Drones and self-driving basicsMiddleLesson 2817 min read

How a drone stays in the air

Four spinning rotors push air down, and the drone goes up. Learn how thrust, weight and tiny speed changes keep it level, turn it and move it.

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In 60 seconds

How a drone stays in the air

Four spinning rotors push air down, and the drone goes up. Learn how thrust, weight and tiny speed changes keep it level, turn it and move it.

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In simple words

Hold a table fan and switch it on. You feel it push back against your hand. That push is called thrust. A drone has four small fans pointing down. When they push hard enough, the drone lifts off. When one side pushes a little harder, the drone tips and moves. A tiny computer changes the fan speeds again and again, very fast, so the drone does not fall over.

The most common small drone is a quadcopter: a frame with four motors, each spinning a propeller. A propeller pushes air down, and by Newton's third law the air pushes the propeller up. That upward push is thrust.

Gravity pulls the drone down. The pull is its weight: mass times gravity, about mass × 9.81 newtons on Earth. To hover, the total thrust of all four motors must equal the weight. More thrust and the drone climbs. Less and it sinks.

Why two rotors spin one way and two the other

A spinning propeller also tries to twist the drone the opposite way. If all four spun the same way, the whole drone would spin around like a top. So two diagonal rotors spin clockwise and the other two spin anticlockwise. Their twists cancel out, and the drone stays pointing in one direction.

How a drone moves with no steering wheel

A quadcopter has no rudder and no wheels. It moves only by changing the speed of its four motors.

To do this	The flight controller does this
Go up or down	Speeds up or slows down all four motors together
Tilt forward (pitch)	Back motors a little faster than front motors
Tilt sideways (roll)	Motors on one side faster than the other side
Turn on the spot (yaw)	Clockwise pair faster than the anticlockwise pair, or the other way round
Fly forward	Tilt forward: part of the thrust now points backwards and pushes the drone ahead

Tilting has a cost. When the drone tilts, only part of the thrust points up. To keep the same height, the motors must push harder. That is why a drone uses more battery when it flies fast or fights wind.

Python: how much extra thrust a tilt needspython

import math

weight_n = 1.0 * 9.81   # a made-up 1 kg drone

for tilt_deg in [0, 10, 20, 30, 45]:
    tilt = math.radians(tilt_deg)
    needed = weight_n / math.cos(tilt)      # total thrust to keep the same height
    forward = needed * math.sin(tilt)       # the part that pushes the drone sideways
    print(f"tilt {tilt_deg:2d} deg: total thrust {needed:5.2f} N, "
          f"forward push {forward:5.2f} N, extra {100 * (needed / weight_n - 1):4.1f}%")

The flight controller keeps it balanced

A quadcopter is unstable on its own. A small gust tips it, and without correction it would flip. A small computer called the flight controller reads its motion sensors many times every second. If the drone tips left, it speeds up the left motors a little to push it back. This fast loop of "measure, compare, correct" is called feedback control. A common method is a PID controller, which reacts to the error now, the error building up over time, and how fast the error is changing.

Thrust-to-weight ratio

Makers compare the maximum total thrust with the weight. This is the thrust-to-weight ratio. At 1.0, the drone can only just hover at full power, with nothing left to climb, turn or fight wind. Many hobby builders aim for about 2 to 1 or more, so the drone hovers at around half power. Racing drones use much higher ratios.

1
Weigh everything
Frame, motors, propellers, battery, camera, wires. Battery is often the heaviest part.
2
Work out weight in newtons
Mass in kg × 9.81.
3
Find maximum thrust per motor
From the motor and propeller maker's test table, with your battery. Treat it as a rough guide.
4
Compute the ratio
Motors × thrust per motor ÷ weight.
5
Check the margin
Too close to 1 means a sluggish, unsafe drone. Remove weight or change parts.

Note

Batteries are the big limit. A bigger battery gives more energy but also adds weight, which needs more thrust, which drains the battery faster. This is why most small battery drones fly for minutes, not hours.

Watch out

Spinning propellers can cut skin badly. When you test motors on a bench, remove the propellers unless the test needs them, and keep hands, hair and loose clothes away.

Try this. Run the code and look at the 45 degree row. Why does a drone that is already near full power fall out of the sky if it tilts too much?

Watch and read more

Lab

Three small functions that check whether a drone design can hover and how much it can carry.

~20 min

The problem

Everything runs in your browser. The drone numbers are made up.

