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Old Jun 20, 2011, 11:46 PM
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Melbourne, Australia
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Can your provide a link to the specification of 18650 cells that deliver 8Wh at 0.5C?
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Old Jun 21, 2011, 02:00 AM
I enjoy the voices
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Perth, Australia
Joined Apr 2007
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Originally Posted by KiloOne View Post
Great car project, how many kwhr of batteries do you have?
Sorry for the slow response, been away. That should have been 23 kWhr of batteries (45 x 160 Hrs LiPos at 3.2 v each) - 70 kW peak power.

rebell, yes it's a BMW 318i Bauer Carbolet. Had it for 2 years, cost of conversion around $20k.

But this all a sidetrack from the real thread - keep up the good work KiloOne
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Old Jun 21, 2011, 03:44 AM
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Dale,

Re. thermally conductive resin: this provides 2 functions - discouraging hot spots and conducting heat to the stator (& thence away via a heatsink and/or stator ribs in airflow). We found in our burnt-out windings that some parts were still fine; the problem is that a hot spot increases resistance and then gets hotter still - so good uniformity reduces thermal runaway. The thing is that, to be effective, you need a vaccum rig to impegnate the resin into the windings. Once applied though, the air gaps between adjacent pole windings are pretty much unchanged - its mainly the air gaps between the windings and to the stator steel which are filled.

I think the Joby motors look a really good match for the Lazair. Guesstimating from the motor dimensions, you should be able to get ~50% more torque (& power) from these than from the Turnigys. I can't quite tell from their photo, but it looks like the windings are varnished anyway - so that should help the heat transfer. Personally, I would still think about a radial fan to draw air though the insides - it looks like the outside cooling fins will get some prop wash anyway. Very best of luck with it - I look forward to hearing how they perform.

Paul
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Old Jun 23, 2011, 01:18 PM
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Maybe someone can help me out here because I don't see how you can get close to 2 hours duration with 96 4S batteries. Here is my understanding:

Weight of the plane = 475 lb loaded
L/D on the order of 12-14 to 1 (I used to own a Lazair and recall this to be the l/d - am I correct?)
So the thrust required is on the order of 475/12 ~ 40ish lbs

Looking at the data for the Turnigy motor this equates to about 90 amps for cruise flight or 90 Ah per hour. Using 80% battery capacity means you need 90/0.8 = 112.5 Ah of battery capacity per hour of flight time. Two motors producing only 20 lbs each looks like a little less amps, but in the same range.

If there are 5 Ah packs, then 112.5/5 ~ 22 packs * 4 batteries/pack ~ 90 batteries for a single hour of flight time not considering take-off power requirements.

What am I missing? My expertise in this is somewhat limited, so I'd like to know where my calculations are wrong because I'm working on a project to electrify a paramotor and these Joby motors look very promising.

Thanks for your help.
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Old Jun 23, 2011, 02:04 PM
Dale
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RE: Weight and balance

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Originally Posted by Fltofancy View Post
Dale,
What does your weight and balance look like in this flying configuration?

Fltofancy.
Gene,

After the first taxi hop I decided to try with a 4 inch cushion behind my back. The next taxi hops felt OK and I flew at that CG. It was 17.5 inches behind the leading edge with the boom level. I think I will move the seat and rudder pedals forward a few inches before I fly again.

Dale
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Old Jun 23, 2011, 02:14 PM
Dale
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Quote:
Originally Posted by hybridplane View Post
Dale,

Re. thermally conductive resin: this provides 2 functions - discouraging hot spots and conducting heat to the stator (& thence away via a heatsink and/or stator ribs in airflow). We found in our burnt-out windings that some parts were still fine; the problem is that a hot spot increases resistance and then gets hotter still - so good uniformity reduces thermal runaway. The thing is that, to be effective, you need a vaccum rig to impegnate the resin into the windings. Once applied though, the air gaps between adjacent pole windings are pretty much unchanged - its mainly the air gaps between the windings and to the stator steel which are filled.

I think the Joby motors look a really good match for the Lazair. Guesstimating from the motor dimensions, you should be able to get ~50% more torque (& power) from these than from the Turnigys. I can't quite tell from their photo, but it looks like the windings are varnished anyway - so that should help the heat transfer. Personally, I would still think about a radial fan to draw air though the insides - it looks like the outside cooling fins will get some prop wash anyway. Very best of luck with it - I look forward to hearing how they perform.

