Motor selection is where drone datasheet mythology is thickest. The good news: for a startup, motor sizing is a lookup exercise wrapped in three concepts — KV, stator volume, and hover throttle. Get those, and manufacturer thrust tables do the rest.
Start from the prop, not the motor
The propeller determines the torque and RPM the motor must deliver; the motor exists to serve it. If you haven't fixed a candidate prop diameter from your frame and mass budget, do that first — sizing a motor in a vacuum produces the classic beginner build: a screaming high-KV motor lugging a big prop, cooking itself amp by amp.
KV demystified
KV (RPM per volt, unloaded) is not a quality score — it's gearing. The back-EMF constant it encodes ties RPM, voltage and torque-per-amp together: low KV = more torque per amp = big props spun slowly; high KV = less torque per amp = small props spun fast.
| Aircraft class | Typical prop | Battery | Typical KV |
|---|---|---|---|
| 5-inch racing/freestyle | 5×4.3 | 6S | 1600–1900 |
| 7–10 kg industrial quad | 15–18 in | 6S–12S | 320–170 |
| Heavy-lift / 25 kg class | 22–30 in | 12S–14S | 150–90 |
| VTOL lift motors | 16–22 in | 6S–12S | 280–150 |
| Fixed-wing cruise motor | 10–14 in high pitch | 4S–6S | 700–360 |
Note the pattern: as prop diameter rises, KV falls and battery voltage rises. Higher voltage means less current for the same power — thinner wires, cooler ESCs, happier batteries.
Stator volume: the torque reservoir
Motor names encode stator diameter × height in millimetres (2806 = 28 mm × 6 mm). Torque capability scales roughly with stator volume, and heat dissipation with surface area. Two motors of equal KV but different stator volume are different machines: the bigger one turns the same prop cooler and survives the hot day your demo lands on. When in doubt between two adjacent sizes, take the larger stator and spend the ~30 g per motor from the growth margin.
The sizing procedure
- Per-motor hover thrust: MTOW ÷ number of motors. A 12 kg quad needs 3 kg hover thrust per motor.
- Per-motor max thrust: hover × your thrust-to-weight target. At 2:1, that quad needs 6 kg max per motor.
- Shortlist from thrust tables: reputable manufacturers (T-Motor, KDE, MAD, SunnySky and others) publish measured tables of prop + voltage + throttle → thrust, current, power. Find rows delivering your max thrust at 100% throttle with your candidate prop.
- Check hover throttle: in the same table, your hover thrust should fall at 45–55% throttle. Much higher and you've no control headroom and poor efficiency; much lower and you're carrying dead motor mass and the control loop gets twitchy.
- Check hover efficiency: the table's g/W at hover point feeds straight into your endurance estimate. For large props, expect 8–14 g/W; for small fast props, 3–6 g/W.
- Verify current: max-throttle current × motor count must fit your battery C-rating and pack wiring with margin.
Manufacturer data comes from a clean thrust stand at 25 °C at sea level. Real aircraft lose several percent to airframe blockage, prop wash interference and voltage sag; hot-and-high conditions cost more (air density falls ~3.5% per 300 m). Derate table thrust by 10% for design, and confirm on your own stand — it's also the evidence trail your flight test program will want.
Thermal reality
Motors die by heat, and heat comes from current. Copper losses scale with current squared, so a motor at 80% load runs far hotter than at 50%. Design habits that keep windings alive: hover at mid-throttle (see above), prefer voltage over current for power growth, give motors airflow (cowlings that look sleek and cook motors are a recurring VTOL disease — lift motors that stop in cruise still soak in sun), and log motor temperature during test flights if your ESC telemetry supports it.
Quality signals that matter
When comparing vendors: measured thrust tables (not simulated), stated maximum continuous vs burst current with time limits, bearing type and replaceability, winding insulation temperature class, and IP-rated options if you'll fly weather. Motor cost is 2–4% of your bill of materials; motor failure is 100% of a crashed aircraft. This is a poor line item to economise on.
Frequently asked questions
What does the KV rating of a drone motor mean?
KV is the motor's unloaded RPM per volt. A 900KV motor on a 4S battery (~14.8 V) spins about 13,300 RPM unloaded. Lower KV means more torque per amp for turning large propellers slowly; higher KV suits small, fast props.
What thrust-to-weight ratio does a drone need?
Design for at least 2:1 total maximum thrust to maximum take-off weight for a working multirotor, meaning hover sits near 50% throttle. Camera platforms can accept 1.7:1; windy environments, agile flight or VTOL transition safety want 2.5:1 or more.
What do drone motor size numbers like 2806 mean?
The first two digits are stator diameter in millimetres, the last two are stator height. A 2806 motor has a 28 mm wide, 6 mm tall stator. More stator volume means more torque capability, roughly in proportion.