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
Bart, probably realistic. I have a few ideas I'm working on to improve the efficiency. But I don't put a lot of importance on FM myself. It's just one small aspect of UAV design.
Hi Guys. This looks like a good step forward. Have you seen the Physical Package Protocol project (formally Pigeon Post)? Your system is the closest I've seen to the proposed spec.
http://www.physicalpackageprotocol.com
With following assumptions:
- you drained batteries for 80% of their capacity
- battery voltage per cell is on average 3.8V
- air density = 1.2 kg/m³
- efficiency between battery power and mechanical power is 60%
- 10% of the power is needed for the tail rotor
you get 48.6% for the first case and 57.2% for the second case, more or less what we would expect I think?
For comparison: an APC SF 10x4.7 can have a figure of merit of about 75%
It's hard to extrapoloate what the maintenance would be at this point. We don't have enough data as there really aren't many people doing it. I've got about 5 hours of flight time on my mapping heli, and it seems like new. Eventually I'll have to do a preventative maintenance on the belt, I just don't know when yet.
It's really hard to compare to other systems, as they also don't have a lot of information on commercial operations. I know I've talked to people using foam flying wings such as X8 for commercial mapping operations in rough areas, and one of them said he only counts on a lifespan of 20 flights for an airframe before being replaced. The problem is the landings are very hard on them. That's where I saw a big opportunity for helicopters to do a better job. Again, a gas powered helicopter could easily fly for an hour, and landing it in a small forest clearing is trivial compared to landing a large flying wing.
Where is this theory explained? I've heard a similar theory that is used in places like RCG, where the conclusion is the most efficient multirotor is when you have 1/3 battery mass, 1/3 payload, and 1/3 frame weight. The problem is the math that was used to create the theory was nonsense.
But yes, for a given size of helicopter, it will fly for longer and longer as you keep loading it up with batteries, unlike a multirotor. It can't fly for an infinite period of time with an infinite battery load, that's not what I'm saying. But a 700 heli can lift 40lbs. It's maximum flight time is going to be somewhere between it's standard 1kg battery load, and a theoretical 20kg battery load. Actually, the 20kg load might fly the longest, but is totally impractical obviously. The mechanics would be under so much strain that it's not they wear out fast, etc. Again, more work here required.
Yes, that is the concern. I don't question at all that a "small" helicopter is more dangerous than a multirotor (or foam airplane) of a similar mass. But at some mass, they're all deadly and shouldn't be used around people anyway. For example, a very large Octocopter with 29" propellers if very very dangerous, and shouldn't be used near people any more than a large helicopter. It becomes an argument over which can "kill you more".
I did try calculating that a little while ago. I can't remember the result, but I do remember turning up a mathematical issue with the way it was being done. The problem is the efficiency estimation. It's just an estimate, but it's not being done very well.
Anyway, two datapoints for you:
In initial testing, my mapping heli ran:
- 1.8kg AUW
- 970mm rotor diameter
- 17 minutes flight time
- 4S 5000 mAH battery
After adding the camera subframe and an extra battery I'm at:
- 3.2kg AUW
- 1020mm rotor diameter
- 20 minutes flight time
- 4S 10,000 mAH battery
You'll note that I almost doubled the mass, and doubled the battery capacity, but it flew actually longer... completely contrary to the typical result for multirotors. This isn't magic, but it's because a lightly loaded helicopter has a very low Ct/Sigma, which results in low efficiency despite good Momentum Theory equations of lift figures (ie: low induced velocity). The low Ct/Sigma wastes power resulting in an overall low figure of merit.
Adding more battery mass to the heli increases the Ct/Sigma, which improves the figure of merit despite falling momentum theory numbers.
Anyway, you can try calculating FM if you like, but the motor and ESC efficiency would just be a guess.
I prefer to exclude that when comparing aircraft designs anyway. The electrical efficiency of the power system is not related to the worthiness of the airframe design. If your motor is inefficient, fix the motor, don't scrap the airframe design.
This is completely dependent on the quality of the motor. I've had motors only last a few mintues before coming apart (Turnigy NTM). Others have very high quality and should last a very long time (KDE, iPower, T-motor, etc.).
I don't know that anybody has thoroughly characterized the good motors. You should be able to calculate the theoretical limit, assuming everything is perfect, but looking at bearing life curves, available from industry.
He didn't state they were more efficient than multi-rotor propellers of the same diameter. They're not. What he said was, the system efficiency can be higher because it so easy to get such a large diameter.
That's the thing about helicopters compared to multirotors. Multirotor frame designs necessarily result in very large and heavy frames. It's not really fair to compare a helicopter to a multirotor of a given rotor area. Because in what use-case would that matter? Most use-cases would compare for empty weight, or possibly frame dimensions.
