Final answer:
The force experienced during sideward flight in rotary wing aircraft, when an accelerative force acts shoulder-to-shoulder, is the Centrifugal force, which is a type of inertial force.
Step-by-step explanation:
The appropriate force that occurs when an accelerative force acts across the body in a shoulder-to-shoulder direction during sideward flight in rotary wing aircraft is B) Centrifugal force. This force is experienced as if pushing the occupants away from the center of rotation due to their inertia. While in an inertial frame, there's no actual force pushing outward, in the non-inertial, rotating reference frame of the aircraft, passengers would experience what's called a fictitious or inertial force, commonly referred to as centrifugal force. It arises because the occupants’ bodies want to continue in a straight line due to their inertia, but the aircraft is changing direction, which gives occupants the sensation of being pushed outward. This is similar to what people experience on amusement park rides or when turning sharply in a car.