# Electric Power Systems

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#### bmcj

##### Well-Known Member
HBA Supporter
Ok, since I am relatively naive in the area of electric propulsion, I would like to pose a question at a very basic level.

Ignoring finer points like charge time, How much would it cost to put together a homemade e-power system? For starters, I am assuming (maybe incorrectly) that you would want a brushless DC motor.

This is not for a specific goal, but for general education and perhaps to inspire someone down a path they had not considered. I would like to see your estimates on the following (for battery purposes, let's say we are shooting for 2 hours flight time):

30 HP
Cost: motor = ?, batteries= ?, controller= ?
Weight: motor = ?, batteries= ?, controller= ?

60 HP
Cost: motor = ?, batteries= ?, controller= ?
Weight: motor = ?, batteries= ?, controller= ?

100 HP
Cost: motor = ?, batteries= ?, controller= ?
Weight: motor = ?, batteries= ?, controller= ?

The three power ratings will help show whether the relationships are linear.

Don't be afraid to go out of the box in search of lower cost... i.e. - can a washer motor and DC/AC inverter be used?

#### RonL

##### Well-Known Member
There used to be a rule of thumb when changing ICE power to electric, that a motor half the horsepower of the ICE could be used. Do you think that would have any bearing on what you are asking ?

Ron

#### BJC

##### Well-Known Member
HBA Supporter
There used to be a rule of thumb when changing ICE power to electric, that a motor half the horsepower of the ICE could be used. Do you think that would have any bearing on what you are asking ?

Ron
Replace a 200 HP ICE with a 100 HP EM and expect similar results?

Where did that rule of thumb come from? Must refer only to starting torque, and not power.

BJC

#### henryk

##### Well-Known Member
1HP=75 kg * 1 m /1 sec \equal for IC and Electric motors\

=electric motor have allmost constant torque \N*m\
=IC engine=have small torque at low rpm,and is growing with rpms...

#### RonL

##### Well-Known Member
Replace a 200 HP ICE with a 100 HP EM and expect similar results?

Where did that rule of thumb come from? Must refer only to starting torque, and not power.

BJC
I don't know how to explain it, I think it has to do with the almost 100% instant availability of maximum torque at any RPM.

#### BJC

##### Well-Known Member
HBA Supporter
Makes sense if starting torque is an issue, and a flywheel is not used, but power is power, and startiNg torque will not make up for having only half the power.

BJC

#### RonL

##### Well-Known Member
Makes sense if starting torque is an issue, and a flywheel is not used, but power is power, and startiNg torque will not make up for having only half the power.

BJC
Consider the gas spring characteristics of compressing air and combustion of fuel as a piston moves through a cycle. If electric could absorb heat, just think how different energy would be ?

But maybe we are drifting too far.

#### DeepStall

##### Well-Known Member
EV Hangar | Electric flight spoken here! (some juicy details paywalled) may answer some questions. www.pingbattery.com has specs on large LiFePO4 packs. Cost and weight got discouraging for me pretty quick, and I never found a suitable source for large motors that weren't seriously overbuilt (=HEAVY!) for industrial applications. I'm interested to see what anyone else has in their bookmarks.

##### Well-Known Member
Try Emrax and Front Electric Sustainer.

Motor power is highly dependent on duration (short bursts) and battery max drainage. For light batteries, power density, not energy density is the limit. Count on about 10 kg/kW for batteries...

Price doesn't really seem to scale, I get to about 5K for motor+controller and about 5K per 30 kg of batteries. For a 2 hour flight time in a low-drag sailplane (using 15 hp for cruise @ 100 mph), we're talking about 230 kg of batteries, no reserves, ZERO climb. Include climb and reserve and we're at 750+ lbs. For your typical Cessna that's another 10-fold more.

#### NoStealth

##### Well-Known Member
Here's a looooong thread on Dale Kramer's elektric lazair:
100% scale, Electric Lazair - RC Groups

As autoreply said. electric is do-able but not particularly practical or comparable for GA without an order of magnitude increase in battery energy density or aircraft drag clean-up.

For comparison, the energy density of gasoline is approximately 12.5kwh/kg (1kg=~.36 gallons) and 4stroke engines are roughly 25-35% efficient. (0.5#/hp/hour =~ 26%) check my numbers.
A lot of electric motors being used are roughly 90% efficient.