Paul
Paul,

Good points, I don't really expect to see much more power out of the Jobys since the air gap diameter and the magnet length are very similar, I just want the extra cooling that their design can provide.

Sounds like the Jobys will be delayed a few days as a FedEx package of motor parts to Joby has vanished.

Dale
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Old Jun 23, 2011, 02:56 PM
Dale
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Quote:
Originally Posted by jlorenco View Post
Maybe someone can help me out here because I don't see how you can get close to 2 hours duration with 96 4S batteries. Here is my understanding:

Weight of the plane = 475 lb loaded
L/D on the order of 12-14 to 1 (I used to own a Lazair and recall this to be the l/d - am I correct?)
So the thrust required is on the order of 475/12 ~ 40ish lbs

Looking at the data for the Turnigy motor this equates to about 90 amps for cruise flight or 90 Ah per hour. Using 80% battery capacity means you need 90/0.8 = 112.5 Ah of battery capacity per hour of flight time. Two motors producing only 20 lbs each looks like a little less amps, but in the same range.

If there are 5 Ah packs, then 112.5/5 ~ 22 packs * 4 batteries/pack ~ 90 batteries for a single hour of flight time not considering take-off power requirements.

What am I missing? My expertise in this is somewhat limited, so I'd like to know where my calculations are wrong because I'm working on a project to electrify a paramotor and these Joby motors look very promising.

Thanks for your help.
My duration estimate was based on my guess that 3 kw would keep the Lazair in the air. Unfortunately that was probably a low estimate based on Series I and II airplanes that were up to 100 lbs lighter than my Elite Electric Lazair and also when I weighed 80 lbs less than I do now.

Actually, two motors at 20 lbs each use substantially less amps than one at 40 lbs. With this setup it would be 3000 watts vs 4000 watts (http://www.rcgroups.com/forums/attac...mentid=3895621).

Anyway, AmpHrs are not what keeps a plane in the air. Power is a more appropriate unit. If 40 lbs of thrust would keep the Lazair in the air at 30 mph, that is about 3.2 hp (1hp=550 ft-lbs/sec) or 2.4 kw (at 747 watts/hp).

Lets say 70% efficient prop ups that to 3.4 kw and 90% efficient motor ups it again to 3.8 kw.

I have 7.2 kwhr of batteries so an endurance of about 1.9 hrs.

I would be happy with that but I think my endurance will be less than that. I think I will be lucky to get 60% prop efficiency and it is probably taking more than 40 lbs to keep me in the air. Time will tell.

Personally, I do not think endurance should be listed at 80% pack capacity.

Endurance in an aircraft uses all usable fuel, reserve amounts are left to the pilot to decide.

Dale
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Last edited by KiloOne; Jun 23, 2011 at 04:42 PM.
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Old Jun 23, 2011, 03:14 PM
Dale
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Electric Ultralight Weight Issue

I have come up with the below analysis of the weight issues regarding electric ultralights. I believe that anyone considering a project like mine should use this approach in determining their allowable component weights as no one has yet found a flaw in the analysis and its implications will allow electric ultralights to be developed with reasonable endurance using todays technology.

Here is my analysis:
I have read Part 103 and its advisory circular thoroughly and I believe that the electric Lazair, as I have constructed it, meets the definition and its interpretation as described in both documents.

The 254 pound weight limit is defined as empty weight without fuel and then, fuel is limited to 5 U.S. gallons.

A U.S. gallon is a measurement of volume, not weight. 5 gallons of diesel, Avgas and Jet A all have different weights and, without question, all three would be acceptable fuels for a Part 103 ultralight.

There is no mention or limitation as to the type of fuel that can be used. Lacking an FAA definition of fuel, I see that the Wikipedea definition of a fuel is any material that can release energy and electric Lipo cells certainly do that.

My fuel is the 384 Lipo cells, each with a volume of 2.9 cubic inches. My total fuel volume is 1114 in^3. 5 U.S. gallons is 1155 in^3.

My fuel volume is therefore less than 5 U.S. gallons and the regulation is met. In effect, the rule allows us to carry 1155 in^3 (5 U.S. gallons) of fuel regardless of its density.

