It has taken almost 100 years to go from the first multicopter created in 1924 to where we are today. In that time, we’ve not only made it possible to fly mutlirotor drones. We have also to increased their range, their payload, and had them deliver pizza. But how does all this work? Let’s explore some basic physics and flight dynamics to understand this problem.
Keeping it as simple as possible, we’ll use just a quadcopter as an example. Your basic quadcopter has a base with four arms to support 4 motors. There are two basic structural configurations, the X-Frame and the H-Frame.
The X-Frame design is what most people typically think of as a quadcopter. Their advantage lies within their symmetry. This makes them great for carrying payloads positioned directly in their center of mass.
H-Frames are useful when you prefer better aerodynamics along one axis. Currently, the most notable usage is for drone racing. They also perform well when you need to support uneven weight distribution.
The direction that each motor spins is crucial to its stability. Take a look at the quadcopter diagram above. Notice that motors 1 and 4 spin in one direction while 2 and 3 spin the opposite direction. It doesn’t matter which direction each pair spin as long as they keep this relationship.
So why do quads have to have a counter rotating system you may ask? Well, let me tell you a secret grasshopper. Newton’s third law states that for every action, there is an equal and opposite reaction. Pretend you had a helicopter without a tail rotor. With only one propeller, a force we call torque reaction would cause the fuselage to spin out of control. Add a second rotor (a tail rotor for a helicopter), and you have created an anti-torque system. That is, the rotation of one propeller cancels out the torque caused by the other propeller.
All other motion of a multicopter is just a combination of varying the amount of speed applied to each motor. That includes yaw, pitch, roll, thrust and getting Slurpees at 7-Eleven.
So, to pitch forward, we would decrease motors 2 and 4 while also increasing speed to motors 1 and 3. To roll (or lean) left, the drone would increase speed to motors 1 and 2, and it would reduce speed to motors 3 and 4.
Yaw is a little more interesting. To yaw (or turn) a quadcopter, you need to increase/decrease motors on the diagonals. For example, yawing left requires a speed increase to motors 1 and 4 while decreasing 2 and 3. The exact opposite needs to happen to yaw right.
Did you notice that when yawing, the drone turns in the opposite direction of what you may think? That is, turning left means increasing speed to the motors that turn right. What the deuce?
If you guessed torque reaction, then go grab yourself a cookie, my friend. I prefer white chocolate macadamia, but you get whatever cookie you like. The same torque reaction that makes a helicopter spin out of control without a tail rotor is at work here. Multirotors control the torque reaction to turn left or right. This is why most drones have an even number of motors. An even number naturally tends to counter torque reaction.
So here is an easy one for you. How do we make the drone fly straight up?
All we have to do is increase the speed to all four motors. Bingo! To descend, we do the exact opposite and lower the speed to each motor. Now you’re getting it. Ok, so that explains quadcopters, but what about these hexacopter and octocopter varieties? We’ll it’s the same thing on a larger scale.
Hexa, Octa and Tricopters
Take a look at the hexacopter below.
You can see it is just like the quadcopter. Each blade rotates opposite the direction of its neighbor. As a result, the motion for yaw, pitch, roll and thrust are the same as well. An even number of motors keeps the torque forces in check just like the quad. And for the octocopter, it is more of the same.
You say, “but wait, I know I’ve heard of tricopters before. How do those work if they have an odd number of motors?” That’s an excellent question. Let’s look at that.
As you can see, a tricopter has two front motors that turn the same way and a tail motor that spins opposite. If all motors are the same size and spin at the same speed with this setup, what would happen? The result is that it would begin to turn in the opposite direction of motors 2 and 3. This is because there are an unequal amount of motors which lead to an unequal force in one direction. To counter this, the speed of motors 2 and 3 must be less than motor 1. Or to put this another way, the torque rotation of motors 2 and 3 must equal that of motor 1.
The Flying Wing
Not all drones are multicopters though. A design becoming more and more prevalent is the flying wing.
Often shaped like a B2 Bomber, the flying wing has a giant wing-like body with one or more motors fixed at the tail. While motors control forward thrust, a centrally placed rudder controls direction. Some flying wings opt for two tail rudders instead.
The plane stays airborne using Bernoulli’s principle. Faster airspeed and lower pressure above the wing causes the vehicle to lift. Thus, the physics are the same as a modern airplane.
That’s all there is to basic flight dynamics for multicopters. Now it should be a lot easier to understand topics like rate mode and auto stabilization.
What kinds of problems or new techniques have you seen with flying drones? Are there any drone concepts you are having a hard time understanding?
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