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This is part of a series of articles put here, written by me to try and
explain electic flight to a newcomer. These articles are taken from the
spad combat
forum
in the electric section.
Glow to electric
conversions
What kind of equipment do I need for [insert type of plane here]
Motor
Glow substituting
Start with the size of electric motor you need. Then you can figure out
ESC/battery requirements to fit.
If you know what horsepower your glow motor would be, you’re
doing well. Here’s a way of converting horsepower to electric
motor size:
Horsepower X 745 X (100/effiency in percent) = power needed in watts.
(A rough guideline motor efficiency is roughly 85% although this will
increase around the midpoint and decrease at max throttle due to wasted
heat energy.)
If you had a 2.2 horsepower glow motor and you wanted equivalent
electrical power with a motor that is 90% effiecient, this is what you
do:
2.2 X 745 X (100/90) = 1821 watts. This sounds like a lot, but
remember, horsepower ratings are often grossly overrated by the
manufacturer and are obtained after precise tuning on a small prop. It
gives you a ballpark figure to aim for, however.
I’ve used this horsepower conversion idea to try to make a
set of formulas for converting DIRECTLY from glow size in CI to
electric power in watts. Glow size is the CI of the engine multiplied
by a hundred, i.e. a .15 has a “glow size” of 15.
Hot BB tuned glow motor with pipe: (glow size/35) X 745 X
(100/efficiency in percent) = power in watts needed
Tired BB or hot bushed glow motor: (glow size/45) X 745 X
(100/efficiency in percent) = power in watts needed
Tired bushed glow motor: (glow size/55) X 745 X (100/efficiency in
percent) = power in watts needed
These are ROUGH GUIDES. Remember, brushless outrunners often swing very
large props at slower speeds, giving them better thrust but less top
speed. More often than not an electric motor with mathematically
equivalent power to a glow motor will give out more power, because
there is no problem with tuning, loss of compression or different fuels
etc. Electric motors will also always perform better at maintaining rpm
in high G turns.
Now you know the power of motor you will need to reach an equivalent
glow engine you need to have a careful think.
Do you want your motor to be able to deliver this power continuously,
or will you be using it in bursts. I personally would prefer to set up
all my equipment so that everything is operating well within limits,
that maximises efficiency and minimises risk of blowing something up.
Power, current
and voltage – choosing a battery pack
Your motor might say something like: Efficiency current: 12-24 amps,
Max burst current: 32 amps.
You need to use a prop that will draw your chosen number of amps. You
can do this by trial and error with an ammeter, or choose a prop
recommended by the motor manufacturer.
It’s reasonable to choose a number slightly more than the
efficiency current for your max throttle setting, depending on how hard
you are going to push things, quality of the motor etc. I’d
set my limit at 28 amps but it’s really up to you.
You want to stay within the limits of the current draw (between 12 and
28 amps in this case) whilst attaining the power you need. Say you need
300 watts.
You need a voltage that will give you the right power whilst not
pushing too many amps.
Watts = amps X volts so watts/amps = volts, and watts/volts = amps.
To get your motor to produce 300 watts on 2S lipo cells (2 X 3.7 = 7.4
volts) you would need:
300/7.4 = 40 amps. WAY to much, try a higher voltage.
More voltage and less current is nearly always better, because more
current produces more heat and wastes energy.
Try 3S lipo cells (11.1 volts). 300/11.1 = 27 amps. This is above the
max effiency but would work ok.
If the motor is able to handle 4S (14.8 volts) then you could use this,
and:
300/14.8 = 20 amps. This lower current draw means your motor will be
more efficient overall because it falls smack in the middle of the
motor’s efficiency rating. Don’t exceed the
motor’s maximum voltage rating though. I would use 4S on this
motor if the specs said it could handle it.
The right prop
The next thing you need to do is make sure your motor draws the amount
of current (amps) that you have planned it to. The ONLY way to do this
is to hook it up to an ammeter and try different props until you find
one the right size that draws the right amount of current. Remember, a
bigger prop draws more current than a smaller one. Normally if you
follow the seller’s recommendations with battery pack and
prop size, you will get the right current draw automatically.
