Last academic year at KU Leuven, we designed, built and test flown a VTOL UAV: the VertiKul. During this project we gratefully made use of the info and support of the DIY Drones community and therefore we would like to share our results on this project.
The VertiKul is designed for automated aerial transport of small packages and is optimized for maximum range and payload capability. The innovative design makes use of the benefits of both multi-rotors and fixed-wing airplanes. For take-off and landing, the VertiKul hovers like a quadrotor and for forward flight, the VertiKul pitches 90° and flies like an airplane.In airplane mode, the attitude is also controlled by differential thrust of the motors. Therefore, no additional control surfaces are required, reducing the number of moving parts, risk of failure and maintenance cost. The structure is made out of three carbon fiber tubes in a ‘H-configuration’ allowing an easy accessible space for a 10x15x20 cm package of 1kg. The tubes are connected using laser cut multiplex wood and wings are constructed using a polystyrene-balsa sandwich structure, covered with Oracover. For a good directional stability, the wings are slightly swept-back and winglets, that also help reducing the induced drag, are added. Since the wings introduce a high moment of inertia and strong moments because of wind around the yaw-axis, the propellers are tilted 10° to improve the yaw control.
Because of the two different flight modes and the transition in between, a new control strategy is needed. This strategy contains three levels. The first level, or low level, is the angular rate control as in “Acro mode”. Because of the -90° pitch in forward flight, it becomes hard for a human pilot to control the VertiKul since a roll command results in a yawing motion and a yaw command makes the vehicle roll (in counter-intuitive direction, yaw to the left results in roll to the right!). To make the control more intuitive, a mid-level controller is designed around the angular rate controller. This controller acts as “Stabilize mode” when the VertiKul is in hover and makes an automatic transition to forward flight when a switch is turned on the transmitter. The transition to forward flight takes around 5 seconds and gradually decreases the pitch angle to build up the speed required for enough lift of the wing in forward flight. Any input from the pilot is ignored during this phase. A quaternion representation was required in order to avoid the ‘Gimbal lock’. In forward flight, the pilot inputs are only the desired altitude and heading, making it easy to fly by inexperienced pilots. Finally, the high-level controller generates a trajectory between two base stations and commands flight mode, altitude and heading to the mid-level controller.
In order to have a fully autonomous system, we also developed a docking system. The system includes an optical precision lading system, based on a PX4FLOW unit and a docking station at which a package or battery can be swapped. The VertiKul starts from one docking station with a fully charged battery and a package of 1 kg and then flies to its destination, 30km further, based on GPS. Once arrived at location, the VertiKul makes a precision landing on the docking station at that location. The battery is replaced with a full one and a new package is loaded so that the VertiKul can continue to its next destination.
The PX4FLOW camera we use for this autonomous precision landing is re-programmed in order to detect the center of the marker on the docking station and sends these coordinates to the autopilot on the VertiKul. Based on the altitude, roll and pitch angle of the VertiKul, the position of the marker is calculated and a position controller navigates the VertiKul to the landing spot. In order to be able to land at night, the marker is illuminated by leds under the surface of the translucent marker.
Check out the video here: http://youtu.be/omaxgFVDUWg
We haven’t yet been able to test the full performance of the VertiKul because of the limited test area where we can fly. During test flights we experienced a lot of influence of the wind on the big wings, making automatic landings very hard. Also the battery and package swap is not yet automated, leaving us with enough work to continue this project.
When descending in hover mode the wing gets reverse airflow. If the plane wants to correct its flight path to the front, it will pitch to the front, like a quadrocopter does. But the wing will create a force to the rear. Same in the opposite direction and also for bank (roll). If you descend very slowly, the wing effect es weak, the quadrocopter effect dominates and the control works. If you descend fast, the wing effect dominates, you lose control or even get control reversal and crash.
The guys of QuadShot "solved" this problem by instructing pilots not to descend fast in hover mode, but to descend in cruise mode, transition at low altitude to hover mode and then descend very slowly. Not very pretty. It would be nice to be able to transit at safe altitude, say 30 meters AGL and then descend fast.
