China Civil UAV Theory Questions — Questions 366–375

Study note: when you see X, choose it. In the “X” layout, adjacent motors are arranged diagonally: motors 1 and 3 lie on one diagonal, motors 2 and 4 on the other. When the aircraft needs to turn or adjust attitude, the diagonal motors can spin in opposite directions, giving faster response and better maneuverability. So the “X” mode is generally considered better for agile maneuvering or fast turning.

Rotor rotation directions are always described viewed from above; the specific direction is by convention.

Study note: the side you move toward slows down; the opposite side speeds up. When a quadcopter moves forward, the rotor lift must have a forward component, so the rotor plane tilts forward; the two rear axes must therefore speed up to produce more lift.

Study note: the side you move toward slows down; the opposite side speeds up. Moving left requires the rotor plane to tilt left so lift gains a leftward component. Therefore the right propellers speed up and the left propellers slow down.

With throttle held constant, motor power stays the same.

In fast forward flight, a vertical tail provides better stability. But in hover, it increases lateral airflow disturbance and weakens hover stability.

The frame mainly protects the rotors from bumps; it cannot improve propeller efficiency.

Retractable landing gear and airframe morphing — see the DJI Inspire series — mainly improve the onboard mission equipment's field of view, so the camera does not capture the landing gear or airframe.

In forward flight, the relative airflow has speed, so blade efficiency is higher than in hover, lift is greater, and the aircraft can fly above its ceiling. (Translator note: the source stem is truncated; translated as written.)

Study note: when GPS is an option, choose GPS. Multi-axis aircraft generally do not carry barometric altimeters; they use GPS for altitude information. Multi-axis flight altitudes are usually low, where barometric altimeters have large measurement errors, and for UAVs needing high precision a barometric altimeter may not be accurate enough. Barometric altimeters infer altitude from air pressure: pressure drops as altitude rises, so altitude can be estimated from pressure change, but at low multi-axis altitudes pressure is strongly affected by the environment and precision is insufficient. Ultrasonic altimeters measure distance by emitting ultrasound and timing its return; they are generally used only at very low altitudes, such as hovering or landing, not during autonomous flight. GPS obtains three-dimensional position from satellite signals, giving real-time precise altitude, position, and speed without extra equipment.