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Jan 10, 2013, 06:43 PM
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EddieWeeks's Avatar

Gas Generator... Build Log...

This is a build log of 23 cc gas engine.... Here are a few post from another tread for back ground...

Dec 23..
My friend and I ran a 32 heli engine with a rubber hose connected to a 700 watt brushless motor as a generator.... The wires then went to a 100 amp rectifier, same kind on everyones car... It worked... I ran the .32 engine with an air hose blasting on it, making all kind of racket...
While he connected it to a ESC and then ran an engine with a prop on it... he turned the knob and the prop turned... the alcohol motor slowed down and we sent 100 watts to the props.... Its was so noisy, messy and it was not long before the 700 watt brushless motor started smoking, all for a lousy 100 watts that we could not maintain... I needed 1000 watts... I think a 50 cc Gas twin with a very low KV motor may work but the motor and generator will need serious cooling... With funding I think it could work but on a somewhat larger scale.. maybe 5-10 hp gas…


Dec 29
Your going to have a hard time getting enough power down those belts.... It would take a large belts and then large belts have a lot of inefficiencies
Along with the very inefficient flat props or blades…. Those things are far less efficient than a proper propeller…
By far the most reliable system would be a power package all enclosed in a box with two wire coming out of it…
Inside the box would be a 25cc gas engine (3.5 hp or 2625 watt, 760 grams)… a large brushless electric motor.. (3-4000 watt)…
Rectifier… a governor….
The key to this…If the gas engine stopped…. Its ok… there will be a battery on board for 2-3 min of flight…
Its clear from the post above, the priority is not reliability… The single IC with 4 belts, variable pitch could work fine… but there is too much to go wrong…
And if any of it failed, the copter crashes…
The Gen-copter could be a octo so the generator and a motor could fail and it could still land…
So… 2625 watts from the gas engine at 9000 rpms.… say %50 of that gets to the flight motors… that is ~1300 watts… and an inefficient copter at 7 grams / watt
That would be 9100 grams of lift....… I think that could be done since the gas engine only weighs 760 grams…
I don’t know if a 4000 watt electric motor can be a generator and hold up..maybe with good forced cooling…. I know it needs to be low KV.. 400kv would give ~20 volts @8000 rpms…
The key to this is it could be attached to working, reliable copters... the two wires would parallel the batteries…
Eddie Weeks
Last edited by EddieWeeks; Jan 10, 2013 at 06:51 PM.
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Jan 10, 2013, 06:50 PM
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EddieWeeks's Avatar
Just today... Ran the beast up to who knows what RPM... it Starting charging a POS really old 4S 5000 at 5 amps... cool...

Powered up the hex and started to pull a load on the junk battery... battery voltage dropped to 3.5.... power up the gen to putout
20 amps... battery voltage came back 4.0... left it sit there for a while at 20 amps... no problem...

This was only a test of the shaft coupler... did not plan on much more....

my friend power up the copter even more...... generator starting putting out 25 amps... so I powered the gas engine up....
got it to 30 amps... stable.... 5-10 sec... I look over and he has the throttle at like %80 and the Hex has got a case of the Jimmy leg...(strapped to the table)....... so I pull on the gas some more.... 40 amps... for 5 sec... then 0 amps... Gas engine spools up and hex stops...
... the diodes fried... we know those little diodes would not work for long but thats ok... Next is the 100 amps rectifier and a servo on the throttle of the gas motor…

So.. today made ~600 watts for 5 sec... and the generator was bearly warm.. good sign....

The big question is how much voltage differental is required to get the amps up... I think that like a battery, the generator has
an internal resistance... that should have something to do with how many extra volts it makes vs how many amps I get out...
not going to worry about it... just going to run the gas motor faster and faster...
Last edited by EddieWeeks; Jan 10, 2013 at 07:07 PM.
Jan 10, 2013, 07:17 PM
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rimshotcopter's Avatar
Wow, Looking on with excitement!!
Jan 10, 2013, 10:59 PM
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very interesting, and good luck!

Now that you are talking a much larger motor it is definitely starting to sound heavy!

Again, if it is concern of flight time, I would still go with the bet or shaft driven... But having a Gas gen octo would be more reliable I suppose, as long as electronics hold up......
Jan 11, 2013, 12:16 AM
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gitit20's Avatar
you know some of them small nitro engines put out 2 hp or so wonder how good one of them would work?
Jan 11, 2013, 03:37 AM
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topgearuk's Avatar
Interesting! I was following the previous thread loosely.. Are you using the gen to power a charger to control voltage/amps the on board lipo battery gets?
Jan 11, 2013, 08:08 AM
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Couple questions:

Is that a magnetic clutch in the picture between your shaft coupler and the generator?

