Selecting Motors, Pinions, ESCs & Battery Packs
Phil Noel (copyright)
(N.B. This also applies to any other electric)
There seems to be a lot of mystery about how to match motors, battery packs and ESC’s to the Swift and other electrics. As it is all about simple numbers, I will try to make it as simple as possible.
First we need some basic information:
1 – The number of teeth on the maingear: - 96 for the Swift. This number will help us later when selecting the best motor pinion for the head speed we want to turn
2 – The KV of the motor we intend to use: - there are two good possibilities from Century, the CNE271 550+ motor that is rated to have a 1470 KV and the more powerful CNE274 600+ motor rated at 1110KV. The KV rating of the motor is important, because it tells what rpm the motor will turn when one applies 1 volt to it. So the 1470KV motor will turn 1470 rpm per volt while the 1110KV will turn at 1110 rpm per volt.
NOTE: do not confuse this rating as a power rating. It is not. It is only a number to tell us what rpm they will turn per volt. This will be used later to help calculate for rotor speed.
3 – The rotor speed we want to fly at, noting that the higher the rotor speed, the faster will be the battery drain.
a) 1500 to 1600 – for more gentle flying and longer flight times (good for training and scale flying). The wattage required for these rotor speeds and their related type of flying will be from 400 to 470 watts.
b) 1600 to 1800 – results in faster cyclic response needed for more aggressive aerobatics and sport type of 3D. The wattage required for these rotor speeds, and their type of flying will be from 470 to 600.
c) 1800 to 2100 – for even faster response needed for the more aggressive to the more extreme type of 3D flight. These head speeds will require 600 to 800 watts with fraction of a second peaks as high as 1275!!!
Looking at the watts required, one can see that the if he is using the same power system (motor, ESC & battery pack), setting it up to fly gently at a 1500 head speed will yield twice the flight time as setting it up to fly at the 2100 head speed.
4 – The battery packs we would have to use to get the performance we want. Here you can use any good battery that can provide the wattage listed above, These would be batteries from a 14.8 Volt 4S/3300 lipo, through the 18.5 Volt 5S/5000 or on up to the high power 22.2 Volt 6S/5000 pack. All will fly the heli, only at different head speeds and for differing flight times. Just be sure they have a good honest 20C or better rating.
For the remainder of this article, I will use the most common 600+ motor and the 80A ESC because of their greater flexibility through the use of various pinions from 9T to the 14T. I will also assume an overall rather standard power system efficiency rating of 90%. I will also use LiPo packs for these calculations, as they will deliver their nominal 3.7V/cell rating at the higher current draws associated with these high wattage demands. Other battery technologies, like the A123 cells, while rated at a nominal voltage of 3.3V/cell, will actually only deliver about 2.7 at these high current demands. SO if you want to calculate for use with these, use the more realistic 2.7 volt figure.
First, let me examine what we would need for a more gentle trainer or scale ship set-up, with a head speed around 1500. And let us assume we are on a tighter budget, and only want to use the less expensive 4S/4000/20C packs.
Step #1 - I will have to find what RPM I can expect my motor to turn at, with this pack, and the ESC set at its’ most efficient setting of 100%, which is much like it was a straight wire. As the motor is rated at 1110 KV, that means my 14.8V pack will result in a rotor speed of about 16428 (1110 x 14.8). As we are using an overall efficiency of 90% for these calculations, we can assume that will drop to about only 14785 (16428 x 0.90). The formula is simply RPM = KV x Volts x Efficiency
Step #2 - I must calculate the gear ratio that would be needed at this motor rpm to get the desired rotor speed. The formula for desired ratio is GR = RPM / RHS. In this case it is 14785 / 1500 = 9.86.
Step #3 - Next we determine the required pinion gear needed, by dividing the number of main gear teeth by the gear ratio. The formula is simply PN = MT / GR. So in this case it is 96 / 9.86 = 9.7 teeth. As a pinion cannot have fractions of teeth, we will round up to one with 10 teeth. This will give a real ratio of 9.6 (96 / 10).
So our final resultant head speed will be 14785/9.6 = 1540. Pretty close to what we want.
Now let’s examine the HOT set up for about a 2100 head speed using a 6S pack of 22.2 volts.
Step #1 – 1110 x 22.2 = 24342 @ 90% = 24342 x 0.9 = 22178 net motor RPM
Step #2 – 22178 / 2100 = 10.56 gear ratio needed
Step #3 – 96 / 10.56 = 9.09 teeth on pinion
Step #4 – using 9T pinion resultant gear ratio is 96 / 9 = 10.67
Step #5 – net rotor speed is 22178 / 10.67 = 2080 which is close enough to 2100.
For good all around sport 3D flying at 1900 using a less expensive 5S pack:
#1 – 1110 x 18.5 x 0.9 = 18481 net motor rpm
#2 – 18481 / 1900 = 9.72 gear ratio
#3 – 96 / 9.72 = 9.88 teeth
#4 – using the 10T pinion the gear ratio is 96 / 10 = 9.6
# 5 – net rotor speed is 18481 / 9.6 = 1925
Of course at these rotor speeds you want to be sure to be using good quality 515mm to 560mm composite blades like the FK or RotorTech’s.
Also of note, is that you can use a pinion with a higher number of teeth and set your ESC at a lower setting to maintain a lower head speed. But I would not recommend going lower the an 80% setting, as you start losing too much efficiency, and your motor and ESC will run hotter.
I hope this has taken a lot of the mystery out of what is needed in selecting motors and pinions for your Swift – HAPPY FLYING