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.
Comments
Hi Peter.
The counter clockwise props has to tilt to the clockwise direction (radius of the yaw axis of the your VTOL), and the clockwise props to the ccw direction
Hope that it helps, if not let me know
Hi Colugo,
It is not a tail sitter and no motor tilt. The 2 forward thrust motors are fixed and the 4 lift fans are fixed, I just power up the two forward thrust motors when I want to start the transition to flight gradually powering down the 4 lift fans until I am purely on lift from the wings.
I like your VTOL, thanks for sharing the link.
Hi Peter
Nice VTOL, Hard to see how it works though. it doesnt look like a tail seater
Its look like the you doing motor tilt, dont you?
Btw look at this
https://vimeo.com/126641676
Hi Bart,
Love your project.
I have built something with similar design goals: https://www.youtube.com/watch?v=HjPvx_TMmBk
but I use elevator and ailerons for flight mode, but would like more yaw authority in the hover as the aircraft is so big and heavy that the power differential does not give me as much yaw authority as I would like.
From your experience, I have a couple of questions:
1. Did it matter that you inclined the motors outwards by 10 degrees or could they have been tilted in the other direction, i.e. front motors tilted backwards 10 degrees and rear motors forwards 10 degrees?
2. Could you have gone for less motor tilt, i.e. 5 degrees and still have gotten the same amount of yaw authority or was 10 degrees the optimum?
Any insights you might be able to give, I would greatly appreciate.
Thanks - Peter
For 'airplane roll' , which is quadrotor yaw, motor torque only was not sufficient for a stable behavior. Therefore we tilted the motors slightly for greater yaw authority.
How did you roll? motor momentum only?
Do you have the code for transition? i mean from vertical to horizontal
thx for the info and tips Gary !:)
Nathan, I did know about pigeon post project, how is it going? Do you have connections with Matternet?
A quick comment on motors and reliability.
Except for the lower hobby grade motors, bearings are the biggest problems and the smaller the motors are and the faster they spin the bigger a problem they are.
The KDE motors have a triple bearing stack and they use top quality bearings that are as big as they can stuff in.
Almost every body else uses 2 bearings on the majority of their motors.
There is also a difference in how well various bearings are protected against foreign materials.
And unfortunately I don't know of any motors that actually use a proper thrust bearing (like in many high end heli heads).
All they use are Conrads a nice general purpose bearing that works better in radial loading than in thrust.
A properly designed motor should incorporate a significant thrust bearing because you are hanging the copter from it.
Most good motors use ABEC bearings which could be swapped out for ceramic or carbide bearings which certainly under some circumstances could provide considerably extended life.
But they are also problematic in various ways and I have not been able to find any information from anyhbody who has actually tried this.
But for now at least the smaller and faster the motor, generally speaking the shorter the bearing life.
(A bearing ball the size of a grain of sand has a very rough life.)
Even the Big TMotor U8s which take quite a good size bearing recommend replacing the bearings every 60 to 80 hours.
Actually increasing the size of the winglets would be even more problematic wind wise, because edge on to the wind during landing (takeoff) vertically they represent a long lever arm trying to twist the copter.
In one of your pictures (the one on the table facing upwards) I thought I saw a bit of dihedral, but looking more closely I can see it was just a misperception.
I agree that removing sweep wouldn't be significant, but I can't see why you can't use differential thrust to compensate for a lack of rudder if you were to get rid of it, especially with the amount of thrust you have available to do vertical flight.
And getting rid of those would go a long way to allowing you to better handle the vertical flight in wind and gusts.
I was thinking of something kind of like a Skywalker X8 but with your motor setup and lose the winglets (because they are a long lever arm trying to twist the aircraft wings into the wind).
That could present a minimal surface edge on to the wind vertically and a small twisting moment.
It would certainly place limits on the shape of your "payload", but it could still be consistent with a common shipping box, very few of which are actually cubical.
I also agree that the Heli blades are not as efficient as the same size fixed pitch multi rotor blades, what I was saying was that the Heli can end up being more efficient by having a much larger rotor disk than the equivalent size multi rotor craft and that the larger disk efficiencies considerably outweigh the intrinsic inefficiencies of the heli rotor blade and head design.
Of course that is true for conventional helicopters and multirotors and has very little to do with your hybrid device, and certainly not in horizontal flight.
I actually think it is a great design, I was just trying to make a few suggestions as to what sorts of things could be looked at to improve its vertical stability and controlability in windy or gusty conditions.
Best Regards,
Gary
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