I. The Flight Principles of Drones

Rotors, much like wheels, represent a remarkable invention.

Quadcopters, in particular, have evolved into aerial photography devices, fulfilling many ordinary people's dreams of the skies.

The reason rotors can fly is something anyone who has flown a bamboo dragonfly should understand: when the hand's motion imparts rotational speed to the dragonfly, lift is generated, enabling it to take flight.

Multirotor drones fly the same basic way—their motors spin, spin the propellers, and boom—lift is born! Take quadcopters, for example: when the combined lift from all four propellers is exactly equal to the drone’s weight, lift and gravity do a perfect balance act (no tipping, no falling)—and the drone just hangs out mid-air, like it’s frozen in place, totally steady.

Remember those Doraemon comics from when we were kids? I’d flip through them for hours, obsessed with how Doraemon and Nobita zipped around the sky with those tiny bamboo dragonflies on their heads. I’d zone out daydreaming: what if I could do that too? Soaring high above the earth, peeking down at the world like a curious bird—total childhood fantasy, am I right?

But if someone were to invent an identical bamboo dragonfly today, I certainly wouldn't want to wear it. Because the actual flying experience would be like this:

The propeller spun wildly, and the man spun wildly in the opposite direction...

Doraemon was completely dizzy from all the spinning—how could he possibly enjoy the scenery with Shizuka now?

According to Newton's Third Law, as the rotor spins, it simultaneously exerts a reaction force (counter-torque) on the motor, causing it to rotate in the opposite direction. This explains why modern helicopters feature a ‘tail rotor’—applying a horizontal force to counteract this reaction torque and maintain airframe stability.

Returning to quadcopters, their propellers generate similar forces. To prevent uncontrolled spinning, the four propellers are arranged such that adjacent pairs rotate in opposite directions.

As illustrated below, the triangular red arrows indicate the aircraft's nose orientation. Propellers M1 and M3 rotate counter-clockwise, while M2 and M4 rotate clockwise.

During flight, the counter-clockwise reaction forces (counter-torques) generated by M2 and M4 cancel out the clockwise reaction forces (counter-torques) produced by M1 and M3. This allows the aircraft's body to remain stable, preventing it from spinning wildly like Nobita.Not only that, but the forward, backward, lateral, and rotational flight manoeuvres of multi-rotor aircraft are all achieved through the speed control of multiple propellers:

Vertical take-off and landing

Let me tell you—this part is so easy, you’ll wonder why you ever thought it was tricky! Imagine your drone’s got a little “up button” in its brain: when it wants to climb higher (gain altitude), all four propellers hit the gas together, spinning faster than a kid on a sugar high. They churn out extra lift, like four tiny superheroes lifting the drone up nice and steady—like it’s floating up to wave at the birds or snap a better pic from above.

Want it to come back down? No panic, no crash—all four propellers hit the brakes in perfect harmony. None of ’em cheat by slowing down too fast or lagging behind; they ease off together, dialing back the lift just enough. The drone drifts down gently, like a balloon with a tiny leak (but way more intentional—no sad deflating here!).

But hey, don’t skip this next part—it’s *super* important! That “all together” rule? It’s not a suggestion, it’s a drone commandment. Keeping those propellers spinning at roughly the same speed is how you keep your drone from doing a wobbly backflip mid-air—think of it like a group of friends carrying a couch: if one person slacks off, the whole thing tips over. Later, I’ll spill the tea on why this stability stuff is make-or-break for how your drone flies—and I swear, it’s way more interesting than it sounds!

Rotation in Place

Like I mentioned before—no fancy jargon! When all the drone’s motors spin at the same speed, their counter-torque cancels out, like two people pulling opposite ways equally—no spin, just steady hovering. But to make it spin in place (stationary rotation), we use that counter-torque: speed up clockwise motors (M2, M4) and slow down counter-clockwise ones (M1, M3), and it spins smoothly like a tiny high-tech top.

Altitude control is just as easy: to climb, all four propellers speed up together for extra lift; to descend, they slow down in sync. That “all together” rule is non-negotiable—it keeps the drone stable, like friends carrying a couch evenly—slack off and it tips! I’ll explain why this stability matters later, I promise it’s not boring.

Horizontal Movement

Unlike the passenger aircraft we commonly board, multi-rotor aircraft lack propellers oriented perpendicular to the ground. Consequently, they cannot directly generate horizontal thrust for lateral movement.

This poses no challenge to us, however. Taking the quadcopter from the diagram above as an example: when advancing in the direction of the triangular arrow, the propellers driven by motors M3 and M4 increase their rotational speed, while those driven by motors M1 and M2 decrease theirs. As the lift generated at the rear of the aircraft exceeds that at the front, the aircraft's attitude tilts forward.When tilted, the side-on view is as shown below. At this angle, the lift generated by the propeller not only counteracts the aircraft's gravitational pull vertically but also produces a horizontal component. This horizontal force imparts forward acceleration to the aircraft, enabling it to fly in a straight line.Conversely: when motors M1 and M2 accelerate while motors M3 and M4 decelerate, the aircraft tilts rearward, thereby flying backwards.

By the same token: when motors M1 and M4 accelerate while motors M2 and M3 decelerate, the aircraft tilts leftward, thereby flying left;

When motors M2 and M3 accelerate while motors M1 and M4 decelerate, the aircraft tilts to the right, thereby flying right.

Explained this way, doesn't the flight principle of multirotors seem rather straightforward?~

In truth, before multirotors, aerial photography relied on far more complex fixed-wing aircraft and helicopters.

However, fixed-wing aircraft demand highly specific take-off and landing sites, cannot hover, and lack vertical take-off and landing capabilities – their limitations are simply too great.Whilst helicopters boast substantial payload capacity and high speed, their structure is exceedingly complex and intricate. The thousands of components involved present considerable challenges in both commissioning and maintenance.By comparison, the flight principles of multi-rotor drones are simpler, resulting in more straightforward and reliable airframes. Consumers can quickly master flying them without excessive calibration or maintenance, which is why multi-rotor drones rapidly captured the aerial photography market.