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Old Jun 28, 2015, 11:20 AM
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Quadcopter with (central) petrol power generator

Let me start of by saying I am not going to pursue those impossible designs like using petrol engines to directly power the propellers. Neither am I going to do a pulley design where all propellers have variable pitch setting.

Introduction to the forum:
As this is my first post here I will give a little introduction to who I am and why I am here. My name is Maarten I'm a 19 year old Dutch applied physics student at the Hague University of applied sciences. I'm currently in my second year and will be entering my third after the 4 week holiday. In this holiday I'm planning to build myself a quadcopter and my main aim will be to build one with long flight times.

I have never built a quadcopter before so this may seem like jumping into the deep a tad too fast. This is however not how I myself feel about it. Building something that is basically just bolting it together and has no personal touch is not something I really like.


Introduction
Since I have been lurking in the shadows on this forum for a couple of years already I know that these ideas have come up many times before. Mostly by people proposing to either use the petrol engines connected to the propellers with servos attached to the gas handle. An idea which is nearly impossible to execute, quadcopters as unstable as they are need fast responding electric motors.
I also know that various people have suggested the same generator idea I am proposing right now. Pulling it of may appear to be a mammoth undertaking to some but I think this problem can be solved.

The way I see it with Lithium-sulfur and metal-air batteries not yet commercially available hydrocarbon based fuels are the only way to overcome the extreme power density limitations that Lithium polymer brings. The best way to go about this from an engineering point of view would be to use either a free piston engine or a solid methanol fuel cell. But those things are just as hard to get your hands on without building form scratch. In my build I will focus on using things that are mostly available for purchase at this moment in time.

However I will be making some parts by hand with the CNC portal mill I will be picking up in 2 weeks from now.


Design philosophy
For the engine I need one with a high power to weight ratio with high efficiency. I will be losing a lot during the various conversion steps. My choice at this point is a model petrol engine either the ones used on fixed wings or on RC cars.

To convert the mechanical rotational energy to electric energy I will be using a high kv high power electric motor. The reason for a high kv motor is to match the 3 phase output voltages at a certain angular rotation to the voltage range I need.

The 3 phases will be converted back to DC using a 3 phase 6 diode bridge with an array of parallel super capacitors at the end. Not only will those capacitors turn the it back into ripple free DC again but will also serve as a tiny reservoir in which energy can be stored. Capacitors are the ultimate choice for this as they are essentially not rate limited nor pose the threat that all kind of chemical batteries pose. They have however one major downside which is the the abysmal energy density. The power density on the other hand makes them the only real choice.

The critical design choice to use capacitors will mean that in case of a failure of the petrol engine the charge on the capacitors is only enough to keep the electric motors running for a couple of seconds. During the couple of seconds the voltage will drop as charge from the capacitor is displaced further decreasing the power the electric motors provide. A solution I have to look into would be an emergency circuit which puts the motors on an alternative power source such as the lithium poly pack I plan to include powering the vital components such as flight controller, video transmitter, servos and radio receiver.

Once the switch to the alternative power source happens the plane should land immediately as the lipo battery won't last long either powering the humongous electric motors keeping it airborne.

wrap up
As a start this seems like enough information on where I want to take this idea. I by the way have already got something which is the flight controller I went with the Pixhawk because autonomous flight is something I find really amazing. I have also gotten in a Ublox m8n which can make use of Glonass, GPS, Beidu () and Gallileo () satellites. I tested its accuracy and it is not stunning about 2 meters of. But I have also tested it while walking with the laptop and the poll rate is high so it should suffice for use in a quadcopter.

My list of concerns at this early stage consists of the following:
Engine vibration affecting the quadcopter.
Choice of spectrum for radio and video links.

Another concern would be the stricter regulations the government will soon impose on us. Which are a result of people buying a pre built model and flying it without knowing the rules or any regards for safety. I can't really blame the government I feel like those rules were bound to follow after the mainstream got their hands on these in their hands dangerous models.

The main limitation is that I will have to keep the mass of the quadcopter down to 4 kg from 25 kg. Due to new regulations. I doubt this will form a problem as currently I think the mass distribution per part will look something like this:
Aluminium frame with fuel tanks 500 gr
electric motors (4 + 1) 1000 gr
petrol engine 700 gr
super capacitors 200 gr
Lipo batteries (~60Wh) 300 gr
Diode bridge, flight controller, ESCs and wiring 300 gr
Petrol 1kg

For a total of 4 kg which is still within the new legal limit.
I would've rather gone big moving up to a larger engine with multiple electric motors attached to it. While running bigger propellers on larger electric motors and a much bigger fuel tank. But that won't be possible here in the Netherlands under new regulation. Flying with large amounts of fuel presents an extra amount of danger but flying will only occur above farmland an the like.

