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-   -   Clueless about Electric (https://www.rcuniverse.com/forum/electric-general-discussion-106/11687663-clueless-about-electric.html)

jimmy7432 02-11-2021 06:16 PM

Clueless about Electric
Been always involved with glow and gas airplanes. Big surprise, right? Batteries have been pretty basic just powering receiver and servos. At least it has always seemed that way until now. After reading several articles and watching several videos I for some reason am having a hard time comprehending everything about the set up, charging, and compatibility between all components on an electric plane. The later being the most confusing. Most of the material I have studied involves a lot of description of the numbers involved. The thing I struggle with the most is using those numbers to know what engine should be used for what battery, or what battery should be used for what engine, what speed controller to use, what servo and or receiver to use. I built all of my planes from kits and of course the first plane I decided to build from plans is electric. I also do not know how much if any ventilation for cooling is needed for the battery. For me I guess its kind of like what came first the chicken or the egg. What do I start out with the battery or the engine. I started with the engine. The plane is a sport model, 52" wingspan, 3lbs 10oz. 4 channel. I got a Power 25 Brushless outrunner motor 870Kv. The motor will be used as a pusher with an 11x7 3 blade pusher prop. What ever help I could get with what the other components suggested are and why these are suggested. Please let me know what other info is needed to make these decisions.

rgburrill 02-12-2021 05:28 AM

First thing to learn is it's a motor, not an engine. Second thing you got right already, start with the motor. The motor specs should give you three numbers, max voltage, max current and max watts. These numbers are used to select the ESC and battery. The higher the voltage the faster the prop will turn (that's where that 870Kv comes in, 870 RPM per volt)..Incidentally, the higher the voltage, the lower the current which can help in battery cost, and wiring. Third thin is there are several good rc electric motor thrust calculators on the Internet. I use ecalc.ch. It can get confusing but can help in selecting your components. I've seen others on this site comment on it as well - perhaps they will comment here.

ron_van_sommeren 02-12-2021 06:33 PM

Originally Posted by jimmy7432 (Post 12662901)
... What do I start out with the battery or the engine. ...

One way is to start with the choice of prop and desired rpm. Those two together determine power needed. You need to have a good idea about prop and rpm though.
Or ... you start with power needed based on plane mass and type, see the quote below.

Voltage does not matter, Kv does not matter, but their product matters, greatly.
A 1000rpm/volt motor on 10volt will try to run just as fast as a 500rpm/volt motor on 20volt.
Their are practical and equipment considerations of course.

Note that the velocity konstant Kv says nothing about max.power and maxcurrent rating, rpm, torque, rpm range, quality, efficiency, ...

Below an excellent quote about motor selection.
brushless motors Kv?.

Originally Posted by scirocco (Post 27290157)
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

ron_van_sommeren 02-12-2021 07:22 PM

Originally Posted by rgburrill (Post 12662960)
... the higher the voltage, the lower the current which can help in battery cost ...

The plane uses a certain amount of energy for a complete flight. Energy '=' batterymass, voltage has no effect on total batterymass.
A 2s 2000mAh packs weighs the same as a 4s 1000mAh pack, both story same energy and both weigh the same, both cost about the same.

Vriendelijke groeten ;) en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

ron_van_sommeren 02-12-2021 07:25 PM

Without a watt-meter you are in the dark.
Until something starts to glow :( ;)
Vriendelijke groeten ;) en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

skyrydr2 03-14-2021 01:33 PM

I'm glad to see I'm not the only one finding electric stuff a mind numbing experience.
I am wondering about charging batteries and can I charge my LiFe batteries with my lipo charger if I adjust the max voltage and current?
I'm just getting into electric stuff because I can't fly my big stuff yet... I am just now converting my lipos to LiFe 6.6v for all my fuel powered planes and hope I can charge them with my existing equipment?

ron_van_sommeren 03-15-2021 02:41 PM

No, that is not possible. Not only have LiPo and LiFe different cell voltages but also different charge algorithms and battery full detection.

If you have a LiPo charger it probably also has LiFe, NiMH/NiCd and Pb/lead charging programs. Charger make/type?

skyrydr2 03-15-2021 03:34 PM

I know the voltage is different at peak but I don't believe the charger I have is new enough to have the algorithm to charge LiFe batteries as it is an old Duratrax intelipeak ice.

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