CW and CCW Technical Breakdown for Outrunner Motor and Propeller Matching

In UAV propulsion systems, the matching of CW (clockwise) and CCW (counter-clockwise) goes far beyond a simple "forward and reverse" rotation. It involves coordinated engineering across multiple domains, including aerodynamics, gyroscopic effects, motor commutation logic, and flight controller control laws. This article breaks down the technical aspects of these "rotational twins" from an engineering perspective.

I. The Essential Definition of CW and CCW

From an engineering viewpoint:

CW (Clockwise): Viewed from above the motor, the rotor rotates in the clockwise direction.

CCW (Counter-Clockwise): Viewed from above the motor, the rotor rotates in the counter-clockwise direction.

These two directions determine three critical aspects:

Propeller blade design: CW propellers have their airfoil lift surface oriented for clockwise cutting, while CCW propellers are a fully mirrored design

Motor commutation sequence: The rotation direction of a brushless motor depends on the energizing sequence the ESC applies to the three phase windings

Counter-torque direction: Each rotating propeller applies an opposite-direction torque to the airframe

II. Why Must Both CW and CCW Exist?

Core Principle: Conservation of Angular Momentum and Counter-Torque Cancellation

According to Newton's Third Law, when a motor drives a propeller to accelerate clockwise, the propeller applies a counter-clockwise reaction torque to the airframe—this is the counter-torque.

If all four motors rotated in the same direction, the counter-torques would superimpose in the same direction, causing sustained horizontal rotation that the flight controller could not compensate for even with maximum output. Specifically, when all four motors generate counter-torque in the same direction, the flight controller must continuously output differential thrust to one side to compensate—consuming additional power and limiting available thrust.

The standard quadcopter uses a "diagonal same, adjacent opposite" configuration for one reason:

The counter-torques from two CW and two CCW motors cancel vectorially in the horizontal plane

The flight controller only needs fine adjustments to each motor's speed for precise yaw control

It is important to note that counter-torque magnitude is not fixed. During actual flight, motor speeds change continuously—all motors increase synchronously during climbs, while diagonal motor pairs spin differentially during yaw maneuvers. Due to the nonlinear relationship between propeller inertia and air resistance, counter-torque is approximately proportional to the square of rotational speed (τ = k·n²). Therefore, the flight controller must continuously adjust motor speed differentials to compensate for the dynamic changes in counter-torque across different flight states, rather than simply achieving static "two-by-two cancellation."

III. Propeller Aerodynamic Characteristics for CW/CCW

Relationship Between Airfoil and Rotation Direction

The cross-section of a propeller is essentially an asymmetric airfoil (convex on top, flat on bottom). When the blade rotates in its intended direction:

Accelerated airflow over the top surface → pressure decreases

Decelerated airflow over the bottom surface → pressure increases

Pressure differential → upward thrust is generated

If a CW propeller is installed on a CCW motor, the airfoil's angle of attack reverses, and the airflow no longer generates lift but rather produces downward negative thrust—the drone doesn't take off; it gets "pressed" into the ground. This is known as a negative angle of attack condition.

How to Identify Propellers

Viewed from above, locate the higher edge of the propeller blade:

Engineering mnemonic: Right-high clockwise, Left-high counter-clockwise

IV. Commutation Logic and Direction Control in Outrunner Motors

An outrunner brushless motor is essentially a three-phase permanent magnet synchronous motor, and its rotation direction is determined by the six-step commutation sequence output from the ESC.

Relationship Between Commutation Sequence and Rotation Direction

For three phase windings (A, B, C), the ESC energizes them in a specific sequence to create a rotating magnetic field around the stator, which then pulls the permanent magnet rotor to follow. Swapping any two phase connections reverses the rotating magnetic field direction, and consequently the motor's rotation direction reverses as well.

This is why the motor itself doesn't have a fixed "CW/CCW"—the direction is determined by the wiring.

Standard Practice for Professional Manufacturers Like BG Motor

For mass-produced motors, to standardize production, we uniformly adopt the A-B-C wiring sequence (defined as the default clockwise direction). During actual installation, customers can adjust the rotation direction through:

Method 1: Swapping any two phase wires between the ESC and motor

Method 2: Using ESC firmware (such as BLHeli, AM32) to set the motor rotation direction at the firmware level without physical rewiring

Standard Verification Process During Assembly

Propeller-less bench test: Test each motor individually through the flight controller (e.g., Betaflight, PX4, ArduPilot) to confirm that the rotation direction matches the preset layout in the flight controller

Confirm layout correctness: Standard quadcopter configuration — M1↻, M2↺, M3↺, M4↻ (viewed from above)

Install corresponding propellers: Based on the confirmed rotation directions, install the matching CW or CCW propellers

Final airflow verification: Run the motors at low throttle with propellers installed. Place your hand under each propeller—all propellers should generate downward airflow

V. Standard CW/CCW Layout for a Quadcopter

It should be noted that this configuration simultaneously determines the flight controller's control allocation matrix. The attitude control outputs—pitch, roll, and yaw—are ultimately mapped to independent speed variations for the four motors: pitch controlled by the front/aft motor pair differential, roll by the left/right motor pair differential, and yaw by the counter-torque differential between the CW and CCW motor pairs. The CW/CCW layout is deeply coupled with the control allocation matrix, so altering the rotation configuration means restructuring the mixer algorithm within the flight controller.

VI. Engineering Summary

CW and CCW matching is essentially the coordinated alignment of motor rotation direction, propeller airfoil, aerodynamic thrust, and counter-torque management:

During actual assembly, follow two golden rules:

Matching Rule: CW motor with CW propeller, CCW motor with CCW propeller (motor direction is determined by ESC wiring)

Layout Rule: Diagonal motors share the same direction; adjacent motors are opposite—ensuring yaw moment balance

To summarize it in one sentence: CW and CCW are not a factory "pre-set" for the motor, but rather a system-level matching result of ESC wiring and propeller selection. When the direction is correct, the airflow goes downward; when it's wrong, the harder you push the throttle, the harder the drone gets pressed into the ground. Mastering this logic means you won't make directional mistakes in UAV propulsion system selection and assembly.