Drone crash investigation – why my drone doesn’t fly

The said question

Why does my drone fail to work? Its the number one question for anyone who has tried to build their own drone.
From the frustration of failing to arm, to the unexpected crash. All the possible problems are solvable but there are plenty of possible causes. I have built every part of my drones from simple components and materials. After 3 years and almost every problem conceivable my multi-rotor drones are totally reliable. In this blog I hope to cut some corners for those who choose the hobby and not the RTF option.

Prediter down

Even the most expensive

The answers to the question, are logical and derivable but only if you understand how these craft are supposed to work. Once understood you can build any craft you wish, with confidence it will fly.

Start with the flight control

So how does the FC controls the aircraft. The core of flight controllers is a very simple algorithm called PID. The primary function of this algorithm is to find center. For any aircraft using a flight control the parameters for this algorithm need to be tuned. The parameters stand for proportional, integral and derivative. Occasionally derivative is not required. The only real way to tune these parameters is in manual or acro mode. So proper tuning would require that you can fly in one of these two modes.

I plan to do an article on how to tune in Acro/Manual mode at a later date.

Using PID the flight controller responds to data from gyroscopes, accelerometers and other sensors. The flight controller will only cause the aircraft crash if the information from these devices is incorrect or the PID parameters are wrong.

The three main causes

Crashes will be caused by motor ESC, propeller failure or wiring.

1. Motors generally don't fail however bearings in them do overheat shafts are bent and the wiring to them is sometimes faulty. It is important to always check the temperature of motor after a flight. If the motor is hot there's a very good chance that the bearings are overheating. Motors also overheat if they don't have sufficient torque to turn a large prop. In this case all the motors will be overheating.
2. Propellers often fail because they are not properly attached to the shaft or they aren't balanced. Unbalanced props not only create problems from onboard cameras, but problems for the flight controller, as well. It's not always obvious that the propeller is not securely attached to the shaft. A Slipping prop will slip more and more as a Flight progresses, ultimately causing crash.
3. ESCs generally give the least problems. They generally fail on the first use if they are unsuitable. It's important to check the temperature of your ESCs after each flight. It's also important that the heatsink is functional. The heatsink on the ESC must be pushed firmly against the power transistors. If not the transistors will fail on the first, second or third flight. An ESC should be matched to the maximum amperage of the motor.
4. Faulty wiring is a killer. Broken wires and dry solder joints. Looking for these is often the easiest. When you first arm your copter the motor affected by a dry solder joint or broken wire, will often hesitate. Test suspected wires with a multimeter for resistance and physically test wires by tugging them. Visually, checked for corrosion or large amounts of flux. It only takes one broken wire to ruin your day.

Navigation trouble

The GPS is a major source of data for most fight controllers that use them . It's not usually the GPS itself that's the problem. These days, most GPs also contain the magnetometer or compass . Unfortunately nearly every component in a multirotor produces a magnetic field. It is very very important that the compass stays away from these magnetic fields. That ugly stork that often holds the GPS on commercially made drones, is often overlooked by the people building their own machines. This ugly looking stand is not a gimmick. They often get broken, so when people building their own drones, they try to leave them out. Just Don't!

Broken gyros. It's really unlikely that gyros are faulty. Issues with a magnetometer the compass are often mistaken for problems with gyroscopes or accelerometers. Go back to the compass before messing with these two.

When people build their own drone they generally follow the specs of pre-existing drones. The motors, the ESCs and the frames are generally recommended by the hobby companies selling these products. This is a perfectly valid way to go, if you don't understand why they all go together.

Wants the working combination

The whole packaging is actually a balancing act. It often sounds reasonable to add a bigger battery, or to add larger props. I often hear people say, “I have added larger propellers because they're more efficient”. You could just as easily say I put large wheels on my car because it will give better fuel efficiency, but we know that wouldn't work. The same goes for a drone. So, it's just best to stick to the recommended motor, ESC and propeller combinations recommended by the hobby store. And look at the maximum thrust that this combination is likely to put out. If the maximum trust is less than double the expected weight, don't go this combination. The calculation has to include the weight of the battery. Adding a bigger battery won't necessarily get you longer flight times. The resulting overworked thrust systems, will cause crashes.

The only thing that I haven't mentioned is the frame. Airframes generally don't cause drones to crash, but they do sometimes. It's usually a result of something becoming loose or falling off. The easiest way to prevent this is to use Loctite and to check all screws and bolts before you fly. Motor screws are notorious for becoming loose because the vibrations create. So check these regularly.

Motor bearings worked really hard. Generally when they wear, the motor gets hot. Obviously, the best way to prevent wear is to lubricated beings. The tiny amount of oil in these bearings doesn't last very long. So you need to add some on a regular basis. I usually turned the whole of the multirotor upside down, to do this. Note: Don't get oil on the motor shaft where the propeller attaches.

One crash to another

Fixing drones have a whole stack of other potential problems. I will cover this in a later article. If you had any questions please respond to my blog. Thanks for reading.

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