Quadcopters and drones

This page covers flying toys as well as larger and more serious drones.


In recent years, drones, both amateur and professional, and flying toys of various kinds have been made possible at reasonable cost by the availability of lithium batteries with a high power to weight ratio, and cheap control electronics and sensors. Particularly with toys, damaging crashes are common and spares are available for the more popular brands. The coreless motors used in toys and cheap drones have limited life but are replaceble.


Be sure you know how to fly your device safely. Rotating propeller blades can be very dangerous, especially the larger ones.
There are strict laws in many territories governing the use of drones and quadcopters, as well as radio controlled model aircraft of all types. See Legal below.

Basic Principles

Aerodynamic Principles

The idea of a machine which screws itself into the air dates back to Leonardo da Vinci in 1493, but it was another 450 years before the first practical helicopter took to the air.

Unfortunately for Leonardo, Sir Isaac Newton had yet to formulate his Laws of Motion (published only in 1687). According to his 3rd law (action and reaction are equal and opposite), in forcing its rotor blades to turn against air resistance, the engine of a helicopter necessarily applies and equal and opposite turning force to the aircraft's body. So nearly all helicopters have a tail rotor to counteract this turning force. If a helicopter looses its tail rotor it spins out of control - the chances of surviving such an accident are very slim indeed.

Just a few helicopters and nearly all drones and flying toys counteract the turning force in a different way. They have two or more rotors spinning in opposite directions. The Boeing Chinook is an example. Some helicopters and many toys have two counter-rotating rotors on the same spindle. Quadcopters have 4 rotors but you can have any number, provided its an even number, and provided you make half of them spin one way and the other half spin the other way in order to balance out the twisting force.


A great advantage of having multiple rotors is that you can very simply manoeuvre your drone by deliberately unbalancing the rotors slightly. Applying a little more power to either the two rear or the two front rotors of a quadcopter causes it to pitch, i.e. to tip forward and so start moving forwards or tip back and move backwards. Equally, you can apply slightly more power to two side rotors make it roll and move sideways, or the two clockwise rotors to unbalance the twisting forces and make it turn anticlockwise, or vice versa. This is known as yaw.

A hand-held controller with two joysticks is normally provided. Typically one controls forwards, backwards and sideways motion, and the other the overall power and yaw. A radio link transmits commands to the device.

Model helicopters have two concentric counter-rotating rotors. Yaw is achieved by increasing the power to one and decreasing it to the other. A small propeller on the tail pointing upwards (unlike nearly all full size helicopters) tips the craft forwards or backwards to control the motion.

You may come across a flying toy simply consisting of a ball with a pair of counter-rotating rotors. An infra-red sensor in the bottom detects the proximity of the ground and boosts the power to keep it airborne.

The more sophisticated quadcopters and drones have a variety of sensors. A quadcopter is naturally unstable and would be very difficult to control without an accelerometer which (probably) even the most basic include, and which they use to automatically maintain level flight. A camera is common, which may record to an SDCard or transmit real-time video to the controller or a smartphone app. An air pressure sensor allows the device to maintain (roughly) a given height. This makes it quite a lot easier to control. There may be a magnetometer to enable it to maintain a given magnetic compass heading, or a proximity sensor to enable it to make an automatic soft landing and avoid obstacles.

A GPS module may be included, allowing it to follow a predetermined path or to automatically return to base. A "follow me" or "active tracking" facility is included in many serious amateur drones. This either uses sophisticated image processing software to track the target, or a GPS device that you have to carry which transmits your position to the quadcopter, or both.


Common faults are physical breakages, unstable flight, and low power. Spares for the more popular models are availble from Far-Eastern suppliers such as AliExpress or Banggood and through eBay shops, if not locally.

Breakages and Mechanical Faults

Crashes are common, especially amongst toys and in the hands of youngsters, and these can lead to breakages. To minimise damage, read the instructions carefully and make sure you know how to kill the rotors dead in the event of a crash.

Cheap quadcopters are often so light that they can fall out of the sky with relative impunity, but if the rotors continue to receive power after becoming entangled or jammed, then damage may result.

It should be possible to repair damage to the plastic shell with 2-part epoxy glue.

Often a set of spare propellers and guards are provided, and for the more popular brands, spares are easily obtained. In fact the smaller propellers are fairly indestructible but nevertheless can get pitted with rough handling. They may simply be a push-fit on the motor spindle and in a crash, they can sometimes come off and get lost. The clockwise and counter-clockwise propellers should be easily distinguishable, for example marked A and B. Make sure you replace an A with an A or a B with a B, or it will be impossible to fly.