As a practical matter, I submit that since it is difficult to physically remove only the fuel (384 Lipo cells) from my aircraft, my aircraft should be weighed with fuel (Lipo cells) and then the Lipo fuel weight should be subtracted to obtain its empty weight.

The 384 Lipo cells weigh 90 lbs.

Since my electric Lazair currently weighs 282 lbs with fuel, I believe that the empty weight without fuel is 282 - 90 lbs OR 192 lbs which is significantly below the Part 103 maximum empty weight of 254 lbs.

I would also like to point out that, under the 2011 NASA Green Challenge rules, these 384 Lipo cells only carry an energy equivalent of less than 1/4 of a gallon of auto fuel.
Dale Kramer
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Last edited by KiloOne; Jun 23, 2011 at 04:40 PM.
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Old Jun 23, 2011, 04:41 PM
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United States, CA, Shingle Springs
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Originally Posted by KiloOne View Post
My duration estimate was based on my guess that 3 kw would keep the Lazair in the air. Unfortunately that was probably a low estimate based on Series I and II airplanes that were up to 100 lbs lighter than my Elite Electric Lazair and also when I weighed 80 lbs less than I do now.

Actually, two motors at 20 lbs each use substantially less amps than one at 40 lbs. With this setup it would be 3000 watts vs 4000 watts (http://www.rcgroups.com/forums/attac...mentid=3895621).

Anyway, AmpHrs are not what keeps a plane in the air. Power is a more appropriate unit. If 40 lbs of thrust would keep the Lazair in the air at 30 mph, that is about 3.2 hp (1hp=550 ft-lbs/sec) or 2.4 kw (at 747 watts/hp).

Lets say 70% efficient prop ups that to 3.4 kw and 90% efficient motor up it again to 3.8 kw.

I have 7.2 kwhr of batteries so an endurance of about 1.9 hrs.

I would be happy with that but I think my endurance will be less than that. I think I will be lucky to get 60% prop efficiency and it is probably taking more than 40 lbs to keep me in the air. Time will tell.

Personally, I do not think endurance should be listed at 80% pack capacity.

Endurance in an aircraft uses all usable fuel, reserve amounts are left to the pilot to decide.

Dale
Thanks for the reply Dale. I was using the 80% not as fuel reserve, but because you cannot use 100% of the battery capacity without destroying the batteries. I use 80% discharge capacity for RC applications as this seems to be a commonly accepted amount to safely discharge Lipo batteries.

The whole kwh concept for batteries has always confused me because of the voltage component. You can use full pack voltage to calculate the capacity, but the voltage changes as the pack is discharged. That is why I usually default to amps. My logic being that if it takes say 80 amps to produce enough thrust to stay in the air, then it will take 80 Ah of battery capacity in one hour. Since battery capacity is rated as mah, this is to me a more direct method to obtain duration. This seems like it will actually over estimate the duration because as the battery discharges the voltage will drop and it will require more amps to get the same thrust (or power).

If I use this method for your application using the data for the Turnigy motor, I get the following:

20 lb thrust requires 40A or 80A for both motors
This is 80 Ah for each hour of flying at cruise
80 Ah/0.8 = 100 Ah (since you can only effectively use 80% of the pack)
100 Ah/5Ah per 4 battery pack = 20 packs, or 80 batteries
So, for 96 batteries this is about 1.2 hours at cruise and this does not account for the voltage drop in the batteries, so it optimistic. It seems like one would be doing good to get 1 hour out of this system.

If I've missed the boat here, please let me know. I'm not trying to shoot holes in your project, I think it is great and have been watching with great interest. I just want to fully understand all this so I can have a realistic prediction of flight duration for my project.

Jeff
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Old Jun 23, 2011, 05:26 PM
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San Diego, CA
Joined Mar 2009
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Originally Posted by jlorenco View Post
...you cannot use 100% of the battery capacity without destroying the batteries. I use 80% discharge capacity for RC applications as this seems to be a commonly accepted amount to safely discharge Lipo batteries.
You cannot use a lithium polymer battery at all without destroying it.

100% DOD destroys the battery at rate x.