Rob, thanks for the info! My remarks:
- “100 flights of 5 minutes before needing significant maintenance” is a maintenance needed after 20 flights of 25 minutes, that’s very high for a system that should operate fully automated at low costs.
- I agree bringing down the RPM of a heli increases the efficiency, however I believe flight time can be increased (for the same rotor diameter! ) by adding more battery weight, maybe rpm has to go up to still be able to lift the heavy battery. For a given disk diameter, theoretically, the battery mass should be 66.6% of the total mass for the longest flight time ( not the most efficient! )
- “heli rotor could easily kill you”, seems one of the biggest concerns about using a heli instead of a multi-rotor for commercial applications with people involved.
- Small question: do you have an idea on of what the Figure of Merit is of your heli’s? I would be very interested to know and compare these to those of multi-rotors. Or do you have perhaps a list with [ Thrust, Amp,Voltage ] for some rotor diameters that you use? ( and an idea of your motor+esc efficiency J )
Redemptioner:
You are right about heli’s that are certainly not the best choice for an automated aerial delivery system. But it’s very interesting to compare their performance, never say never, for example this Belgian company uses an electric heli for professional aerial filming: commercial and people involved. 30 minutes flight time with 5kg payload. (ok the take-off weight is 25kg so let’s not compare it to our VTOL ).
FD:
- The only big potential to increase the range is battery energy density (which I expect to increase with the market for electric cars that’s coming up) , also the back side where the package is inserted is an aerodynamic disaster, but easy to insert and remove package and battery so it will be hard to change it.
- Does anyone has an idea how long you can use a well-balanced motor+propeller on a multirotor before the bearings are completely useless (less performance is acceptable, failing not )
Gary:
- We need the rudder for directional stability, but we can get rid of them by sweeping the wings further (getting harder to insert payload ) and increasing the size of the winglets.
- We do not have dihedral, perhaps you mend sweep but I don’t see why removing sweep would be beneficial
- I totally agree in reducing the “wetted area” or flat plate area, especially at points far away from the COG ( big lever arm)
- Fixed chord,continuous pitch are not more efficient when you take them for the same diameter, it would be more fair to compare performance of a heli to a multi-rotor with the same total disk area
- Thanks for the useful comments !
Rob’s comments have been very useful. Please stay a little bit on topic, we are here for fun and common interests, right? ;)
Redemptioner, I'm not really sure why you decided to pick this fight here. But you're losing again. You should just delete you comments like you did last time.
I guess you missed the part where the OP requested to compare the performance of a helicopter to his airplane:
And so I gave him an example.
Really ambitious project guys,
I know that the big wings and wind often are a problem acting in vertical mode.
Flat plate area side on to the wind is intrinsically problematic for this kind of design.
A few suggestions which might help.
Get rid of the flat plate (rudder) on your motor spars.
Use a flat wing profile (get rid of the dihedral and the inclined winglets.)
Make your fuselage flatter and more rounded on all edges. (thinner in relation to wing).
Land with now vertical wing edge on to the wind (yaw so that this is the case).
This will present minimum side profile flat plate area to wind.
You can control equivalent rudder in horizontal flight by differential application of each side motor pair plus elevons for that matter.
You really need to minimize fuselage and flat plate profile as it relates to wind.
A round section has approximately half the equivalent flat plate area of a flat section and an aerodynamic section can have a vanishingly small flat plate area.
Attacking Rob as incompetent in any way on this topic is not a tidbit of useful information, but is an indication of the ignorance of the attacker.
I am absolutely positive that no other developer on the DIYDrones sites better understands helicopter aerodynamics and efficiencies better or better in relation to multi or fixed wing than Robert.
Helis can actually be very aerodynamically efficient because they permit the largest possible rotor which is really the bottom line for hover - flight.
In spite of the inefficiencies involved with a fixed chord rotor blade and continuous pitch change, you can always make a case for them being as or more efficient than any conventional fixed prop multicopter because of the relatively huge swept blade area.
And modern slow rotor head methods are leading to very high efficiencies and reduced vibration for photo and video use.
And I am a died in the wool multicopter - in fact quadcopter guy.
I think the method presented here can be made to work efficiently, but I think you need something sleeker and less like a box kite with a minimal side profile.
Best Regards,
Gary
Robert, I do appreciate the information presented above including empirical values. I hope Bart as OP doesn't mind that the discussion deviating a bit from the core topic.
For me I'd start with payload vs. range. Simplest / cheapest concept wins. If you'd take your 600 heli stretched to 800, carrying 1kg payload, how far would you get? At which flight time / speed? Only electric please.
