Formula Used
Pack voltage: Cell count × voltage per cell.
No-load RPM: Motor KV × pack voltage × throttle fraction.
Loaded RPM: No-load RPM × (1 − RPM load drop percent ÷ 100).
Pitch speed: Loaded RPM × pitch inches × slip factor ÷ 1056.
Total current: Current per motor × number of motors.
Input power: Voltage × current.
Shaft power: Input power × motor efficiency.
Static thrust: Ct × air density × revolutions per second² × prop diameter meters⁴.
Thrust to weight: Total thrust grams ÷ aircraft weight grams.
Flight time: Usable battery amp-hours ÷ total current × 60.
How to Use This Calculator
Enter the battery cell count and nominal voltage per cell.
Add the motor KV rating and your expected throttle setting.
Enter current per motor from a wattmeter, datasheet, or test estimate.
Add ESC, battery capacity, and C rating values.
Enter propeller diameter and pitch in inches.
Use the default thrust coefficient when exact propeller data is unknown.
Press calculate and review current, thrust, speed, and margin results.
Confirm final choices with a wattmeter before flying.
R/C Motor Propeller Planning
Choosing a propeller is not guesswork. A small change in diameter or pitch can change current quickly. It can also change thrust, pitch speed, and motor heat. This calculator gives a practical first check before a bench test. It combines motor, battery, propeller, and aircraft weight inputs in one place.
Why the Match Matters
An R/C power system works as a chain. The battery supplies voltage. The motor converts electrical power into shaft power. The propeller converts shaft power into air movement. When the propeller is too large, current rises. The ESC may overheat. The battery may sag. Flight time can fall sharply. When the propeller is too small, thrust may feel weak. The aircraft may need more runway or more throttle.
Reading the Results
Pitch speed estimates the best possible forward speed. Real models fly slower because of drag and propeller slip. Static thrust estimates pulling force while the model is still. It is useful for takeoff checks, hovering models, and climb estimates. Thrust to weight ratio helps judge launch comfort. A trainer may fly with modest ratio. A 3D model needs a much higher ratio. Tip Mach warns when blade tips approach noisy and inefficient speeds.
Using Advanced Inputs
The thrust coefficient lets experienced users tune the estimate. Larger slow propellers may use a different coefficient than small high speed propellers. Motor load drop represents RPM lost under propeller load. Propeller slip adjusts the pitch speed estimate. Battery usable capacity gives a better flight time estimate than full capacity. Current margins show whether the ESC and battery have enough reserve.
A balanced setup also protects connectors, solder joints, and packs. It makes flying calmer. It reduces emergency landings. It lets pilots compare propellers without buying every possible size on the bench.
Safe Testing Advice
Treat every result as an estimate. Propeller brands, blade shapes, air density, and motor timing can change readings. Always test with a wattmeter. Secure the model before running a propeller. Keep hands, wires, and loose tools clear. Start with short throttle runs. Check motor, ESC, and battery temperature. Stop testing if anything becomes hot, smells unusual, or vibrates strongly. The calculator helps narrow choices, but real measurements confirm the setup.
FAQs
What does motor KV mean?
Motor KV means RPM per volt with no load. A 1000 KV motor may spin near 1000 RPM for each applied volt before propeller load is added.
Why does a larger propeller draw more current?
A larger propeller moves more air. That increases torque demand on the motor. Higher torque demand usually raises current and heat.
Is calculated thrust exact?
No. Static thrust is an estimate. Propeller shape, air density, RPM accuracy, and test stand design can change the real value.
What thrust to weight ratio is good?
Many trainers fly below 1:1. Sport models often feel better near 1:1. 3D aircraft may need much more for vertical performance.
What is pitch speed?
Pitch speed estimates forward speed from propeller pitch and RPM. Real flight speed is lower because of drag and propeller slip.
Why include ESC margin?
ESC margin helps show whether the controller has current reserve. Low margin can cause heating, shutdown, or component failure.
Can I use full battery capacity for flight time?
It is safer to use only part of capacity. Many pilots leave reserve capacity to protect packs and avoid sudden power loss.
Should I still use a wattmeter?
Yes. A wattmeter is strongly recommended. It confirms voltage, current, and power under real propeller load before flight.