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   INFORMATION - The truth about Motor KV, Amps, Watts, RPM, ESC's & Lipo life

All 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.

Please feel free to rate this Tips & Tricks entry to help 
me to ensure these are as good as possible - Thanks!

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