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Old Feb 20, 2007, 11:13 AM
SG Talon... Super Genius.
Lenox, Michigan
Joined Nov 2005
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How big can you make a Brushless motor??? or... Can we fly real planes with these?

I have been toying around with an idea for a while now.

Just how big can you make a DC Brushless motor? With Lithium Polymer Batteries you can get trememndous amounts of power with very low weight. The question is.... can it be translated into full size aircraft?

10 years ago you could barely get an electric plane off the ground. Now you can build a plane with a $25 brushless motor that has 2 pounds of thrust. So why isn't anyone exploring this for full size aircraft? I have been searching but no luck.

So could you build a motor that makes 200 pounds of thrust? Put in a big LiPo and fly a plane like an ultralight? Think of the potential with using LiPo batteries in the wings instead of fuel tanks.

Anyhow. Just thought i would throw it out there. It seems the people on RCGroups are way more interested in serious advancements in electric motor tech than most other places.
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Old Feb 20, 2007, 11:38 AM
Figure Nine Champ
madsci_guy's Avatar
North Texas
Joined Nov 2002
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Quote:
Just how big can you make a DC Brushless motor?
As big as you want. Some synchronous motors are 6 feet in diameter.

Quote:
The question is.... can it be translated into full size aircraft?
Yes, but internal combustion still gives the most power for the weight.

Quote:
Now you can build a plane with a $25 brushless motor that has 2 pounds of thrust. So why isn't anyone exploring this for full size aircraft?
1. Cost. Lipoly costs $10/oz.
2. Internal combustion engines still produce the most power for the weight.
3. Duration. Your 2 lb thrust plane carries no cargo and only flies for 30 minutes max. That's not feasible for a full size aircraft, that need a 30 minute reserve to fly IFR.
4. Full size planes need FAA certification. Hard to do with innovative designs. Just ask Burt Rutan.

Quote:
So could you build a motor that makes 200 pounds of thrust? Put in a big LiPo and fly a plane like an ultralight? Think of the potential with using LiPo batteries in the wings instead of fuel tanks
The measurement number that we would use to compare gas vs. batteries is joules (watts times seconds)

Gasoline has 1.3xE8 joules per gallon (6 lbs). For a two hour flight, that would work out to 18055 watts as available power.

For your basic 3S Lipo that would mean a 1626 amp hour (Ah) battery. at 3.2 oz/Ah that means 325 lbs of battery or so. Boy! I didn't realize the numbers were THAT bad.

So if I use gasoline, it's six lbs vs 328 lbs for an equivalent lipo battery. Unless I've slipped a cog somewhere. As we all know, light planes fly much better than heavy. I think the difference is so great, you would never see a full sized Lipo powered plane. Of course the engine weight vs. motor weight would also have to be factored in, but it's hard to see how one could make up the difference in weight.

Edit: Oops! You'd need two batteries, not just one. So that's 650 lbs of battery for a two hour flight if you wanted to match the gasoline performance.
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Last edited by madsci_guy; Feb 20, 2007 at 11:45 AM.
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Old Feb 20, 2007, 11:51 AM
SG Talon... Super Genius.
Lenox, Michigan
Joined Nov 2005
3,163 Posts
But what about the efficiency of the motors? Electric Motors can have up to what 95% efficiency? IC engines are maybe 30%? Sure the energy you get from gas is so many joules but how does that translate into thrust/time

I saw an article on the internet somewhere. Some College kids in Japan built a nice big pedal powered type airplane and powered it with AA alkaline batteries. It flew for like 500 feet or something. A LiPo has way more energy than an alkaline. They were also using a standard off the shelf brushed motor.

I also found this.... http://www.kitplanes.com/magazine/engines/176-1.phtml

They want to use a fuel cell to power it.

I am just speculating here but an Ultralight would not have all the issues that you are talking about. They can only weigh 254 pounds dry. They have none of the restrictions that regular FAA approved aircraft require. And they regularly fly with 20-50 hp motors.

