Control Tricks For Tailsitters

An RC VTOL aircraft always makes for a compelling project, but ensuring the transition between hover and forward flight can be quite challenging. In the video after the break, [Nicholas Rehm] demystifies of the flight control algorithm required for a VTOL tailsitter.

Tailsitters are one of the simplest VTOL arrangements, the testbed here being a simple foam KF airfoil wing with two motors and two servo-controlled elevons. As with almost all his projects [Nicholas], uses of his open-source dRehmFlight flight controller to demonstrate the practical implementation of the control algorithm.

Three major factors that need to be simultaneously taken into account when transitioning a tailsitter VTOL. First off, yaw becomes roll, and vice versa. This implies that in hover mode, elevons have to move in opposite directions to control yaw; however, this same action will make it roll in forward flight. The same applies for differential thrust from motors — it controls roll in hover and yaw in forward flight. Nevertheless, this change of control scheme only works if the flight controller also alters its reference frame for “level” flight (i.e., flips forward 90┬░). As [Nicholas] demonstrates, failing to do so results in a quick and chaotic encounter with the ground.

With these adjustments made, the aircraft can transition to forward flight but will oscillate pitch-wise as it overcorrects while trying to maintain stable flight; this is due to PID gains – 3rd factor. The deflection required by control surfaces is much more aggressive during hover mode; thus PID gains need to be reduced during forward flight. A final improvement involves adding a brief delay when switching modes for smoother rotation.

For more interesting VTOL configurations, check out [Tom Stanton]’s RC V-22 Osprey, and this solar recharging trimotor

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Will The Lilium Jet Work? A Deep-Dive Into The Physics Behind EVTOL Aircraft

The Lilium Jet is a proposed eVTOL (electric Vertical Take Off and Landing) aircraft that the German company Lilium GmbH has claimed it will bring to the market ‘soon’, which would made it the first eVTOL aircraft in the world to enter into commercial service. As anyone who has any experience with VTOL knows, it’s a tricky subject to engineer, let alone when you want to do it fully electric. In a deep-dive video on the Lilium Jet and eVTOL in general, [John Lou] goes through the physics behind VTOL take-off, landing and flight, as well as range and general performance.

It is clear that Lilium’s presented aircraft concept has many issues, some of which are due to new and unproven technologies, while others seem to be founded in over-promising and likely under-delivering. With Lilium having signed a number of contracts to deliver the first Pioneer Edition Lilium Jets and commercial service promised by 2025, it’s hard to ignore that the first full prototype of the 7-seater Lilium Jet is supposed to fly this year.

Although as [John] points out in the video, eVTOL is not an impossible concept, it is important to remain realistic about what is physically possible, and not seek to push the boundaries. When the UK introduced its first mass-produced VTOL jet in the form of the Harrier, it too faced an uncomfortable time as bugs got ironed out. As these eVTOL aircraft would be carrying real human passengers, it’s a good place to realize that although you can pick a fight with physics, you will never come out on the winning side.

Hopefully Lilium realizes this too, and these sleek, battery-powered aircraft will truly take to the skies in a few years.

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[Tom Stanton] Builds An Osprey

The V-22 Osprey is an aircraft like no other. The tiltrotor multirole military aircraft makes an impression wherever it goes; coincidentally, a flight of two of these beasts flew directly overhead yesterday and made a noise unlike anything we’ve ever heard before. It’s a complex aircraft that pushes the engineering envelope, so naturally [Tom Stanton] decided to build a flight-control accurate RC model of the Osprey for himself.

Sharp-eyed readers will no doubt note that [Tom] built an Osprey-like VTOL model recently to explore the basics of tiltrotor design. But his goal with this build is to go beyond the basics by replicating some of the control complexity of a full-scale Osprey, without breaking the bank. Instead of building or buying real swash plates to control the collective and cyclic pitch of the rotors, [Tom] used his “virtual swashplate” technique, which uses angled hinges and rapid changes in the angular momentum of the motors to achieve blade pitch control. The interesting part is that the same mechanism worked after adding a third blade to each rotor, to mimic the Osprey’s blades — we’d have thought this would throw the whole thing off balance. True, there were some resonance issues with the airframe, but [Tom] was able to overcome them and achieve something close to stable flight.

The video below is only the first part of his build series, but we suspect contains most of the interesting engineering bits. Still, we’re looking forward to seeing how the control mechanism evolves as the design process continues.

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Winged Drone Gets Forward Flight Capability

Drones are pretty common in the electoronics landscape today, and are more than just a fun hobby. They’ve enabled a wide array of realtors, YouTubers, surveyors, emergency responders, and other professionals to have an extremely powerful tool at their disposal. One downside to these tools is that the power consumption tends to be quite high. You can either stick larger batteries on them, or, as [Nicholas] demonstrates, just spin them really fast during flight.

We featured his first tests with this multi-modal drone flight style a while back, but here’s a quick summary: by attaching airfoils to the arms of each of the propellers and then spinning the entire drone, the power requirements for level flight can be dramatically reduced. This time, he’s back to demonstrate another benefit to this unique design, which is its ability to turn on its side and fly in level flight like an airplane. It’s a little bizarre to see it in the video, as it looks somewhat like a stationary propeller meandering around the sky, but the power requirements for this mode of flight are also dramatically reduced thanks to those wings on the arms.

