Why most drones have even-numbered propellers?

FD1600 multi-rotor drone

FD-1660 fly 80mins with 3kg payload long endurance drone

You may have noticed that the number of propellers of multi-rotor drones is mostly even, and the number of odd ones is relatively small. Why is this? It's very simple. Take the 4-rotor drone in the picture as an example. When the propeller rotates, it will generate a reverse torque. If there is no corresponding design on the drone to offset this reverse torque, it will keep spinning. , it is useless. After making an even number of paddles, the steering directions of the two motors (1, 3) on the diagonal line are set to be the same, and the steering directions of the two motors (2, 4) on the other focus line are also the same, but the rotations of the odd and even groups are the same. The steering is reversed, so the opposing torques generated by the two sets of opposing propellers as they rotate cancel each other out, and the drone doesn't spin.

How can a drone with odd-numbered propellers solve the problem of spin? In fact, the principle is similar to that of a multi-rotor helicopter. Taking a 3-propeller drone as an example, a reverse propeller is installed under each propeller, so the 3 propellers become 6 propellers, and the reverse torque is also offset, but this way A 3-propeller drone requires 6 motors, and the cost is higher than that of a 4-propeller. Therefore, there is another solution. Set one of the propellers of the 3-propeller drone to an adjustable angle, so that it generates a lateral component force to offset the spin, so that only a motor for adjusting the angle of the propeller needs to be added.