First of all if your looking for a guide how to actually build a quad copter then use the search at the top and you'll find plenty of my projects. This blog article is aimed at covering the absolute essentials I discovered the hard way in hope that you can benefit and skip the failure stages I endured.
Creating a quad copter is actually fairly simple if you follow some simple rules and just want to get flying, learning the essentials of developing your own design.
CoG - Centre of Gravity
Alike anything that flies having the CoG is critical. However on multi rotor models its as simple as making sure the CoG falls exactly in the middle so all motors have an equal load between them - this is critical. However where is best in terms of vertical alignment? (lower/ center /higher).
Well this is where designs can vary depending on what your objectives are:
- If you want a really agile model capable of flips and such then sticking the EXACT center of the model is absolutely best.
- If you want a model that is highly stable for say exploration FPV flying then having CoG slightly lower will act like a slight pendulum making the model want to "self" balance itself. However having a lower CoG is a matter of design and tweaking to find the balance that is not only right for your style of flying but also the model itself and how stable it is.
- Higher CoG is extremely unstable and should be avoided. If your design is becoming top heavy then your design is flawed and you need to rethink.
Fixing CoG problems
When it comes to tweaking your CoG position your options are limited by your design. The most obvious components to move around would be your Battery and in some cases your ESC (speed controllers) when they happen to be mounted on the top of arms - move them to the bottom.
If your still running into CoG problems then calculate what your total lifting capacity is and see if it is possible to use a bigger battery pack to help offset weight.
Length of motor arms from center, smaller or longer?
This is a tricky one as both have their place. For example if you want a super stable model then making the arms longer would help but in retrospect the vibrations from the motors can be extremely amplified. In these cases making sure that motor and propellers are well balanced is essential.
Shorter arms will make your model much more agile, responding very quickly to input and have an amazing short turning circle. It's this type I personally like best but it is not for beginners as over-compensating your corrections while flying WILL lead to a crash. However that said the amount of vibrations allowed to resonate across the frame will be drastic reduced, this makes the design amazing for dealing with some knocks and bumps while having fun and not go crazy from one slightly imbalanced propeller.
Self levelling, drones and AP (auto pilot)
I cannot really speak volumes on this subject as I seriously dislike AP and set-ups that "help you fly". I don't want help, I want to fly the model as it wants to fly and learn the characteristics and new methods of recovery.
I personally feel the art of learning a model is lost when you hand over elements of control to a chip. You learn nothing except that your model flies.
Worse still I hate AP on these models not only because of the bad reputation it is bringing the hobby but it is my opinion that using AP is like spending lots of money on a model then hiring someone to fly it for you.
Too often I see people come to the field to fly their model to watch it take off and fly about itself, then they go home. Er... I could do the same thing on a flight simulator except without spending hundreds on a model.
Self levelling boards (multi rotor controllers with gyro assistance) are a good idea for beginners and letting go of the controls will make the model self level itself. However it will not keep its own position so learning to stay in one place in wind and other such skills are still being learnt by the pilot, it's for this reason I really like these types of boards.
Hopefully from reading this you will have thought about some new ideas for whatever your own build is. These types of models much like planes can be as easy or as complicated as you want and better still don't have to cost hundreds to get flying when you have places like HobbyKing.