Kv vs prop diameter — calculate, don’t guess

“On 5 inch everyone runs 1900 Kv” is at best a starting rumour. Kv is the no-load constant (rpm per volt). Whether that motor works with your prop, cells and AUW is a load problem — in eCalc: Prop-KV-Wizard plus a full calculate pass.

What Kv actually is

Kv ≈ no-load rpm / voltage (no prop). Under a prop, rpm drops; current and losses rise with aerodynamic load (PConst, diameter, pitch).

Chain:

1. Cells → pack voltage
2. Kv × voltage → rough rpm scale
3. Prop diameter & pitch → torque demand / current
4. Hover at AUW → required rpm and throttle

Too high Kv + large prop + many cells → peaks and heat. Too low Kv → no thrust, high throttle, weak reserve. So: prop for frame first, then Kv band, then a database motor.

Prop-KV-Wizard instead of gut feel

Per eCalc tutorial/help:

1. Frame and configuration
2. Largest sensible prop
3. Estimate AUW
4. Wizard → recommended Kv range
5. Search/select motor
6. Calculate; check hover throttle, T/W, temperature, peaks (Sizing)

Changing cell count shifts the sensible Kv a lot: more volts → often lower Kv for a similar prop.

Worked example: same 5” prop, 4S vs 6S

Assumptions (both):

• Flat quad 220 mm, prop 5×4.3, AUW 600 g
• Explorer goal: linear hover throttle < 50 %, T/W ≥ 1.8

Variant A — 4S: Wizard typically shows a higher Kv band (often roughly ~1800–2200 Kv class — read the exact interval in the tool).

Variant B — 6S: same prop/AUW → lower Kv band. Blindly reusing 4S Kv on 6S often hits peak/heat limits (Marginal setup).

Run both with real DB motors; compare hover current, max current and est. temperature.

Steps in xcopterCalc

1. Open xcopterCalc.
2. Set frame, rotors, prop D×pitch, cell count.
3. Enter AUW.
4. Run Prop-KV-Wizard — note the interval.
5. Pick a motor in range (custom only with measured Kv, Io, Rm).
6. Calculate; if gauges go red, change prop, pitch or Kv — don’t “add Kv and hope”.
7. Optional: ±1 cell and re-run the Wizard.

Measuring Kv: help describes a short no-prop run on 2S/3S (<10 s), Kv = rpm/volt — respect voltage limits.

More practice: common mismatch pairs

Recurring mistakes:

• High Kv + large diameter + high cell count — peak current and temperature climb into the red quickly.
• Low Kv + small race prop — little thrust, hover throttle too high despite a “strong” stick feel.
• Copying Kv across pack voltages — blind 4S→6S reuse is the classic trap.

Method: change one variable (prop or Kv or cells), recalculate, compare gauges. You see cause and effect instead of three forum opinions at once.

If the Wizard suggests a band and no motor sits dead-centre: aim for the middle of the band and fine-tune with pitch rather than picking extreme Kv. Manufacturer current/voltage limits still rule — eCalc advises; it does not replace the label.

For custom motors (help): don’t spoil Rm with a simple ohmmeter (use Kelvin/4-wire); measure Io and Kv only briefly without a prop. Bad parameters mean bad results — prefer database motors.

FAQ

Is higher Kv always “stronger”?
No. It shifts the rpm scale. With an oversized prop it mainly means more heat and current.

Change pitch instead of Kv?
Both change load. Keep pitch/D under ~2/3 when you can.

Database vs custom motor?
Prefer DB entries; custom only with manufacturer or measured data — never guess from “similar” motors (help).