Feb 19, 2007
The cyclogyro is a kind of aircraft using cycloidal propellers to generate lift and thrust. The cycloidal propulsion system is composed of two or more blades that rotate around an axis parallel to the blades. When the aircraft hovers, the blades travels along a circle and when the aircraft moves backward or forward, the blades travels along a cycloid. With the pitch angle controlled by certain mechanisms, usually the eccentric, the desired direction of the total aerodynamics force can be obtained. When the position of the eccentric is moved, the direction of the total force vector can be varied 360deg. This provides the aircraft 360deg vector trusting, which greatly improve the maneuverability of the airplane.
Fig 1. The basic principle of cyclogyro
Fig 2. The motion of the blade in hovering mode
(The blue vector shows the velocity of the blade leading edge and the green vector shows the velocity of the blade trailing edge)
Fig 3. The motion of the blade in forward flight mode
Fig 4. The motion of the blade in backward flight mode
The primary advantages of the cyclogyro are as follows:
The cycloidal propeller can provide 360deg of vector thrusting at ease by simply moving the eccentric. This enables the cyclogyro fly in any direction and hover. Thus cyclogyro has very good maneuverability and can obtain very fast response on the pilot control command;
There is no strong blade tip vortex since all sections of the cycloidal propeller travels at the same speed. Therefore the noise of the cycloidal propellers will be much lower than the screw propellers.
If a tail rotor is installed to balance the main rotor torque, the tail rotor generates useful lift. There are no power lose caused by trimming the aircraft. But for helicopters, the tail rotor generates side force which has no contribution on improving the performance. Typically 10% power is wasted to balance the main rotor torque.
The disadvantages of the cyclogro are as follows:
The structure of the cycloidal propeller is quite heavy compared to the screw propeller. It needs big struts to support the blades;
The control mechanism is complex and heavy;
To evaluate the aerodynamics forces, we developed software called UVLMFW. The unsteady vortex lattice algorithm is used since the panel method can obtain reasonable results without very high computation cost. We can also observe the wake structure to predict the performance of the propeller.
Fig 5. The layout of the software
Fig 6. The GUI of the software
Fig 7. The simulation on a two bladed design using unsteady vortex lattice method
The first generation of our control mechanism is composed of a cam and a slider moving on the blade soprting strut. The blade soprting strut drives the blades and the cam mechanism controls the blade pitch angle so that the blade can travel at desired angle of attack. This type of control mechanism can not work at all because the cam mechanism has high friction and breaks down soon after the propeller starts to rotate.
Fig 8. The transition and control unit with motor installed
The second generation control mechanism is a mixed 4-bar and geared 5-bar control mechanism. This kind of mechanism is derived from the 4-bar control mechanism (Fig 9.). The propeller using 4-bar control mechanism generates aerodynamics forces with the same concept as the propeller with cam mechanism. The desired AOA is obtained by varying the offset of one end of the control rod as shown below.
Fig 9. The four-bar control mechanism
If we define the end of control rod that is not linked to the blade as control end. Then the control end shall be a big ring to avoid the control rod intersects with the shaft of the blade suporting strut (Fig 10). The control ring in this case acts as a hinge of the control rod and shall be a big bearing. For the propellers that have several blades, each control end of the control rod will have a big control ring and this result in very bulky structure. To solve this problem, we developed a mixed four bar and five bar control mechanism, as shown below.
Fig 10. The four bar control mechanism with control ring
Fig 11. The mixed four bar/five bar mechanism
As shown above, all blades share one control ring, which is the fixed on one end of a control rod. The rest of control rods hinges on to the control ring. Therefore, for the blade with the control rod fixed on control ring, the mechanism is the 4-bar control mechanism. For the rest of blades, they are five-bar control mechanism. Since all control rods share only one control ring, the system is greatly simplified and the weight is reduced. This generation of control mechanism is also not successful. It is a bit heavy and bulk. Since the control ring, or the eccentric moves in a plane that is perpendicular to the main shaft and parelell to the symetrical plane of the fuselage, it causes many problem when the servoes try to move it freely. The control mechanism often jamed and damaged the servo.
Fig 12. The structure of a 6-bladed propeller with mixed 4-bar and 5-bar mechanism
To solve the problems occured on the 2nd generation of control mechanism, t he 3rd generation control mechanism was invented. Compare to the 2nd generation control mechanism, the 3rd generation is lighter, more compact and reliable. The tests show that the 3rd generation control mechanism works quite well and can change the pitch angle of the propeller very fast. Therefore, the 3rd generation of control mechanism is very successful.
The gearbox was also re-designed for the third generation cycloidal propeller. It is more compact and lighter.
Fig 13. The gear box with a brushless motor installed
(CAD model on the left, actual gearbox on the right)
We developed 3 generations of cyclogyro. And the last one is the most successful one.
Fig 14. The 3rd generation of cyclogyro
Since the blade of cycloidal propeller rotates around an axis parallel to it, it will experience very large centrifugal force along with the aerodynamics forces. The design of blade structure can be a challenging task. Therefore, the blade supporting structure has to be carefully designed to reduce the weight and promote the reliability.
There are 3 types of blade supporting structure available. They are the "T" shape, "pi" shape and "n" shape respectively (Fig 15).