Finish hover_thrust_per_motor: the weight in newtons, shared equally by the motors.
Finish thrust_to_weight: all motors' maximum thrust added up, divided by the weight in newtons.
Finish max_payload_kg: the heaviest extra load that still keeps total thrust at least ratio times the total weight. Return 0.0 if the empty drone already misses the ratio.
Run it. Then write two sentences: is the example drone a good design for carrying a small camera? What would you change?

Hint for step 3: the heaviest total mass is total thrust ÷ (ratio × G). Subtract the drone's own mass.

Practice lab

Edit the code and run it. Python runs in your browser — nothing is sent to our server.

Ctrl + Enter to run

G = 9.81  # gravity, metres per second squared

def hover_thrust_per_motor(mass_kg, motors=4):
    """Thrust (newtons) each motor must give so the drone just hovers."""
    # TODO: total weight is mass_kg * G, shared equally by the motors
    return 0.0

def thrust_to_weight(max_thrust_per_motor_n, mass_kg, motors=4):
    """Total maximum thrust divided by weight. Hovering needs at least 1."""
    # TODO
    return 0.0

def max_payload_kg(max_thrust_per_motor_n, mass_kg, motors=4, ratio=2.0):
    """Heaviest extra load (kg) that keeps total thrust >= ratio x total weight.
    Return 0.0 if even the empty drone does not meet the ratio."""
    # TODO
    return 0.0

# A made-up small drone: 1.2 kg, four motors, each can push up to 6 N at full power.
mass = 1.2
per_motor = 6.0
print("Weight (N):", round(mass * G, 2))
print("Hover thrust per motor (N):", round(hover_thrust_per_motor(mass), 2))
print("Thrust-to-weight:", round(thrust_to_weight(per_motor, mass), 2))
print("Max payload at 2:1 (kg):", round(max_payload_kg(per_motor, mass), 3))

Output

No output. Add print(...) to see values.

You are done when

The three functions give the right numbers for the example drone (hover about 2.94 N per motor, ratio about 2.04)
The payload function returns 0.0, not a negative number, for a drone that is too weak
The write-up notices that the example drone has almost no spare capacity at 2 to 1
The suggested change is concrete: lighter battery or frame, stronger motors, or fewer extras

Hard questions

Try to answer before you reveal. If you can answer these, you understood the lesson.

Q1Why can you not just add a much bigger battery to fly twice as long?Reveal

A bigger battery is heavier. The motors must make more thrust to hover, which draws more current, which drains the battery faster. The thrust-to-weight margin also shrinks, so the drone becomes sluggish and less safe. Past some point, adding battery gives very little extra flight time, and can even reduce it.

Q2Your drone hovers at 50% throttle at the seaside. On a hot day in a hill town it needs 65%. Why?Reveal

Air is thinner at height and when it is hot. Each propeller pushes less air per turn, so it makes less thrust at the same speed. The motors must spin faster to hold the same weight, leaving less spare thrust and draining the battery faster. Plan for this before flying in hills or summer heat.

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Questions people ask

Why do drones have four rotors and not one, like a helicopter?

A helicopter controls itself by changing the angle of its blades as they spin, which needs complex moving parts. A quadcopter uses fixed propellers and just changes motor speeds. That is mechanically simple and cheap, and the computer does the hard work. Drones with six or eight rotors also exist; they can carry more and may keep flying if one motor fails.

Can a quadcopter glide if the battery dies?

No. It has no wings, so without thrust it falls. That is why good drones warn you early about low battery and can land or return on their own before it runs out.

What is a fixed-wing drone?

It flies like a small aeroplane: wings give lift as it moves forward. It is much more efficient, so it can fly longer and farther, but it cannot hover and needs space to take off and land. Some designs combine both.

Does altitude or heat change thrust?

Yes. Thin air, which you get high up and on hot days, gives the propellers less air to push, so each rotor makes less thrust at the same speed. A drone that hovers easily at the seaside may struggle in the hills in summer.

Lesson test

5 questions. Get 3 right (60%) to pass and complete this lesson.

How a drone stays in the air

How a drone stays in the air

Why two rotors spin one way and two the other

How a drone moves with no steering wheel

The flight controller keeps it balanced

Thrust-to-weight ratio

Weigh everything

Work out weight in newtons

Find maximum thrust per motor

Compute the ratio

Check the margin

Watch and read more

The problem

Practice lab

You are done when

Hard questions

Questions people ask