Static equilibrium in flight requires which of the following?

Static equilibrium in flight requires which of the following?

• A. Zero net force
• B. Zero net moment
• C. Both zero net force and zero net moment
• D. Neither

Answer: (C)

In static equilibrium, the aircraft isn’t speeding up, slowing down, or turning. That means two things must be true: the total force acting on the airplane must be zero, and the total moment (torque) about the center of mass must be zero. In steady flight, lift must balance weight to prevent translational acceleration, and thrust must balance drag. At the same time, the aerodynamic and control forces must balance so there’s no turning tendency. If only one of these is zero, you’d either get translational acceleration (net force) or rotational acceleration (net moment). So the only way to be truly in static equilibrium is having both the net force and the net moment equal to zero.

In static equilibrium, the aircraft isn’t speeding up, slowing down, or turning. That means two things must be true: the total force acting on the airplane must be zero, and the total moment (torque) about the center of mass must be zero. In steady flight, lift must balance weight to prevent translational acceleration, and thrust must balance drag. At the same time, the aerodynamic and control forces must balance so there’s no turning tendency. If only one of these is zero, you’d either get translational acceleration (net force) or rotational acceleration (net moment). So the only way to be truly in static equilibrium is having both the net force and the net moment equal to zero.