What is the thrust needed to maintain straight and level unaccelerated flight referred to as?

The thrust needed to maintain straight and level unaccelerated flight is referred to as thrust required. This concept is fundamental in aerodynamics, as it represents the amount of thrust that an aircraft's engines must produce to counteract drag and maintain a steady altitude and speed without any acceleration.

In straight and level flight, the forces acting on the aircraft are balanced: lift equals weight, and thrust equals drag. Therefore, thrust required is specifically the amount of thrust needed to resist the drag forces acting on the aircraft at that particular flight condition. This concept is used in performance calculations to ensure that an aircraft can achieve and maintain its desired flight path under varying conditions.

The other terms, such as thrust available, power required, and power available, represent different aerodynamic and power-related measures that do not specifically define the thrust needed for unaccelerated flight conditions. Instead, they refer to either the capability of the engines to produce thrust (thrust available) or the power levels required and available for various flight operations.