100% scale, Electric Lazair - Page 10 - RC Groups
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Jun 10, 2011, 11:56 AM
rebell's Avatar
Yellowbird, do you have pictures of how you use a duct for cooling? I try all sorts of methods and combinations thereof to cool my motors and am always willing to try different methods.
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Jun 10, 2011, 01:39 PM
Registered User
Here is a pic I have on hand of an out runner and one of a baffle I added to my Neu motor. The motor face is set back behind the cowl face. The dummy radial which is mounted on thin ply fits tight around the motor. The only air that can go through the cowl has to travel through the stator. This made a huge difference on motor temp. This also works well on my P47 and P39. If the the motor is 90% efficient and peak watts are around 6000 then 600 watts of heat is produced. Sure it is not that simple of an equation but you get the idea. ESC cooling has to be monitored but Castle only requires 5 mph airflow so it has never been an issue for me.

I did something similar to my Neu motor set up because it was getting hot and it worked great. Motor now comes down almost ambient. Helped that the built in fan blades departed from the shaft at some point.
Jun 10, 2011, 11:47 PM
Registered User

Battery Choice?

G'day Dale. I'm hoping to make the pilgrimage to Oshkosh this year (I'm from Sydney Australia) & see your creation.
In the meantime I'm intrigued by your battery choice. I've been loitering-with-intent at the fringes of electric cars. Conventional wisdom in the home brew end of this genre uses big 3.7v 40-100Ah submarine batteries (Thundershy etc) that work out to about $350/kwh & about 11kg/kwh
I like the Tesla approach better. Ordinary 2Ah 18650 format commodity cells (laptop batteries) come in at $250/kwh & 5.6kg/kwh around 2/3 the price & half the weight. They don't have the peak discharge of the traction batteries but run OK at the 0.5C rate for a 2 hour drive.
If I read your post correct your 14.8v 5Ah 430g zippys @ $32.89 ea work out to $440/kwh & 5.8kg/kwh? Given the energy density is about the same as the regular 18650s I'm thinking you paid extra for the higher discharge capability, for take offs?
Jun 12, 2011, 08:28 AM
Dale Kramer
KiloOne's Avatar

You would be surprised at how much prop noise there is still. It is quiet but still makes me consider wearing a helmet with ear cover.


I agree completely that cooling systems are lacking in these motors. The CA120-70 motors that I have been using has very little room for airflow through the stators and the aluminum bearing hub that the stator is bonded to does not have many cooling passages either. The cowl-less installation I have helps out a bit but makes it difficult to add ducting to force air through the center of the motor where it is needed. I still believe that these HK motors could be made to fly the Lazair with some more attention given to cooling but I just do not want to rewind them again and the Joby inrunner design appears to me to address the cooling issues directly. I am glad to see that ducting has worked so well for you and if I have to add some on the Joby motors, the inrunner design will make this easier.


Hope to see you at Oshkosh assuming I get there.

Isn't the 18650 designation a cylindrical size specification (18 mm dia x 65 mm long)?

Can you tell us where to buy the 18650's at $250/kwh & 5.6kg/kwh and point us to charge/discharge curves?

In my installation I am not pushing the discharge rate of my cells since even at full power I am only discharging at 3-4c.

Jun 12, 2011, 10:26 AM
Registered User
Right, 18650 is 18mm x 65mm and the A123 18650 cells are nominally 1100mAh but really closer to 900mAh. My understanding is that the Tesla uses the A123 26650 2.3Ah cells which retail at around $8 but can be sourced from China for less.

Considering each cell to be: $8, 2.3Ah, 2.8V under load and 70g that gives around 6Wh/cell => $1300/kWh and 12kg/kwh.
Jun 13, 2011, 01:27 AM
Zero Expo
typeRA's Avatar
Originally Posted by kgfly
...2.8V under load...
That sounds awfully low for a 1C discharge rate.
Jun 13, 2011, 03:08 AM
Registered User
You are right, I use them at more like 15C to 20C. So call it 3.2V at 1C discharge, that doesn't change the figures that much:

$8, 2.3Ah, 3.2V @ 1C load and 70g gives around 7.3Wh/cell => $1100/kWh and 10kg/kWh
Jun 14, 2011, 07:03 PM

Motor Cooling

KiloOne (Dale)