I live in the middle of nowhere and part of my family owns large stretches of farmland so I have the space to fly. Even though new regulation states I can no longer fly above 120 m down from 300 m.

I am looking forward to responses and suggestions as a new member on this forum I'm excited to finally roll into this hobby
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Old Jun 29, 2015, 03:53 PM
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Heh, so pretty much a Quad-Copter hybrid... I'm interested to see how that works out. I'm not so sure about your generator system, it seems like there should be some way make a dirt simple one that would work.

The cool thing is the fly time could be way longer than battery alone. Downside is the weight may make it handle like a pig, LOL.

It just may work though, I'm interested for sure!
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Old Jun 29, 2015, 04:27 PM
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Cool idea. I see you've probably come upon the video where the motor does directly power the props:
IncredibleHLQ - Heavy Lift Quadcopter - EngineTest (3 min 54 sec)
they are still working on it, and yeah it's not a great way to lift mass, as I've seen an electric quad lifting 33 kg also on youtube and a much simpler design.

The hybrid approach sounds interesting.
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Old Jun 29, 2015, 05:00 PM
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Hmmm, what about a "Penta-copter" with a central gas engine hooked to both prop and generator in the center?

The gas engine could provide some non-directional "Bulk lift" with the electric motors for pan/tilt directional control. That way the electric motors don't have to be as strong.

Just spitballing some ideas out there...
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Old Jun 30, 2015, 12:19 PM
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I don't see why 4 gas engines is impossible .
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Old Jun 30, 2015, 02:20 PM
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Quote:
Originally Posted by TroyO View Post
Heh, so pretty much a Quad-Copter hybrid... I'm interested to see how that works out. I'm not so sure about your generator system, it seems like there should be some way make a dirt simple one that would work.

The cool thing is the fly time could be way longer than battery alone. Downside is the weight may make it handle like a pig, LOL.

It just may work though, I'm interested for sure!
Most regular generators don't use permanent magnets but have powered coils so they can change the output voltage of the generator accordingly to the needs of the appliance. In my case a 3 phase electric motor with permanent magnets will fill that roll. It reduces complexity versus a powered anchor approach and we are fine with running the engine at a certain set throttle setting. If we want to increase fuel efficiency at all load levels either the engine throttle has to increase with the throttle on the electric motors. I think a set throttle level at first is a good start.

Starting of with a couple of different electric motors and a small engine to do some open loop voltage measurement and with the caps in parallel to do some load voltage measurements.

In the final setup I'm considering spinning the nut to power up the engine by a internal setup. This is necessary because I won't have a prop to swing as all is packed densely together. Of course I could suffice with tightly wound rope or something obscure like that but considering there are far better ways to deal with the problem I will go those ways.

This remote start can be seen here. Whether I will order one of their kits, CNC my own or simply temporarily power the electric motor that is already attached to nut to start it. I have not yet decided.

Quote:
Originally Posted by JWilliams2 View Post
Cool idea. I see you've probably come upon the video where the motor does directly power the props: https://www.youtube.com/watch?v=nwO4ncWLZnk they are still working on it, and yeah it's not a great way to lift mass, as I've seen an electric quad lifting 33 kg also on youtube and a much simpler design.

The hybrid approach sounds interesting.
I have seen that video it is quite impressive that they got it to work and be as stable as they have got it.

That is indeed that idea with variable pitch. The mechanical complexity and all the transmissions make that thing very prone to failure in my eyes. I hope choosing this hybrid approach will pay of. But I will find out how viable it is on the way.

The symmetric frame I will be building and the fact that the engine, caps, electrics, electronics and battery will be in the center means that I have the ability to somewhat displace the center of gravity to lie under the propellers. Which should help with stability.

Quote:
Originally Posted by TroyO View Post
Hmmm, what about a "Penta-copter" with a central gas engine hooked to both prop and generator in the center?

The gas engine could provide some non-directional "Bulk lift" with the electric motors for pan/tilt directional control. That way the electric motors don't have to be as strong.

Just spitballing some ideas out there...
I have considered your approach to be honest. But I quickly disregard the idea as just too difficult and too dangerous. But it is a very interesting idea so I will explain why I disregarded it back then.

The main problem with having the main engine for generating force perpendicular to the quad's bottom in the y-direction is that the force will not be perpendicular once the quad tilts. Once this tilt occurs the force will have a vector not only in the y-direction but also in either or both x and z direction.
This force vector would be so large that the electric motors would have a very hard time to have enough force to level the quad again.

