electric RC brushless motor
am going to try to keep this as simple as possible and present you with
just the needed information. some of this information you may already
know and may wish to skip sections with "BASICS"
RC brushless motor has 3 wires. The voltage to these wires are PULSED
in a particular pattern that requires a microcontrolled Electronics
Speed Controller (ESC). The ESC takes the power from your
flightpack and controls these voltages by the input comming from the
servo wire that goes to your Rx. The ESC May or May not have
built in Battery Eliminator Circuit. If not, you need to
the Rx with power through a seperate battery or seperate BEC. Often
smaller ESCs will have a Linear Regulator BEC which is inexpensive but
can heat up (research this seperatly). External BECs are often a
"switcher" type which heat up less.
motors do not have brushes or commutators. They have wires and magnets.
The wires are energized through the power from the ESC and
create an electromagnetic field that opposes and attracts the magnets
on the motor. One of the more commonly used motor types used
(which I focus on) is the outrunner. The outrunner has the
windings on the inside and the magnets on the outside. The outside
spins while the inside is stationary. The Inner core is
to one end of the motor where the wires attach and where the motor
mount connects to. the outter core is often called the
Because if you pull the motor apart, the outter part looks like a can
with an open top (BELL).
can mount an out runner from One side only. The side where the wires
are. The main part of the motor (BELL) will spin. But there
are 2 directions you can face the motor from
the mounting point.
1) If you have a prop adaptor that mounts to the BELL side, then you
often use an X mount and mount the otherside to a firewall.
If you have a prop adaptor that mounts to the stationary side, then you
have to have a long shaft on the stationary side and poke it through a
firewall (or special motor mount) and then attach the prop
from the front.
sizing your ESC, you need
to think about a few things.
size (will it fit where you want to put it?).
Max continuous current (will it handle the amount of current you will
put through it?), To figure out max current, you first need to size
your prop and calculate the current draw. It is best to over
your ESC to deal with situations where your current draw exceeds
calculations (bent shaft, out of balance prop, ground strikes etc).
3) Max LiPo Cell count (if you want to put a 4S lipo to it, it had
better support up to 4S Lipos)
Does it have a built in BEC (see ESC Basics) or will you want to supply
external power. Many people agree that anything over 3lbs of aircraft
or over 4S flightpack require a seperate switching BEC or Rx battery.
5) Airflow, If you don't have good airflow across the heatsink, you may
want to over rate the ESC to compensate.
6) Look to get an ESC that is 20-30% more than the max continous
current you will run.
LIPO Battery info Basics
Click here for some LiPo info
SIZING MOTOR FOR YOUR AIRPLANE
we finally get to the good stuff. What you have been wanting to know.
The first thing you need to know is the approximate weight of your
aircraft and how you want to fly it. This will help determine
target Wattage you will want. Here is a general table of Size
Wattage that many people go by.
Sport/Training : 75-125 Watts per lb
Aerobatic/Warbird flying : 125-150 Watts per lb
Extreme 3D flying : 150-200 Watts per lb
numbers are very generic and can be massaged with your prop selection.
For example, go a larger diameter prop and get more low end
thrust and vertical climb. Go with a higher pitch prop and lower
diameter and get more top end speed. You can make a plane go
on low diameter and high pitch on 100W/lb , but it won't climb
vertically as well as a higher diameter/low pitch prop using the same
wattage. What is high diameter or high pitch?
I tend to give
the breaking point at a 2x ratio. If the diameter is greater than 2x
the pitch, then it begins the high diameter range (12x6). When the
Pitch is passing 75% of the diameter (12x9) then you are getting into a
high pitch prop. I have a few light Foamies/funflies that run
an 11x3.8 slow fly prop and they climb forever on little wattage, but
it does not go fast at all horizontally and comes down in a steep dive
almost at the same speed it goes horizontally. When you get
the prop calculator below, look at how changing diameter vs. Pitch
affects static thrust and max level flight speed. Increase Pitch one
degree and decrease diameter and see how the static thrust and max
level flight speed changes.
OK, so you have an approximate
Wattage you want to look at. The next part is to get familiar
with a Motor Calculator program. I like to use one of thse calculators. There are
others (including ones you can buy) but these are free and gets me close
For my favorate Airplane motors (Monster Power and Tacon Big Foot)
I pick out a motor
that needs the wattage requirements I have estimated.
1) Start with the ADAMONE
calc and set the ESC to custom and a very large Current rating.
Otherwise the warnings get annoying.
2) Set the battery to Custom and select your first guess at cell count.
We will tweek the battery later.
Select a motor that closely matches the motor you have selected.
In this case for example, we'll size a Monster Power 32.
the E-flite Power 32 motor from the Motor drop down box.
The E-flite Power 32 is close but not the same Kv as the Monster Power
32. Now Select Custom Motor and the E-Flite 32 numbers stay
there, but you can now change some parameters. Switch the Kv
match the motor you chose (770 in this case). (Note: true Kv
the motor may differ by up to 10%). You can change some of the other
parameters but the difference is not that significant compared to the
error of using a calc to begin with.
