Thrust in a fixed-pitch propeller is directly proportional to which of the following?

Thrust in a fixed-pitch propeller is directly proportional to the shaft torque. This relationship stems from the mechanics of how propellers generate thrust. When the engine applies torque to the propeller shaft, it causes the blades to rotate. The amount of torque applied directly influences the power delivered to the propeller, which in turn affects the thrust produced.

It's important to understand that while revolutions per minute (RPM), propeller diameter, and fuel consumption can influence thrust, they don't have the direct proportionality of torque. RPM is a measure of rotational speed and, while higher RPMs can result in greater thrust due to increased blade area interacting with water, it is not the sole factor. Propeller diameter affects the overall thrust potential but doesn't directly correlate to thrust without considering other factors such as speed and pitch. Fuel consumption relates to engine power and efficiency rather than directly determining thrust produced by the propeller. Thus, shaft torque is the accurate choice for understanding thrust generation in fixed-pitch propellers.