Converting a MONI motor glider to electric

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helno
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Converting a MONI motor glider to electric

Post by helno » Jun 19, 2017 8:51 pm

It was suggested that this project should have it's own thread.

I have a MONI single seat motorglider. It is not a particularly high performance glider and is more of an aerobatic touring glider.

This is a very long term project and as far as electric power goes it in the back of the envelope stage. I am hoping to keep the conversion cost below $10k (probably wishful thinking) and would like to have 30 kw available for take off. Range is not an issue as this is a playing around aircraft.

Image

These little planes have been flown with electric power previously using a Lynch brushed motor but performance appears to be pretty mild and it appears that this is pushing the limits of that motor.

https://www.youtube.com/watch?v=t2IrNIExHbM

There have been substantial improvements in power systems since this one was converted so I hope that I can develop a working formula that would work for other similar conversions.

I'll have to get out the scale and the weight and balance calcuations for this individual aircraft but it would appear that there is between 160-180 lbs that could be budgeted for a power system. The current power plant is a 25 hp KFM 107e with about 50 lbs of ballast attached to it. I am working on getting the KFM running at the moment to fly the aircraft in it's original form to get a feel for its performance and handling.

My back the envelope planning is looking at the Emrax 188 turning a 38" prop at around 6000 rpm. This is less than ideal but prop clearance is an issue with this aircraft. The high voltage versions seems capable of running at these speeds at the desired power level.

Battery would be an 80s10p pack of 3 ah 18650's arranged in blocks of 20s10p.

Probably going to spend quite a bit of time talking to the few electric airplane people at Oshkosh.

My background is that I work in the power generation industry as an instrumentation tech so electricity is something I am pretty familiar with.

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jonescg
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Re: Converting a MONI motor glider to electric

Post by jonescg » Jun 19, 2017 9:49 pm

So would you be towed up with power or do you wish to take off with power? If so, I would think you'd need some reasonably powerful cells (high discharge) so 18650s might not be the best. But if you were to be towed up, the continuous drain would be low enough that you could use it to supplement any thermals you find.

I'd definitely avoid a brushed motor in this situation - they just don't have the efficiencies you need at full power.

Very cool anyway. Farfle here on the forum is building an electric single-seater with a Zero powertrain. Should be interesting!

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Re: Converting a MONI motor glider to electric

Post by Ohbse » Jun 19, 2017 11:52 pm

Cool project!

Why the HV option? You could run the same 20s modules in parallel with the LV winding, achieve the same RPM/power/torque but have a dramatically more simple battery monitoring solution with a lot more inherent safety. See something like the Mobipus 72600 which will comfortably run 30kw continuous and a lot more peak. Would need to see the updated Emrax 188 spec sheet with all the missing values filled it to confirm, but should work fine.

Battery wise, you're talking about 800 cells. As a rule Samsung 30Q cells are capable of 70 watt bursts on a regular basis, however they probably will get warm under a 38 watt constant discharge. Still workable, but on the verge of too much. Design for a 30w/cell constant upper limit, that's obviously independent of whatever voltage you specify. I've burst these cells to ~120w 4 second peaks with no long term ill effects, but you definitely don't want to get them too hot.

helno
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Re: Converting a MONI motor glider to electric

Post by helno » Jun 20, 2017 8:09 pm

I am looking at the high voltage versions of the motor because it puts me in the sweet spot for the target RPM. The lower voltage windings want to spin much faster to make rated power. I'd like to keep it direct drive for simplicity.

The 30 kw figure would only be for takeoff and climbout would probably occur at under 25 kw since the prop unloads once the plane is moving, cruise power would be around 6 kw. So it should be within the range of possibility for the cells you mentioned. I would love to make a pack out of larger pouch cells but it seems that nothing beats 18650's for energy density, it is going to be a lot of work to build this pack.

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Re: Converting a MONI motor glider to electric

Post by Ohbse » Jun 20, 2017 8:28 pm

helno wrote:I am looking at the high voltage versions of the motor because it puts me in the sweet spot for the target RPM. The lower voltage windings want to spin much faster to make rated power. I'd like to keep it direct drive for simplicity.
I don't understand your thinking here, the KV of the LV wind is 88, the KV on HV is 22. With your proposed pack no load RPM would be literally identical at 80s/HV vs 20s/LV? All of the power ratings at various RPM's are identical according the datasheet I'm reading. It makes sense, given that it's just a variation on the termination of the same copper quantity. Why would you want to deal with potentially very lethal voltage levels when you can achieve the same outcome on much less? Not to mention the lower voltage controllers are substantially more affordable. Only downside I can think of is dealing with more substantial conductors, but that's really minor in the scheme of things.