5) Now select a prop from the
drop down list. APCe prop and APC slow fly are the most
used props. Most electric props will behave like an APC E prop or an
APC Slow Fly anyway. If you don't see the prop you want to
then select something close, then switch to CUSTOM for the prop
selection and change the Prop diameter/Pitch.
thing to keep in mind on prop selection is to make sure you can BUY the
prop you are selecting. There are more props available than what is
listed and you may want to inventory props at your local hobby shop or
go online to a supplier like Graves RC to see what they have as
you are doing your prop selection.
6) As you are tweeking your prop selection, keep some things in mind.
Now, you have some numbers to look at. In general you want to
purchase an ESC that allows a continuous current that exceeds the
continuous current draw that you will run. Since the
is not perfect, you may find that your current draw (and RPM) may be
ABOVE what the calculator says on a fresh battery pack. A good rule of
thumb is to add 20%-30% more ESC than you need. (calc shows 30A? buy a
40A ESC). There is only the weight and size pentaly of a
ESC. Using a 60A ESC when you only need 30A is not going to
the motor or give you less power (it might actually give you more power
because the bigger ESC will have a smaller resistance, but not much).
- Total input Wattage
- Static Thrust
- Static RPM
- Motor/ESC current
TWEEKING the numbers.
Here are some things to look at tweeking.
Battery affects the motor performance MORE than people think. A LiPo is
3.7V per cell and that is all that matters righ? WRONG. A
discharges during use and the battery drops from a resting voltage of
4.20V per cell when charged down to a resting voltage of around 3.65V
when nearly 90% depleted. (Table
of generic resting voltage vs. remaining capacity).
The battery voltage in the motor calc should be set to the resting
voltage of the LiPo. Notice how if you set it to 4.0V,
increases. As pointed out by the LiPo
Internal resistance plays a key role in motor output.
default Internal resistance in the ADAMONE calc is pretty high compared
to most of the LiPos that people are using today. It is hard
not impossible to find Internal resistance values on vendor or MFG
websites. ADAMONE shows some packs and the numbers seem
accurate (but there are some tweeks I'll note). NOTE that the
batteries are shown by capacity (mAh) and C rating. YOU SET
cell count. Internal resistance is per cell, so as you change
cell count, so does the calc respond. In general, here is
suggest you do: Select the closest matching battery capacity
the Hyperion Section. If your battery is a Zippy Flightmax,
Lipo, Turnigy, Blue Lipo or other generic budget
brand, then leave it alone. If it's a Gens ace, Turnigy Nanotech,
Thunder Power, Hyperion or Rhino, then set the battery back to custom,
and then tweek the Internal resistance value down a littl (say decrease
30%). You can alter the capacity and then view how that
flight time in the Prop Results section of ADAMONE at the bottom.
the motor resistance is published for the motor, you can tweek that.
Remember, you have to select a comparable motor first, then set it to
custom to chang Kv and Motor resistance.
Static Thrust vs. Approx
Max level flight speed.
could spend all day pondering different props in order to look at
different values for Static thrust and level flight speed.
max level flight speed is not how fast the plane will go.
will depend on the drag of the plane and how much the motor unloads in
the air. A higher pitch prop will unload MORE than a lower
prop as the speed increases and the load on the prop decreases.
The calc does not show this. Some will
tell you it
is a 5-10% decrease in current in flight and an increase of about 5%
more RPM. A
loggeer is often needed to determine how much unloading is going on.
It is however safe to say, that unless your airplane is
backwards,it will unload in the air and get MORE RPM, LESS
current/wattage in flight.
people believe it is safe to over Prop a motor. This is where
select a prop/battery that will give MORE Wattage pull and/or current
draw than the motor is designed for. Heck, I've done it
Often motor mfg will show a BURST current draw for a motor.
can take that also as a BURST wattage draw. Say the motor is rated for
50A max and then says 60A burst. Well, that means it can
another 20% wattage for a small period of time. (sometimes 10-15 sec).
This can be done during high speed passes or
climbing maneuvers. But should be followed by a low/half throttle
period of time to cool back down. You will see BURST ratings on ESCs as
well. What does burting really affect? Heat! Heat is the
motors as well as Nitro Engines. You can get away with more
behavior when you ventilate (air comming in and having a place to exit)
as well as flying in cooler sub 90F weather.
looking to size a motor, the Wattage of the motor is one of the
important things to look at, but achieving the desired wattage is more
time than not, an experimenting game. The
variables involved are many and affect the outcome greatly.
the actual motor you have match the specified Kv? (often they vary by
10%). What is the Internal resistance of the LiPo? Try changing or
doubling the IR of a battery in the motor calc and see how much it
changes! Is the prop one of the ones listed or did you pick
similar? I often pick a few different props and test with a Wattmeter
like the P0 or P1 from Hobbypartz
which is reasonably priced and can save your plane from damage when
properly used during testing should you accidently over prop it.
Ramping that throttle up slowly as you check your current and wattag on
the meter can save you more than the cost of that wattmeter.