The somatogravic effect—the non-visual sensations of aircraft attitude. The arrows indicate the magnitude and direction of the resultant force vector generated by powered fixed-wing aircraft and rotary-wing aircraft during acceleration, constant velocity flight and deceleration. In the fixed-wing aircraft (a), the resultant force is derived from the addition of two forces—the lift on the wings and the force associated with acceleration or deceleration in the line of flight. To the aircraft occupants, the resultant force establishes a sense of the vertical that is no longer aligned with the true vertical. In consequence, a powered aircraft when accelerating tends to feel more pitched up and when decelerating, more nose down than it actually is. In the helicopter (b), the lift of the main rotor is the only source of force for both lift and forward acceleration or deceleration. Forward acceleration can only be achieved by putting the helicopter into a nose-down attitude. However, the force from the rotor remains predominantly vertical with respect to the aircraft. In consequence, a helicopter feels to be in a level attitude whether it is accelerating, at constant velocity or decelerating. A fixed-wing aircraft (c) can also feel to be in a level attitude as a consequence of a possibly inadvertent nose-down or nose-up attitude when unaccompanied by any change in engine power setting since the change of attitude alone results in acceleration or deceleration in the line of flight. This phenomenon is similarly experienced in an unpowered glider.