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Re: Converting a MONI motor glider to electric

Post by helno » Jun 20, 2017 10:15 pm

Anything over 50 volts at these power levels is lethal.

Perhaps it is just because in the field I work in lower current is always preferred over lower voltage. Any load over 200 hp jumps from 600 VAC to 4160 to keep the fault currents manageable.

Heavy gauge wiring aside it just seems that a lower current is easier to deal with but I may be completely off base when it comes to brushless motors in this size.

jonescg wrote:So would you be towed up with power or do you wish to take off with power? If so, I would think you'd need some reasonably powerful cells (high discharge) so 18650s might not be the best. But if you were to be towed up, the continuous drain would be low enough that you could use it to supplement any thermals you find.

Very cool anyway. Farfle here on the forum is building an electric single-seater with a Zero powertrain. Should be interesting!
Plan is to be self launching. Have you got a link to Farfle's build thread?

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Re: Converting a MONI motor glider to electric

Post by jonescg » Jun 20, 2017 10:49 pm

helno wrote:Anything over 50 volts at these power levels is lethal.

Perhaps it is just because in the field I work in lower current is always preferred over lower voltage. Any load over 200 hp jumps from 600 VAC to 4160 to keep the fault currents manageable.

Heavy gauge wiring aside it just seems that a lower current is easier to deal with but I may be completely off base when it comes to brushless motors in this size.

jonescg wrote:So would you be towed up with power or do you wish to take off with power? If so, I would think you'd need some reasonably powerful cells (high discharge) so 18650s might not be the best. But if you were to be towed up, the continuous drain would be low enough that you could use it to supplement any thermals you find.

Very cool anyway. Farfle here on the forum is building an electric single-seater with a Zero powertrain. Should be interesting!
Plan is to be self launching. Have you got a link to Farfle's build thread?

He's on facebook: https://www.facebook.com/photo.php?fbid ... =3&theater But I have pestered him to start a build thread here.

As for energy density - yes 18650s win but their power suffers. It's the old asymptotic relationship between power and energy. I think 180-190 Wh/kg is in the sweet spot for acceptable C-rates (10 C cont, 20C peak) in pouch cells. The reason I will always err on the side of pouches is I have better control over the pack's total impedance. I can use thicker busbars for more power and the cells don't suffer excess heating. With 18650s, you will always be limited by the ability for the pack to deliver current through nickel strips. I guess in a fall-or-fly situation I'd always appreciate access to power.

And fear high voltages not (best Yoda impression). If done properly and isolation is incorporated into the design, you can safely manage up to 1000 volts without any exotic components. Like you, I decided on the high voltage option because it made the best use of the motor and controller's potential.

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Re: Converting a MONI motor glider to electric

Post by helno » Jun 21, 2017 9:21 pm

jonescg wrote:If done properly and isolation is incorporated into the design, you can safely manage up to 1000 volts without any exotic components.
I work on high voltage equipment for a living. I get to climb inside and inspect the hydrogen seals on 24 kv bushings about twice a year. The neutral point on our generators carry around 21000 amps at rated power.

Lets just say that I have a very healthy respect for high energy levels. To be honest I kind of look forward to getting rid of the 10 gallons of avgas that sits between my legs in this plane.

Sadly I cannot view your buddies facebook post.

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Re: Converting a MONI motor glider to electric

Post by jonescg » Jun 22, 2017 5:15 am

helno wrote:
jonescg wrote:If done properly and isolation is incorporated into the design, you can safely manage up to 1000 volts without any exotic components.
I work on high voltage equipment for a living. I get to climb inside and inspect the hydrogen seals on 24 kv bushings about twice a year. The neutral point on our generators carry around 21000 amps at rated power.

Lets just say that I have a very healthy respect for high energy levels. To be honest I kind of look forward to getting rid of the 10 gallons of avgas that sits between my legs in this plane.

Sadly I cannot view your buddies facebook post.

It's okay, he's here now :)
https://endless-sphere.com/forums/viewt ... 7#p1298367

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