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56-416: Cyclorotors produce thrust by combined action of a rotation of a fixed point of the blades around a centre and the oscillation of the blades that changes their angle-of-attack over time. The joint action of the advancement produced by the orbital motion and pitch angle variation generates a higher thrust at low speed than any other propeller. In hover, the blades are actuated to a positive pitch (outward from

112-683: A 32 ft boat in Washington, which eliminated the need for a rudder and provided extreme manoeuvrability. 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

168-400: A blade pitch mechanism should adjust for these diverse flow angles. High rotational velocities makes it difficult to implement an actuator based mechanism, which calls for a fixed or variable shape track for pitch control, mounted parallel to blade trajectory, onto which are placed blade's followers such as rollers or airpads - the pitch control track shape reliably determines blade's pitch along

224-591: A built-in flight computer that automatically prevents the aircraft from increasing the angle of attack any further when a maximum angle of attack is reached, regardless of pilot input. This is called the 'angle of attack limiter' or 'alpha limiter'. Modern airliners that have fly-by-wire technology avoid the critical angle of attack by means of software in the computer systems that govern the flight control surfaces. In takeoff and landing operations from short runways ( STOL ), such as Naval Aircraft Carrier operations and STOL backcountry flying, aircraft may be equipped with

280-502: A chord line of the whole wing may not be definable, so an alternate reference line is simply defined. Often, the chord line of the root of the wing is chosen as the reference line. Another choice is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis). Some authors do not use an arbitrary chord line but use the zero lift axis where, by definition, zero angle of attack corresponds to zero coefficient of lift . Some British authors have used

336-413: A combination of sonic tip speed and retreating blade stall . As the advance ratio increases, the relative velocity experienced by the retreating blade decreases so that the tip of the blade experiences zero velocity at an advance ratio of one. Helicopter rotors pitch the retreating blade to a higher angle of attack to maintain lift as the relative velocity decreases. At a sufficiently high advance ratio,

392-460: A high lift coefficient. This unsteady lift makes cyclorotors more efficient at small scales, low velocities, and high altitudes than traditional propellers. It is otherwise evident that many living beings, such as birds, and some insects, are still much more efficient, because they can change not only the pitch but also the shape of their wings, or they can change the property of the boundary layer such as sharkskin . Some research tries to acquire

448-418: A particular airspeed . The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor , the center of gravity of the aircraft and other factors. However, the aircraft normally stalls at the same critical angle of attack, unless icing conditions prevail. The critical or stalling angle of attack is typically around 15° - 18° for many airfoils. Some aircraft are equipped with

504-402: A test. https://www.cyclotech.at/ Angle-of-attack In fluid dynamics , angle of attack ( AOA , α , or α {\displaystyle \alpha } ) is the angle between a reference line on a body (often the chord line of an airfoil ) and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack

560-487: Is a reduction of angle of attack at which stall conditions are reached. 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. It is then evident that a cyclorotor is much more energy efficient than any other propeller. Actual cyclorotors bypass this problem by quickly increasing and then decreasing blade angle of attack, which temporarily delays stall and achieves

616-435: Is also influenced by the wing shape, including its airfoil section and wing planform . A swept wing has a lower, flatter curve with a higher critical angle. The critical angle of attack is the angle of attack which produces the maximum lift coefficient. This is also called the " stall angle of attack". Below the critical angle of attack, as the angle of attack decreases, the lift coefficient decreases. Conversely, above

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672-489: Is little inertia associated with blade pitch change, thrust vectoring in the plane perpendicular to the axis of rotation is rapid. 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 fly forward,

728-411: Is more separated and the airfoil or wing is producing its maximum lift coefficient. As the angle of attack increases further, the upper surface flow becomes more fully separated and the lift coefficient reduces further. Above this critical angle of attack, the aircraft is said to be in a stall. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below

784-579: Is necessary that the stalling blade increases the pitch angle to keep some lift capability. This risk puts constraints on the design of the system. An accurate choice of the wing profile is necessary and careful dimensioning of the radius of the rotor for the specified speed range. Slow speed cyclorotors bypass this problem through a horizontal axis of rotation and operating at a comparatively low blade tip speed. For higher speeds, which may become necessary for industrial applications, it seems necessary to adopt more sophisticated strategies and solutions. A solution