Fig 15. The 3 types of blade supporting structure
Fig 16. The centrifugal force acting on the blade
In our application, the blade is a 1mm uniform thin plate. The blade supporting hinge line is located on the blade mass center, so that the forces transferred to the servos will be reduced. Since the centrifugal force is much larger than the aerodynamics force according to the calculation results, the distributed force acting on the blade is assume to be uniform distribution.
Now the blade structural analysis is quite simple. The blade can be deemed as a plate sustaining uniform distributed centrifugal force. There exists one deisgn such that the maximum deflection of blade reaches minimum. And it can found that the blade maximum displacement reaches minimum when the distance between supporting hinges equals 56% of blade span. In this case, the blade deflection is only 4.58% of that of "T" shape design and 12.45% of "n" shape design. Hence the "pi" shape design can solve the problems of large blade deflection caused by large centrifugal force.
Fig 17. The maximum blade deflection for various blade supporting strut distance
However, despite of the blade deflection, other factors, such as the control mechanism also influent the main shaft vibration and hence the efficiency of the propeller. Therefore, further comparisons have to be made before the best configuration is selected.
On the "T" shape propeller, the blade support is located on the center of the blade. It has the shortest main shaft and only one support strut for each blade. But this type of blade will experience very large deflection and possibly structure damage if the blade span is too long (We did have a broken main axis for "T" shape propeller). Moreover, the hinge that links the blade and the support strut has to be very strong. Because the control link, which is used to vary the blade pitch angle, will push and pull the blade and the control link hinge usually is not located on the center of blade, the control link force will cause bending moment that tilts the blade sideward, if the blade supporting hinge are not stiff enough. The high bending moment at blade hinge will also result in high friction and hence reduce the mechanical efficiency. In our early stage test models, big and heavy blade supporting hinges and blade supporting struts was installed on the "T" shape propeller. This also causes larger aerodynamics drag compared to other configurations. Worst of all the eccentric that controls the blade pitch angle is far away from the aircraft fuselage. The force caused by varying blade pitch angle will be transferred onto the main shaft by control mechanism. This force is caused by blade aerodynamics force and propeller centrifugal force since in reality the blade mass center do not located on the blade hinge line exactly. The component of this force that is perpendicular to main shaft will bend the main shaft. It is large enough to cause main shaft vibration. If the main shaft is too long, the force will result in fatigue stress and even break the main shaft. The vibration also dramatically increases the power consumption and reduces the propeller efficiency.
The "n" shape propeller has the longest main shaft and two blade supporting strut. Therefore it is the heaviest configuration. It has moderate blade deformation since the two ends of blade are linked on the supporting strut. Since the distance between the two supporting strut are very far, the force caused by control link will not cause bending moment on the supporting hinges as that of "T" shape propeller do. Because the main spar is too long, the stiffness of the main shaft will be low and vibration will be heavy. The vibration will undoubtedly increase the friction force on the hinges and bearings hence will reduce the efficiency of the propeller. To solve this problem, the main shaft have to be very thick and heavy. The advantage of this configuration is that the distance between the eccentric and the fuselage is the shortest one in the three configurations and hence the weight of control mechanism can be saved and vibration caused by control mechanism can be reduced.
The "pi " shape propeller has moderately long main shaft and two supporting struts. It is heavier than "T" shape propeller and lighter than "n" shape one. But the blade deformation is the smallest of all. Therefore the blade span can be increased without very thick main shaft. The main shaft is not very long and hence vibration can be reduced. The distance between fuselage and the eccentric is also not too long therefore the vibration caused by control mechanism will not be heavy if the main shaft root area is strengthened. There is also no bending moment caused by control link acting on the blade hinge. Therefore low profile hinge can be used. The vibration problem can be alleviated and hence the over all efficiency is high. Therfore the "pi" shape propeller is the best of all. Based on this conclusion, the new generation of "pi" cycloidal propeller was designed and built. The experiments show that the blade is almost straight in full rotation speed range.
To find propeller with higher hovering efficiency, blades with various shape and propeller with various size were tested. The hovering efficiency of the cycloidal propeller was doubled through the experiments and this is the basis for us to build a successful cyclogyro.
According to the tests, the following phenomena were observed.
1. As had expected, the propeller can vary the direction of thrust very fast.
2. Similar to the screw propeller, the hovering power loading (thrust generated per unit power) is mainly affected by disc loading (thrust divided by disc area). With disc loading low enough, the power loading can very high.
3. The blade solidity can affect the efficiency of the propeller. There exists one blade solidity value that results in propeller with best efficiency.
4. The blade hinge near the centre of the blade cord line is more efficient. In this case, the C.G of the blade also locates near the hinge line and the large centrifugal force will not cause large force transmitted to servo.
After the propeller with high efficiency was found, a cyclogyro equipped with 2 cycloidal propellers and one tail rotor was built and tested. The cyclogyro weights 358g and is powered by two brushless electrical motors. The battery is a 720mAh Li-Po package rated at 12V. The maximum thrust generated by this power unit is 520-540g. Therefore there are enough lift force for the cyclogyro to hover. During the test, it was found that the cyclogyro can hover with only half throttle.
For the reason of security, a tethered flight was performed. The video is shown bellow.
Now, we are preparing the free flight without the string.