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May 03, 2005, 11:56 PM
Registered User

Micro Generator for Hybrid UAV

Well there is some great information in this forum I hope it stays around for a long time. UAV's need extended flight times in the 4-24 hour range especially for the configuration I am thinking of using it for. Batterys do not operate that long especially when you are trying to power avionics and computers. I do not care what size Lipo you use it will become way to combersome. I want to develop a Hybrid UAV that uses a micro generator to power the avionics, computers and brushless propulsion system as well as charge the battery packs for the hybrid function. The power supply needs to be compact, light as possible and mated to a super fuel efficient 2 or four stroke power supply. 2 and 4 stroke powers supplys are a bunch but micro generators are no where to be found in the sizes and weights I am thinking about. I want to build a generator for my project if anyone knows of the ideal method for building an electric generator i.e brushed or brushless, magnets, type of winding, gauge of wire and how to optimize for amps and voltage please respond. There are a host of other concerns and engineering hurdles for this project but lets start here please focus on the this particular question before jumping to the other obvious ones. This power suply would ideally make from 0-2kw at full throttle.
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May 04, 2005, 12:17 AM
Dead Reckoner
On this similar vein, whose feet do I have to kiss to buy the fuel cell in the Aprillia bike?
Last edited by sesat; Feb 28, 2016 at 08:27 PM. Reason: personal information removed
May 04, 2005, 12:35 AM
Registered User
Originally Posted by MrLift
This power suply would ideally make from 0-2kw at full throttle.
Well, the 0KW is easy, the 2KW is going to be large and would take quite an airframe and power plant indeed.

For your hybrid idea, you could use a brushless PM motor with a shaft at each end. One side for propulsion and the other side would be your gas engine with an over-run clutch. The electric motor would be a simple 3-phase generator when the gas engine is running. When the electric motor provides propulsion, the over-run clutch prevents the gas engine from turning and wasting power. You might want an electric clutch so you could use the motor to start the engine still. You could probably make 200 Watts or better with the available RC power plants.

Still I think your overall efficiency would not be too great, but the energy density of liquid fuel is better than that of current batteries for sure. It should certainly keep your electronics going...
May 04, 2005, 01:00 AM
Registered User
LukeZ's Avatar
I'm glad MrLift started a thread on this, I think it's a real interesting topic. Here's a link to a brief thread over on RC-Cam on generators that nevertheless has some good info.

I guess I'm wondering, given that a lot of people are winding their own brushless motors these days, if it would be possible to wind one that would be better suited to the generating function than a stock brushless you'd buy for the 'powering' function. Even if only to make it larger in diameter to maximize the flywheel effect of the rotating magnets. I've wondered if with a fly-wheel, and a steady (not surging) current draw, if a small engine in the .061 to .15 size might not be able to put out quite a bit of energy: though I've not done any testing.

Also, I've wondered if several smaller brushless motors running on the same shaft might be more efficient than one large one: can't say I know why that should be, I've just wondered it.

I've never flown brushless or even played around with them, so it could be I'm not even asking the right questions. These are just some things I've been pondering. Whenever I can quit spending money on other electronic stuff for my plane I aim to put some into this project...

Until then, if anyone more knowledgeable than I can throw out some math or other information, we might be able to constrain the problem on paper, which wouldn't cost any of us anything.
May 04, 2005, 01:31 AM
Plane & Computer crasher
ElectroLawndart's Avatar
I brought this up a couple of weeks ago on another thread dealing with endurance in UAV's. It's been eating at the back of my brain since.

I would think that two brushless motors would be necessary. One to drive the prop, the other to act as starter motor/generator on the IC engine. I would shy away from mechanical clutches and such. Trying to make things small and light tends to eat away at reliablity and durability.

The first issue I would tackle is how to start a .049 diesel converted motor remotely.
Then find out what altitude that sucker maxes out at.
The reason I would choose a diesel is two-fold. Fuel economy and no glow plug to power-up on startup.

The basic airframe would be based on a soaring glider with a big variable-pitch prop on the front.