A result would be that it would start making flips and spiral into the ground. However after having done this mind experiment the solution to this problem would be to come up with a design where the force vector can never have anything other than a y component. The way I would go about this is put the gas engine in the center underneath the wings. And to keep the engine pointing towards downwards rather than follow the angle of the plane I would use ball joints.

This would however still mean that despite having solved the problem there would be a huge prop hanging underneath the quadcopter. A terrifying idea because you're basically flying with a circular saw blade underneath. Another problem is that you would have to have pretty long legs on the quad located underneath the electric motors since you can't place them at the center as that is where the big prop and engine is. (or should I say death saw )

But yeah it is an interesting idea but something I consider too dangerous and too difficult for now. As for more than 4 arms it is good for control ability to have more arms in case of motor/ESC failure or when loosing a prop but they are generally less efficient. I may go with 6 to 8 but I am more keen on starting of with just 4. As for the electric motors the Foxtechfpv website has efficiency graphs with certain propellers I aim for 10g/W at 4 kg total of thrust. Their graphs are made at ground level where you create more thrust than high up in the sky so I will go as efficient as possible. But I will try to keep the quad compact so I think the propeller span should be no bigger than 50 cm (~20 inch) in diameter.

Quote:
Originally Posted by swervo View Post
I don't see why 4 gas engines is impossible .
It is very much possible but a quad is inherently unstable and gas engines don't provide the same thrust at certain nozzel settings the way electric motors do. Another problem is they can't be driven fast enough to balance the plane.

By altering the pitch at a set throttle the thrust can be changed according to the needs. This is the same thing helicopters do. But when you start doing that you are better of going with a helicopter like design anyways as it is less work, more efficient and proven.

4 gas engines are in most cases going to be less efficient than a bigger one. This is considering both are well built and both run leanest as possible.
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Old Jun 30, 2015, 11:00 PM
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"a quad is inherently unstable" . i disagree . most multirotors have the cg below the prop plane making them inherently stable or at least neutral . your anti petrol reasoning doesn't make sense to me but its your project and maybe you just don't like petrol motors .
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Old Jun 30, 2015, 11:32 PM
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Quote:
Originally Posted by TroyO View Post
Hmmm, what about a "Penta-copter" with a central gas engine hooked to both prop and generator in the center?

The gas engine could provide some non-directional "Bulk lift" with the electric motors for pan/tilt directional control. That way the electric motors don't have to be as strong.

Just spitballing some ideas out there...
I have seen a similar design. Central gas prop motor with regular quad motors for pitch and roll. I don't know if it will fly longer than a full electric quad but it sure is a LOT noisier. Pretty much defeating the purpose of these types of aircraft.
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Old Jul 01, 2015, 05:47 AM
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Here is another gas quad currently on kickstarter. Seems way more viable than the "heavy lift" one that I mentioned earlier with a single motor: https://www.kickstarter.com/projects...?ref=discovery Some tall claims in that kickstarter.
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Old Jul 01, 2015, 07:25 AM
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Originally Posted by swervo View Post
"a quad is inherently unstable" . i disagree . most multirotors have the cg below the prop plane making them inherently stable or at least neutral . your anti petrol reasoning doesn't make sense to me but its your project and maybe you just don't like petrol motors .
It needs realtime adjustments to keep it from falling from the sky. That is very much unstable compared to for example a fixed wing.

Petrol is a great dense power source but using petrol engines directly on the wings would require some way to compensate for their high latency and non repeating throttle settings. The Yeair as mentioned below has those electric motors to compensate for the petrol engines this way they avoid having to have a variable pitch on 4 propellers.

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Originally Posted by RCvertt View Post
I have seen a similar design. Central gas prop motor with regular quad motors for pitch and roll. I don't know if it will fly longer than a full electric quad but it sure is a LOT noisier. Pretty much defeating the purpose of these types of aircraft.
With the major power density petrol has even at 10% mechanical efficiency it would be able to fly longer. I don't really care about the noise though and there will be nobody else around to complain about it so that would be fine.

My high school end project needed a compressor to operate now that is what I would call annoying noise. During test firing we had it in a barn located about 40 m away but it was still annoying and loud.
1 (0 min 47 sec)



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Originally Posted by JWilliams2 View Post
Here is another gas quad currently on kickstarter. Seems way more viable than the "heavy lift" one that I mentioned earlier with a single motor: https://www.kickstarter.com/projects...?ref=discovery Some tall claims in that kickstarter.
Their approach of using equal powered electric motors rather than pitched props could work wonders. But it is clearly not a very efficient way to do things though it should still yield much better results than electric only. If they meet their goals that is.
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Old Jul 01, 2015, 09:55 AM
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"If we want to increase fuel efficiency at all load levels either the engine throttle has to increase with the throttle on the electric motors. I think a set throttle level at first is a good start."

