Quadcopters and drones

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This page will cover 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, but other faults may also arise.


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 on 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 the two rear rotors of a quadcopter causes it to tip forward and so start moving forward. You can do the same with the rear two to make it move backwards, or two side rotors to make it move sideways. Equally, you can apply slightly more power to 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 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 it to roughly maintain a given height, making 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 enale it to make an automatic soft landing. A GPS module may be included, allowing it to follow a predetermined path or to automatically return to base. A variety of sensors may be included to implement a "follow me" function to film your sports activity.


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 ones 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 may be possible to repair damage to a plastic shell with 2-part epoxy glue.

Often a set of spare rotor blades and rotor guards are provided, and for the more popular brands, spares are easily obtained. In fact the smaller rotors are fairly indestructible but may simply be a push-fit on the motor spindle. In a crash, they can sometimes come off and get lost. The clockwise and counter-clockwise rotors should be easily distinguisahble, 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.

If the problem is unstable flight, try flicking each rotor in turn. They should all spin equally freely. If not, this is likely to be the cause of the problem.

Hairs can get tangled around the spindle between a rotor and its motor, causing additional friction and loss of control. A magnifying glass and a pair of tweezers may be helpful in removing it, but this will get difficult if the hairs get into the motor bearing.

If the quadcopter has been disassembled (or has never flown well) there could be something else 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 rotors are driven directly from the motor spindles whereas in others they are driven via gears to 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. 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:

The smaller ones use coreless motors. These have two wires, and whilst you can make them spin in the reverse direction simply by swapping the wires, the arrangement of the brushes is normally optimised either for clockwise or anticlockwise service. Failure of the brushes is probably the most common problem. 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.

You can test a coreless motor 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.

Larger drones invariably use brushless motors. These have 3 wires and rely an electronic circuit 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, if you get two of the wires reversed in replacing such a motor it will spin in the wrong direction.

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 burned out.


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