Good luck with your electric developments & congrats. on your maiden flight. We had some trouble with motors overheating too (our's is the UK electric Lazair) - we found that model motors just can't take the sustained power required for larger aircraft (we let smoke out of a couple !) until fitting a centrifugal fan on the back - made from FR4 PCB material. See detail of fan on back of Predator 30 motor. We can run these at about 5kW continuous with 32x12 3-blade props. - oversized for the motor but work well at ~4,000rpm. We also have auto power control to maintain the motor windings below 150 deg.C. We have done the same thing on a Hacker 200, which is on a hybrid gas/electric Alatus motorglider - this burnt out and had to be rewound with more copper and thermally conductive epoxy potting to get the heat out. Seems good now for about 8kW continuous at 7000rpm. We went for large LiPo cells - I'd be interested to hear how you get on with lots of small ones; I'm thinking of going that route for higher power/weight ratio. Jobys motors look promising too - good that we're getting more choice as time progresses; may still need forced-air cooling though for high continuous power.

Paul (R)
Jun 15, 2011, 01:39 AM
rebell's Avatar
The difference with using the same motor in a model is that you stand on one place and fly only short distances within line of comfortable vision with short power bursts with lots of low power in turns. With man carrying planes you fly long distance at sustained power setting which makes a big difference. How good the model motor might be, it is not designed for that.

I like your cooling system. It looks like you have a double aluminium plate at the back for transferring the heat to for cooling. If it is that, I did that on some of my high stressed motors in models as well.
Jun 19, 2011, 09:12 AM
Registered User

Motor cooling

I agree with Hybridplane. The thermal conductive epoxy is a big plus for heat proplems. I've used it in industry with large motors. They where under alot of high torgue stress and heat would do them in. I went with the epoxy and had it vacumm inpregnated. Our heat problems with the windings subsided. liquid cooling would solve even more.

KiloOne (Dale)
I have a virgin Lazair III . It has never been flown. Always thought a electric powerd Lazair would be possible.This has my creative jucies flowing. Good luck . Looking forward to see it at Oshkosh!
Jun 19, 2011, 09:57 AM
Dale Kramer
KiloOne's Avatar
Hi Paul,

Great to hear you are still developing your electric Lazair. Wish I had a temp input to my Jeti controllers that would limit power based on temp. They do have an internal temp limit that is software adjustable, tried to figure out a way to parallel that sensor with another that would effectively limit both controller and motor but could not get access to the components.


I've spent a good part of my life taking things that are not supposed to do something and then making them do it. A lot of motors would work fine on a J3 Cub model and not on a screaming 3D. No one knows the limits of these HK motors. I am probing those limits without being hindered by the warning 'but they are not designed for that'. I still believe I could plod on and make the HK motors work fine for the Lazair (and may rewind them someday, I just bought the wire).


I do hope that the Joby motors will work well enough to bring the Lazair to Oshkosh. Still on track for starting their courier journey to me late this week from CA.

Last edited by KiloOne; Jun 19, 2011 at 10:19 AM.
Jun 19, 2011, 10:08 AM
Dale Kramer
KiloOne's Avatar

Thermally conductive potting

As this is being mentioned lately, I did consider it when I rewound my motors.

I ended up just spraying a green McMaster Carr supplied 'Sprayon' varnish for two reasons, the first was expediency and the second was that I could not come up with a simple solution to heat dissipation from the potting compound to the airflow.

Sure the potting compound would draw heat from the windings but where would it go? The more potting compound applied, the less airflow there would be around it. I agree that the potting compound would help greatly as long as the compound ends up transferring the heat to some sort of heat sink in the airflow.

The Joby motors appear to have that heatsink and if the supplied heat flow path to the heatsink from the windings is not adequate then I think a good thermally conductive potting compound will be in order.

Is this thinking flawed?

Jun 20, 2011, 11:00 AM
Fltofancy's Avatar

Weight and Balance

What does your weight and balance look like in this flying configuration?

Jun 20, 2011, 09:25 PM
dalbert02's Avatar
I have not been this excited about a build project since the "Pink Floyd" foamie several years ago. Way to go Dale! Awesome!
Jun 20, 2011, 10:26 PM
Registered User
I can only get 8wh out of the 18650 Li polymer generics if I keep the discharge rate to 0.5C Dale. Anyway hope you can get to Oshkosh on your zippys, sounds like you need a bit circular heatsink for those motors like the ones we used to clip onto 2N3055s transistors only bigger.

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