Brushless Motor Prop Calculator

Calculator Inputs

Example Data Table

Formula Used

The calculator uses a simplified electric motor and static propeller model.

• Pack voltage = cells × cell voltage × (1 − sag percent ÷ 100)
• Effective voltage = pack voltage × throttle percent ÷ 100
• RPM = KV × (effective voltage − current × resistance)
• Shaft power ≈ (current − no load current) × back EMF voltage
• Prop power = Cp × air density × n³ × diameter⁵
• Static thrust = Ct × air density × n² × diameter⁴
• Torque = prop power ÷ (2πn)
• Pitch speed mph = pitch inches × RPM ÷ 1056

Here, n means revolutions per second. Diameter is converted to meters.

How to Use This Calculator

1. Enter battery cells, loaded cell voltage, and expected sag.
2. Add motor KV, winding resistance, no load current, and ratings.
3. Enter ESC rating and battery discharge data.
4. Add propeller diameter, pitch, Cp, Ct, and air density.
5. Enter aircraft or model weight for thrust ratio checks.
6. Press Calculate and review current, thrust, heat, and margins.
7. Export the result as CSV or PDF for build notes.

Brushless Motor Prop Selection Guide

Why Prop Matching Matters

A brushless motor prop calculator helps match a motor, propeller, battery, and ESC before a bench test. It gives a quick static estimate. It also shows risky combinations. The estimate is useful for drones, planes, boats, and test stands.

How Prop Load Changes

A propeller is not a fixed load. Its power demand rises very fast as diameter and RPM increase. A small change in pitch can also raise current. This calculator uses propeller coefficients, air density, and a simple motor model. It then estimates loaded RPM, thrust, torque, current, heat, and pitch speed.

Motor Model

The motor model starts with pack voltage. It applies throttle and voltage sag. It then uses motor KV, phase resistance, and no load current. The script searches for a current where motor shaft power matches propeller power. This gives a practical operating point.

Propeller Model

The prop model uses standard static equations. Power is based on air density, revolutions per second, diameter, and a power coefficient. Thrust is based on a thrust coefficient. You may edit both coefficients. Use measured prop data when available. Generic values only give planning estimates.

Electrical Limits

Battery and ESC limits matter. High current can heat windings and reduce magnets. It can also trip an ESC. The calculator compares estimated current with motor, ESC, and battery limits. It also checks motor power margin and thrust to weight.

Safer Inputs

Use conservative inputs first. Enter the true loaded cell voltage, not a fresh charge peak, when checking safety. Add sag if the pack is old or small. Use a lower coefficient for thin efficient props. Use a higher value for aggressive props.

Bench Testing

Static thrust is only one part of the system. Real flight changes airflow. Forward speed unloads the prop. Cooling also improves with motion. Treat the result as a bench estimate. Confirm it with a wattmeter and tachometer.

Comparing Setups

The tool is most helpful when comparing options. Try a larger diameter with lower pitch. Then try a smaller diameter with higher pitch. Watch current, heat, pitch speed, and thrust. Choose the setup with enough thrust, safe current, and acceptable efficiency.

Use Case Notes

For multirotors, hover thrust margin is important. For airplanes, pitch speed and watts per pound matter more. Keep notes for each prop. Small records improve future selections and reduce failed tests later.

FAQs