Cyclorotor aircrafts

 

††Wings on rotors of liftoplane are in cycloidal motion under movement of the aircraft. And so this kind of aircraft is referenced as cyclorotor aircraft. History of cyclorotor aircrafts has a long trend from beginning of twenty century. The trend began simultaneously with trend of helicopters. Finally the trend of cyclorotors wasn't fruitful instead of the trend of helicopters. I suppose, wrong understanding of possibilities of the cyclorotor aircraft mainly caused it. In many cases there were intentions to build a cyclorotor aircraft with ability of vertical flight. Adherents of this kind of aircraft were lured by the advantage in motion of wings in cyclorotor relative to motion of wings in helicopter. Indeed, wings in cyclorotor are moved in parallel manner with same speed over their entire length, instead of wings of helicopter, which have low speed near center of rotor. But this advantage isn't a main factor for vertical flight. Prior end of nineteen-century a moment theory of actuators was developed, which implies area of an actuator as main factor for an efficiency under desired thrust. Low area of actuator under fixed high thrust induces very high inflow, which alters the base flow, creating a high drag. Only increasing rotation speed of the actuator, which can increase the efficiency a bit, can decrease the drag. But nevertheless it cannot alter the inflow at all and so outflow. This outflow is a non-overcoming limitation for entire thrust or propulsion efficiency of any kind of actuator. Also a gliding wing can be considered as an actuator with downwash as the outflow. The thrust specific area of typical cyclorotor aircraft is significantly lower than the thrust specific area of a helicopter of same scale. Cyclorotor should have its rotation speed much higher, than for case of low inflow of a helicopter. And so it encounters a number disadvantages on this way. Main such disadvantage relative to helicopter is the direction of centrifugal forces. They always have radial direction, which is direction of weakness for wings of cyclorotor and direction of strongness for wings of helicopter. Other disadvantage is induced by first: the centrifugal forces on a helicopter induce additional rigidness for its wings for applied aerodynamic forces. It acts as some multiplicative coefficient. But for the cyclorotor aircraft this feature acts as an oscillated superposition of two forces: centrifugal and aerodynamic. Finally, the cyclorotor aircraft never can be on same level of efficiency for vertical flight as a helicopter. More than, only a building such kind of aircraft of full-scale size with some efficiency is a great challenge, also using contemporary advanced materials. The wrong intention also was reflected in naming of actuators of such aircrafts. They until now are referenced as cycloidal propellers, and the trend still continues.

 

Also I suppose, there was an additional factor, which can prohibit building of cyclorotors for horizontal flight. It is high value of rotation moment upon powering of the rotor. It follows from the "flying elevator" conception. . The cyclorotor can be considered as a drum of the elevator upon winding wire. And force on the pivot of a wing will be equal to force on the wire in case the wing is in the forward position, and only it provides the sustain. In real case there are four wings, which provide 90 percents of sustain on the forward side. So the total force in such pivotsí radial position of both rotors will be about of half of entire weight of the aircraft. And a moment on the rotor can be represented by ratio of such force to entire weight of the aircraft. I reference it as particular case of a Moment Ratio (MR), when internal aerodynamic moments of wings are discarded. Also this particular case can be referenced as an External Moment Ratio (EMR). And so that EMR can be too high for powering the aircraft. Indeed, also for a helicopter exists the problem. Helicopter uses a spoor gear with a pinion for cope such moment. Also it uses a high-pressure oil pump for decreasing a wearing action in this kind of transmission. For the liftoplane the problem is resolved by other way: I don't use the power transmission at all. Instead it, I use an electrical engine with high torque, permitted by its high area of magnetic air-gap. And this electrical engine is directly connected to the rotor's shaft.

 

Nevertheless, some people tried to adapt the cyclorotor for horizontal flight. They related boundary between two kinds of flight by pair of operation modes of the rotor. Those two kinds of flight mainly differed by kind of cycloid, which their wings follow. A rotor, operating as a propeller with low airspeed, has low advance ratio relative to air on infinity, which is known as a True Aerodynamic Speed (TAS), and significantly higher advance ratio relative to the airspeed in its vicinity, where an inflow exists, and which can be referenced as a Local Aerodynamic Speed (LAS). The advance ratio is simple a ratio of the airflow speed to the linear speed of wings, where last I reference as a winding speed (WS). It is very useful in realm of propellers. I use it in other form for characterization operations of the liftoplane. I use it in the form of reversed ratio as Winding Ratio (WR), since the presented aircraft can simple glide, without motion of its rotor at all. In this case it has the WR equal to zero, instead of infinity, if I kept the old referencing. Also it is always referenced relative the LAS.

