In order to calculate the number of T for your motor pinion, there are 4 things you need to know:

1. Targeted head speed: This is the round per minute (rpm) of your RC helicopter rotor blade spin required to make your RC helicopter fly as you intended. This target head speed is not the same for all RC helicopters depending on many factors such as weight, motor, gear, etc. Normally, the RC helicopter manufacturer recommends a certain head speed for each RC helicopter. Check the owner manual for recommended head speed. In addition, the head speed is set depending on your own flying modes. For example, if you just want to hover and do basic flights, your target head speed would be different than if you do 3D or aggressive flying activities.    
2. The number of T of the main gear: This is easy to find – just count the teeth on your main gear. Well, not that easy – just try it, you’ll know what I mean. I get a different number every time I count it. Again, you may find the number of your main gear T in the owner manual.
3. Motor kv - the rpm (head Speed) produced by a motor per volt applied. Usually you can get the motor kv from the motor specification sheet.
4. Voltage: This is the voltage of your battery. You may get this information from your battery specification sheet or label.
5. Efficiency rate: This is the percentage of your RC helicopter motor efficiency when you are flying with your set up. It is the norm to use 80% (.8) or 90% (.9) depending on many factors though you run 100% on your throttle curve.

Once you get these 4 numbers, the only thing left to do is to calculate it by using the formula below: 

Pinion Teeth = Targeted Head Speed (1) * Main Gear Teeth (2) / Motor kv (3) / Battery Voltage(4) / Efficiency (5)

For example, I set up my MX400 with JGF 400HD (KV: 4000-4400). I am going to use 3 cells Li Po battery that produces 11.1 voltage. T of MX400 Helicopter main gear is 138. Based on what I heard people say, my target head speed would be 2200 just for basic hover.   

2200 * 138 / 4000 / 11.1 / .8 = 8.5. I would round it down or up depending on whether I want more head speed or I want longer flight time.  For example, if I round it down, it would be an 8T pinion. That will give me less head speed than 2200. Check here to see how to calculate exactly what is the head speed for an 8 T pinion. If I round it up, it would be a 9T pinion. That will give me more head speed than 2200. Check here to see how to calculate exactly what is the head speed for an 9 T pinion. You take your pick. Of course more head speed means more current drawn from your battery. On the other hand you need good head speed to fly.

If I use JGF 450TH  (KV: 3000) for the same set-up and battery, my calculation would be 2200*138/3000/11.1/.8 = 11.39. I can go with either an 11T pinion or 12T pinion.

If I use Align 430L 430L (3550kv) for the same set up and battery, my calculation would be 2200*138/3550/11.1/.8 = 9.63. I would go with a 10T pinion.

I hope you get the concept. If you know how to calculate the T of your pinion, you will be able to also calculate your estimate head speed. Please click here.  

If you know how to calculate head speed without using a tachometer, click here.

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