Is that generator in the picture only 760 grams?

What will you do about the 13% ripple you get from the rectified 3-phase signal?

what is your goal for power? was it only 1000 watts?

What are you using to measure current? OSD/current sensor? ESC data logging?

- I am just curious, because sometimes I like to compare what my OSD measured and what castle link tells me. One will say 890 watts, while the other tells me 1100 watts. (which to trust? who knows...)

I am along for the ride though, cool project. Good luck!
Jan 11, 2013, 09:49 AM
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EddieWeeks's Avatar
Is that a magnetic clutch in the picture between your shaft coupler and the generator?

No... its a centrifugal clutch... works great..

Is that generator in the picture only 760 grams?

I think so ... that is just some hobby king motor...

What will you do about the 13% ripple you get from the rectified 3-phase signal?

Ignore it... I am hopeing the battery will smooth things out...

what is your goal for power? was it only 1000 watts?

My goad is 1300 watts... or 1/2 the power that engine can produce...
If I get 1300 watts for 5 min... I will build a copter to fly this thing..

What are you using to measure current? OSD/current sensor? ESC data logging?

I am using some inline amp meter... it outputs milli volts to my fluke.. each milli volt is an amp..

I have a feeling to get to 1300 watts I will need to move to 6S and this generator has a too high KV... I don't think I can produce that many amps at lower voltage... The motor is rated at 90 amps and I need 87 amps to make 1300 watts... that is too many amps... the wires and plugs will get hot and may melt.. I think I will need a 200 KV motor... about 2000 watts...
Last edited by EddieWeeks; Jan 14, 2013 at 10:36 AM.
Jan 12, 2013, 12:13 AM
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EddieWeeks's Avatar
Jan 11th..

Ran the generator with a new 50 amp rectifier… was testing a program mix to apply gas power via a servo...To try to match the load of mid size hex which was providing the load…
Everything was working great… As throttle was applied to the hex, with the crappy battery, the gas engine would spool up to match… The clutch would kick in right about the time the hex motors turned on and I tweaked the P_Mix so the voltage Stayed constant at 3.8 volts all the way up to 20 amps….

Then the brand new 50 amp rectifier smoked.. !!! LOL… I was hoping / expecting to smoke generators not that…

This was after 5 min of running at low power.. oh well…

Going to get a 200 amp rectifier on ebay… I guess...

What would capacitors do in this situation. .. They could be on the 3 phase side… or on the DC side....
I know they tend to smooth things out… but would that help here.. ?

Jan 12, 2013, 12:41 AM
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Caps wont help your rectifier as they go on the DC side. All they do is discharge to fill in the "holes" of the ripple and smooth out the signal to look more DC.

I would say a good heat sink with thermal paste on the rectifier would do the trick here. Without a good heat sink, the rectifier will smoke FAST, even at 1/3 - 1/2 its rated load.

If you are already running a heat sink, then yeah - better use a bigger rectifier.
Jan 12, 2013, 12:59 AM
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EddieWeeks's Avatar
ok good advice... yeah... I had the smoked one screwed to a wooden table.... opps...

Later I am sure I can find a place to screw it to the aluminum frame I will make later...
Jan 16, 2013, 04:12 PM
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i am amuzed at how i had the exact idea today, but with subtle differences.

I thought the use of a small nitro engine connected to a DC or brush-less motor via a regulator would be sufficient.

If i ever manage to get any money to afford this sort of project, i will try it out!

Perhaps you should try using Nitro as the small engines would vastly reduce weight
Jan 16, 2013, 05:00 PM
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EddieWeeks's Avatar
Well the idea is to fly for an hour or more... When I flew Nitro I would burn 1 gal / day no problem...
The energy density for gas is far more than nitro….

4 stroke gas would be the best… but those motors are very heavy and make little power..
Jan 16, 2013, 05:10 PM
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rimshotcopter's Avatar
Unless one has a turbocharger, but that is just a dream at this scale.
Jan 16, 2013, 08:06 PM
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EddieWeeks's Avatar
Bought a 300 amp rectifier on ebay…. I swear it was smaller in the picture.. !!! LOL…

Oh well… this will never fly, but it will be good for testing

The motor I am using now is missing one magnet... it still works and flew on the 5000 watt
tri copter but you could tell it was not right...

I have a 200 kv motor on the way... much testing this weekend..


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