All the propellers should turn equally freely. For a quick test, flick each in turn and see how long it spins for. A slightly better test might be to blow on each propeller in turn, or better still, hold your quadcopter in front of a fan. If you are able to start the propellers spinning without taking off (or hold the quadcopter down - but beware of the spinning propellers!) then turn it off, check whether any of the propellers stop spinning significantly quicker than the others.

A cause can be hairs getting tangled around the spindle between a propeller and its motor, resulting in additional friction and loss of control. A magnifying glass and a pair of tweezers may be helpful in removing it, but this is likely to be difficult if the hairs get into the motor bearing.

If the quadcopter has been disassembled (or has never flown well) there could be something rubbing on a spindle. For example, there might be a rubber boot over each end of the motor to make it fit snugly in its housing, and this could be rubbing against the spindle if not correctly fitted.

In some quadcopters the propellers are driven directly from the motor spindles whereas in others they are driven via gears which reduce the speed of rotation. Check that the gears are undamaged, clean and free of anything which might obstruct them.

Electrical Faults

The electronic controls should be highly reliable but wires and solder joints can get broken. Wires can easily get pinched between two halves of the shell on reassembly.

Motors can fail, either completely or they may loose power as the brushes get worn. Fortunately, replacements are available from Far Eastern sources and they are relatively easily fitted, though it may require fine soldering.

There are two distinct types of motor:

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The brushes from a failed coreless motor. The far brush has completely worn away.

The smaller ones use coreless motors. These have two wires, and whilst you can make them spin in the reverse direction just by swapping the wires, the arrangement of the brushes is normally optimised either for clockwise or anticlockwise service. Failure of the brushes is a common problem since they are made of springy metal which gets eroded with the heavy current and inevitable sparking until they fail completely, as shown in the accomanying photo. As a result they may have a limited life.

Sets of four, two clockwise and two anticlockwise are often advertised, commonly distinguisable by one pair having red and blue wires and the other pair white and black. If you need to replace one it's probably worth replacing them as a set as the others may not have much life left in them. This will also avert any problems if the new set are not identical to the old.

You can test a coreless motor (having disconnected it from the drone) simply by connecting it to a 3V battery, but the fact that it spins won't necessarily confirm that it can spin as freely as it should. They can be tricky to disassemble and are not generally repairable, but fortunately they are cheap to replace especially from Far Eastern sources.

Larger drones invariably use brushless motors. These have 3 wires and rely an electronic circuit on the main control board or often in a separate package, one for each motor, to drive a current through different pairs of wires as the spindle turns. Simply by reversing the phasing of the currents, it can be made to spin in the opposite direction. In fact, it will spin in the wrong direction if you get two of the wires reversed in replacing such a motor.

You can't so easily perform a functional test on a brushless motor, but the resistance of each of the 3 pairs of leads as measured with a multimeter should be very low indeed. If any is open circuit then the motor has probably burnt out.

Flight problems

If your drone or quadcopter fails to take off, it could be that the battery is getting weak. Even if it measures 4.2V on full charge it may no longer be able to deliver the heavy current demanded of it. Cheap replacement batteries may not last long, or may not be able to deliver the necessary power even from new. Check that the battery does actually reach 4.2V on full charge (or 4.2V per cell for a multi-cell battery pack). If not, perhaps the charger is faulty.

Also, double check that the clockwise and anti-clockwise propellers are fitted to the right motors. If not it will fail to take off, or immediately flip over and crash.

Check, too for pitting of the rotor blades, which could cause an increase in their air resistance and unstable flight, and check for free rotation of the blades as described earlier. Disassembly may be required in order to see and remove any hairs or other foreign matter around the spindles.

Read the instructions carefully and pay particular attention to calibration and trim. The accelerometer may need calibration to ensure that its idea of down really is down, and not skewed off slightly. Trimming whilst in flight may be tricky, especially if you're indoors in a relatively confined space, outdoors with any wind, or if it's not flying well. If you know which way it needs to be trimmed it may well be possible to adjust the trim before taking off, or to reset the trim if you only seem to be making it worse.

Finally, understand the ground wash effect. When near the ground the propellers give extra lift as the air has nowhere to go except sideways. If it's near the edge of a table or on an uneven surface this effect will be more pronounced on one side than the other and it may scoot off sideways and crash when you try to launch it. In the same way, if you fly it low over an edge or uneven surface it may be hard to control.


Legal restrictions on the use of airborne devices such as drones will vary from one country to another.

  • In the UK you need a licence if your device is over 250g. See Drones: how to fly them safely and legally for the official Governmet guidance. More informally, you might like to look at this Quick Start Guide from RC World.
  • In the EU, from 2021 you will have to register yourself as a drone owner/operator unless your device comes under the official definition of a "toy".