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
Not sure how those record flights scale down to "real-life" + payload, because most of them are using these NCR18650B batteries that go only up to 1C or 2C.
The rope idea is interesting, but maybe a bit too complicated if you'd like to have a few landing pads in the field. Guess a balloon would be to wind sensitive? Or maybe it doesn't matter if the rope is at an angle during strong wind?
Maybe one or more long poles (those oil-palm harvesting poles we have here are up to 15m long) would be easier, with or even without ropes. Probably hard to fully automate as you say, but for me THAT would be the full prize.
For sure there are use cases where some operator can help. In my opinion however the objective should be full automation.
Not so sure about the heavily loaded / fast flying issue though. 1kg is already a good payload size, sensors get smaller every day. So let's say the AUW is 10kg. Speed profile, I am guessing, 60-100km/h cruise should be fine for most applications. Let's assume conservatively cruise/stall speed is 2:1 so that's 40km/h to pick the middle value. That'll be around 600J if I didn't miscalculate. Not something to easily grab with your hands, but not impossible to deal with using a dedicated mechanism.
Still quite a challenge to establish a simple, affordable and reliable system.
Over two hours is very impressive. I think that confirms that 30 min should be easily achieved, even with 1kg payload and 40km/h forward flight.
My best idea for a vertical precision landing of a fixed wing aircraft is a rope landing. This can be enhanced compared to the proven system of the ScanEagle by letting a Copter carry the rope. Then no crane is required and the rope landing can be performed in greater height. I had this idea last year, am very excited and but unfortunately only find little time to develop a system. This system could be automated partially, but probably not fully.
I think a fully automated solution is quite a challenge. Especially for heavily loaded fast flying long range UAV. For slow flying light UAV there are some easier solutions considerable. The status of research is to let quadrocopters find their "docking station" to charge their batteries. Mostly precision navigation is done with computer vision. But I haven't seen many working systems yet.
The longest flight of an electric helicopter that I know of is 2:31 have a look here:
http://www.twheli.de/modelle/weltrekord-logo-600-se/ (sorry only german)
Not sure how much that would be with 1kg payload.
In my point of view the winged VTOL has potentially a larger development potential if the main challenges are overcome. Not sure if wind is such a big issue if the landing pad site is properly chosen / modified - see "wind protected area" discussion.
Which leads me to another question: if we assume that fixed wing flight is always more efficient than rotary wing flight to cover a distance, what's the best method for fully automated precision take-off and landing for FW aircraft?
Some ideas to inspire thought:
- could the landing pad have some simple (!) grab (for landing) and throw (for take-off) mechanism?
- can some extra take-off equipment be dropped off (back to landing pad) right after transition to stable forward flight?
- can the aircraft "crash" / deep stall / etc. right into a landing pad component designed for that? Think precision bomb drop into a soft cushion, after all the overall mass is not that high...
Dunno, just some thoughts...
You are right, there is not much information around concerning performance of multi-rotors in forward flight. I see two reasons for this:
- Most multirotors are used for hover flight. Forward flight is not a typical flight pattern, especially not fast or far forward flight.
- Multirotors perform badly in forward flight. The rotors are not designed for that. The fuselage often isn't either. The multirotor guys don't like to admit and document that.
To me it is clear that a conventional helicopter performs significantly better in forward flight than a multirotor. I don't know any scientific or field proven numbers. I guess there must be a difference of factor two or so.
You are right about safety, I rather have 4 small propellers than one big. To improve landing and take-off by choosing for a conventional helicopter, I would think going to a conventional multi-rotor improves these qualities even further. But I suppose a conventional helicopter (same mass, dimensions ) can fly further/longer/faster than a multi-rotor? There is actually a big lack of information about multi-rotors in forward flight (speed, power consumption, drag, flight behaviour ).
A well designed VTOL will fly faster and further than a conventional helicopter. E.g. the Osprey flies about 50% faster and 50% longer therefore about double the distance of a comparable helicopter. But it suffers serious stability problems in hover and is complex and expensive.
You want to fly 30km at 60km/h, therefore have a flight time of 30min. I am sure a conventional helicopter can be easily designed for 30min flight time at 40km/h. That is a bit less, but might be overcompensated by higher stability to wind at landing. Keep in mind that a well designed electric model helicopter can fly over an hour, I think the world record is almost two hours. Most RC helicopters only fly 10-15 minutes because they are designed for 3D flight. A a sky crane with appropriate blades or even a funcopter already fly significantly longer and can carry heavy loads.
You are right, the swash plate mechanics are quite complex. But they are extremely mature, very reliable, hardly need any maintenance, mass produced and cheap. Might be a down side of a conventional helicopter, but from my point of view not very critical.
Another issue comparing quadrocopters and helicopters is safety. The rotating blades of a helicopter are quite dangerous. The propellers of a quadrocopter a bit less, especially when you have a protecting frame around them, like many quadrocopters have. But for VertiKul a protection frame a not ideal as it deteriorates flight performance significatly.
I'm not sure whether a conventional helicopter has a better performance in carrying a payload of 1kg at a speed of around 60 km/h. A helicopter scores definitely lower in terms of complexity, price and maintenance because of the swash plate mechanics.
It would be very interesting to compare the performance of the VertiKul with those of a conventional (electric, to keep it quiet and environmental friendly ) helicopter. Does anyone know such a helicopter?
I think a steep or ideally vertical approach is valuable for your application. For parcel delivery you will typically land in urban areas with lots of obstacles, not in wide open areas. Therefore I like your idea of the deep stall into the wind. The strategy could be:
- Circle over the landing spot at 30m AGL to determine the wind direction and strength
- Calculate the optimal approach path for the deep stall
- Perform the deep stall into the wind from 30m to 3m AGL
- At 3m AGL over the landing spot transition to hover
- Descend slowly in hover mode to the landing spot
This would be a real innovation, I don't think anyone has done this before!
One could argue that a standard helicopter could perform such a landing even better, being less prone to wind, perfectely maneuverable and maybe even doing it with no power and no propwash problems in autorotation. But I guess you want to stick to your VTOL concept ...
Reto, we didn't bump into this problem yet, we always descent slowly ( like any other multi-rotor ). If we want to minimize the time in " quadrotor mode " we'll have to descent faster. A solution might be a descent with the wing in deep stall and the nose pointing to the wind direction?
Hi guys,
Very cool innovation & research topics!
Sometimes, I'm proud to be Belgian :)
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