One of the most popular segments of RC flying is a genre called “3D aerobatics”. If you’re not familiar with the concept of 3D flight, just imagine airplanes and helicopters performing low-altitude maneuvers that seemingly cheat physics. It was only a matter of time before multi-rotors evolved to be able to execute the same types of daredevil stunts. That time has arrived, and the kits you can build for 3D flight are pretty great.
What is 3D Flying?
With fixed-wing aircraft (airplanes), the definition of 3D flying is relatively simple. It includes any (intentional) maneuver performed below the airplane’s stall speed. A vertical component of the propeller’s thrust augments the lift from the wing to keep the airplane flying. During tricks such as a torque roll, the propeller is providing all of the lift.
Rotary-wing 3D flight includes maneuvers such as inverted, backwards, sideways, and pirouetting flight.
The definition of 3D flying is not so clean with rotary-wing aircraft (helicopters and multi-rotors) since they do not really have a stall speed. According to the ace RC pilots that I introduce below, rotary-wing 3D flight includes maneuvers such as inverted, backwards, sideways, and pirouetting flight. Bluntly stated, 3D is in-your-face flying that flaunts the unearthly abilities of the pilot and his machine.
Whether talking about fixed-wing or rotary-wing aircraft, 3D aerobatics require a machine with tremendous power and aggressive maneuverability, not to mention a fearless pilot with finely-tuned flying skills. Here's what's available for those daredevil RC pilots today.
Unleashing the Beast Within
One of the things we covet most about multi-rotors is their extreme stability. This not only makes them great camera platforms, but it also lets beginners get in on the action with less of the traditional RC training regimen. Much of that stability is artificial, being tied to the feedback of gyros and accelerometers. When we alter or disable this artificial stability, the true aerobatic potential of multi-rotors begins to emerge.
By turning off the attitude stabilization on my DJI Phantom (it’s controlled by a switch), I am able to execute flips forward, backward, or to either side. That is pretty much the extent of the Phantom’s aerobatic chops. Even these simple maneuvers require a rather tall column of space to pull off. More graceful quads may be able to perform flips in a smaller area, but they are still the only moves on the menu. Moving up to the realm of 3D aerobatics requires a feature that the Phantom and most other multi-rotors simply do not possess: the ability to create lift while inverted. Let’s call it “negative lift”.
Sustained inverted flight for the purposes of 3D aerobatics is no easy technical challenge.
The challenge of creating negative lift with a multi-rotor is no trivial thing. Even the most direct path introduces a daunting new level of mechanical complexity to these otherwise simple aircraft. Designers have the Sophie’s choice of either adding mechanical devices to vary the pitch of the rotors or implementing electronics that allow the motors to reverse direction in flight. Either method will allow sustained inverted flight, but there are subtle differences in how each system performs.
Leave it to die-hard RC helicopter guys to plow through the barriers and bring 3D-capable multi-rotors to market. The Stingray 500 and Invertix 400 are two new quad-rotors developed by world-class helicopter pilots. Each designer chose a different path in addressing the negative lift dilemma. Let’s examine both quads in detail and discuss the advantages of each design.
Stingray 500: The Mechanical Approach
Curtis Youngblood is to RC helicopters what Michael Jordan is to basketball. He has been flying competitively since the early 1980’s. If there is a helicopter trophy to be won, Curtis has it on his mantel--probably several. He also designs and sells RC Helicopters through his company, Next-D. The Stingray 500 is Curtis’ hat in the 3D quad-rotor ring.
Conventional quads utilize 1-piece rotors (propellers) that do not change during flight. The motors spin faster or slower to alter the amount of lift each rotor produces. In contrast, the Stingray utilizes a variable pitch system on the four rotors. In this set-up, each rotor is an articulating device that maintains a constant rpm throughout the flight. An external servo changes the pitch of the rotor blades (the angle of the blade in relation to its plane of rotation) to alter the produced lift. Positive pitch angles produce positive lift, with the opposite being true of negative angles.
