Wings are great for cruising over long distances and carrying heavy loads, but they aren’t that great if your aircraft needs vertical agility. Rotors, on the other hand, are great for vertical agility, but they aren’t that great for long distances and heavy loads. Any aircraft that wants to fly efficiently can be designed for cruising or hovering, but not both.
Lots and lots of people have tried to figure out a way of making some sort of compromise work. Mostly, this involves stapling as many vertical rotors as you have a budget for to a fixed-wing aircraft and just calling it a day: When you want to go up or down, you use the vertical rotors, and the rest of the time, you use whatever other rotors you can afford to have mounted horizontally. If you’re very clever, maybe you come up with a design that uses one set of rotors for both vertical and horizontal flight, either with some kind of rotating wing or with a vehicle that can pitch over in flight; but the fact remains that your design is wasteful—either you have useless rotors when flying horizontally, or useless wings when flying vertically.
At ICRA this year, researchers from the Singapore University of Technology & Design introduced a new kind of flying robot called THOR: Transformable HOvering Rotorcraft. THOR manages to achieve very high structural efficiency by using all of its aerodynamic surfaces in both vertical and horizontal flight modes, transforming from a flying wing into a sort of whole-body spinning bicopter thing that you really need to see to believe.
At first glance, THOR has a lot in common with monocopters, which are single airfoils with one motor on the end that generate lift by spinning. THOR doubles up on that idea to give itself two modes of flight: hovering mode and cruising mode. In hovering mode (which the researchers call H-MOD), THOR spins in place with its airfoils rotated 180 degrees from each other, like the rotors on a helicopter. In cruise mode (C-MOD), the airfoils are both aligned in the same direction, and you get a flying wing.
The THOR aircraft in hover configuration. Image: SUTD
The tricky part is of course in the transition: Getting from spinning hover to (hopefully) non-spinning forward flight and then back again. You can see this happen in the video, but essentially, all that has to happen is that each wing rotates by 90 degrees, driven by an actuator in THOR’s center section. Here’s how the researchers describe their experiments:
We brought the craft up to an altitude of about 2 meters in either mode and attempted a transition, producing some interesting results. In the transition between C-MOD to H-MOD, it was found that by putting the C-MOD into a climb, the craft is able to reliably maintain altitude while switching its wings into its H-MOD configuration. We suspect that, given strong enough motors, the craft operates akin to a bi-copter during the transition step after-which the rotating wings take over as the primary source of lift. In transition from H-MOD to C-MOD, the initiation of an upwards facing C-MOD was once again useful in maintaining craft altitude during transition. In fact, the motors were strong enough to almost instantaneously break the craft out of H-MOD rotation and push the craft into translational flight.
When the drone is in hover mode (left), it can rotate its two servos by approximately 90 degrees (middle) to reorient itself for cruise mode (right). And when in cruise mode, it can do the reverse to switch to hover mode. Image: SUTD
With the exception of the servo and bearing used for wing rotation, THOR uses every other structural component in both hovering and cruising modes, making it highly efficient relative to hybrid designs. It’s very much a prototype, and both the hardware and the software controller need some optimization, but we love how innovative the design is.
For more details, we spoke with lead author Jun En Low and Professor Shaohui Foong via email:
IEEE Spectrum: Why is structural efficiency important for aircraft, and what kinds of advantages does THOR have over more traditional hybrid VTOL platforms?
While we work on this novel system, we use structural efficiency as a preliminary metric to measure energy efficiency. This way, we are not constrained by conventional design. As to the advantages the THOR has, we feel it has the potential to be more efficient than any other hybrid system that is currently in use simply because there will be no redundant or obstructive systems on the craft in either flight mode (and hence no redundant mass). This also makes the platform potentially more scalable as the number of actuators and control surfaces are kept to a minimum.
What makes THOR a more efficient or effective platform than a monocopter?
We see the THOR as more of an innovation and evolution of the monocopter concept. In the monocopter, we have a terrific system when it comes to hovering, only that its payload has to constantly rotate and more critically, it doesn’t have range. The THOR overcomes this by fusing the monocopter concept to a fixed wing craft (or vice versa). There are also lots of interesting dynamics involved in analyzing and optimizing such a unique platform.
Can you describe the process for the transition between hovering flight and forward flight (or forward flight and hovering flight)?
A cool feature we came across was that the positive angles of attack of both modes of operation mean that a simple rotation of the two servos by approximately 90 degrees flips the craft onto the right side up for cruising. The craft then breaks out of the rotation and maintains altitude using raw motor thrust, after which the wings return to being the primary lift source.
What kinds of unique applications could THOR be useful for?
Anything that requires both long range and an agile hover, and because of its inherent potential to scale, it can be made smaller than other hybrid platforms, which will unlock many possibilities where current hybrid UAVs are too big or bulky to operate! These include agriculture, surveillance, and package delivery, all of which are hot topics in drone development as of the moment.
What are you working on next?
A more robust switching mechanism and non-linear flight/transition control to allow for tighter and aggressive transitions. And also a failsafe feature that takes advantage of the natural autorotational property of the monocopter’s inspiration, the samara fruit. Safety is a very big thing now in drones and if we can prove this system is safe to use in high density population places, I think we’d really get the ball rolling on further development of this platform.