This was an easy decision. Since this was going to be my first multirotor, I threw out the idea of a 6 or 8 motor design right away. There's already too much complexity in even a very basic design. No need to make the job any harder! I was going to keep it really simple and then go on from there.
Tricopters have been very successful. But I really hate the idea of the yaw (directional) stabilization servo in the back. I see it as a weak point. Just like a traditional helicopter, it is another weak point susceptible to mechanical failure.
So in the end the decision was easy. I went with a quadcopter design. This is a very simple design where the motors are spinning counter-rotating propellers.
I knew that choosing the flight controller needed to come next. After all, quadcopters replace mechanical parts with gyros. If you think about it, it is really just like a computer game!
I'm a terrible helicopter pilot. I can barely control a coaxial beginner's helicopter. Being able to use a flight controller was a big part of the appeal of a multirotor platform.
The simpler flight controllers only have angular rate of turn sensors. These sensors are what we call gyros. They can sense when you are turning, but that is all.
A feature that I really wanted the flight controller to have was accelerometers. An accelerometer can tell in which direction is down by measuring the acceleration due to gravity. With this auto leveling feature I can let go of the control sticks when I get into trouble and it will bring it back to perfectly level attitude. This was very important to me!
Next I needed to decide what the frame was going to look like. The size of the motors was determined by the size of the frame and the payload. In other words, the size of the camera I wanted to use was a key factor to how strong the quad needed to be.
What material to use for the frame? Carbon? Aluminum? Fiberglass? We always want to use the best at first, but reality soon sets in.
I watched the video of another quadcopter made out of wood that flew beautifully. Wood was inexpensive and strong enough to meet my needs, and it only cost about $5!
There are many ways to build the frame. It would take far too long to describe all the decisions I had to make. We need to get to the flying!
In the end I decided that designing my own custom frame was the way to go. I wanted to be really proud of my quad.
Deciding which specific motors to use was a hard decision. Knowing roughly what I needed was easy. If you look around, you quickly learn that there are really only two sizes of quadcopters out there.
There are medium-sized quadcopters that don't carry a payload and are designed for 3D flying. These use electric motors of about 45 grams (1.6 oz) each. I already knew mine was not going to be one of these.
Then there are the heavier quads with onboard video cameras that need motors of about 60 grams (2.1 oz). This was me!
Unlike an airplane, if you have a motor failure in a quadcopter you are dead. There are no inexpensive top quality motors, so I decided to spend what I needed to spend to get the quality I wanted.
Low Kv Motors
I need to explain why a low Kv (motor voltage constant) was so important for the motors. A speed control (ESC) is not infinitely variable. When you move the throttle stick, the ESC might not be able to exactly match the amount of movement requested. Having low Kv motors effectively makes these incremental steps smaller. When the software of the flight controller needs to maintain the quadcopter level, having this finer control makes its job easier. You then will have a smoother flying quad, which is best for shooting video.
I also wanted low Kv motors because they can then turn bigger propellers, which are more efficient.
A good rule of thumb is to use speed controls that can handle one and a half times the current needed to maintain a hover. My two pound (1 kg) quad needs 150 watts to hover, which is approximately 40 watts per motor. With three cells in series in the battery, this is less than four amps per speed control (there is one per motor). I originally chose a 25 amp ESC because I was afraid of overloading it. In the future, I would use a 10 to 15 amp ESC for a quad like that.
I used a 10x4.7 propeller for a simple reason. It is very common and lots of other quadcopters use it. It is also easy to find in reverse rotation versions. A larger-diameter low-pitch propeller is a good choice for a quad.
If the Kv of the motors is too low, you may need to increase the number of series cells in your battery pack. That is what happened in my saucer quadcopter. Three cells in series were not enough to provide the power I wanted. When I switched to a 4S battery pack it came alive.
This is also a highly demanding application, with constant high current flows. I needed a quality battery with at least a 30C rating.
If you lose power while flying a quadrotor, you are dead. It is not like an airplane where you can glide to a safe landing. With a quad, you need to keep a close watch on the flying time and land long before the battery runs out. Don't try and beat a flying duration record! A low battery on-board alarm might be a wise investment, too.
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