Reading a motor datasheet

KV is not power

KV is revolutions per minute per volt, measured with no propeller attached. A 1900 KV motor on a 6S pack at 22.2 V spins at roughly 42,000 rpm unloaded. With a propeller on it, far less.

KV tells you how fast, not how strong. It is a winding choice: the same motor is sold in several KV ratings, wound with different numbers of turns of different wire. More turns gives lower KV, more torque per amp and a bigger propeller. Fewer turns gives higher KV, less torque and a smaller propeller spun faster.

The practical rule: match KV to the propeller and the cell count, not to how fast you want to go. Too much KV on too big a propeller draws current until something burns.

Stator size is the real capability

The four-digit stator code is diameter and height in millimetres. 2207 is 22 mm across and 7 mm tall. This, and not KV, is what sets how much torque the motor can make and how big a propeller it can drive.

Stator volume scales roughly as diameter squared times height, and thrust follows it. Going from a 2207 to a 2807 is a 60% increase in diameter squared and a large step in what it can swing. Going from 2207 to 2205 is a small reduction in height and a modest one.

Conventional pairings, which are exactly that and not laws:

• 1103 to 1404: 2 to 3 inch propellers, whoops and micros
• 2205 to 2306: 5 inch, the freestyle and racing standard
• 2806 to 2812: 6 to 8 inch, cinelifters and long range
• 3520 and up: 10 inch and beyond, survey and heavy lift

Thrust tables are the only real data

A thrust table lists, for one propeller at one voltage, a series of throttle settings with the current, power, thrust, rpm and efficiency at each. It is the only part of a datasheet that is a measurement rather than a claim.

Read it for three things:

• Thrust at 100%, which sets your thrust to weight
• Current at 100%, which sizes the ESC and the battery
• Efficiency at about 50%, which sets your flight time, because that is where a correctly sized aircraft hovers

If a motor is sold with several propellers, the table changes completely for each. There is no such thing as "the" thrust of a motor.

Grams per watt is the endurance number

Efficiency for a propulsion system is thrust divided by electrical power in, in grams per watt. It falls as throttle rises, which is the single most important fact about multirotor endurance.

A typical 5 inch setup manages 4 to 6 g/W at hover. A large low-KV motor on a 15 inch propeller reaches 10 to 14 g/W. That difference is why survey aircraft are big and slow, and it is a disc area effect: a bigger propeller accelerates more air more gently, and gentle is efficient.

hover time ≈ (battery Wh × usable fraction) ÷ (AUW in g ÷ g/W)

Use 0.8 for the usable fraction. Running a LiPo flat damages it, and the last 20% of a pack sags too much to be useful anyway.

Figures to distrust

Maximum power is often quoted as a continuous rating and then contradicted by the manufacturer's own thrust table. One catalogued motor advertises 2,281 W while its test data shows 8,842 W at full throttle. Both numbers are in the same PDF.

Maximum thrust is quoted at the largest propeller the motor will physically take, on the highest cell count, for a few seconds. It is real and it is not a number you should design a hover around.

Weight is sometimes with the wires and sometimes without, and the difference on a small motor is 10%. Where a figure here says how long a wire it includes, that is because the manufacturer said so.