Pinion vs Headspeed

What is a pinion ?
The pinion is that small gear you put on the motor

What is headspeed ?
The RPM of your helicopters rotor blades

How does the pinion play a role in the headspeed ?
The pinion will drive the main gear of your helicopter, and based on the number of teeth on the main gear and pinon you will get different headspeeds

So if I what less headspeed what then ?
That you will change your pinion to one with less teeth

So if I what more headspeed then what ?
Then you will change your pinion to one with more teeth

Are other factors playing a role ?
Yes, what kind of motor you using and number of cells on your battery

Tell me about how the motor play a role ?
Every motor will have a KV rating, if a motor has a rating of 2000 KV, it means it will turn 2000 rpm, if you supplied 1 volt. and the motor had 100% efficiency, but with a brushless motor I always go with only 90% efficiency

Got it. that's why the number of cells play a role ?
You are right, as higher number of cells means more volt to the motor, and therefore higher RPM of the motor.

But will the voltage of the battery not drop during use ?
Your right. It will go from 4.20 volts pr cell to 3.4 at the end of the flight. so the max headspeed will go down during flight.

Give me the math!
Rotor RPM = (Motor RPM/V * Battery Voltage) / (Main gear Teeth / Pinion Gear Teeth) * (% Efficiency /100)

Can you come with a example ?
Ok, lets say you buy the HK-450 CCPM 3D helicopter kit with a Turnigy Typhoon 2215H brushless 450-size motor.
First the helicopter specs give us.
Main gear teeth: 150
The KV rating of the motor is: 3550
You don't get any pinion in this kit, but let use a 13 tooth pinion.
We will use a 3 cell battery and assume 4 volts per cell.

Let put on the math hat
Rotor RPM = (Motor RPM/V * Battery Voltage) / (Main gear Teeth / Pinion Gear Teeth) * (% Efficiency /100)
Rotor RPM = ( 3550 * (3*4)) / (150 / 13) * (90 /100)
So rotor RPM with 13 tooth pinion is: 3323
but with a 12 tooth pinion it will become: 3067
This is with 100% throttle

There is a super simple online calculator made by Fredrik Melin, that I use at the moment, and so can you (he also makes a smartphone app)
http://heli.dacsa.net/calcv3/

As always leave a comment.