Now here’s a useful little trick. Once you know what prop
size you want (let’s use a 14x3) you can convert this into
any diameter of prop and alter the pitch angle to maintain the same
load on the motor. This is useful because you might know that a 14x3
might draw the right amount of current, but because the pitch is so
shallow, you will hardly get any top speed.
If you want more top speed and less acceleration, use a smaller prop.
If you want lower top speed and more acceleration, use a bigger prop.
Right, our example 14x3 prop gives a certain load value. This prop
moves 14 X 14 X pi X 3 = 1847 cubic inches of air per revolution
(theoretically). We can ignore pi when comparing props because it is a
constant and cancels out, so we can get a “load
rating” for any prop by doing the following:
Diameter X diameter X pitch = load rating.
The load rating of a 14x3 is 14 X 14 X 3 = 588
Any other prop with a load rating of around 588 will draw the same
current and put the same load on the motor. You could use a 13x3.5, a
12x4, a 11x5, a 10x6, a 9x7, an 8x9 etc etc etc. You can choose any
prop which has roughly the same load characteristics.
Battery pack
capacity
Now you know what voltage you need, on to capacity.
When deciding about capacity, the first thing you need to ask yourself
is “how long do I want this plane to fly for”.
Say you want it to fly for 10 minutes. Now you need your average
throttle usage. Say it is on about 60% on average, well, you need to
multiply the time you want by the percentage so 10 minutes X 60% = 6
minutes at full throttle. Gettit? You need to find the equivalent time
your motor will be at full throttle and work from that.
Our example will be a combat round, you want it to last at least 6
minutes on full throttle. We will use our 300 watt example motor on 4S,
so it draws 20 amps at full throttle.
The battery pack you use must therefore be able to supply 20 amps for 6
minutes. Here is a simple formula to find what mah rating you need.
TIME IS IN MINUTES!!!
Amps X (time/60) X 1000 = MAH rating
So for this example, 20 amps X 6/60 X 1000 = 2000mah. For this motor to
do what you want it to, you need a 2000mah pack.
Now you’ve got enough voltage and enough capacity, but you
need enough current delivery rate, or C rating. To find out what C
rating you need, follow this formula:
Max amps / (MAH/1000) = C rating. For this pack, you would need
20 / (2000/1000) = 10C. This pack would have to have a 10C or greater
max discharge rate. It’s probably best to just make sure and
go a couple over, say 12C or more. It prolongs the life of the lipo
pack if it is working well within its limits.
There you have it, now you know what motor and battery pack you need.
Last but not least, on to the ESC
Choosing the
Right ESC
You need an ESC that can handle MORE current than you will be using
with the motor at maximum power. If you are drawing 30 amps, a 30 amp
ESC won't cut it. You need a 35 amp ESC or more, just to make sure you
have enough leeway. Pushing equipment to its limits will just result in
overheating, poor performance or breaking stuff
So for our example power system, a 300 watt motor with a 2000mah 4s1p
lipo pack, drawing 20 amps, a 30 amp ESC would work just fine.
That's all you need to think about, pretty simple compared to most of
the other stuff. Just don't load too many servos on a receiver and
expect the BEC on the ESC to cope with it - they can only manage up to
about four standard servos before they overheat.
Other bits
The only other stuff you need to worry about is how you connect it all
together. You can't go wrong with 4mm gold connectors, they're good up
to about 60 amps and have less resistance than 12 guage wire. T
connectors are also a good choice because you can't get them the wrong
way round.
I hope this helped somebody. If anybody would like to know what
equipment they need for a certain glow size/flight duration/plane type,
just ask, and I'll try to help.
Disclaimer
I don't claim to be an expert. All of this information is offered on an
"as is" basis. All text here is hereby released under the GNU FREE
DOCUMENTATION LICENSE for anybody to use, copy or alter, commercially
or otherwise, as long as, AND ONLY AS LONG AS THIS TEXT IS PRESERVED.
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