推进与推力

Open operation propulsion systems generate thrust by accelerating ambient fluid to high velocity. They draw fluid from a large upstream area and exhaust it downstream in a narrow jet. Using a control volume approach, the thrust is produced because the outflow velocity is much higher than the intake velocity. Since mass flow rates in and out are balanced, the momentum difference manifests as a force on the propeller.

Thrust efficiency is the ratio of thrust generated to input power supplied to the propulsion system. For example, if an electric motor drives a propeller with known electrical input power, dividing the measured thrust by that power yields the thrust efficiency. This metric indicates how effectively a propulsion system converts input energy into useful thrust force.

A static thrust test stand uses a rigid lever arm balanced on a pivot point. The propeller mounts on the long arm, and a precision scale measures force on the short arm. Torque from the scale balances thrust-generated torque, with the moment arm ratio amplifying force measurements for accuracy. This setup allows direct measurement of thrust at various input voltages and currents.

The outflow area is smaller than the intake area because of conservation of mass and the need to accelerate fluid to high velocity. Since mass flow rates in and out must be equal, and densities are approximately the same, a smaller outflow area forces the fluid to exit at much higher velocity than it enters. This velocity increase is essential for generating thrust.

Fluid propulsion systems extend beyond vehicles to stationary applications like HVAC equipment, which uses propulsion systems to drive fluid circulation. Natural propulsion systems also exist in living organisms, including jet propulsion from cephalopods, fins on fish, and flagella on amoeba. Additionally, windmills and turbines apply the same principles in reverse, extracting energy from moving air.

Measured outflow velocity can predict thrust using the thrust equation from momentum conservation. First, calculate the approximate outflow area by subtracting the hub area from the propeller area. Then multiply this area by the measured outflow velocity to estimate thrust. Due to velocity variation across the outflow and measurement uncertainty, this prediction should be interpreted qualitatively.

Thrust efficiency varies based on device design and operating conditions. Large propellers maintain relatively constant efficiency across power ranges, while smaller propellers and PC fans show efficiency drops at higher input powers. The PC fan produces the highest absolute thrust due to higher maximum input power, but efficiency patterns differ because of design differences in blade geometry and operating regimes.