There are a few downsides to this design, namely that the vertical wing only adds drag in level flight, so it’s not as efficient as some bi-wing designs, but it compromises for that loss with much more effective hover capabilities. He also plans to demonstrate the use of a camera during spin-hover mode as well in future builds. It’s an impressive experiment pushing the envelope of what a multi-rotor craft can do, and [Nicholas] still has plans to improve the design, especially when it comes to adding better control when it is in spin-hover mode. We’d expect plenty of other drones to pick up some of these efficiency gains too, except for perhaps this one.

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Optimising An RC Tilt-Rotor VTOL

There are a variety of possible motor configurations to choose from when building a fixed-wing VTOL drone, but few take the twin-motor tilt-rotor approach used by the V-22 Osprey. However, it remains a popular DIY drone for fans of the military aircraft, like [Tom Stanton]. He recently built his 5th tilt-rotor VTOL and gave an excellent look at the development process. Video after the break.

The key components of any small-scale tilt-rotor are the tilt mechanism and the flight controller. [Tom]’s tilt mechanism uses a high-speed, high-torque servo that rotates the motor mount via 3D printed gear mechanism. This means the servo doesn’t need to bear the full load of the motor, and the gearing can be optimized for torque and speed. [Tom] also used the tilting motors for yaw and roll control during forward flight, which allows him to eliminate all the other conventional control surfaces except for the elevator.
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Radio Control Joby Aircraft Uses Six Tiltrotors To Fly

eVTOL (Electric Vertical Take-off and Landing) craft are some of the more exciting air vehicles being developed lately. They aim to combine the maneuverability and landing benefits of helicopters with the environmental benefits of electric drive, and are often touted as the only way air taxis could ever be practical. The aircraft from Joby Aviation are some of the most advanced in this space, and [Peter Ryseck] set about building a radio-controlled model that flies in the same way.

The design is inspired by the Joby eVTOL test vehicle.

The result is mighty complex, with six tilt rotors controlled via servos for the utmost in maneuverability. These allow the vehicle to take off vertically, while allowing the rotors to tilt horizontally for better efficiency in forward flight, as seen on the Bell-Boeing V-22 Osprey.

The build uses a 3D-printed chassis which made implementing all the tilt rotor mounts and mechanisms as straightforward as possible. A Teensy flight controller is responsible for controlling the craft, running the dRehmFlight VTOL firmware. The assembled craft only weighs 320 grams including battery; an impressive achievement given the extra motors and servos used relative to a regular quadcopter build.

With some tuning, hovering flight proved relatively easy to achieve. The inner four motors are used like a traditional quadcopter in this mode, constantly varying RPM to keep the craft stable. The outer two motors are then pivoted as needed for additional control authority.

In forward flight, pitch is controlled by adjusting the angle of the central four motors. Roll is achieved by tilting the rotors on either side of the plane’s central axis, and yaw control is provided by differential thrust. In the transitional period between modes, simple interpolation is used between both modes until transition is complete.

Outdoor flight testing showed the vehicle is readily capable of graceful forward flight much like a conventional fixed wing plane. In the hover mode, it just looks like any other multirotor. Overall, it’s a great demonstration of what it takes to build a successful tilt rotor craft.

We’ve seen tilt rotor UAVs before, and they’re as cool as they are complicated to build. Video after the break.

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VTOL Tailsitter Flies With Quadcopter Control Software

Quadcopters are great for maneuverability and slow, stable flight, but it comes at the cost of efficiency. [Peter Ryseck]’s Mini QBIT quadrotor biplane brings in some of the efficiency of fixed-wing flight, without all the complexity usually associated with VTOL aircraft.

The Mini QBIT is just a 3″ mini quadcopter with a pair of wings mounted below the motors, turning it into a “tailsitter” VTOL aircraft. The wings and nosecone attach to the 3D printed frame using magnets, which allows them to pop off in a crash. There is no need for control surfaces on the wings since all the required control is done by the motors. The QBIT is based on a research project [Peter] was involved in at the University of Maryland. The 2017 paper states that the test aircraft used 68% less power in forward flight than hovering.

(Editor’s Note: [Peter] contacted us directly, and he’s got a newer paper about the aircraft.)

Getting the flight controller to do smooth transitions from hover to forward flight can be quite tricky, but the QBIT does this using a normal quadcopter flight controller running Betaflight. The quadcopter hovers in self-leveling mode (angle mode) and switches to acro mode for forward flight. However, as the drone pitches over for forward flight, the roll axis becomes the yaw axis and the yaw axis becomes the reversed roll axis. To compensate for this, the controller set up to swap these two channels at the flip of a switch. For FPV flying, the QBIT uses two cameras for the two different modes, each with its own on-screen display (OSD). The flight controller is configured to use the same mode switch to change the camera feed and OSD.

[Peter] is selling the parts and STL files for V2 on his website, but you can download the V1 files for free. However, the control setup is really the defining feature of this project, and can be implemented by anyone on their own builds.

For another simple VTOL project, check out [Nicholas Rehm]’s F-35 which runs on his dRehmFlight flight control software. Continue reading “VTOL Tailsitter Flies With Quadcopter Control Software”