How Does a Quadcopter Rise, Tilt and Turn with Four Rotors?

• Drone seen from above
• Horizontal direction in the drawing
• Vertical direction in the drawing

What you will learn

• Distinguish total lift from force differences used to control direction.
• Trace a motion request into different rotor outputs.
• Explain why releasing the sticks in Position differs from holding attitude alone.

Before you start

No flight experience is needed; it helps to picture how a combined force determines the motion of the whole drone.

How does total thrust make the drone rise?

Rotors turn propellers, accelerate airflow downward and receive an upward reaction force. When total upward thrust equals weight, the drone can hold altitude under ideal conditions; thrust above weight accelerates it upward, while less thrust accelerates it downward. So “more throttle” first changes force; it is not itself an automatic altitude command.

Thrust is not just rotational speed. A propeller acts on the air flowing through its disk; NASA describes the force on that disk as depending on pressure change and area, while momentum theory relates thrust to the mass of air and its change in velocity. Blade shape, air and load also affect the result. The same motor command therefore does not guarantee the same altitude in every situation.

How does a force difference tilt the frame?

To tilt, the controller creates a force difference between rotors on opposite sides. In this illustrative geometry, for example, increasing force on the left and decreasing it on the right creates a roll torque to the right. As the frame tilts, the direction of total thrust tilts too; part of that force then pulls the drone sideways. That is why a drone usually tilts before it translates horizontally.

This is a geometric example, not a fixed motor diagram for every frame. Rotor positions and body axes determine which combination causes roll or pitch. PX4 uses configured geometry and output mapping to turn desired torque and thrust into actuator signals; control allocation computes commands for that configuration.

How do torque differences turn the drone?

Each spinning propeller creates reaction torque on the body. Arranging two rotors clockwise and two counter-clockwise lets their torques cancel when the groups are balanced. To yaw around the vertical axis, the controller increases force from one spinning group and reduces it from the other: the remaining torque imbalance turns the body. Yaw changes heading without requiring a sideways tilt.

The three requests—increase total thrust, make a force difference across the frame, and make a difference between spin directions—can happen together. The allocator must find the closest combination within motor, frame and configuration limits. When a motor reaches its limit, not every requested torque remains achievable.

How does a rightward command pass through the system?

Suppose a learner asks a simulated quadcopter to move right while holding altitude. The position controller turns the horizontal request into a desired acceleration; the attitude controller requests a tilt angle; the angular-rate controller requests torque; control allocation converts the requests into force levels for individual rotors. As the body tilts, the combined force has a horizontal component and the drone starts moving right.

When it reaches the target velocity, the system can reduce the tilt so it does not keep accelerating. To stop, it needs to tilt the opposite way, reduce velocity, then return close to upright. This is a continuous feedback chain, not a one-time command to “switch on” one side's motors. The names and details of each stage depend on the flight stack and control mode.

How is Position different from attitude hold?

Position uses estimated position and velocity to reduce drift and hold a three-dimensional location when the controls return to centre. Attitude hold alone only makes the body track a desired angle; wind can still move the whole drone if no position loop corrects it. Mode names and prerequisites, such as position information, depend on the flight stack; PX4 documents the behaviour of each mode separately.

A simulator walkthrough

Proposed illustration, not a measured result. Start with a stationary virtual quadcopter and inspect total force and each rotor's force. Increase all four forces together to observe the vertical response. Then create a left-right difference and observe roll angle and horizontal acceleration; finally, make a difference between the counter-rotating groups and observe yaw. Read three separate signals: altitude, tilt angle and heading. Do not infer a specific command level for real hardware.

Common misunderstandings

Tilting right by itself means the drone has already moved right. Tilt changes the direction of total thrust; horizontal motion also depends on acceleration and existing velocity. All rotors spin in the same direction to lift. Opposite spin directions help balance yaw reaction torque, while the propellers are chosen to push air in the same direction.

Frequently asked questions

What does a drone need to rise instead of hold altitude?

Total upward thrust must exceed weight to produce upward acceleration; when they balance, vertical acceleration is zero in the idealized model.

Why change force between the rotors?

A force difference across the frame creates roll or pitch torque, tilting the frame and pointing thrust sideways.

Why do two rotor groups spin in opposite directions?

Balanced reaction torques keep the body from turning by itself; changing that balance creates a yaw command.

Does Position hold altitude and location the same way on every drone?

No. Behaviour depends on the flight stack, state estimates and selected mode; verify behavior for that configuration.

Summary

Total thrust controls vertical acceleration; force distribution across the frame creates roll and pitch; torque difference between spin directions creates yaw. Tilting also gives total thrust a horizontal component. Position adds position feedback to reduce drift, which an attitude loop alone does not do.