840-456: Is only indirectly related to stall behavior. Some military aircraft are able to achieve controlled flight at very high angles of attack, but at the cost of massive induced drag . This provides the aircraft with great agility. A famous example is Pugachev's Cobra . Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft is not capable of either aerodynamic directional control or maintaining level flight until

896-455: Is rotating slowly, the advance ratio of its propeller(s) is a high number. When the vehicle is moving at low speed or the propeller is rotating at high speed, the advance ratio is a low number. The advance ratio is a useful non-dimensional quantity in helicopter and propeller theory, since propellers and rotors will experience the same angle of attack on every blade airfoil section at the same advance ratio regardless of actual forward speed. It

952-404: Is stated to overcome most of the traditional limitations of traditional Darrieus VAWTs. 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 in any direction parallel to the plane of the water surface. In 1922, Frederick Kirsten fitted a pair of cyclorotors to

1008-421: Is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air. In aerodynamics , angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist,

1064-446: Is the independent actuation of the blades which have been recently patented and successfully tested for naval use by use on hydraulic actuation system. The horizontal axis of rotation always provides an advancement of the upper blades, that produce always a positive lift by the full rotor. These characteristics could help overcome two issues of helicopters: their low energy efficiency and the advance ratio limitation. The advancement of

1120-434: Is the inverse of the tip speed ratio used for wind turbines. The advance ratio J is a non-dimensional term given by: where The advance ratio μ is defined as: where Low Advance Ratio (J < 1): When the advance ratio is low, the vehicle is moving forward slowly relative to the propeller speed. This usually happens at low speeds or when the propeller is turning very fast. High Advance Ratio (J > 1): When

1176-648: The Northwestern Polytechnic Institute in China. Since then, universities and companies have successfully flown small-scale cyclogyros in several configurations. 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

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1232-491: The Second World War. Today, the same company sells the same propeller for highly manoeuvrable watercraft. It is applied on offshore drilling ships, tugboats, and ferries. A cyclogyro is a vertical takeoff and landing aircraft using a cyclorotor as a rotor wing for lift and often also for propulsion and control. Advances in cyclorotor aerodynamics made the first untethered model cyclogyro flight possible in 2011 at

1288-530: The US Navy seriously considered fitting of six primitive Kirsten-Boeing cyclorotors to the USS ; Shenandoah airship. 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 20 m (66 ft) cyclorotor airship demonstrated improved performance over a traditional airship configuration in

1344-408: The action of an elevator. Lift and thrust had to be created by paddle wheels consisting of 12 blades, established in pairs under a 120° angle. The blades of a concave shape were changing an angle of incidence by the means of eccentrics and springs. In a bottom of the craft 10 hp engine was arranged. Transmission was ensured by a belt. Empty weight was about 200 kg. "Samoljot" was constructed by

1400-401: The advance ratio is high, the vehicle is moving forward quickly compared to the propeller's rotational speed. This typically occurs at higher speeds or when the propeller is turning more slowly. The advance ratio is critical for determining the efficiency of a propeller. At different advance ratios, the propeller may produce more or less thrust. Engineers use this ratio to optimize the design of

1456-403: The aircraft of speed very quickly due to induced drag, and, in extreme cases, increased frontal area and parasitic drag. Not only do such maneuvers slow the aircraft down, but they cause significant structural stress at high speed. Modern flight control systems tend to limit a fighter's angle of attack to well below its maximum aerodynamic limit. In sailing , the physical principles involved are

1512-458: The angle of attack of a fixed-wing aircraft increases, separation of the airflow from the upper surface of the wing becomes more pronounced, leading to a reduction in the rate of increase of the lift coefficient. The figure shows a typical curve for a cambered straight wing. Cambered airfoils are curved such that they generate some lift at small negative angles of attack. A symmetrical wing has zero lift at 0 degrees angle of attack. The lift curve

