Cyclorotor
A cyclorotor, cycloidal rotor, cycloidal propeller or cyclogiro, 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 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 or cyclocopter. A unique aspect is that it can change the magnitude and direction of thrust without the need of tilting any aircraft structures. The patented application, used on ships with particular actuation mechanisms both mechanical or hydraulic, is a Voith Schneider Propeller.
Operating principle
The blades revolve around the central axis while individually cycling back and forth to engage and disengage. By adjusting their angle-of-attack they maximize the net force. This joint action generates a higher thrust at low speed than any other propeller design.In aircraft hover, the blades adjust to a positive pitch on the upper half of their revolution and a negative pitch over the lower half inducing a net upward aerodynamic force and opposite fluid downwash. By varying the phase of this pitch the force can be shifted to any angle. Increasing the pitching kinematics amplitude magnifies thrust.
History
Samoljot
The cyclorotor propeller emerged in Russian aeronautics. Sverchkov's "Samoljot" or "wheel orthopter" is the first vehicle thought to have used this system. Its scheme came near to cyclogiro, but is difficult to precisely classify. It had three flat surfaces and a rudder; the rear edge of one surface could be bent, replacing the action of an elevator. Lift and thrust had to be created by paddle wheels consisting of 12 blades, set in pairs at a 120° angle. The blades were concave. The angle of incidence was controlled by eccentrics and springs.At the bottom of the craft a 10 horsepower engine was arranged. Transmission was by belt. The empty weight was about 200 kg. It was constructed by military engineer E.P. Sverchkov under the Main Engineering Agency. It was demonstrated at the Newest Inventions Exhibition and won a medal. However, it could not pass preliminary flight tests.
In 1914, Russian inventor and scientist A.N. Lodygin proposed a cyclogiro-like aircraft, similar to Samoljot, but the project was not carried out.
Adolph Rohrbach
In 1933, Adolf Rohrbach experimented in Germany with a paddle wheel wing arrangement. Oscillating winglets cycled from positive to negative angles of attack during each revolution, and their eccentric mounting could, in theory, produce nearly any combination of horizontal and vertical forces. The DVL evaluated Rohrbach's design, but the aviation journals of the time cast doubt on the design preventing funding, even with a proposal as a Luftwaffe transport aircraft. No evidence indicates that this design was ever built. Platt in the US designed by 1933 his own independent Cyclogyro, based on Rohrbach's work. His arrangement was awarded a US patent, and underwent extensive wind-tunnel testing at MIT in 1927. Despite this, Platt's aircraft was never built.Voith-Schneider
The first functional design was developed at Voith in the 1930s. Its origins date to the decision of the Voith company to focus on turbine transmission gear assemblies. The Voight propeller was invented by Ernst Schneider and enhanced by Voith. It was launched as the Voith-Schneider Propeller for commercial marine vessels. It significantly improved ship manoeuvrability as demonstrated in sea trials on the test boat Torqueo, in 1937. The first Voith Schneider Propellers were put to work in the canals of Venice, Italy. During the 1937 World Fair in Paris, Voith was awarded the grand prize – three times – for its propellers and turbo-transmissions. A year later, two of Paris' fire-fighting boats started operating with the system.In 2025, ABB announced its Dynafin cyclorotor system, which promises up to 85% efficiency and precise maneuvering. Dynafin follows the Azipod system introduced in the 1990s. The Dynafin uses an electric motor to rotate a disc on the underside of a ship or boat, which spins at between 40 and 80 RPM. Five independently controllable blades are mounted at equal intervals on the disc. The record efficiency could allow the use of more compact propulsion systems. The relatively low-pressure pulses and blade-tip speeds create less turbulence than conventional props. Independent testing reported that Dynafin managed energy savings of 22% compared to conventional configurations. The concept was evaluated on the Carlton Ilma cruise yacht, saving power, weight and space.
Design
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 redirecting the entire device to alter the thrust direction. This realignment 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 this requires rotating the entire vessel. In contrast, cyclorotors need to vary only the blade pitch. Since little inertia is associated with blade pitch change, thrust vectoring in the plane perpendicular to the axis of rotation is rapid and efficient.Advance ratio thrust and symmetric lift
Cyclorotors can produce lift and thrust at high advance ratios, which, in theory, enable a cyclogyro to fly at much faster subsonic speeds than single rotor helicopters.Forward speed of single rotor helicopters is limited by a combination of retreating blade stall and sonic blade tip constraints. As helicopters fly forward, the tip of the advancing blade experiences wind velocity that is the sum of the helicopter forward speed and the rotational speed. This value cannot exceed the speed of sound if the rotor is to remain efficient and quiet.
Keeping the rotational speed down avoids this problem, but presents another. In the traditional method of the composition of velocity the wind velocity seen by the retreating blade is the vector composition of the blade rotation velocity and the freestream velocity. In this condition in the presence of a sufficiently high advance ratio the air velocity on the retreating blade is low. The flapping of the blade changes the angle of attack. It is then possible for the blade to reach the stall condition. In this case the stalling blade must increase the pitch angle to maintain lift. This constrains the wing profile and requires careful dimensioning of the rotor radius for the specified speed range.
Slow speed cyclorotors avoid this problem by switching the rotation axis to horizontal and operating at a lower blade tip speed. For higher speeds it is necessary to adopt more sophisticated strategies. One approach is to independently actuate the blades via hydraulic actuation. The horizontal axis of rotation always advances the upper blades that always produce a positive lift by the full rotor. These characteristics could help overcome helicopter's low energy efficiency and the advance ratio constraint.
Aerodynamics
The revolving and oscillating blades are the cyclorotor's two dynamic actions, generating complex aerodynamic phenomena:- delaying the blade stall;
- increasing the maximum blade lift coefficient at low Reynolds numbers.
At low Reynolds numbers little turbulence is present and laminar flow conditions can be achieved. A traditional wing profile minimizes the speed difference between upper and lower face of the wing, reducing both lift and stall speed. A consequence is a reduction of stall condition attack angle.
In this regime, conventional propellers and rotors must use larger blade areas and rotate faster to achieve the same propulsive forces, while losing more energy to blade drag. Thus, a cyclorotor is more energy efficient than any other propeller.
Cyclorotors quickly increase and decrease blade attack angle, which delays stall and achieves lift. This unsteady lift makes cyclorotors more efficient at small scales, low velocities, and high altitudes than traditional propellers. However, birds and some insects are still much more efficient, because they can change both the pitch and the shape of their wings, or the boundary layer.
Research aims for the same level of efficiency of wings or surfaces. One direction is morphing wings. Another relates to the introduction of boundary layer control mechanisms, such as dielectric barrier discharge.