Going back to vertiKUL, how much potential is there to increase range, maybe using a more efficient wing & propulsion for distance.
Actually, I'd expect the vertiKUL concept to win against traditional heli in pure payload vs. range.
On a sidenote: what about maintenance in remote areas with dust, rain, maybe saltwater spray, etc. Wouldn't the traditional heli suffer from this in a commercial use? I'd expect a quad (with wings) to do much better, basically you've just the bearings in the motors to care about as moving parts.
Re: maintenance: 100 flights at 5min each, that's less than 10 flight hours. Hm not much for a delivery system. Not sure though how many hours you can keep a quad flying without doing anything to the motor bearings.
BTW: Personally, I do not appreciate preemptive, personal attacks on other people warning of incompetence. Don't feed the troll.
I wouldn't try this here, it won't work. When you say things like:
Or state that 2 batteries going into a single wiring system and a single Hobby King BEC is a reliable "redundant" power supply system... you don't have much credibility.
Hey guys, I didn't want to bring up the helicopter comparison, but since you did....
The hardest part about comparing the two systems is making sure we're comparing apples to apples. So what size helicopter do we use? I tend to look at folded/stored volume, as this is usually an important factor for any machine that does not have a permanent base of operations. If it needs to be carried in a vehicle, or on plane, or just in a backpack, that should be an important design criteria. What is your wingspan? The degree to which we allow disassembly needs to be considered here. A helicopter typically has easily folding blades, and that's it. The tail could fold at the mid-section if using a direct drive tail motor.
Or, do we look at MTOW? This gets tricky.
My Protos 500 mapping heli weighs in at 3.2kg currently, but it probably could get off the ground with 6.5kg. It would just take a lot of power. It wouldn't fly long like this. Disk loading is just too high. But at 3.2kg, it flies for 20 minutes. I am looking at extending this to 30 minutes using better batteries (not 18650's) and blades.
But, I don't think this machine is really competitive to yours.
However, a 600 heli, typically weighs in around 3.5kg. These normally only fly for about 5 minutes, but this can easily be extended to 15-20. Lifting an extra Kg of payload is trivial for this machine. Right now I have a 600 that is flying a whopping 7.5 kg, including about 1kg of camera/gimbal, as well as a double battery load, and a heavy subframe. This machine could probably be configured with a similar 6.5 MTOW as your machine, using a triple battery load. It should fly for 30 minutes like this. 60 km/h is pretty trivial for this heli. It should be able to do 80 km/h without trouble. I'm not sure what the max efficiency cruise is, but it's going to be around 50-60 km/h.
This machine could also be stretched to 700, even 800 size, while adding virtually no weight (1-200g) This would make it more efficient still.
The key to helicopter efficiency is slowing down the rotor. There are lower bounds to this. Somewhere around 900 rpm on a 700, while maintaining good stability and manoeuvrability. Normally these run about 2200 rpm.
My 600 flies about 20 minutes with two batteries at 1800rpm and 7.5kg. The reason I don't run the rotor slower is because it's a video machine, and vibration becomes an issue at lower speeds. This wouldn't be a problem for a cargo lifter.
As you've stated, the big thing with helis is complexity and safety. I think the complexity is often a bit overstated. It is true that there are more moving parts. But the designs are VERY mature, and you can easily purchase high quality parts that are well over-rated for the task you'd be asking of them. Also, modern helicopter mechanics are far, far simpler than they were in the past with way less moving parts. I have an acrobatic 500 heli, that weighs about 2.2 kg, and can pull 6g accelerations without failing. People typically thrash these for 100 flights of 5 minutes before needing significant maintenance. So my 500 mapping heli at 3.2kg with no shock loads, the parts are all well under-stressed.
Safety, is equally complicated to define. A 600+ size machine with rotor running at full speed can easily kill you. But at 900 rpm... that changes quite a lot.
I'd love to build a twin-rotor 500 size machine. I think it could lift what you are lifting, would be compact, and 500 size rotors at reduced RPM would be not terribly dangerous.
I think at the end of the day, it really is going to come down to simplicity/safety vs. flight performance. I don't think they will ever meet in the middle, and different applications will choose which compromise is best for a given roll.
And as a final point: I'm currently building an 800 size gas powered heli. It's about 50" long, AUW of 5.5kg, will fly for about 30 minutes. Getting it to lift 1kg of payload, would be trivial. 5kg would be more reasonable. And flying to 1 hour at 80 km/h is very possible. Again though, simplicity and safety need to be considered. But in an "industrial" application like running parts around a mining operation or the like, it seems to be a superior platform.
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