This is a characteristic of VTOL planes in this configuration, therefore also applies for VertiKUL. Do you have a strategy to handle this?
Additionally you have the effect of descending into your own propwash. Any quadrocopter or helicopter suffers this effect. For safety reasons no full scale helicopter pilot therefore ever descends fast vertically. But from my experience for model quadrocopters this is not a major issue and rarely causes crashes.
Why does QuadShot have bad descent characteristics in hover mode?
Another issue to consider is descend characteristics in hover mode. The QuadShot has experienced quite some problems due to instability. The wing counteracts the quadrocopter in this flight phase. Ailerons make it even worse. The Wingcopter configuration should work better.
Okay, nice to know! Thanks ;-)
Wingcopter is very cool indeed! I like their video where the plane takes off in the forest and flies to an open field. Very maneuverable. I'd like to know how well it handles wind...
I like your approach "simple is beautiful". Simplicity is the silver bullet to reliability.
In stabilized mode the QuadShot acts exactely same as you describe the VertiKUL. Both having a full feature autopilot on board they both fly even completely autonomously. The QuadShot is only difficult to fly in aerobatics mode.
You might share some experience with the guys of Wingcopter. They have a pretty cool 3m composite prototype flying and also use APM.
Awesome work guys!!
IMHO, to do a full automatic flight, I think you have to set up a simple control guide which consist of a windsock and airspeed sensor placed at the base station to provide data to the VertiKUL so it can choose how to approach the base station safely.
And if you can fly up to 30 km, I think the ability to land 5cm is not essential, and it is already very good.
Also the automatic battery and package swap is not very necessary.
We were indeed familiar with QuadShot and Atmos. However, QuadShot has ailerons, VertiKUL has not. Atmos has two tilting props, VertiKUL has not. Absence of servos and tilting mechanisms was for us a basic design feature to have higher robustness and to be innovative.
I also think QuadShot was less of a success because it's difficult to fly. Unfortunately I never flew it, so I'm not sure. However VertiKUL can be controlled intuitively. In hover it flies like a quad in AltHold. At the switch of a button the transtition is done automatically by the autopilot. In forward flight you only use two sticks. One the set your desired climb rate and one to set a radius of curvature for taking a turn. With no input it will fly straight, at constant altitude and stable.
Indeed the propulsion system is always a trade-off with this concept.
Thanks for your input ;-)
Cool project and concept.
The QuadShot and the Atmos did this already in 2012 using Paparazzi. Check out the review on FliteTest.
Unfortunately the QuadShot was no commercial success. My guesses are:
- The advantages in flight time, speed and distance compared to a quadrocopter or helicopter are too small
- The flight controller and ground station is too complex
- The landing is not stable enough, especially in wind and turbulence
Such a transition concept has the following inherent problems:
- You need about 10 time the propulsion power for hover compared to level cruise flight. In cruise flight you will have a propulsion system that is 10 times overdimensioned. It will not be efficient.
- In hover you need high thrust at low speed. In cruise flight you need low thrust at high speed. There will be no propulsion system providing both efficiently.
- The landing is prone to wind
There are some solutions that help a bit, like:
- Variable pitch prop
- Separate propulsion systems for hover and for cruise
- Other precision landing system, like rope, deep stall or maple seed
- Standard helicopter instead of a quadrocopter
Looking forward to your progress!
We took several configurations into account. X- or + configurations are very good from a structural point of view. However, if you want your payload easy to swap and at the center of gravity ( so that different payloads to not influence the center of gravity ), a H-configuration is more suitable. We didn't choose for a double wing ( in X or just like a bi wing airplane ) because of the drag penalty. However, this would have resulted in a less wind-sensitive VTOL
What about 4 wings like the X-wing (from star wars) ? Besides the coolness/crazyness, that might give sweatest stability in yaw at the expense of weight and drag. Just getting excited by your idea and dreaming about the potential of VTOL & long range FPV.