The aerobatic advantage of this system is that it provides precise and immediate control response. Tradeoffs for this snappy responsiveness are the necessary servos and mechanical gadgets required to actuate the variable pitch mechanisms. These add-ons introduce additional potential failure modes as well as greater maintenance overhead.
While it isn’t required, variable pitch multi-rotors have the option of utilizing a single motor to spin all of the rotors. Curtis exercised this option with the Stingray 500. A single brushless motor powers all four rotors via a system of shafts, belts and pulleys. While the Stingray has four more servos than a traditional quad (which has zero servos), it has three fewer motors and their requisite Electronic Speed Controls (ESC).
Aside from omitting the mass and complexity of multiple motors and ESCs, the Stingray’s single motor layout has an ancillary benefit. With the motor, servos and battery all mounted very near the lateral midpoint of the Stingray, its moment of inertia in the roll axis is very low. You can imagine the difference in comparison to standard multi-rotors with a rather heavy motor positioned at each extremity. The Stingray’s low moment of inertia translates into less torque being required to start and stop rolling maneuvers, further contributing to the quad’s responsiveness.
In spite of its aerobatic intent, the Stingray does not forfeit the more common multi-rotor duties. It has a location for mounting a GoPro camera to shoot aerial video and/or First Person View (FPV) flying. The primary difference is that you may need to take a Dramamine before watching the gyrating video captured from a Stingray!
Invertix 400: The Reversing Approach
Bobby Watts has only been flying RC helicopters since 2003, but he has an impressive list of accomplishments on his resume, most of them related to his 3D flying skills. Like Curtis, Bobby is also involved in the development of new rotary wing designs. Most recently, Bobby helped design the Invertix 400, a 3D-capable quad-rotor that uses reversing motors. This type of design is called “fixed pitch”.
The Invertix looks similar to most other quad-rotors because it has the same core components. It does not require extra servos, belts or pulleys to generate negative lift. What it does have are ESCs that are capable of reversing the direction of its brushless motors extremely quickly and reliably. If that doesn’t seem like a big deal, pause for a moment to consider the angular momentum of an 8” propeller spinning at 15,000rpm. Now imagine the power required to stop the propeller and get it spinning at 15,000rpm in the opposite direction--all in a fraction of a second. Standard reversing ESCs (such as those found on some RC cars) simply aren’t up to the demands imposed by a 3D quad. Bobby’s team focused much of their R&D effort on perfecting the Outlaw Power Reverse3D ESCs found on the Invertix 400.
A further challenge imposed by reversing motors is that standard propellers are designed to operate in one direction. Examining the cross section of a typical propeller blade would reveal an airfoil shape much like that found on an airplane wing. This makes the propeller very efficient at producing thrust when spun in its intended direction. While it will blow air when spun in reverse, it is decidedly less efficient when doing so. For the Invertix, Bobby’s team developed propellers that are equally efficient in either direction. To accomplish this, they had to sacrifice some efficiency in the forward direction. This loss, however, is compensated by much improved performance in reverse.
The obvious advantage of the fixed pitch approach is simplicity. While it certainly has its challenges, it appears that they are all in the development stage. The end user gets a quad with the same look and feel of a standard ship. The Invertix 400 just has a much larger repertoire of maneuvers in its playbook. The tradeoff for this simplicity is less immediate control response compared to a variable pitch set-up. While they operate very fast, there is some latency as the motors change rpm or direction in response to control inputs. The difference is subtle, perhaps not even perceptible to most of us. The Invertix certainly has no trouble tumbling about the sky.
What to Look Forward to
Neither variable pitch props nor reversing ESCs are new to the RC world. The Stingray 500 and Invertix 400 are unique in that they pull these technologies out of the tinkerer’s workshops and make them available to a much broader audience. It will be interesting to see whether both approaches to 3D flight are embraced by the multi-rotor community, or if one will go the way of the Betamax. Both designs show plenty of potential at this point. However, the huddled masses have a way of fleshing out design limitations and long term reliability issues. One thing is for sure: 3D aerobatics is the next logical step in the evolution of recreational multi-rotors. One way or another, hobbyists will find ways to make their machines and their maneuvers ever more radical.