INFORMATION

The truth about
Motor KV, Amps, Watts, RPM, ESC's & Lipo lifeAll helicopter pilots today need to
understand something about electrics when flying electric powered
helicopters. This is because it is a minefield of disinformation
and advertising hype, especially in regard to LIPO power, the 'C'
ratings, Watts or Amps available/required and that most crucial of all
setting, the maximum rotor head speed your particular helicopter needs
to fly at it's best.
Therefore I felt it was about time we
defined all of these terms in detail, and showed YOU that YOU can do
these calculations quite easily, and get the very best from the
electrics and electronics on your helicopter.
By definition this is a
slightly complex subject, and it will therefore be quite long, so please
read it through slowly and carefully, and maybe print it out so that you
can go through it in your own time several more times.
Before we start, we are going to have to
use some fairly simple maths, and a few very useful equations to help us
sort it all out, so here they are first so that you can look back and
refer to them :
To find out how many motor rpm = 1 rpm of
the main gear :
Main gear Teeth / Pinion gear teeth eg:
140 / 10 = 14 rpm
To find out the theoretical maximum motor
rpm possible for a given LIPO battery :
LIPO Voltage * motor KV rating eg:
11.1v * 3800KV = 42,180 motor rpm
To find out the maximum Amps available in any given
LIPO :
Lipo KV * LIPO 'C' rating / 1000
eg: (1800 Mah * 15C)/1000 = 27Amps
To calculate the Watts of power available,
given that we know the Volts and Amps
Volts * Amps = Watts eg: 11.1 *
27 = 299.7 Watts
To calculate the Amps available, given that
we know the Volts and Watts
Amps = Watts /Volts eg:
299.7 Watts / 11.1 Volts = 27 Amps
MOTOR
The most obvious thing to look at first is that most
essential of items, the motor in the helicopter.
In almost every advert you will see at least one
important fact quoted, and that is what is known as the 'KV' rating.
This is just a fancy name for a simple concept, it stands for the
number of revolutions the motor will turn at at full throttle for each
VOLT that it has available to it.
Therefore, if we have a LIPO rated at 11.1V (the
standard for smaller helicopters) and a LIPO that is rated at 3800KV, we
potentially have the ability to spin our motor
at up to 11.1 * 3800 =42,180 rpm. Scary eh.
Another rating you need to know about any motor is
the AMPS (current) it can draw. As you will read later on, there
is a direct relationship between Volts and Amps, so the maximum amperage
draw a motor is capable of is a very important value for us to
know. A typical motor may be rated at around 30Amps, but can be
much higher. Some motor manufacturers quote a Watts value instead
of Amps, but as you will see later on, that's fine as we can calculate
the Amps from the Volts and Watts.
But before we go further into such calculations,
lets look at the other factors that effect this.
THE LIPO BATTERY
VOLTS (Pressure)
Most of us fly using 11.1 Volt LIPO cells, which is
admittedly a strange number for the Volts, but these LIPO's are almost
always made up of three separate cells, all rated at 3.7 Volts,
therefore 3 * 3.7 = 11.1 Volts. For the 22.2 volt systems used by
the larger electric helis, the reach 22 Volts by using 6 separate cells
in the LIPO 6 * 3.7 = 22.2 Volts
Sadly it is a fact of life that nothing works at its
optimum level, so we will not actually get 11.Volts from our batteries
for very long at all, if ever, so it is wise for the purposes of
calculations to allow for this, and most of use a value of 10.5
Volts as a reasonable level to work with. Remember
this point for later on in this discussion. This means in our
motor rpm calculation in the first section above, we will really have
10.5 * 3800 = 39,900 rpm.
AMPS (Current)
Now another rating given for a typical LIPO is it
Mah (MilliAmp) rating. This is what is called the total CURRENT
stored in electrical terms. A typical 450 size helicopter will use
a LIPO rated at anything from 1800 up to maybe 2500 Mah or even more in
some cases.
'C' RATING
The final, and crucial rating for a LIPO is what is
called it's 'C'' rating. This simply means you can multiply the
Mah by C Amps as the potential maximum available current draw that the
battery is able to provide continuously to a motor.
Using the formula (AMPS * C) / 1000. Say we have a
2200 Mah LIPO rated at 20C, it will theoretically be able to provide a
continuous CURRENT of 44 AMPS (2200 * 20 = 44000 Mah = 44 Amps)
Equally a LIPO rated at 30C would be able to provide us with 66
AMPS (2200 * 30 = 66000 = 66 Amps)
This 'C' rating does NOT EFFECT the
maximum rpm of the motor, which is still controlled solely by the KV
value.
Watts
The Watt value is another very useful reading that
we like to know. A Watt is really the classic measure of the power
a device will give, such as a light bulb, which you buy on the basis of
Watts as the higher the Watts value, the brighter the bulb will be (more
powerful)
To calculate Watts is relatively simple, as it is
always Volts * Amps = Watts, so from our example above, if we have 11.1
volts at 44 Amps we will have 11.1 * 44 = 488 Watts.
