Technology
How does a drone hover?
A quadcopter has no wings, no tail and no moving control surfaces, yet it hovers, turns and darts sideways. Everything comes from changing the speed of four propellers, and from the computer that keeps adjusting them.
In short. Each propeller pushes air down; when the four together push as hard as the drone weighs, it hovers. Two spin one way and two the other, so the drone does not spin. To move, it tilts by making one side push harder, and part of the thrust then pushes it sideways. A quadcopter cannot balance on its own: a flight controller adjusts the motors hundreds of times a second. And hovering costs energy every second, which is why small drones fly for minutes.
Lift
A propeller is a spinning wing that throws air downwards; the reaction pushes the drone up. Spin all four faster, and the drone climbs; slower, and it sinks. Hovering is the point where the total push equals the weight.
Not spinning: the torque trick
Every spinning propeller also twists the drone the opposite way. A helicopter cancels that twist with a tail rotor. A quadcopter cancels it by spinning two propellers clockwise and two anticlockwise. To turn on the spot, it speeds up one pair and slows the other, keeping the total lift the same: the twists no longer cancel, and it rotates.
Moving: tilting the push
To fly forward, the rear propellers push a little harder than the front ones, so the drone tips forward. Its thrust now points slightly forwards as well as up, and that forward part moves it. Rolling left or right works the same way with the sides.
Why it needs a computer
A quadcopter is unstable: left alone, a small tilt grows into a flip. Gyroscopes and accelerometers sense its motion, a filter blends their readings (what a Kalman filter is), and a flight controller corrects the four motor speeds hundreds of times a second. Better software holds it steadier in a draft and closer to its path. For the full maths, Robert Mahony, Vijay Kumar and Peter Corke’s tutorial on modelling and control of quadrotors is the classic starting point.
Why small drones fly briefly
A plane glides on its wings; a quadcopter pays for every second in the air. Physics makes small propellers less efficient: they must throw a small amount of air very fast to make the same lift, which costs more power than throwing a lot of air slowly. A light drone also carries a light battery. The result is flights measured in minutes, and docks that charge or swap batteries: a dock that swaps the battery.
Indoors, the air fights back
Near the floor, the downwash cushions the drone; near a ceiling, it can be pulled upwards; near walls and in drafts from fans or open windows, the air pushes it sideways. Flying a few centimetres from furniture is a control problem as much as a sensing one: flying a drone indoors. Changing shape in flight adds another: drones that fold.
How Alkyon is designed
Alkyon, our concept for a flying home companion, is a quadcopter with guards round its four propellers and arms that fold to pass narrow doors. In our simulation, its controller is tested against drafts, delays and sensor noise before anything flies for real: why robots are tested in simulation first. It is a concept in development, for Europe only: an autonomous indoor drone.
Source, read 4 November 2026: Mahony, Kumar and Corke, multirotor aerial vehicles: modeling, estimation, and control of quadrotor, IEEE Robotics & Automation Magazine, 2012. General information.
Questions
How does a quadcopter turn without a tail?
Two of its propellers spin clockwise and two anticlockwise, so their twisting forces cancel out. Speeding up one pair and slowing the other leaves a small net twist, and the drone turns.
Why do small drones fly for so short a time?
Hovering costs energy all the time, and small propellers need more power for the same lift than large ones. A light drone also carries a small battery, so flights last minutes.