The flight profile I would choose would be an ascent with the diesel running to charge the NiCads or NiMH batteries and excess power going to the prop. Get up to altitude where the IC motor has difficulties and shut it down. Roll the prop RPM's back and crank up the pitch to start a powered extended glide. I like NiMH batteries because they can be fast-charged. Might have to have a controllable cooling vent to cool the batts while charging while keeping them warm during discharge.

Yeesh! I just started rambling on. Just might have to get an .049 motor.

I hope this thread goes on. There is alot of potential here.

May 04, 2005, 02:16 AM
Registered User

Motor Design

I really am looking forward to tackling this topic and building a proto type. Thank you so much Mr. williams541, lukez and Electrolawndart you all have some great input. I was concerned about efficiency in the design like williams541 brought up and do understand fuel has a high energy density which makes the hybrid attractive. I all ready have problems with the overrun clutch if the motor is being used to spin the prop it is no longer being used to power the avionics and computer I do not plan to use batterys for these functions.

If anyone has any equations to demonstrate the efficiency issues or equations to demonstrate the relationship of generator design to its actual output would be a great place to start. LukeZ I think if you put two motors together I think you would loose some efficiency in relationship to drag with multiple bearings but the biggest impact would be wiring the outputs in series upping the voltage like multiple battery packs in series. Also why three phase can it be built to work in single phase?
May 04, 2005, 07:29 AM
Registered User
typicalaimster's Avatar
Have you checked this Generator out?

I was thinking about picking one up to charge my camera battery.

--Scott Fuller
May 04, 2005, 01:32 PM
Scale Robotics Inc.
radiohound's Avatar
Originally Posted by typicalaimster
Have you checked this Generator out?

--Scott Fuller
I believe that was the one used in the first uav/rc trans-atlantic flight by Maynard Hill.

Here is an article on the 1900 mile record breaking flight:
May 04, 2005, 06:18 PM
Professor of Wood
kd7ost's Avatar
Originally Posted by radiohound
I believe that was the one used in the first uav/rc trans-atlantic flight by Maynard Hill.

Here is an article on the 1900 mile record breaking flight:

No, Maynard simply coupled to a standard DC motor and used it to keep the battery pack charged.

2 thousand watts? Are you sure thats what you meant to say? Thats a honkin lot of power.

The Sullivan system listed above puts out 5 watts per every coil you locate around the magnet ring. You can put up to 5 coils around it, (according to Sullivan) for up to 25 watts. That equals .03 HP consumed.

Check out Sullivan's other products for generating power here.

May 04, 2005, 09:59 PM
Registered User
typicalaimster's Avatar
Originally Posted by kd7ost
No, Maynard simply coupled to a standard DC motor and used it to keep the battery pack charged.
Dan is correct here. He used a standard DC brushless motor. Looked like a HiMax or something when I saw his display.

May 04, 2005, 10:44 PM
Registered User
Originally Posted by MrLift
I all ready have problems with the overrun clutch if the motor is being used to spin the prop it is no longer being used to power the avionics and computer I do not plan to use batteries for these functions.
I think I understand what you are after now. An electric propelled craft with an on-board APU (Aircraft Power Unit). The APU would need to be of sufficient size to provide propulsion power, charge the batteries and run the avionics. I suppose you could idle the APU and then really crank it when charging your propulsion batteries.

Wouldn't it be simpler to just power the craft with a gas engine and run a small generator (off same engine) for the Avionics and small back-up battery pack? Just a thought.

Originally Posted by MrLift
Also why three phase can it be built to work in single phase?
The 3-phase output of a PM generator is very simple to rectify into DC for running your avionics (6 diodes make a nice full wave bridge rectifier for this purpose). As a minimum, you will need a capacitor bank to handle the filtering and surges. You would also want a good regulator that can handle the full voltage and current of the generator and output the supply voltage(s) you want.

Keep it simple and go with an off-the-shelf PM brushless motor. Find one that has a many poles as possible to reduce cogging. Decide on how many watts you want to generate and select a motor capable of this power rating without over heating. Next you would want to match the RPM of your drive engine to the rated RPM of the motor/generator. You can always run the electric motor/generator slower than its rated RPM, but the output voltage will be lower. You can also use a belt and pulley connection and make everyone happy.