The 85kwh telsa car battery pack is made up of 7-8000 individual 18650 format lithium batteries that weigh roughly 1000# including wiring, packing, cooling, armor and fire protection schmoo.
Amazing Core Tesla Battery IP - 18650 Cell

The last time I looked, a lot of radio control batteries were in the $350-500 and 5-7kg / kwh. Thundersky batteries were$400 and 10kg / kwh
It will take about 80 x high end panasonic Li NCR18650B (3.7v 3400mah 47g) batteries to get a kwh at a little less than 4kg + wiring and packaging.

If and when some of these battery breakthroughs result in affordable metal air batteries hitting the air, then things might change.

The upcoming Phinergy aluminium air primary (non-rechargeable) battery is supposed to produce 3.5kwh / kg. Simply replace the aluminium plates being used as fuel.
Stuck in a small airport and outa aluminium, no problem, a few minutes with snips and a neglected spam can and you're good to go

#### Thalass

##### Member
Batteries are the main problem area. Motenergy make brushed DC motors with around 23kW peak output (1 minute) and around 11kW continuous output - which is probably where the whole "half the horsepower" thing came from. IC Engines are rated in peak power, electric motors are rated in continuous power (with peak given with a time limit). I suppose if you could cool the motor you could get the peak power out of it for longer, but that adds weight etc.

Specifically this is the Motenergy ME1003, which I think(?) would compete with the Hirth F-33 engine. I've been looking at the FP-202 lately...

Unfortunately the batteries aren't light, or cheap. A 72V pack of winston 40Ah cells weighs 34kg! A 72V pack of headway 10Ah cells will have 25% the energy, but only weigh 8kg. Unfortunately the winston pack will let you cruise at max continuous for 12 minutes, and the headways for... 3 minutes. I'm not 100% sure I haven't screwed that up though.

There was an article recently about Aluminium/graphite cells that have quite a bit of promise. And there was also an article about a Siemens motor that weighed 50kg and produced 260kW - which is impressive. So the future looks pretty good but is not quite there, yet.

I know he uses pretty small motors, but I'd like to know what kind of batteries the E-Lazair has.

Also: Thanks for that EV Hangar link, Deepstall!

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#### Highplains

##### Well-Known Member
There used to be a rule of thumb when changing ICE power to electric, that a motor half the horsepower of the ICE could be used. Do you think that would have any bearing on what you are asking ?

Ron
This might be true if we were talking about model airplane engines. The glow engines are rated at rpms's that they will never reach with normal sport sized props. So at the actual operating speeds, they produce about half the published power. Electrics typically use larger diameter props than the glow motors, so that adds to the overall efficiency. Some of the older model designs have suboptimal directional stability when converted to electric power due to too much disk area ahead of the CG, but often the RC pilot can't tell the difference due to lack of skill.

#### Nims11

##### Well-Known Member
There used to be a rule of thumb when changing ICE power to electric, that a motor half the horsepower of the ICE could be used. Do you think that would have any bearing on what you are asking ?
This rule of thumb was established by the auto conversion guys, but it does somewhat apply to electric aircraft. In an ICE auto, max power is rarely ever reached or used because of the transmission. When you're cruising down the highway in fifth gear, or whatever, your well below your max power. Max power in an ICE might be at around 6000 rpm, but you rarely go above 3000 rpm because of the tranny.

Unlike an ICE, electric motors rate their power by two measurements: peak power and max continuous power. Your peak power should be the same as your ICE conversion, but your max continuous power can be less, maybe half, potentially at least.

In an ICE powered airplane, you need max power at takeoff and steep climb, but in cruise (similar to a car) you are operating at less power. So your electric motor would need peak power for takeoff and climb but at cruise it would just need mcp.

So in other words, the rule of thumb should be that your electric motor's mcp can be half of the ICE max power.

#### Thalass

##### Member
I'd say your max continuous power should be whatever your aircraft's normal cruise power would be. The peak power might be only for 1 minute or something, but if you're somewhere between max continuous and peak power you ought to be able to sit there for longer. Say max peak for takeoff, then throttle back for climb to a point where you can sit there for ten or twenty minutes without overheating. Then cruise for the bulk of the flight. A decent aviation motor controller would take that into account, warning of overheat as well as warning for coming up to the time limit for your power setting.