But remember from our previous comments that we will
not get 11.1 volts, so the equation is more likely to be 10.5 * 44 = 462
Watts.
Most electric 450 sized helicopter need around 300
Watts as a good median power level.
PHYSICAL FACTORS (Head
Speed)
This is the final part of the data we need to be
able to calculate the right sized Motor, LIPO battery, ESC (Electronic
Speed Controller) and the motor pinion we need.
All helicopters come fitted with a main gear that
has xx teeth, We cannot really vary that very much, so it is one
"constant" in our equations. A typical main gear has around
140 teeth. The motor also has a pinion that meshes with the main
gear, but we can vary the number of teeth on the pinion if we wish to by
changing it for a different pinion.
Therefore it is often a good idea to start with this
calculation to see what rpm we will get based on the KV of the motor and
the Voltage of our LIPO.
The first part therefore is to check the number of
teeth on our current motor opinion. A typical value is around 11
teeth. so we can easily see by dividing the # of teeth on the main gear
by the # of teeth on the pinion gear we will get a value that will be
the number of motor rpm we need to make the rotor head turn ONE RPM
Lets assume our 3800 KV motor, an 11.Volt LIPO, a
140 tooth main gear and an 11 tooth pinion for this calculation.
Using main gear teeth / pinion gear teeth we get :
140 / 11 = 12.72 (rounded down) So for each 12.72
rpm of the motor, our rotor head will rotate exactly once.
However, a good average rotor head speed for a 450
size helicopter is 2300 rpm, so if we multiply 12.72 * 2300, we get
29,256 motor rpm which is more than we will need our motor to produce to
make our rotor head spin at 2300 rpm.
But if you look back to the discussion on the motor,
you will see that we have already worked out what our motor is actually
capable of, which is 10.5V * 3800KV = 39,900 rpm, so right now, at least
in theory, our motor will give us too many rotor head rpm. 39900 /
12.72 = 3136 rotor head rpm
Are you still with me so far ?
Lets try to work it back the other way,
working from our required head speed back to the motor rpm we need.
The head rpm we are getting right now is 3136, we want 2300, so 3136 /
2300 = 1.36 which is the ratio we need to reduce our pinion gear down
to. 11 / 1.36 = 8 so we really need an 8 tooth pinion to work with
this setup.
Lets just prove that by calculating it the other way around  just for
completeness and hopefully clarity:
Still assuming our 3800 KV motor, an
11.Volt LIPO, a 140 tooth main gear but now an 8 tooth pinion for this
calculation.
Calculate the pinion to main gear ration
using the 8 tooth pinion :
140 / 8 = 17.5 so for each 17.5 rpm
of the motor, our rotor head will rotate exactly once.
Divide our required rotor Head RPM by this
new value :
2300 / 17.5 = 131 so we need a
gearing that will give us 131 rpm of the motor to one rpm of the rotor
head
As our motor spins at 10.5 * 3800 = 39,900
rpm the final calculation is :
39900 / 17.5 = 2280 rpm, which is very
close to what we want at 2300 rpm, so an 8 tooth pinion would work fine
with the motor and main gear we have.
MOVING ON TO THE ADVANCED
AREA
We have done a basic calculation to see
what head speed a motor of a certain KV will produce, and discovered
that changing the # of teeth on the motor pinion gear allows us to
adjust the maximum rotor head rpm we can achieve with that motor.
There is (OF COURSE) another way to
approach this, and that is to use a motor with a LOWER KV rating, which
you now know will reduce the maximum rpm available...
Lets repeat our
calculations with a 3500KV motor
So assuming a 3500 KV motor, an 11.Volt
LIPO, a 140 tooth main gear and an 11 tooth pinion for this calculation.
140 / 11 = 12.72 (rounded down) So for each
12.72 rpm of the motor, our rotor head will rotate exactly once.
10.5V * 3500 Mah = 36,750 max motor rpm.
36750 / 12.72 = 2889 Rotor Head rpm  still
too high.
Lets try a 3200KV then :
10.5V * 3200 Mah = 33,600 max motor rpm.
33600 / 12.72 = 2641 Rotor Head rpm  still
too high.
DO YOU SEE WHERE I AM GOING WITH THIS YET ?
Well, actually the calculations we have
been through above were just to get you in the mood for some simple
math in case you
were goofing off school the day they covered this stuff.
If only it were quite as simplistic, it
would not be a problem as you have seen, but there are several other
factor we must take into account.
One of them is the efficiency of the
systems we are working with, which most certainly do not come close to
100%, so we need to assume a value of around 85% efficiency as a 'norm'
for the motors and the mechanical efficiency and drag created by the
mainshaft and rotor head and tail drive, so we should loose another 10%
to allow for this.
Lets do our original calculation again
factoring these value in to see what happens.
Still assuming our 3800 KV motor, an
11.Volt LIPO, a 140 tooth main gear but and an 11 tooth pinion for this
calculation.
