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How to Calculate RPMs on a 450 Heli

RPM (revolutions per minute) is an important statistic for model helicopters. It tells you how hard the motor is working to keep the helicopter hovering. While you could easily count the number of times it turns in a minute at a slower speed, once the 450 helicopter is spinning fast, it could easily be around 2200 RPM. The best method to figure this number is to calculate it using information from your helicopter.

Things You'll Need

Marker
Motor spec sheet
Calculator

Instructions

- 1
  Find the number of teeth on your motor pinion. The motor pinion is the gear attached to the top of the motor. Put a small dot on the first gear tooth you count, then count the rest of the teeth. Record this number as "mT."
- 2
  Calculate the number of teeth on the main gear. This is the gear where the motor pinion attaches. Put a dot on the first gear to mark it, then count the number of teeth. Record this number down as "gT."
- 3
  Retrieve the motor specification sheet to find the motor's kv rating. This is the number of RPM the motor puts out per volt sent in. This number may also be on the motor housing. Record the number as "kv."
- 4
  Write down the voltage on the battery label. This information may also be on the specification sheet of your helicopter. Record this as "bV."
- 5
  Estimate the efficiency rate. Standard practice is to use the number .80 (80 percent) or .90 (90 percent). This is how much energy from the battery turns into energy spent lifting the helicopter off the ground. There is no exact way to calculate this, so your RPM number will be an estimate. Record this estimate as "Eff."
- 6
  Calculate the following equation: RPM = ( kv * bV / ( gT / mT ) ) * Eff. Plug in the numbers you have already recorded to solve the equation. As you know from "order of operations," solve the inner-most parentheses first. The solution is your estimated RPM.

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