Chris Collins

Tools · rc

RC power setup — motor, prop and battery

Work out what a motor, propeller and LiPo will do together — RPM, current, voltage sag, power, efficiency, static thrust, pitch speed, thrust-to-weight, hover or cruise throttle and flight time — with warnings for the motor, ESC and battery, a throttle sweep chart and up to three setups side by side.

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How to use it

This is the sum an eCalc-style drive calculator does: put a motor, a propeller and a battery together and find where they settle. Enter the motor’s figures from its data sheet, the prop, and the pack, and it shows the operating point at full throttle and at the throttle that holds the model in the air.

How to use it

  1. Load a preset close to what you’re building — a 5″ freestyle quad, a 3″ cinewhoop, a 7″ long-range quad, a 1 m foam trainer or a 3D profile foamy — and change the numbers to your parts.

  2. Multirotor or Plane. A multirotor shows the hover throttle (total thrust = all-up weight). A plane shows the cruise throttle: in level flight thrust equals drag, which is roughly weight ÷ lift-to-drag ratio (about 4 for a 3D foamy, 6 for a trainer, 10–15 for a glider), at the cruise speed you enter. If a plane has more static thrust than weight it shows a 3D hover throttle too.

  3. Read the warnings: motor current and power over its rating, the ESC within 15% of its rating or over it, the battery past (or close to) its C rating, and the pack sagging below 3.3 V per cell under load.

  4. + Compare copies the setup so you can change one thing — a prop, the cell count — and see the two (or three) side by side and on the same chart. Share link (top right) copies a link with all of them.

The flight times are to 80% of the capacity at a steady throttle; real flights with climbs, punch-outs and wind come in lower.

How it works

  • Battery: S cells at the resting voltage you give (3.8 V a cell is a mid-flight LiPo; 4.2 V is a fresh one) behind an internal resistance of S × IR per cell ÷ P plus the leads. Use the IR your charger reports — it is usually lower than the pack behaves under a 100 A punch.

  • ESC: the throttle is the PWM duty; the motor sees duty × the sagging pack voltage, and the battery supplies duty × the motor current (switching is taken as lossless, plus the resistance you enter).

  • Motor: the standard three-constant model. Voltage = current × Rm + RPM ÷ Kv; shaft torque = Kt × (current − Io) with Kt = 60 ÷ (2π Kv). Io is measured at a test voltage; it is scaled with √RPM to stand in for the iron and bearing losses at other speeds.

  • Propeller: thrust = Ct ρ n² D⁴ and power = Cp ρ n³ D⁵ (n in revolutions per second, D in metres). Air density ρ comes from the field altitude and temperature.

  • The operating point is the RPM where the motor’s torque equals the prop’s; the solver finds it by bisection, then works out the throttle for hover or cruise the same way. The page’s tests check that the energy balances: what leaves the cells = battery, ESC, copper and iron losses + prop power.

The propeller model is an approximation

Ct and Cp depend on the blade’s shape, its Reynolds number and how much it flexes; two 5×4.3 props from different makers can differ by 20%. When you leave Ct and Cp blank the tool uses a small generic model: static coefficients from the pitch/diameter ratio, fitted by eye to the ballpark of two-blade sport/slow-fly props in the UIUC propeller database (an APC 10×4.7 SF comes out near Ct 0.10, Cp 0.045), scaled up for more blades and given a little more power for props under 8″, which run at low Reynolds numbers. In forward flight thrust falls to nothing near an advance ratio of pitch/diameter + 0.1, and power falls more slowly. Expect ±25% on thrust and current. If you have a thrust-stand result or the UIUC data for your prop, enter Ct and Cp (static values) and those are used instead. The presets' motors and packs are typical of their class, not a particular product — check your own data sheets.

Ducted props (whoops) make more static thrust than the model gives for the same power.

Browser support

Any current browser. Nothing needs installing or connecting.

Privacy

Everything runs in your browser and your setups are saved in its local storage. Nothing is sent anywhere; a share link carries the setups in the URL itself.

Runs entirely in your browser; nothing you type leaves this page.