Calculate the pinion to main gear ration
using the 8 tooth pinion :
140 / 11 = 12.72 so for each 12.72
rpm of the motor, our rotor head will rotate exactly once.
10.5V * 3800KV = 39,900 rpm, with
100% efficiency so lets reduce that to allow for drag and inefficiency.
We will multiply our total by 0.85 for the
motor inefficiency, and a further 0.90 for the mechanical
drag.
39900 * .85 * .90 = 30523 motor rpm, a far
cry from the original 39,900 rpm !
30523 / 12.72 = 2400 Rotor Head rpm 
Almost exactly correct
No great surprise there really, as the
values we have used are those of the stock Esky Belt CP, so the
combination of values of the stock LIPO & motor should have ended up
giving us around the head speed we wanted.
REVISITING AMPERAGE
Just when you thought it was safe to go
back in the water, we must reintroduce the dreaded Amps, as this is the
true measure of power that we have available. We talked earlier
about Amps when we discussed 'C' ratings for LIPOS, and both Amps and
'C' ratings are VERY CLOSELY INTERWOVEN as far as we are concerned.
Remember we have already calculated that a
2200 Mah LIPO rated at 20C would be able to provide us with 44 Amps of
continuous power.
So lets see what our motor needs in terms
of Amps. All motors should specify their maximum Amperage draw, and a
typical value for a 3800 KV motor might be 25 Amps or so
However, some quote the Wattage rather than
the Amperage, so we then need to use that clever equation to resolve the
amperage. Remember the formula we used was Volts * Amps =
Watts. So if we know Volts and Watts, we can get Amps by some
simple math.
Watts / Volts = Amps  a simple equation
? Therefore general knowledge tells us that a decent 3800 KV
motor is likely to be capable of producing around 270 Watts, so :
270 /10.5 = 25.71 Amps. That is the
Maximum Amps this motor will draw from a battery.
So what you may be saying, we have
calculated our LIPO power, and using an 1800 MAh LIPO we have 1800 * 20
= 36000 Mah = 36 Amps which is quite sufficient. Yep, very true,
but there is one other item we have yet to talk about, and that is
THE ESC (Electronic Speed Controller)
What is it ? simple really, it is an
electronic black box that controls the throttle on the motor, and also
feeds both the motor and the receiver and servos with ALL OF THE POWER
THEY NEED.
There are only TWO values that are quoted
about ESC's and the most important one is the 'C' rating. Yep,
here it is again, but this time in a different disguise. In terms
of an ESC, the 'C' rating tells you (at least in theory) what the
maximum AMPERAGE is that it can handle.
However, you can easily see that from the
Amperage values we have calculated so far, there are no ESC's that come
close to this value, so the common perception is that the ESC you have
MUST be at least equal, if not higher, to the 'C' value of your LIPO.
If we go back to the stock Esky Belt CP,
this works again (well almost) as the ESC provided is rated at 25C, and
the 1800 Mah LIPO is normally rated at 20C. However we have also
calculated the max amperage that may be requested can be a little over
25, which may just answer the question as to why a lot of these Esky
ESC's tend to catch fire. ?
So why have we gone through this second
(Advanced stage) exactly. ?
Well, it is because you need to be aware
that if you have a 25A ESC, and put a 2200 Mah 25C lipo in the
helicopter, and switch the motor to one that has 350 Watts of power, you
are going to have a problem as 350 Watts = 350/10.5 = 33 Amps
Why ?
Lipo power is 2200 * 25 = 55000 = 55 Amps 
No problem
Motor max draw = 33 Amps  so far so good, Battery can power Motor
ESC max = 25A  TOO LOW
You are going to need to grab a 35 or even
40A Esc to run this combination.
SOME FINAL WORDS
I would imagine many of you are confused by
now, and I would not blame you, but here are some final thoughts.
The LOWER the KV value of a motor, the more
TORQUE it is typically capable of, but at lower rpm's so a 3550KV motor
will have more low down punch than a 3800KV, but will not generate the
same rpm, so you will need to adjust the motor pinion to compensate for
this by using a motor pinion with MORE TEETH.
A larger motor pinion takes more effort for
the motor to drive it and the main gear, so the efficiency loss will
need to be increased in all calculations.
The Mah value has little effect on anything
other than how long the LIPO will last. So a 1200Mah LIPO will
only last half the time of a 2400MAh LIPO, assuming the same flying
style.
Power is all about the motor's maximum
possible amperage draw coupled with its KV rating
Rotor Head speed is all about the correct
motor pinion, and then matching a LIPO to suit that requirement
The larger the Mah rating, the longer a
LIPO will last, but it is also heavier, so that will reduce flight times
as well.
Electric motors work most efficiently
when running at around 95% of their maximum rpm.
If you run a LIPO at it's maximum discharge
rate all the time, you will seriously damage it's longevity, so it is a
good idea to only run them at around 80% of their maximum output.
In other words, there is no
really easy, foolproof way to calculate these things, but hopefully the
above will let you understand how to do it sensibly and in an informed
way, and let you experiment with different combinations to reach an
informed decision.