I was thinking of it more like a vehicle hybrid approach..... you basically would be making a constant output battery charger, with steady throttle. A LiPo (or whatever chemistry) battery would run the electric motors from the battery, which would be constantly charged by the petrol power source. So you wouldn't have to throttle in response to load at all, just make sure that the charging rate exceeds the average use rate. Especially since you would need a pack of some kind to support the electronics anyway. The type of generator isn't really a big deal, whatever can output enough power to provide the charge at the lowest weight would be the winner.

Similar thought with using the petrol as a lifting unit... it wouldn't provide enough thrust to lift all by itself, but just say 60% to take some strain off the electrics.

Again, just spitballing ideas.... there's plenty of room for experimentation which is why it sounds like such a fun project, LOL.

I am excited to see what you come up with! :-)
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Old Jul 01, 2015, 01:26 PM
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"If we want to increase fuel efficiency at all load levels either the engine throttle has to increase with the throttle on the electric motors. I think a set throttle level at first is a good start."
Just as a start of course I'm thinking I will use an arduino and a couple of sensors to increase the engine throttle when the voltage drops off due to load between the terminals. A servo, and arduino and maybe some relays should do the trick.

My C is a bit rusty since it has been well over a year that I actually got that class and it was only basic level but I'm sure I can write a working program for the task.

Quote:
Originally Posted by TroyO View Post
I was thinking of it more like a vehicle hybrid approach..... you basically would be making a constant output battery charger, with steady throttle. A LiPo (or whatever chemistry) battery would run the electric motors from the battery, which would be constantly charged by the petrol power source. So you wouldn't have to throttle in response to load at all, just make sure that the charging rate exceeds the average use rate. Especially since you would need a pack of some kind to support the electronics anyway. The type of generator isn't really a big deal, whatever can output enough power to provide the charge at the lowest weight would be the winner.
I'm not really fond of a battery charge design. Sure the energy density is 40x higher than the super capacitors but it needs overcharge and overdischarge protection and won't handle the cycling that well. I probably would destroy the battery after a few flights. Once the arduino thing is worked out it should no longer be a problem.

Lithium polymer cells are very nice but they are so very fragile. And need to be treated with great care. All other battery chemistries have problems too like lead batteries they have hydrogen gassing at too high power and crystal forming at too low power. Whichever you choose they all have very specific ranges and need to be treated with care.

I will use a lipo pack to power vital electronics and as an emergency power source but for now I will keep with the super capacitors. I have worked with lipos before and it was really stressful. Though in the end the results were decent laptop doesn't run out of power anymore with the new 104Wh capacity.

Quote:
Originally Posted by TroyO View Post
Similar thought with using the petrol as a lifting unit... it wouldn't provide enough thrust to lift all by itself, but just say 60% to take some strain off the electrics.

Again, just spitballing ideas.... there's plenty of room for experimentation which is why it sounds like such a fun project, LOL.

I am excited to see what you come up with! :-)
It is doable with the ball joints but whether I use the thrust directly or use it to power just the generator and the electric motors shouldn't make a huge difference. The downside is additional conversion losses the upside is that I can have the thrust at whichever electric motor I like.

Will be picking up the CNC mill this Saturday if everything goes according to plan
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Old Jul 01, 2015, 04:04 PM
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Sound like you have a good flying site for your project. Might look into graphene supercaps if going the cap route.

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...Some tall claims in that kickstarter.
Not sure King Kong could climb that high.
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Old Jul 01, 2015, 05:21 PM
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Originally Posted by maarten12100 View Post
The main problem with having the main engine for generating force perpendicular to the quad's bottom in the y-direction is that the force will not be perpendicular once the quad tilts. Once this tilt occurs the force will have a vector not only in the y-direction but also in either or both x and z direction.
This force vector would be so large that the electric motors would have a very hard time to have enough force to level the quad again.
I don't understand this at all. Why would it be any more difficult for the electric motors to level the quad over tilting it in the first place?
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Old Jul 01, 2015, 05:31 PM
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Just as a start of course I'm thinking I will use an arduino and a couple of sensors to increase the engine throttle when the voltage drops off due to load between the terminals. A servo, and arduino and maybe some relays should do the trick.
Maintaining constant RPM for an IC engine with a changing load has been solved for over a century with centrifugal weights. I'm sure you will have no problem dialing it in perfectly with an arduino.

But it takes time for the IC engine to spool up, and I share your concern over quick repeated charge/discharge cycles for a lipo. What if your lipo were upsized a bit, and run through some heavy duty diodes that would allow it to supply current if the voltage sags like during hard acceleration or maneuvers but not force it to be recharged.

Interesting project. Please do post your progress as you go.
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