World's Smallest Cyclocopter Brings Unique Design to Microdrones

Cyclocopter designed at Texas A&M
Image: Moble Benedict/Texas A&M
It's taken a century to get this thrust-vectoring aircraft off the ground.

A cyclocopter is a weird sort of aircraft that uses airfoils rotating around a horizontal axis to generate lift and thrust. The concept was developed about a century ago, but these things are tricky to build and fly, so they haven’t, er, taken off as much as helicopters have. In fact, there’s only a small handful of research groups working on cyclocopters at all, and at the moment, they’re focusing on small scales. Professor Moble Benedict and graduate students Carl Runco and David Coleman at Texas A&M’s Advanced Vertical Flight Laboratory has been testing the smallest cyclocopter ever developed: It’s just 29 grams in mass, and could be a tiny step towards replacing helicopters and multirotors with something better.

I’m not even going to try to describe this thing, just watch the video:

If it’s still not clear how it actually flies and maneuvers, this diagram might help:

A single cycloidal rotor, or cyclorotor, consists of multiple airfoils attached to a frame that turns around in a circle very fast. The airfoils produce lift and thrust as they move through the air, and because each blade can pivot, that thrust that can be directed in any direction perpendicular to the cyclorotor. Or, as Benedict explains, “With the blades cyclically pitched such that each blade has a positive geometric angle of attack at the top and bottom of the circular trajectory, a net thrust is produced.” The thrust vectoring is instant, making the cyclocopter very maneuverable, and (among other advantages) the vehicle can transition from, say, stable hovering to high-speed forward flight without needing to pitch itself over like a helicopter or multirotor aircraft. The little rotor on the back stabilizes the pitch.

Benedict has been working on cyclocopters for years; we wrote about a quad-cyclocopter that he developed at the University of Maryland a while back. That was, in fact, the first successful flight test of a cycloidal-rotor based aircraft and along with Dr. Benedict, other people involved in that effort were Elena Shrestha, Dr. Vikram Hrishikeshavan and Dr. Inderjit Chopra. At 800 grams, it wasn’t what you’d call large, but cyclocopters get particularly interesting at very small scales because of their combination of very high maneuverability and potential for excellent efficiency. They’re also more stable, more space efficient, and they’re theoretically quieter and capable of a higher top speed than helicopters are. 

Cyclocopters sound pretty great, right? So our first question for Benedict was this:

IEEE Spectrum: Why aren’t we all flying cyclopters right now, instead of helicopters and multirotors?

Moble Benedict: Even though people were trying to explore cyclorotors 100 years back, we have only started looking at this concept seriously now. What happened in the early 20th century is that helicopters became successful before cyclocopters and then people naturally lost interest in pursuing this concept.

One of the biggest structural issues in cyclorotors is the fact that blades have to take large transverse centrifugal bending loads, and 100 years ago, we did not have the materials that had the strength-to-weight ratio to do that. Today, with composites and so on, it is possible, and this is a key enabler for the present cyclorotors. Also, all the successful cyclocopters built so far needed electronic onboard feedback stabilization, unlike helicopters, which can be passively stabilized. So a cyclocopter idea was far too advanced for its time when it was introduced.

Can you describe the characteristics of the cyclocopter that give it advantages over multirotors and helicopters?

A cycloidal rotor can achieve higher hover efficiency than a conventional rotor at smaller scales, because of the uniform aerodynamic conditions along the blade span and favorable unsteady aerodynamic phenomena on the blades. We have experimentally demonstrated the higher aerodynamic efficiency (thrust per unit aerodynamic power) of cycloidal rotors in comparison with conventional micro rotors used in multicopters and helicopters.

Additionally, a cycloidal rotor is capable of instantaneous thrust vectoring, which can potentially make the vehicle more maneuverable. The cyclorotor can perform efficient high-speed forward flight even beyond an advance ratio of 1.0 by a simple phasing of the cyclic blade-pitch schedule. Unlike a traditional hybrid aircraft (e.g., a tilt-rotor), a cyclocopter can transition from hover to high-speed forward flight without any configuration change due to its thrust vectoring capability. Finally, a cyclorotor can efficiently utilize the available 3-D space, and therefore, requires smaller footprint as compared to a conventional rotor, resulting in a highly compact flying vehicle.  

What are some of the challenges in building a cyclocopter this small, and how did you solve them?

Designing and building a rotor at those scales was extremely challenging. We had to come up with innovative carbon composite fabrication techniques to make the rotor blades (0.12 grams each) and pitch links (10 milligrams) and they needed to have sub-millimeter accuracy. We had to custom build a 1.3-gram autopilot (called ELKA, designed by Dr. Vikram Hrishikeshavan at University of Maryland) with triaxial gyros, triaxial accelerometers, a processor, and wireless communications. System integration was challenging. When you scale things down, the dynamics becomes faster, so we had to spend many months trimming and tuning the feedback gains for hover stability. Developing and flight testing the 29-gram cyclocopter took more than 2 years, and was sponsored by the Army Research Laboratory's MAST-CTA Program.

What are you working on next?

I think that the key areas that still need to be improved are: 

  1. Designing ultralight blades that can handle the large centrifugal bending loads at high RPMs
  2. Reducing the weight and complexity of the cyclorotors significantly
  3. Optimization of blade kinematics, blade aerodynamic design, rotor geometry, etc to maximize efficiency in both hover and high-speed flight
  4. Mechanically simpler means of implementing these optimal pitching mechanisms either passively or actively
  5. Investigating more compact cyclocopter configuration and understanding the upward scalability of this concept

We have shown that this concept has the potential at smaller micro air vehicle scales. The next big step in our research is to investigate the upward scalability of a cycloidal rotor to be used on large VTOL UAVs weighing 100s of pounds and maybe even on a manned aircraft. We have a five-year grant from the U.S. Army to investigate the upward scalability of this concept.

More than anything I want more people around the world to be aware of such a concept so that we can encourage them to work on this. One or two groups working on this idea can only make so much progress. I hope that in the future, once this technology is more mature, it will find its place in the next generation of personal air vehicles and flying cars. 

Texas A&M ]

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