1568-579: The angle of attack or Lift Reserve Indicators . These indicators measure the angle of attack (AOA) or the Potential of Wing Lift (POWL, or Lift Reserve) directly and help the pilot fly close to the stalling point with greater precision. STOL operations require the aircraft to be able to operate close to the critical angle of attack during landings and at the best angle of climb during takeoffs. Angle of attack indicators are used by pilots for maximum performance during these maneuvers, since airspeed information

1624-484: 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. 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

1680-409: The blade and the oscillation of the blade (it is a movement somehow similar to the pendulum), which continue to vary its pitch generate a complex set of aerodynamic phenomena: The two effects are evidently correlated with a general increase of the thrust produced. If compared to a helicopter or any other propeller, it is evident that the same blade section in a rotocycloid produces much more thrust at

1736-542: The blade will reach the stalling angle of attack and experience retreating blade stall. Specially designed airfoils can increase the operating advance ratio by utilizing high lift coefficient airfoils. Currently, single rotor helicopters are practically limited to advance ratios less than 0.7. The advance ratio is the inverse of the tip speed ratio , λ {\displaystyle \lambda } , used in wind turbine aerodynamics: In operation, propellers and rotors are generally spinning, but could be immersed in

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1792-418: The blades and oscillations are the two dynamic actions which are produced by a cyclorotor. It is evident that the wing-blades of a cyclorotor operates in different way than a traditional aircraft wing or a traditional helicopter wing. The blades of a cyclorotor oscillate by rotation around a point that rotating describes an ideal circumference. The combination of the advancement motion of the centre of rotation of

1848-403: The blades. 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

1904-435: The centre 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 to any perpendicular angle or even downward. Before blade stall , increasing the amplitude of the pitching kinematics will magnify thrust. The origin of

1960-401: The critical angle of attack, as the angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge. At the critical angle of attack, upper surface flow

2016-444: 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. 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

2072-401: 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

2128-407: The low density of air in the upper atmosphere as well as at low speed at low altitude where the margin between level flight AoA and stall AoA is reduced. The high AoA capability of the aircraft provides a buffer for the pilot that makes stalling the airplane (which occurs when critical AoA is exceeded) more difficult. However, military aircraft usually do not obtain such high alpha in combat, as it robs

2184-513: The maneuver ends. The Cobra is an example of supermaneuvering as the aircraft's wings are well beyond the critical angle of attack for most of the maneuver. Additional aerodynamic surfaces known as "high-lift devices" including leading edge wing root extensions allow fighter aircraft much greater flyable 'true' alpha, up to over 45°, compared to about 20° for aircraft without these devices. This can be helpful at high altitudes where even slight maneuvering may require high angles of attack due to

2240-710: The military engineer E.P. Sverchkov with the grants of the Main Engineering Agency in St. Petersburg in 1909, was demonstrated at the Newest Inventions Exhibition and won a medal. Otherwise, it could not pass the preliminary tests without flying. In 1914, Russian inventor and scientist A.N. Lodygin addressed the Russian government with the project of the cyclogiro-like aircraft, his scheme was similar to Sverchkov's "Samoljot". The project

2296-520: The narrow canals of Venice, Italy. During the 1937 World Fair in Paris, Voith was awarded the grand prize – three times – for its exhibition of Voith Schneider Propellers and Voith turbo-transmissions. A year later, two of Paris' fire-fighting boats started operating with the new VSP system. 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

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2352-435: The orbit regardless of the blade's RPM. 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. Wind turbines are a potential application of cyclorotors. They are named in this case variable-pitch vertical-axis wind turbines , with large benefits with respect to traditional VAWTs. This kind of turbine

2408-499: The propeller and the engine, ensuring that the vehicle operates efficiently at its intended cruising speed, see propeller theory . For instance, an airplane's propeller needs to be efficient both during takeoff (where the advance ratio is low) and at cruising altitude (where the advance ratio is higher). Similarly, a boat's propeller design will vary depending on whether it's designed for slow-speed maneuvering or high-speed travel. Single rotor helicopters are limited in forward speed by