80% DOD destroys the battery at rate y.

x > y
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Old Jun 23, 2011, 05:48 PM
Dale
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Joined Jul 2004
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Jeff,

I have convinced myself that discharging these cells to 3.3v (nominal 100% discharge level) at my expected cruise current does not significantly affect their life. I have discharged to 250 cycles and have retained 75% of their original capacity, this is acceptable to me. I do not plan to discharge to 3.3v in actual use very often and I still believe that actual endurance determination should go to 3.3v/cell.

Watts are used to specifically take the voltage variable out of the equation.

I found that these batteries will not provide 5 amps for 1 hour but interestingly they do provide their nominal whr at <= 5 amps discharge since they start their discharge at nearly 4.2v. Their nominal whr is 5*3.7 or 18.5 whr. This means that the optimism you think you have turns to pessimism since at a 5 amp discharge, they provide more kwhr above 3.7v than below it.

I think that you are pessimistically using 40A for 20 lbs of thrust from my linked data. (also this is static thrust and does not really represent the rpm/amps/volts required to get 20 lbs of thrust at 30 mph). If you look at the graph provided, 20 lbs of static thrust can be interpolated to about 1500 watts. Lets say that this will occur at about 44.5V (another educated guess by looking at the data table). So, 1500/44.5 = 33.7 amps

And at discharge to 100%, using your method would be 67.4/5 or 13.5 times the 4 batteries used in the test per hour of endurance or 54 packs per hour.

And 96 would last 96/54 or 1.8 hrs. This is actually closer than I expected since another large discrepancy between this test data and the 96 pack I have used is that I am using a 16s voltage on the motor and the fact that there are a lot more parallel legs in the 96 pack than the 4 pack of the test. This means that there will be much less voltage sag at the same power levels and the batteries should provide more whr because of this.

I am not as happy with this AmpHr reasoning as my Whr reasoning since more assumptions were made, especially about conditions at 30 mph. In my whr scenario the only use of the test data was to determine that the prop/motor combination seemed to have the efficiency required at the expected cruise thrust.

Am I clearing things up or clouding them over?

Dale
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Last edited by KiloOne; Jun 23, 2011 at 06:01 PM.
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Old Jun 23, 2011, 07:08 PM
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United States, CA, Shingle Springs
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This is good stuff - thanks Dale. So, it sounds like the reason we have to limit discharge in RC applications is due to the very high amp draw from the batteries, and they can actually be discharged much lower at low discharge rates.

Your explanation for the using Whr is quite clear and I understand now. This is good news to me as I was thinking that the weight required to get a decent endurance for the paramotor would be excessive, but using your assumptions it looks much better.

The data I saw on the Joby motor site indicates that those motors are almost 95% efficient which should be considerably better than the Turnigy test data. I'm looking forward to seeing what you come up with in your testing.

Thanks for taking the time to explain.

Jeff
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Old Jun 23, 2011, 07:29 PM
Dale
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Jeff,

I think that the most important data to have for your paramotor would be to have the glide ratio at the speed that you would want your endurance at (something I don't have but should have). Most likely the best L/D speed and then the endurance numbers, for a first approximation, are pretty simple as you can see.

Dale
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Old Jun 23, 2011, 07:50 PM
Dale
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The easiest CG measuring system!

I just made a hook that slides along the boom tube and attaches to the hoist.
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Last edited by KiloOne; Jun 24, 2011 at 09:22 AM.
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Old Jun 23, 2011, 09:32 PM
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United States, CA, Shingle Springs
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Quote:
Originally Posted by KiloOne View Post
Jeff,

I think that the most important data to have for your paramotor would be to have the glide ratio at the speed that you would want your endurance at (something I don't have but should have). Most likely the best L/D speed and then the endurance numbers, for a first approximation, are pretty simple as you can see.

Dale
Dale,

I have approximate thrust data required to maintain level flight. I recorded the rpm required during flight with the existing gas engine, then put it on a thrust test bed on the ground. Since my flying speed is quite slow, about 24 mph, I don't think the difference between flying and static will be significant. This indicated I need about 45 lbs of thrust.

I'm interested in the larger JM2S motor because I need to swing a larger prop to make this work, and that motor turns at 2500 - 3000 rpm. Joby said they have tested it with a 48". That should have a much higher prop efficiency than the small props and I'll need it because the paraglider L/D is only about 6 to 1 with a pilot/motor/cage hanging below it.

Jeff
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