Cyclorotor aircraft

 

The wings on the rotor in the selected configuration are in cycloidal motion during the movement of the aircraft. And so, this kind of aircraft is known as cyclorotor aircraft. The history of cyclorotor aircraft has a long trend from the beginning of the twenty century. This trend began at the same time as the trend of helicopters. Finally, the trend of cyclorotors wasn't successful compared to the trend of helicopters. I suppose that a misunderstanding of the capabilities of cyclorotor aircraft mainly caused that. In many cases, there were intentions to build a cyclorotor aircraft with the ability of vertical flight. Adherents of aircraft of this type were lured by the advantage of the movement of the wings in the cyclorotor, relative to the movement of the wings in the helicopter. Indeed, the wings in the cyclorotor move in parallel with the same speed along their entire length compared with the wings of the helicopter, which have a low speed near the center of the rotor. But this advantage is not the main factor for vertical flight. Prior to the end of the nineteen century the moment theory of the actuators was developed, which implies that the area of the actuator is the main factor for the efficiency under the required thrust. A small area of an actuator under a fixed thrust induces a very high inflow, which alters a base flow, creating a high drag. Only an increase in the rotation speed of the actuator, which can slightly increase efficiency, can reduce this drag. However, this cannot alter that inflow at all, as well as the outflow. A presence of this outflow is required by the entire thrust and is reflected in the propulsion efficiency of any kind of actuator. A gliding wing also can be considered as an actuator with a downwash as the outflow. The thrust specific area of a typical cyclorotor aircraft is significantly lower than the thrust specific area of a helicopter of the same scale. The cyclorotor should have its rotation speed much higher than in the case of low inflow. And so, it encounters a number of disadvantages on this way. The main disadvantage here with respect to the helicopter is the direction of centrifugal forces. They always have a radial direction, which is the direction of weakness for the wings of the cyclorotor and the direction of strongness for the wings of the helicopter. Another disadvantage is induced by the first. The centrifugal forces in the helicopter induce an additional rigidness of its wings for the applied aerodynamic forces. This acts as some kind of multiplier coefficient. But for the cyclorotor aircraft, this feature acts as an oscillatory superposition of two forces: centrifugal and aerodynamic. Finally, a cyclorotor aircraft can never have the same level of efficiency for vertical flight as a helicopter. More than, simply creating such a full-size aircraft with any efficiency is a great challenge, also using contemporary advanced materials. This wrong intention was also reflected in the naming of the actuators of such aircraft for use in horizontal flight. They are until now referenced as cycloidal propellers, and the trend is still ongoing.

 

Also, I suppose that there was an additional factor that could prohibit the building of cyclorotors for horizontal flight. This is a high value of the rotational moment when the rotor is actuated. This follows from the "flying elevator" concept. The cyclorotor can be considered as a drum of an elevator when winding wire. And the force on the pivot of a wing will be equal to the force on the wire, if the wing is in the forward position and only it provides sustaining. In the real case there are four wings, which provide 90 percent of this sustaining on the forward side. So, the total force in the pivots' radial position will be about half the entire weight of the aircraft. The moment of rotation of the rotor can be represented as the ratio of such a force to the entire weight of the aircraft. I reference it as a particular case of the Moment Ratio (MR) of the entire aircraft when the internal aerodynamic moments of the wings are discarded. Also, this particular case can be referenced as the External Moment Ratio (EMR). This EMR may be too high to actuate the rotor. Indeed, also in the helicopter this problem exists. The helicopter uses a spur gear with a pinion to cope this moment. Also, it uses a high-pressure oil pump to reduce wear in this type of transmission. I resolve this problem in the presented invention in another way: I don't use a power gear transmission at all. Instead this, I use an electric engine with high torque that permitted by its large area of the magnetic air-gaps. And this electric engine is directly connected to the rotor shaft.

 

Nevertheless, some people tried to adapt the cyclorotor for horizontal flight. They related a boundary between two kinds of flight to a pair of the operational modes of the rotor. These two kinds of flight are mainly differed by a kind of the cycloid, which their wings follow. The rotor that operates as a propeller with a low airspeed has a low advance ratio relative to the air on infinity, which speed is known as True Aerodynamic Speed (TAS), and a significantly higher advance ratio relative to the airspeed in its vicinity, which can be referenced as Local Aerodynamic Speed (LAS), since here an inflow exists. The advance ratio is simply the ratio of an airflow speed to the linear rotation speed of the wings, which I reference as the winding speed. This ratio is very useful in the realm of propellers. I also use it for characterization the operations of the aircraft that is presented in the invention, but in another form. I use it in the form of the reversed ratio as Winding Ratio (WR), since the presented aircraft can simply glide without a rotation of the rotor at all. In this case, it has the WR equal to zero, instead of infinity, if I kept the former form of the referencing. Also, it is always referenced relative to LAS.