I am not saying "hey all these gas people are nuts" i am just wondering what CAN be done. They have electric cars breaking speed records all over. Heck you can buy an electric car that does 0-60 in 4 sec and has a 200-300 mile range for $100,000 for prototypes on its way to $20,000 for production models. http://www.commutercars.com/

Like I said i am just wondering if it something that can be done. 30 minutes in a silent ultralight would be great!
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Old Feb 20, 2007, 12:43 PM
homo ludens modellisticus
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The Netherlands, GE, Nijmegen
Joined Feb 2001
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The shape of things to come?
http://www.grc.nasa.gov/WWW/RT/2003/...20emerson.html
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Old Feb 20, 2007, 02:27 PM
Figure Nine Champ
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North Texas
Joined Nov 2002
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Aeronautics goes by the numbers. It has ever since the Wright brothers. Untill you can get energy density per pound comparable to gasoline, I don't see battery powered flight except as a novelty or hobby. Sure, you "can" make a battery powered plane, but it wouldn't be practical. And the price would be prohibitive.
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Old Feb 20, 2007, 04:51 PM
X-Era Motors
DavidB.'s Avatar
Newport, NC
Joined Jan 2002
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Energy density actually has to get to "only" 1/3 to 1/4 that of gasoline. Electrics are WAY more efficient.
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Old Feb 20, 2007, 05:53 PM
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Palo alto CA
Joined Oct 2004
716 Posts
Fancy that, im doing just what your saying as we speak!
http://www.rcgroups.com/forums/showthread.php?t=645386
The technology is so here-right now. You can get 18650s cheep! (<2$a cell)
use 4 large chinese brushless motors geared together and you have 120lbs of thrust from a 25lbs set up that cost less then a grand.
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Old Feb 20, 2007, 06:09 PM
Figure Nine Champ
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North Texas
Joined Nov 2002
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Quote:
Originally Posted by DavidB.
Energy density actually has to get to "only" 1/3 to 1/4 that of gasoline. Electrics are WAY more efficient.
Power efficiency is NOT all you look at. In aircraft one must factor in weight. 6 to 650 is a 100:1 fuel energy/pound ratio, that you must overcome. I would certainly put "only" in quotes, as you are a long way from even coming close. Your advanced small turbine (cruise missile type) is 46% efficient. That means you must get to 2:1, in order to compete, merely on similar theoretical performance. That's a 24 lb battery comparing to 12 pounds of fuel, for our two hour trip. And that's a 50 times improvement in current energy density of your battery. As we say in the engineering world, "That's rather aggressive engineering". And of course I used LiPoly in plastic weight numbers. Run the 1.5 thousand amps through the battery that you need to run to fly, and you are talking a LOT of heat that requires cooling, which of course eats up your weight budget. Putting the batteries in series to reduce the amps the motor uses still means the same heating, because the watts consumed by the motor must be the same to drive the prop the same.

And a battery that was 50 times more energy dense per weight than a Lipoly looks more like a bomb than a battery. Just compare Lipo to NiMh flammability and see what I mean.

Oh, and there's another problem. NdFeB magnets lose their magnetism at 100 degrees C, some even lower, like the high MGO (35+) number magnets, that you would have to use for your theoretical high efficiency motor. You climb out for 1/2 hour at full power in summer and probably your magnets will go south, causing your motor to overheat severely, making a great fire complete with very toxic fumes from the NdFeB magnets.
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Last edited by madsci_guy; Feb 20, 2007 at 06:18 PM. Reason: Corrected bad theory
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Old Feb 20, 2007, 06:35 PM
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Ireland, Kildare, Leixlip
Joined May 2002
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On the subject of temperature. Lipos don't like low temperatures very much. That could be a problem at normal full size altitudes if you're spreading the cells out in say the wing. Although on second thoughts you could heat them a little without too much difficulty but it will cut into your overall efficiency.

By the way there is an electric paramotor harness being developed. Looks to be pretty practical. It may even be on sale already. I can't remember where I saw it. I suspect someone on RCGroups supplied me a link.

Aidan
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Old Feb 20, 2007, 09:09 PM
Fly fast and turn left!!
Bruxelles/Uccle
Joined Dec 2001
169 Posts
http://www.lange-flugzeugbau.de/htm/...news/news.html
http://www.lange-flugzeugbau.de/htm/...ropulsion.html


Full size Electric glider is already flying.
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Old Feb 20, 2007, 10:10 PM
X-Era Motors
DavidB.'s Avatar
Newport, NC
Joined Jan 2002
2,225 Posts
Quote:
Originally Posted by madsci_guy
Your advanced small turbine (cruise missile type) is 46% efficient.
Please backup that number, it looks like it's about double the actual figures I've seen.

