A fresh look at the cyclocopter

I recently spoke to Moble Benedict, associate professor at the aerospace engineering department at Texas A&M University, and his student Carl Runco who is working toward his doctorate degree. Moble and Carl have dedicated a good tranche of their academic research to the cyclocopter aircraft concept.

Moble started by describing how flapping-wing flight allows for greater maneuverability and tolerance of gusts compared to other designs. However, reproducing these movements mechanically rarely, if ever, resulted in a durable machine. These limitations led him and his students to study alternative methods to obtain a similar high maneuverability and gust tolerance. The cyclocopter was determined to be one such solution.

I asked Moble to briefly describe the mechanism by which a cyclocopter generates thrust and vectors it accordingly for lift and control. Moble said that unlike a helicopter rotor, in a cycloidal rotor, the blades rotate around a horizontal axis and are cyclically pitched in a once-per-revolution fashion to produce thrust. By changing the phase of the cyclic pitching, the thrust can be vectored instantaneously, making the aircraft more agile. It can also respond more rapidly to wind gusts — a crucial advantage over conventional helicopter rotors.

The concept is not new and has been around since the beginning of the 20th century. However, these early designs were ungainly and fragile, which kept their development and commercialization out of reach. Today, with improved scientific understanding and improvements in materials with better strength-to-mass ratios, researchers like Moble and Carl are taking another look at this technology.

Moble explained that while most research was done at small drone scales, Carl’s mesoscale model and computational fluid dynamics (CFD) studies of even bigger aircraft at higher Reynolds numbers indicated efficiencies equivalent to a figure of merit of around 0.6. This is comparable to mainstream rotorcraft. The key is keeping the weight down because the blades and their supporting structures can amount to about 20% of the total empty mass of the aircraft.

While the challenges of scaling up to passenger-carrying sizes may slow development in that direction, there are several applications of the smaller versions, such as lightweight, low cost, quasi-disposable surveillance drones for security use. Carl explained that one of the big advantages of the cyclocopter is that you can have the aircraft hover at any pitch attitude. This could be used for camera stabilization without the expensive and heavy gimbal systems required on other types of drones.

When using these drones in a security scenario, low noise signature is another desirable feature. Describing the noise produced by the cyclocopter drone, Carl said it was not an unpleasant hum that tends to blend better into background noise than the noise generated by conventional axial rotors. The reason for this is the fact that on a cycloidal rotor, unlike conventional rotors, every point on the blade experiences the same airspeed due to rotation, and therefore, can produce the same thrust at low rotational speeds, which makes it quieter.  

I asked Carl how they are tackling reliability as most cyclocopters have just four thrust units. In this case, loss of one unit would inevitably result in loss of control. He said that at small drone scale, the construction was robust enough to resist falls and bumps. If the drone could be recovered, the repair would be cost-effective and could be done in the field. Moble also added that increasing the number of thrust units on larger aircraft would inevitably add weight and reduce payload. Therefore, overall reliability had to come from the design, construction, and material integrity rather than redundancy in numbers.

We also discussed aircraft behavior in case of total power loss. Here, Moble explained that as with any rotor system, if the rotating assembly kept enough kinetic energy, it could, in theory, continue functioning aerodynamically in an autorotative mode. He said that these blade configurations were in fact used very effectively as wind turbines, which demonstrates how they can harvest and store energy from the airflow.

As we approached the end of our discussion, Moble and Carl explained the next steps they plan for moving their research forward. Although a passenger-carrying aircraft was not off the cards, Moble said that the technology could have many other practical applications.

The cycloidal rotor can also be used in a denser fluid, such as water. Therefore, it is possible to design and build multimedium vehicles that could fly in the air and maneuver equally well in water. Single cycloidal rotor elements could even be used instead of a conventional tail rotor assembly in a mainstream rotorcraft. The thrust vectoring element could be incorporated into the flight control laws that can be used to increase maneuvering options that are simply not possible today with traditional axial tail rotors.

Cycloidal rotors could also be an excellent option for controlling airships at low speeds. The reduction in mechanical complexity would also result in lighter, simpler, and an inherently more reliable design.

Seeing firsthand the passion, rigor and dedication that Moble and Carl put into their work, it will not be a surprise if we see some of these technologies finding their way into several real-world applications in a not-too-distant future.