Hi All,
We have recently discovered a way to build hybrid gas/electric-powered multi-copters and would like to share it with you.
The main advantages are:
1) Heavy lift - due to gas engine power
2) Long endurance - due to high gas fuel power density
3) Safety - power source redundancy for safe landing
For the full description of the idea please have a look at: http://www.gas-copter.com/Description
We are also looking for partners to help us with prototyping and commercializing the idea.
You comments in the blog are welcome ...
Best Regards,
The gas-copter.com team
Comments
Oh, you're talking about throttling the IC engine up and down. I just assumed you weren't, because, frankly, it's very inefficient.
1 kg of fuel has 46 MJ of extractable energy. An IC engine throttling up and down has an average efficiency around 15-20%, depending on how well the motor is sized and what the "mission" will look like. A drive train has ~85% efficiency. So from 1kg of fuel, you will get 46*0.2*0.85 = 7.82 MJ of energy. Alternatively, a generator runs at 100% load at its peak efficiency rpm, giving a constant 35% efficiency. The alternator efficiency will be about 60%, so you'll get 46*.35*.6 = 9.66 MJ of energy. For systems of comparable size, the generator design will perform better.
I thought you were proposing running the IC engine at a constant 100% load, peak efficiency, then using a drive train instead of the alternator - 46*0.85*0.35 = 13.69 MJ, then using electric motors for maneuvering. It's an efficient system, but mechanically very challenging (slip rings or planetary gear system, etc.).
@Tyler McGahee.
Let me disagree.
>You have to force the motors to a higher or lower rpm to maneuver.
Exactly. I do force individual motors to a bit higher or lower rpm - why should anything burn?!
> it's not going to spin faster
It is! To some degree it will do because of the load change on the IC engine. And don't forget you only need to change the speed of all of the 4 propellers in one direction during climb or decent in which case you use IC engine control. All others need acceleration in some propellers and deceleration in others. So no locks - differentials will do the job while brushless motors will control the distribution.
> A reasonable expectation is about 55-60%
Exactly. And reasonable expectation from the mechanical system we purpose is around 85%.
@Armen Well, I saw that diagram. You realize that mechanically, that doesn't function like you want it to, right? If your gas engine is driving your propellers at 3000 rpm, and your electric motors try to spin at 4000 rpm, it's not going to spin faster - something is going to break, probably the electric motor. You need a drive train that can combine the shaft power from the electric motor and the gas motor.
Similarly, the differentials don't work the way you've displayed. Propellers are not analogous to the wheels on a car. When you turn in a car, one wheel naturally wants to slow down. When you turn in a multirotor, there is no natural feedback. You have to force the motors to a higher or lower rpm to maneuver. Vince is right - you need a lock differential, or, even simpler, just a gear box.
As for the alternator a properly sized 400g brushless motor should be able to handle the 2kW shaft input, but you're not going to find any generator that has 70% efficiency. A reasonable expectation is about 55-60%, and you should be able to get that just fine, saving you 8lbs. The one you linked is rated for 150kW input, which is why it's so big.
To Vince Hogg:
>I dont think anyone in their right mind would suggest using a 9lb Lycoming generator.
Can you provide a link to a lighter generator having 2KW power and > 70% efficiency?
Best Regards,
Armen Poghosov
To Tyler McGahee:
On the web-site: description there is a picture. Assume the blue-colored bodies of the motors are fixed to the frame.
Best Regards,
Armen Poghosov
To Vince Hogg:
>Differentials?
>So if one motor demands more lift wont the differential share out the torque to the opposite motor?
Yes, that's the main point! Please read the description carefully - no diff-locks at all ...
Best Rerards,
Armen Poghosov
At the end of the day, like all engineering (well should be), you have to ask what is the easiest / best way to solve a task.
What are you trying to do?
Fly outside as long as you can, in one small area, taking pictures?
-Well lighter then air is best for that
Fly outside side over a large area, taking pictures / sensor sweeps
-Glider type air frame works best
Fly in a limited area, moving often, as long as you can
-T-hawk type air frame will be best
Look at what you want do to, and what you need to do it?
A hybrid system can work, I had it working, but it solved nothing, other them an engineering exercise, which as engineers we should do anyways, to keep our skills sharp :)
"We are using brushless motors which are mounted to the frame. The shafts of brushless motors are directly connected to the bevel gears driving the props."
Could you post a simple diagram of what you mean by this?
Differentials?
So if one motor demands more lift wont the differential share out the torque to the opposite motor? I think you need a diff-lock.
There are so many things to get right and so many things that could fail.
I dont think anyone in their right mind would suggest using a 9lb Lycoming generator.
Im sure someone cleverer than me could come up with a 3 phase rectifier unit so that a large-ish brushless motor could be driven directly from a nitro engine. The whole lot - eng/gen + regulator + fuel could be attached as one unit when required.
Im all for having a go at different ideas but when the solution is not elegant its an uphill battle.
Thanks to everyone who commented on our post and let us clarify the raised issues.
About central generator and "classic" quad-copter design. Let’s compare it with the system we proposed (simple one, without safety feature).
Les’s chose the following IC engine as power source:
http://www.dle-engines.com/dleg0031.html
We’ll use it at 75% of maximum power which is around 2KW. Then we chose lightweight aircraft generator to generate electric power:
http://plane-power.com/AL24_70.htm
Plus we need 4 brushless motors to drive the propellers say like this one: http://www.scorpionsystem.com/catalog/motors/s30_series_v2/SII-3014...
Electric speed controllers: http://www.scorpionsystem.com/catalog/speed_controllers/4s_lipo_bec...
This makes at least 1.1kg (gas engine) + ~4kg (generator) + ~4*130g (brushless) + ~4*40g(ESC) = 5.780g
Now let us do the same calculation for our mechanical design.
Same IC engine. 4 smaller brushless motors like this one: http://www.scorpionsystem.com/catalog/motors/s22_series_v2/SII-2205...
Smaller ESC: http://www.scorpionsystem.com/catalog/speed_controllers/4s_lipo_bec...
3 Differentials:
http://www.ebay.de/itm/Thunder-Tiger-EB4-S2-2-5-RALLY-1-8-Buggy-Dif...
Aluminum housing:
http://www.ebay.com/itm/2Set-Alu-Differential-Gear-Box-Mount-Fits-L...
4 bevel gears pairs - each weighting 30g + shafts 30g.
This makes our power unit weight: 1.1 kg(gas engine) + 4*36g(brushless) + 4*20g(ESC) + 3*150g(Diffs) + 4*60g(shafts+bevel-gears) = 2.014g
So, with the same IC engine, the difference is about 3.7kg which is not acceptable.
About Redundancy: the IC engine+Generator+classic quad-copter design doesn’t have power source redundancy- In case of any failure in electric motors or ESC it will crash.
About efficiency: Each differential will have around 5% loss, and each bevel gear pairs about 1.5%.
http://web.mit.edu/2.972/www/reports/differential/differential.html
So, totally we’ll have 2*5+1.5%=11.5%loss in 3 Diff system and 6.5% loss in 2diff systems.
We couldn’t find any exact data for generator and brushless motors efficiency, however, according to http://ecalc.ch/ and some other sites the lightweight generator efficiency is around 60-70% and brushless motor’s efficiency is 80%. So, total efficiency will be around 50-55%. Compare with 11.5% :)))
About slip rings - there are NO slip rings in the system. We are using brushless motors which are mounted to the frame. The shafts of brushless motors are directly connected to the bevel gears driving the props.
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