This post delves into a key question in flight simulation: Is motion contributing to a realistic experience more than visuals alone or are both needed? Is motion in the absence of other forces, such as centrifugal force and varying g-forces, imparting useful inputs to our brain or getting us accustomed to sensations that are too different from reality?
To understand the effect of motion in a flight simulator it is essential to understand first how the human body works in respect to motion. The ear is responsible for hearing and two additional functions: three-dimensional balance and the detection of forces, including acceleration. The vestibular system, responsible for these two functions, is found inside the inner ear. It consists of three semicircular canals aligned on three different planes and two otolith organs, called utricle and the saccule. The semicircular canals and the structure of the otolith organs are filled with fluid. Each of these canals end in a space called ampulla that has small hair cells in it. Whenever we turn our head, the inner ear turns with it. There is however a delay for the fluid in the semicircular canals and ampullae to follow the movement of our head. This delay bends the sensory hair cells in the ear, creating a signal that motion has occurred. The hair cells then send the resulting information to the brain via nerves which process motion according to the three planes of the canals.
Each of the three semicircular canals is responsible for a specific direction of head movement: One of the canals responds to the head tilting upwards or downwards, one responds to it tilting to the right or to the left, and one responds to it turning sideways.
The otolith organs are found diagonally under the semicircular canals and have a similar function: There are also thin sensory hair cells in both organs. The difference is that, unlike in the semicircular canals, there are small crystals on the hair cells – like pebbles on a carpet. These crystals are called otoliths or “ear rocks.” The otolith organs detect acceleration, including forces such as centrifugal force and gravity. (More on this subject in this NASA article.)
In some situations, for example on a car, ship or airplane, different sensory organs (e.g. the eye) send either concurrent or contradictory messages to the brain. This can cause us to reinforce the information coming from the ear or confuse us, making us feel confused, dizzy or nauseous.
In a fixed-wing flight simulator this sensory information plays a big role in giving us a “realistic” experience.
If we examine motion along the three planes, sideways (bank), up-down (pitch) and left-right (roll) outside visual stimuli, in a motion-equipped simulator, then likely left-right, the effect created by the pedals on an airplane, will be received as a fairly accurate sensory input. Pitching up and down will be always accompanied by an increase or decrease of the effect of gravity (g-forces). In this case, the semicircular canals will detect movement, but the otholith organs will not detect any vertical acceleration. The same is in a bank: in absence of centrifugal force the body will tend to “fall” to the inside of the turn, the canals will detect sideways motion but no compensating effect in increasing outward acceleration created by centrifugal force. In essence, two out of three movements will convey non-realistic information to the brain.
Simulating other conditions that may affect all available motion axis, such as turbulence and vibrations may, because of their movements of actual shorter duration, be accepted by the brain as more realistic than pure turns or climbing-related changes involving longer-lasting gravity or inertial forces.
If visual clues are applied to motion, they will essentially reinforce all three directional changes.
The other scenario has visuals only and no motion. Obviously, in this condition, motion clues related to acceleration, inertial forces, and movement are eliminated. Although low frequencies can be introduced to simulate vibration (through seat shakers or similar devices), the brain will rely solely on visual clues to determine changes in direction along the three axis. While most studies analyze the importance of motion, much less time has been devoted to understanding whether visuals alone are sufficient to induce correct sensations in the brain and avoid false inputs. The article “A brief introduction to the art of flight simulation” by Ron Reisman (here) mentions that: “The primary source of steady motion is the visual system. A good out-the-window display, with no other cues, is capable of inducing a prolonged perception of linear motion (linear vection) and self-rotation (circular vection). In order to be effective, two guidelines must usually be followed: (1) the peripheral, rather than the central visual fields, must be stimulated; (2) background stimulation is more important than foreground stimulation.
Usually, however, there is a delay of 5 to 10 seconds from the onset of the visual stimulation to the perception of motion if no other perceptual systems are stimulated (this is highly variable). This delay can be shortened (sometimes almost eliminated) if vestibular system stimulation is coordinated with the visual cues. Brief vestibular cues in the appropriate direction hasten the onset of vection, and vestibular cues in the conflicting direction delay the onset.” This points to displays that encircle the whole field of vision and have enough depth-of-field clarity to display far objects clearly. It also points out that refresh rates higher then 30Hz are needed to provide the necessary “smoothness”. It concludes “As noted above, there is no hope that ground-based motion systems can duplicate the actual motion of a craft in flight. Instead, the motion systems are often used to stimulate the vestibular system in coordination with visual stimulation.”
From the AATD/FTD manufacturers’ viewpoint, various “realism induction” combinations are used in an effort to maximize training effectiveness and marketing appeal in a number of formulae: Motion with monitor-based display such as some high-end Redbird AATDs or Frasca’s Motion Cueing System, wraparound seamless display with no motion but optional vibrations as Entrol uses, or realistic, projected wraparound displays as found in Alsim models.
From the regulatory viewpoint, motion is required only in Full Flight Simulators (FFS) level A to D. In these simulators, 6 degrees of freedom motion is paired with collimated visuals to eliminate errors in viewing the flying environment regardless of the seats the pilots are in. The view will be correct from both left and right. In simulators certified under the ATD or FTD categories, motion is not a requisite to provide adequate transfer of training.
In closing, a 2005 study titled “The Effect of Simulator Platform Motion on Pilot Training Transfer: A Meta-Analysis” found a slight but statistically significant result favoring motion, but for effective pilot training, many find that items such as a realistic cockpit, cockpit avionics and accessories, sound, vibration and sharp and smooth visuals may be sufficient to convey the sense of realism that fosters the effective reproduction of flight phases, procedures, and abnormal conditions for flight training and proficiency. Improvements in motion techniques and their decreasing cost, however, will continue to make inroads in making flight simulators more and more realistic.