forces and comparatively long time scales since the propeller inertia is considerable, and the rotor gyroscopic forces resist rotation. For many practical applications (helicopters, airplanes, ships) this requires rotating the entire vessel. In contrast, cyclorotors need only to vary the blade pitch motions. Since there is little inertia associated with blade pitch change, thrust vectoring in the plane perpendicular to the axis of rotation is rapid.

Cyclorotors can quickly vector thrust by altering the pattern of blade pitching

High Advance Ratio Thrust and Symmetric Lift

Cyclorotors can produce lift and thrust at high advance ratios, which, in theory, would enable a cyclogyro aircraft to fly at subsonic speeds well exceeding those of single rotor helicopters. Single rotor helicopters are limited in forward speed by a combination of retreating blade stall and sonic blade tip constraints. As helicopters accelerate, the tip of the advancing blade experiences a wind velocity that is the sum of the helicopter forward speed and rotor rotational speed. This value cannot exceed the speed of sound if the rotor is to be efficient and quiet. Slowing the rotor rotational speed avoids this problem, but presents another. The velocity experienced by the retreating blade is the difference between the velocity due to rotation and the freestream velocity. At a sufficiently high advance ratio the retreating blade will stall from excessive angle of attack while flapping to a higher angle to maintain even lift over the rotor disk. Cyclorotos bypass this problem via a horizontal axis of rotation and operating at comparatively low blade tip speed. The horizontal axis of rotation always provides an advancing blade, and the capability to produce lift, on every rotor thereby bypassing the helicopter advance ratio limitation.

Unsteady Aerodynamics

Cyclorotors oscillate blade pitch to produce thrust which delays blade stall and boosts maximum blade lift coefficient at low Reynolds numbers. At low Reynolds numbers, increased relative fluid viscous forces cause wings to stall at lower angles of attack and produce lower maximum lift coefficients. In this regime, conventional propellers and rotors must use larger blade area and rotate faster to achieve the same propulsive forces and lose more energy to blade drag. Small birds, insects, and cyclorotors bypass this problem by quickly increasing and then decreasing blade angle of attack, which temporarily delays stall and achieves a high lift coefficient. This unsteady lift makes cyclorotors more efficient at small scales, low velocities, and high altitudes than traditional propellers. This advantage inspired significant research of cyclorotors for micro-air-vehicle applications.

Quiet Operation

During experimental evaluation, cyclorotors produced little aerodynamic noise. This likely due to the lower blade tip speeds, which produce lower intensity turbulence following the blades.

Hovering Thrust Efficiency

In small-scale tests, cyclorotors achieved a higher power loading than comparable scale traditional rotors at the same disk loading. This is attributed to utilizing unsteady lift and consistent blade aerodynamic conditions. The rotational component of velocity on propellers increases from root to tip and requires blade chord, twist, airfoil, etc., to be varied along the blade. Since the cyclorotor blade span is parallel to the axis of rotation, each spanwise blade section operates at similar velocities and the entire blade can be optimized.

Structural Considerations

Cyclorotor blades require support structure for their positioning parallel to the rotor axis of rotation. This structure, sometimes referred to as "spokes," adds to the parasite drag and weight of the rotor. Cyclorotor blades are also centrifugally loaded in bending (as opposed to the axial loading on propellers), which requires blades with an extremely high strength to weight ratio or intermediate blade support spokes. Early 20th century cyclorotors featured short blade spans, or additional support structure to circumvent this problem.

Blade Pitch Considerations

Cyclorotors require continuously actuated blade pitch. The relative flow angle experienced by the blades as they rotate about the rotor varies substantially with advance ratio and rotor thrust. To operate most efficiently a blade pitch mechanism should adjust for these diverse flow angles. High rotational velocities makes it difficult to implement an actuator based mechanism, and it is challenging to design and construct a mechanical based device. While the pitching motions used in hover are not optimized for forward flight, in experimental evaluation they were found to provide efficient flight up to an advance ratio near one.

Applications

Ship Propulsion and Control

The most widespread application of cyclorotors is for ship propulsion and control. In ships the cyclorotor is mounted with the axis of rotation vertical so that thrust can quickly be vectored any direction in plane with the water surface. In 1922, Kurt Kirstin fitted a pair of cyclorotors to a 32 ft boat in Washington, which eliminated the need for a rudder and provided extreme maneuverability. While the idea floundered in the United States after the Kirsten-Boeing Propeller Company lost a US Navy research grant, the Voith-Schneider propeller company successfully commercially employed the propeller. This ``Voith-Schneider propeller was fitted to more than 100 ships prior to the outbreak of the second world war. Today, the same company sells the same propeller for highly maneuverable watercraft. It is applied on offshore drilling ships, tugboats, and ferries.

Twin Voith Schneider propeller with thrust plate on a tug's hull

Airship Propulsion and Control

A large exposed area makes airships susceptible to gusts and difficult to takeoff, land, or moor in windy conditions. Propelling airships with cyclorotors could enable flight in more severe atmospheric conditions by compensating for gusts with rapid thrust vectoring. Following this idea, the US Navy seriously considered fitting of 6 primitive Kirsten-Boeing cyclorotors to the USS Shenandoah airship. Unfortunately, the Shenandoah crashed while transiting a squall line on 3 September 1925 before any possible installation and testing. No large scale tests have been attempted since, but a 20m cyclorotor airship demonstrate improved performance over a traditional airship configuration in a test by Nozaki et al.

Micro-Air-Vehicles (MAVs)

The performance of traditional rotors is severely deteriorated at low Reynolds Numbers by low angle-of-attack blade stall. Current hover-capable MAVs can stay aloft for only minutes. Cyclorotor MAVs (very small scale cyclogyros) could utilize unsteady lift to extend endurance. The smallest cyclogyro flown to date weighs only 29 grams and was developed by the advanced vertical flight laboratory at Texas A&M university.

Cyclogyros

A cyclogyro is a vertical takeoff and landing aircraft implementing cyclorotors as the principal source of lift, propulsion, and control. Advances in cyclorotor aerodynamics made the first untethered model cyclogyro flight possible in 2012. Since then, universities and companies have successfully flown small-scale cyclogyros in several configurations.

Small-scale cyclogyro In flight

Concept drawing of a cyclogyro.

See also

• Advance Ratio
• Airship
• Cycloid
• Cyclogyro
• Cycloturbine
• Helicopter Rotor
• Micro Air Vehicle
• Retreating Blade Stall