Sounds like you have received a lot of good feedback, good luck with your project! Bill
Last edited by william541; May 04, 2005 at 10:58 PM.
May 05, 2005, 12:01 AM
Registered User
Thanks Williams541 for hitting what I was thinking of doing on the head. I am still leaning towards a pure brushless propulsion system driven by batterys and the generator. The whole purpose is to be able to retard the combustion engine to save fuel when the battery's are full and go into sometype of powered glide for periods the aircraft is not being utilized but flying in a standby mode. This will extend the flight time enough that I beleive it will be worth it.

Uhm I am totally for 3-phase brushless propulsion but plan to build a custom generator strictly single phase. I am just not familiar with how to wind it occordingly and optimize it for maximum output. I would rather focus on building a compact light weight power supply and not half to deal with rectifying if at all possible. Do you have any idea what configuration produces the most power output and do you know of any equation that may help to explain the relationship of physics involved to predict the ouput of different designs.
May 05, 2005, 11:49 PM
Registered User
Originally Posted by MrLift
Do you have any idea what configuration produces the most power output and do you know of any equation that may help to explain the relationship of physics involved to predict the ouput of different designs.
I do not have any hard and fast answers for that one... There are so many variables involved, especially with the internal design of the generator head. Typically for a small lightweight generator, a 3-phase, high RPM (this means high frequency too) generator would give you the most "bang" for the weight.

I'm not sure I follow the reason for wanting the single-phase design... Do you plan on powering AC devices, or DC devices? If they are DC devices, then you are going to have to rectify the AC output of the generator to come up with DC to work with. This is true of single and 3-phase designs. The ripple of a 3-phase design is less and easier to filter. Unless you went with a less efficient brushed DC generator. A brushed generator would generate more interference and still requires regulator circuits, but it does output DC (+/- commutator noise).

I guess the place I would start is with your power budget and go from there. What current and voltage does the APU need to supply? To give you a small idea of what is required for the gas engine to run the generator, you can use the following to get pretty close.

Engine HP required = (electric power output in Watts/746) / 65%

The 65% is a safety factor so you don't kill the engine or burn it up prematurely.

If you want to use an off-the shelf electric motor as the generator head, look at the RPM/Volt rating. An 800rpm/Volt motor will produce a little less than 1 volt of output if turned at 800 RPM. Keep in mind that the output will sag when under load. Also look at the MAX amperage of the motor, this will indicate how much current you can pull from it when used as a generator. To be safe, only draw about 65-70% of this current or you could end up burning the windings. This current rating is valid for any output voltage as it is related to the size of the windings.

If you want to run the propulsion motor, charge the batteries and run your avionics at the same time, then your APU will need to be able to supply all of these loads combined, or you will never catch up with the battery drain. Unless you plan on gliding and riding thermals for extended periods of time.

If it were me, I would start off with an efficient motor for the generator head and just get a system working. Choose one with the highest working voltage and the lowest RPM/volt rating that can also supply the current desired. Test the generator head under different RPM and loading conditions, then work out the proper gear ratio between the engine and the generator. Then later if you want to increase efficiencies for a particular RPM range, then think about re-winding a generator head.

Sorry I couldn't be more helpful, but this is the best I can come up with at the moment. -Bill-
Last edited by william541; May 06, 2005 at 10:26 PM. Reason: Corrected Formula, Thanks Mr Lift
May 06, 2005, 12:38 AM
Registered User
No Bill Thank you very much that is helping a bunch. Just a few questions your equation for horsepower required when you adjust for 65% it actually gives you a lower horsepower required so for 2kw it would require 2.68HP adjusted it would take 1.74HP. So use a smaller motor to save the engine did you really mean add 35% more HP then needed to save the motor?

Also I understand the lowest volt per RPM turned but what relationship does RPM have to Amps or are Amps constant after a certain RPM.
May 06, 2005, 05:58 PM
Always right
Hovertime's Avatar
If you need 2kW of electric power it would have to have about 2.5kW of mechanical power provided TO the generator, and to be on the safe side it will need an engine that has 4kW of power max rating, and run it at 65% of its power. Or get an engine that has rated CONTINUOUS 2.5kW power rating.

As most said-much easier and economical just to use engine to provide 2kW for the props in the first place, skipping all th conversion loss.

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