Curtate mode     Prolate mode

†             Curtate                                      Prolate

 

Returning to the mentioned pair of operational modes of cycloidal propellers, they were divided on a curtate mode, when the rotorís cinematic mechanics is adapted to an operation with the advance ratio below 1, and a prolate mode, when the adaptation is targeted to the advance ratio above 1 (see animations above). And the adaptation itself was an intention of minimizing the powering force reaction normal to the cycloidal path, which reflects intention of minimizing of the powering moment, which I discussed before. For the adaptation it can be obviously, a wing will perform some oscillating relative its pivot for the curtate mode in a rotated referenced frame of the rotor. Simultaneously the wing will perform a rotation relative to its pivot, looking from a steady reference frame outside the rotor in the mode. In the prolate mode there will be opposite picture: the wing will be rotating relative to the rotor and will be oscillating, looking outside. For the last, the rotation of wings inside of rotor is performing in the direction opposite of rotation of the rotor itself, which can be implemented by using a double planetary gear transmission with four gears per wing, where one central gear is common. Kinds of such transmission for keeping pitches of all wings equal were referenced in many inventions related to cyclorotor aircrafts. And those were accompanied with particular solutions of steering wings from neutral position.

 

US patent 2,045,233 of Kirsten et al describes a cycloidal propeller designed for the prolate operation, which utilized the four gears transmission scheme, where one pair of meshed gears uses bevel teeth. And the steering of wing is performed by an additional differential connected to first of the mentioned bevel gears. Those differentials of each wing participate in a common movement by levers pivoted on a common eccentric. Also there exist two handling inputs. One regulates the value of eccentricity and seconds the direction of eccentricity. The last regulation was blocked with the regulation of the common pitch by rotating the central gear. Now from point of view of the PGS state there are: steering of the gain by the level of eccentricity, steering of the skew by the direction of eccentricity and steering of the pitch by the blocking with the skew regulation. So there misses a possibility for changing the pitch independently of the skew. Nevertheless, inventors claimed that as a positive feature, which permits more effective action, having the common control for the center of symmetry and the pitch. Although invertors only guess in that effective action, it exists indeed, but only for propelling, which can be useable for runway operations of SAA. In any case this solution cannot be adapted for the target aircraft, because the steering elements obstruct central area of rotor, which isn't permit to place here a central powering shaft. Also separating the pitch and the skew controls for the scheme needs an additional steady base inside, which leads here to exceptional complexity.

 

US patent 5,100,080 of Servanty describes a cycloidal rotor for horizontal flight, which also utilizes the four gears transmission scheme. In the rotor, steering of each wing is performed by a rotated hydraulic actuator, embedded in a coupling of two intermediate gears of the four gears transmission scheme. The actuator assures a correct pitch for the wing in each instant, which is managed by a special calculator. Also there is mechanics for handling the neutral common pitch. The solution has exceptional flexibility for handling pitches of particular wings, which out of range of the PGS state. Also the solution isn't secure and dangerous. Indeed, the pitch calculated for some instant is correct only in vicinity of its specific phase. In case of an outage of hydraulic pressure or electricity of calculator, the remained or not assigned pitch will wrong for other phase, which can drastically change the overall lifting force, leading to an aircraft incident. And so this example demonstrates additional advantage of the mechanical steering fitted to constraints of the PGS state: In case of a power outage the steering will be continue operating correctly, since the state simply remains as a mechanical state for any intermediate phase of any wing.

 

US patent 6,932,296 of Tierney describes an unmanned aircraft with a cycloidal rotor, having possibilities to operate in curtate mode, prolate mode and with fixed wings with separated fan as a propeller. It uses a tree gears transmissions scheme, which can be considered as particular case of the four gear scheme, where all four gears are equal, so intermediated coupled pair of gears is reduced to one intermediate gear. Also instead of one central gear there is a set of central gears, one per each wing. Those central gears have some elements, which permit switching between the curtate and prolate modes of operation. In the prolate mode the set of central gears is stationary, and in the curtate it is rotating. The steering of wings is performed by moving the entire set of central gears by some XY pair of servos. Also there exists some case of handling the common pitch by a selective griping of the entire set of central gears upon switching to the prolate mode and with possibility of changing it in the fixed wings mode. The system of gears keeps integrity by links connecting their axes pivotally. Also there is some center shaft, to which those links are connected, and it is used for locking the rotor in the mode of fixed wings operation. Gears related to particular wing occupy their own position in depth of the rotor, but links have a common level where they are connected to the central shaft. The rotor is presented for three wings, but placement of these gears and links isn't permit having more than five wings. Also for that, there can be collisions between such links upon steering. Nevertheless, this solution complies with the PGS state in its prolate mode of operation. Remarkable feature of the unmanned aircraft is a demonstrating of the principal limitation of an cyclorotor aircraft based on the law of obeying the "propeller rule" of having minimal projections of lift forces to the direction of rotation: the aircraft is designed operating with high rotation speed upon low powering moment, and when obeying the mentioned law upon increasing the flight speed leads to decreasing of rotation, a propulsion power is decreasing, so it should use the additional fan for propelling in the high speed flight instead of utilizing the lifting power possibility of the primary actuator.

 

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