translation.en: Conventional wisdom says that you need at least four fixed motor propeller pairs in order to fly, because there are four degrees of freedom to control: roll, pitch, yaw and acceleration.
translation.zh: 传统观念认为 至少需要四对固定电机螺旋桨来飞， 因为要控制四个自由度： 滚转、俯仰、偏摆 和加速。