多直升机空气动力学:六轴飞行器上的特征推力

Multicopters use fixed-pitch rotors and control flight by varying rotor speeds, whereas traditional helicopters employ variable-pitch rotors controlled by the pilot. In a hexacopter, some propellers rotate clockwise and others counterclockwise. Flight control is achieved by adjusting individual rotor speeds rather than changing blade pitch angles.

Hexacopter attitude is described using the pitch axis, roll axis, and yaw axis, similar to fixed-wing aircraft. Pitch control involves increasing propeller speed on one side and decreasing it on the other. Roll control creates side-to-side movement using the same differential speed method, while yaw control changes heading by balancing clockwise and counterclockwise rotational torques.

Yaw control, which changes the heading angle, is achieved by balancing the clockwise propeller rotational torques with the counterclockwise propeller rotational torques. By spinning the counterclockwise propellers faster than the clockwise propellers, the opposite net reaction induces a clockwise rotation about the yaw axis.

Thrust is calculated using T = CT × ρ × n² × D⁴, where CT is the thrust coefficient, ρ is air density, n is rotational speed in RPM, and D is propeller diameter. Torque uses τ = CQ × ρ × n² × D⁵, where CQ is the torque coefficient. Both coefficients are determined experimentally from dynamometer data.

A dynamometer measures and calculates individual propeller parameters including thrust, torque, motor RPM, motor current, and throttle command using a step input program with pulsed signals. The system records these values to establish relationships between motor performance and control inputs, providing baseline data for propeller characterization and propeller characterization variations in pitch diameter and blade number performance analysis.

Load cell testing outside the wind tunnel measures static thrust at various throttle commands. Wind tunnel testing then characterizes lift and drag forces under airflow conditions at different pitch angles and airspeeds. Results show that increasing throttle significantly increases lift and drag, while higher airspeed increases drag but not lift substantially.

Since hexacopters lack direct RPM feedback, a polynomial surface is fitted to dynamometer data relating motor RPM to electrical power and throttle command. This relationship enables calculation of actual RPM from control inputs and power measurements, allowing engineers to predict motor performance across different operating conditions without direct sensor feedback.