The Physics Of Multirotor Drone Flight
” When someone sees a multirotor aircraft for the first time, they are usually amazed at how they fly. One question constantly asked about multirotors is, “How do these things fly without any control surfaces?” That is a good question; let’s take a closer look at the physics of how multirotors fly.
Since there are no control surfaces on a typical multirotor, the only way you can maneuver the craft is by changing the speeds of the motors. This, in turn, changes the amount of thrust produced by the motors. This imbalance in thrust is what makes the craft deviate from a stable hover and move in various directions.
In any multirotor craft, some form of flight control board is required to go between the radio receiver and the speed controllers for the motors. The job of the flight controller is two-fold: First, it interprets the command signals that come from the radio receiver in the form of pitch, roll, yaw and throttle and converts them into the variations of throttle command needed to control the craft. Second, when a stability mode is active, it acts as a gyro stabilization module to keep the craft upright and level when no control commands are given. Without a flight controller board, flying a multirotor aircraft is virtually impossible.
To assist in the description of the various flight modes, a diagram of the top view of a Quad-X platform multirotor will be used. This is the most common configuration flying today, so it will be most applicable to the discussion of flight principles. Referring to Figure 1, this illustration shows the quad in a stable hover. The black arrow indicates the front of the craft and the direction of forward travel. The red arrows around each of the motors indicate the direction and speed at which the propellers are turning. In this illustration, all four red arrows are the same size, which indicates that all four motors are spinning at the same speed and the arrowheads indicate the direction that the props are turning. In this condition, each motor is producing an amount of thrust equal to one quarter of the weight of the airframe, and when the thrust of all four motors is combined it is equal to the weight of the craft which neutralizes the effect of gravity that is trying to pull the machine back to earth. With no other forces applied, the craft will stay in level flight at a consistent altitude.”