7.2 Electric Motors, Kv and Current Draw

Brushless Motors: Why RPAS Use Them

Almost every commercial RPA uses a brushless direct-current (BLDC) motor. In a brushed motor, physical carbon brushes press against a rotating commutator to switch current between windings; they wear, spark, generate electrical noise and limit the motor's life. A brushless motor moves the switching into the ESC, which energises three sets of stator windings in sequence to drag the permanent-magnet rotor around.

The advantages that matter to an RPA are direct:

• No wearing brushes, so service life is long and predictable.
• Higher efficiency — less energy lost as heat, which on a battery-powered aircraft is endurance.
• Better power-to-weight ratio, which is decisive when everything you add must be lifted.
• Less electrical noise, though a brushless motor and its ESC still radiate enough interference to disturb a nearby compass or GNSS antenna.

Most multirotor motors are outrunners: the outer bell rotates and carries the magnets, while the stator stays fixed in the centre. Outrunners produce high torque at modest rpm, which is exactly what a large propeller wants.

Kv: The Number on the Motor

Kv is the motor's velocity constant: the no-load revolutions per minute it will turn for each volt applied.

$\text{No-load rpm} \approx K_v \times V$

A 920 Kv motor on a 4S pack (nominal 14.8 V) turns roughly 920 × 14.8 ≈ 13,600 rpm with no propeller fitted. Under load, with a propeller attached, actual rpm is substantially lower — but Kv still sets the character of the motor.

Note what Kv is not. It is not a power rating, not a quality rating, and the "K" is not "kilo" — a 920 Kv motor is not more powerful than a 400 Kv motor. The relationship is inverse:

The reason low-Kv motors pair with large propellers is torque. A big propeller has high rotational inertia and high aerodynamic load; a motor that wants to spin fast but cannot produce the torque to turn it will simply draw enormous current trying, and overheat.

Current Draw: The Three Levers

Schedule 4 topic 3 asks specifically about current draw in relation to propeller diameter or pitch and to propeller load. Three levers control it.

1. Diameter. Current draw rises steeply with propeller diameter — roughly with the fourth power of diameter in the classical propeller relations. Going from a 10-inch to an 11-inch propeller on the same motor and pack is not a 10 per cent change; it is far more.

2. Pitch. A coarser (higher-pitch) propeller takes a bigger "bite" of air per revolution. At a given rpm it produces more thrust and draws proportionally more current.

3. Load. Anything that makes the propeller work harder raises current: a heavier aircraft, a climb, a hover in ground effect versus free air, flight into a headwind, and — the one pilots forget — higher density altitude, where the propeller must spin faster to produce the same thrust in thinner air.

Determining an Appropriate Kv

Selecting a motor is a three-way match between Kv, propeller and cell count. Change any one and the other two must be reconsidered.

The manufacturer's thrust table is the authoritative source, and every serious motor is published with one: for a given cell count and propeller, it lists rpm, thrust in grams, current in amps and efficiency in grams per watt. Use it as follows.

Worked example. A quadcopter has an all-up weight of 2.4 kg. Good practice is a 2:1 thrust-to-weight ratio at full throttle, so the four motors together should be able to produce about 4.8 kg of static thrust — 1,200 g per motor.

Reading the manufacturer's table for a 920 Kv motor on 4S with a 9.5×4.5 propeller:

At 100 per cent the motor makes 1,240 g — enough. Hover thrust needed is 2,400 ÷ 4 = 600 g per motor, which sits at about 50 per cent throttle drawing roughly 6.1 A. Four motors therefore draw about 24 A in the hover, and about 76 A at full throttle. Those two numbers drive everything downstream: the ESCs must be rated above 19 A each with margin, and the battery's continuous C-rating must comfortably exceed 76 A.

Notice the efficiency column. The motor is most efficient at low throttle (6.9 g/W) and least efficient at full throttle (4.4 g/W). An aircraft that hovers at 50 per cent throttle is operating near its efficiency sweet spot; one that hovers at 80 per cent because it is overloaded is both inefficient and out of thrust margin.

Symptoms of a Mismatch

A wrongly matched motor, propeller and pack shows itself in recognisable ways:

• Hot motors and hot ESCs after a short flight — the classic over-propping signature. The combination is drawing more current than the components are rated for.
• Severe voltage sag on throttle-up, sometimes triggering a low-voltage warning within seconds of take-off despite a full pack.
• Very short endurance relative to the manufacturer's figure.
• Sluggish, mushy control response — a low-Kv motor with a small propeller cannot change rpm quickly enough for crisp attitude control.
• Twitchy, oscillating hover — the opposite mismatch, where a high-Kv motor on a large propeller gives the control loop far more authority than it is tuned for.
• Desynchronisation ("desync") — the ESC loses track of rotor position under sudden load, producing a distinctive stutter and a momentary loss of thrust on one arm.

The remedy is always the same: return to the manufacturer's specified motor, propeller and cell count. RPAS propulsion is a matched system, and "it fits" is not the same as "it is correct". If an airframe is being modified for a heavier payload, the correct process is to recalculate the required thrust per motor, read it off the manufacturer's thrust table, and confirm the ESC and battery ratings before flight — not to fit a bigger propeller and hope.