China Civil UAV Theory Questions — Questions 436–445

Multi-axis aircraft are inefficient in flight, with short range and low speed, but are flexible and simple in structure, suiting aerial photography and film shots (e.g., DJI UAVs are commonly used for aerial photography). High-altitude long-endurance reconnaissance and integrated reconnaissance-strike require long-endurance fixed-wing UAVs.

Study note: look for the option without “rotor”. A rotor itself produces rotor thrust; forward thrust requires tilting the rotor thrust direction to create a forward-leaning component.

A multi-axis aircraft's reaction torque is the sum of each rotor's counter-torque; compared with a helicopter, each individual rotor's reaction torque is not large. In stable vertical climb, all rotors' reaction torques cancel each other and do not increase.

A multi-axis aircraft's mechanical structure is relatively simple, and its propellers generally spin at high speed, controlling flight direction by changing rotor tilt angles. This design consumes more energy to overcome air resistance during flight, reducing aerodynamic efficiency. So options A and B are wrong, and option C is correct.

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

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

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.

A rotor relies on blade rotation for lift, so the hub produces no lift; gravity acts at the hub, making the rotor form an inverted cone.

When taking off, a rotorcraft can usually fly a short distance at a relatively low altitude of 1 to 10 meters to accelerate and take off, instead of the longer ground roll of a conventional airplane.

Considering descent inertia and safe altitude: early in descent, altitude is high, so descent can be faster. Late in descent, speed should slow to reduce inertial effects and leave enough time to operate the landing or transition to level flight/hover for precise operation.