2464-420: The retreating blade has a value that is produced by the vector composition of the velocity of blade rotation and the freestream velocity. In this condition it is evident that in presence of a sufficiently high advance ratio the velocity of air on the retreating blade is low. The flapping movement of the blade changes the angle of attack. It is then possible for the blade to reach the stall condition. In this case it

2520-399: The rotocycloid propeller are Russian and relates to the aeronautic domain. Sverchkov's "Samoljot" (St. Petersburg, 1909) or "wheel orthopter" was the first vehicle expressly thought to have used this type of propulsion. Its scheme came near to cyclogiro, but it's difficult to classify it precisely. It had three flat surfaces and a rudder; the rear edge of one of surfaces could be bent, replacing

2576-434: The same Reynolds number. This effect can be explained by considering the traditional behavior of a propeller. At low Reynolds numbers there is little turbulence and laminar flow conditions can be reached. Considering a traditional wing profile it is evident that those conditions minimize the speed differences between upper and lower face of the wing. It is then evident that both lift and stall speed are reduced. A consequence

2632-419: The same as for aircraft—a sail is an airfoil. A sail's angle of attack is the angle between the sail's chord line and the direction of the relative wind. Advance ratio The propeller advance ratio or coefficient is a dimensionless number used in aeronautics and marine hydrodynamics to describe the relationship between the speed at which a vehicle (like an airplane or a boat) is moving forward and

2688-422: The same level of efficiency of the natural examples of wings or surfaces. One direction is to introduce morphing wing concepts. Another relates to the introduction of boundary layer control mechanisms, such as dielectric barrier discharge. During experimental evaluation, cyclorotors produced little aerodynamic noise. This is likely due to the lower blade tip speeds, which produce lower intensity turbulence following

2744-405: The speed at which its propeller is turning. It helps in understanding the efficiency of the propeller at different speeds and is particularly useful in the design and analysis of propeller-driven vehicles.It is the ratio of the freestream fluid speed to the propeller , rotor , or cyclorotor tip speed. When a propeller-driven vehicle is moving at high speed relative to the fluid, or the propeller

2800-466: The term angle of incidence instead of angle of attack. However, this can lead to confusion with the term riggers' angle of incidence meaning the angle between the chord of an airfoil and some fixed datum in the airplane. The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases. As

2856-455: The time cast doubt on the soundness of the design which meant that funding for the project could not be raised, even with a latter proposal as a Luftwaffe transport aircraft. There appears to be no evidence that this design was ever built, let alone flown. Based on Rohrbach's paddle-wheel research, however, Platt in the US designed by 1933 his own independent Cyclogyro. His paddle-wheel wing arrangement

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2912-414: 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. In the traditional method of the composition of velocity it is easy to understand that the velocity experienced by

2968-477: Was awarded a US patent (which was only one of many similar patents on file), and underwent extensive wind-tunnel testing at MIT in 1927. Despite this, there is no evidence Platt's aircraft was ever built. The first operative cycloid propulsion was developed at Voith . Its origins date to the decision of the Voith company to focus on the business of transmission gear assemblies for turbines. The famous Voight propeller

3024-449: Was based on its fluid-dynamics know-how gained from previous turbine projects. It was invented by Ernst Schneider, and enhanced by Voith. It was launched with name of Voith-Schneider Propeller (VSP) for commercial vessels. This new marine drive could significantly improve the manoeuvrability of a ship as demonstrated in the successful sea trials on the test boat Torqueo, in 1937. The first Voith Schneider Propellers were put into operation in

3080-473: Was developed by the advanced vertical flight laboratory at Texas A&M university. Commercial cyclogyro UAVs are being developed by D-Daelus, Pitch Aeronautics, and CycloTech. 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,

3136-471: Was not carried out. In 1933, experiments in Germany by Adolf Rohrbach resulted in a paddle-wheel wing arrangement. Oscillating winglets went from positive to negative angles of attack during each revolution to create lift, and their eccentric mounting would, in theory, produce nearly any combination of horizontal and vertical forces. The DVL evaluated Rohrbach's design, but the foreign aviation journals of

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