Curtate mode     Prolate mode

            Curtate                                      Prolate

 

Returning to the mentioned pair of the operational modes of the cycloidal propellers, they were divided into a curtate mode, when the rotors' steering is adapted to the operation with an advance ratio below 1, and into a prolate mode, when the adaptation is targeted on an advance ratio above 1. And this adaptation itself was an intention of a minimizing the powering force reaction normal to the cycloidal path by adjusting angles of attack, which reflects an intention of a minimizing that rotational moment, which I discussed before. For this adaptation, it can be obvious that a wing will perform some oscillating relative to its pivot for the curtate mode in the rotating reference frame of the rotor. Simultaneously, this wing will perform a rotation relative to its pivot, looking at it, from a steady reference frame outside the rotor in this mode. In the prolate mode, there will be opposite picture: the wing will rotate relative to the rotor and will oscillate, looking from the outside. In the latter mode, the rotation of the wings inside the rotor is performed in the opposite direction of the rotation the rotor itself, which can be implemented by using a planetary gear transmission with four gears per wing, where one central gear is common. Such transmissions, which can keep the pitches of all wings equal, were represented in some inventions related to cyclorotor aircraft. And they were accompanied with particular solutions of the steering the wings from the neutral position.

 

In the US patent 2,045,233 of Kirsten et al. a cycloidal propeller is described, designed for the prolate operation, and which utilizes a four-gears transmission scheme, where one pair of the meshed gears uses bevel teeth. There, steering of each wing is performed by an additional differential connected to the first of the mentioned bevel gears. These differentials 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 the second - the direction of the eccentricity. Also, the latter regulation is combined with a regulation of the common pitch by rotating the central gear. Now, from the point of view of the PGS-state, there exist: gain steering using the eccentricity level, a steering of the skew using the direction of the eccentricity, and a steering of the pitch using the combination with skew control. So, there missed a possibility to change the pitch independently of the skew. Nevertheless, the inventors claimed that this is a positive feature, which permits more effective action, having a common control over the center of symmetry and the pitch. Although the inventors only guess in that effective action, it exists indeed, but only for propelling, which may be useful for the runway operations of an SAA. In any case, this solution cannot be adapted for the required aircraft, because the steering elements obstruct the central area of the rotor, disabling to place here a central powering shaft. Also, the separation of these pitch and skew controls in this scheme requires an additional steady base inside, which leads here to exceptional complexity.

 

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

 

In the US patent 6,932,296 of Tierney an unmanned aircraft with cycloidal rotor is described, capable to operate in the curtate mode, prolate mode, and with fixed wings using a separate fan as a propeller. It uses a transmission scheme with three gears, which can be considered as a particular case of a four-gears scheme, when all four gears are equal, so the intermediate 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. These central gears have some elements that allow to switch between the curtate and prolate modes. In the prolate mode, the set of central gears is stationary, and in the curtate mode it rotates. Steering of the 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 selectively griping the entire set of central gears upon switching to the prolate mode and with the possibility of changing it in the fixed-wings mode. The system of gears keeps the integrity by links pivotally connecting their axes. Also, there is some central shaft to which all inner links are connected, and which follows the rotation of these inner links with some degree of uncertainty. The gears, related to a particular wing, occupy their space in the depth of the rotor, but the links have a common level where they are connected to the central shaft. The rotor is presented for three wings, but the placement of gears and links doesn't allow the use of more than five wings. Above this limit, collisions can occur. Nevertheless, this solution complies with the PGS-state in its prolate operating mode. A notable feature of this unmanned aircraft is the demonstrating of the principal limitation of a cyclorotor aircraft, based on the law of obeying the "propeller rule" of having minimal projections of the lift forces onto the direction of rotation: the aircraft is designed to operate with a high rotation speed upon a low torque, and when the obeying of the mentioned law, upon increasing the speed, leads to the decreased rotation, the propulsion power decreases, so it should use the additional fan for the propelling in the high speed flight, instead of utilizing the lifting power possibility of the primary actuator.

 

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