Pinion Gears and Calculating Head Speed

Calculating head speed for a heli requires a few pieces of information.

  • Pinion Gear Teeth
  • Main  Gear Teeth
  • Motor kv,  rpm per volt of input
  • Battery Voltage
  • ESC head room to maintain speed

head speed = (battery voltage) * (head room multiplier) * ( motor kv ) * (pinion teeth) / ( main gear teeth)

For a 6 cell pack the voltage will vary from:

  • fully charged 4.2V x 6 cells =   25.2V
  • down to 3.4C x 6 cells = 20.4V ( recommended max discharge )

Stock TREX 550e calculations

Motor:  600M/600MX = 1220 Kv
Pinion: 16T
blades: 520mm

21V * .90 * 1220 * 16 /170 = 2170   rpm

My calculations

Motor kv Neu 1515 2.5DF =  1650 kv
Pinion: 13T
blades: 550mm/553mm

21V x .90 ( esc headroom) * 1650 kv * 13T pinion/ 170T main  gear = 2384
21V x .90 ( esc headroom) * 1650 kv * 14T pinion/ 170T main  gear = 2568

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