Quote:
Originally Posted by madsci_guy
Run the 1.5 thousand amps through the battery that you need to run to fly, and you are talking a LOT of heat that requires cooling, which of course eats up your weight budget.
There are planes flying now with lithium cells, this is not really that big an issue.

Quote:
Originally Posted by madsci_guy
Putting the batteries in series to reduce the amps the motor uses still means the same heating, because the watts consumed by the motor must be the same to drive the prop the same.
I have no idea where you're headed with this response..what is your point?

Quote:
Originally Posted by madsci_guy
And a battery that was 50 times more energy dense per weight than a Lipoly looks more like a bomb than a battery. Just compare Lipo to NiMh flammability and see what I mean.
Every power source is dangerous if misused or designed incorrectly, theres no getting around it.

Quote:
Originally Posted by madsci_guy
Oh, and there's another problem. NdFeB magnets lose their magnetism at 100 degrees C, some even lower, like the high MGO (35+) number magnets, that you would have to use for your theoretical high efficiency motor. You climb out for 1/2 hour at full power in summer and probably your magnets will go south, causing your motor to overheat severely, making a great fire complete with very toxic fumes from the NdFeB magnets.
We're assuming the system is designed correctly here, correct?! It sounds like you're digging at the bottom of the ditch to find a response. I must ask, do you have some sort of stock in the oil companies? I mean common, NdFeB and toxic fumes, where do you find this stuff.....
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Old Feb 20, 2007, 11:15 PM
Creations Aberration
Aethertek's Avatar
United States, OR, Newport
Joined Jan 2004
609 Posts
Small gas engine/turbine(better) turns alternator / alternator supplies electric power.
Not battery powered per se, but energy density of gas with propulsive efficiency of electric motor.
Just a thought...
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Old Feb 21, 2007, 01:55 AM
Registered Snoozer
FairlyFaded's Avatar
Palo alto CA
Joined Oct 2004
716 Posts
200 li-ion 18650 cells: 1.6kwh, burst of 7kw: 20lbs <400$
1 brushless motor: http://www.himodel.com/electric/176K...e_A7050_6.html
6.6kw: 180$ 3lbs
cheap chinese ESC: 100$ <1lb
total wieght 24 lbs, max hp 8.75. total cost 680$ plus 150 for a charger...
find me a reasonable gas engine that wieghs much less then 24lbs...
madsci are you ever wrong.... i think people like you are the reason electric vehicles never can get of the ground. (sorry to be such a sinik but comon...)
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Old Feb 21, 2007, 02:05 AM
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Italy, Umbria, Perugia
Joined Sep 2006
85 Posts
It's easy. We just need to utilize the same technology used in regenerative braking in electric vehicles.

All you'd have to do is massive climbs to store energy, then massive dives to spin the prop to generate electricity to charge the batteries!

Or even easier, just mount a wind generator somewhere on the fuselage at least 4 times bigger than your prop, and it will charge itself as you fly!!

I should work for NASA!

PS, I'm also trying to take out a patent for a Prosthetic Head made out of cork for people who've accidentally been decapitated by the prop on their 1:1 scale RC plane, when they plug the HUGE deans connector in, THEN realize their throttle stick was WOT!!

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Old Feb 21, 2007, 03:54 AM
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Aio_1's Avatar
Ireland, Kildare, Leixlip
Joined May 2002
6,441 Posts
Quote:
Originally Posted by Aethertek
Small gas engine/turbine(better) turns alternator / alternator supplies electric power.
Not battery powered per se, but energy density of gas with propulsive efficiency of electric motor.
Just a thought...
You're still using a turbine or piston engine to generate the power so you've already go the loses associated with that and then you're adding the losses of an electric system. I don't see any sense in hybrid drives in aircraft. A car spends it's time accelerating and decelerating, climbing and decending hills, resting at traffic lights etc... the benefit is that you can run the engine at a more constant and efficient rpm and smooth the peaks and troughs in power usage using the cells. Full size planes on the other hand spend their time cruising at pretty constant power levels. I don't see how you gain anything by using a hybrid. Even if take-off were electrically assisted to reduce engine size requirements the weight and volume of associated cells and systems would undoubtedly make this pointless.

Aidan
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