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I'm interested in classifying aerobatic maneuvers by their propensity to cause airsickness. While there will be some variation across a population, I think we can use first principle arguments to show that low-gee loops are much less nausea-, dizziness-, and vertigo-inducing than snap rolls. Likewise, a single 1G barrel roll is much less disorienting than extended inverted flight.

I'm hoping that in a similar vein to how we have documented human physiology during high-acceleration events, there will also be some literature on physiology in aerobatic environments.

(For the purposes of this question, I'd like to put anecdotal experiences aside, or at least limit them to the comment section. Certainly there are outliers who have little trouble with the hardest maneuvers but a lot of trouble with the easiest.)

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    $\begingroup$ Motion sickness usually results from conflicting sensory information. Your inner ear doesn't match what your eyes are seeing. I would guess barrel rolls would be the absolute worst. Your eyes see you spinning around yet your vestibular sense feels that you are upright the whole time. $\endgroup$
    – TomMcW
    Jun 8, 2021 at 18:02
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    $\begingroup$ Amongst other things, it matters a lot whether the person is in control (i.e. the pilot or the passenger). $\endgroup$
    – Zeus
    Jun 9, 2021 at 5:36
  • $\begingroup$ Generally, in my (somewhat professional) experience, your "first principle arguments" are not true. For many people, light and particularly repeating accelerations are more nauseating than strong loads per se. The disconnect between the vestibular and visual information that @TomMcW mentioned is more important. But anecdotally, spin must be one of the worst. $\endgroup$
    – Zeus
    Jun 9, 2021 at 5:44
  • $\begingroup$ Any chain of aerobatics is increasingly likely to induce airsickness as time progresses. I don't think that refutes the first-principles claimed above, since extended periods of snap rolls or spins are going to have an even stronger disconnect between vestibular and visual. I'm hoping to find data which goes beyond "many people" to include "people in general". $\endgroup$
    – Kenn Sebesta
    Jun 10, 2021 at 14:51
  • $\begingroup$ Supporting TomMcW, "Sensory conflict theory holds that, when [optical] perception of self-motion is not corroborated by inertial forces transmitted through the vestibular system, simulator sickness is likely." $\endgroup$
    – Camille Goudeseune
    Jun 17, 2021 at 18:56

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The most widely accepted explanation for motion sickness (which includes Airsickness, seasickness, carsickness, simulatorsickness and since recently selfdriving car sickness) is the sensory conflict theory. This theory states that if you sense a different motion via your vestibular system (the inner ear), then what you see via your visual system (your eyes) which does not align with your experience, then you will encounter motion sickness. It is important to notice that this mismatch is matched with your personal experiences of motion, therefore you can get acustomed to certain types of motion after having experienced them a couple of time. In other words: You learn to accept a given motion, which then will not make you motion sick anymore (or in some cases, not as much). Additionaly, people are especially suceptible to certain frequencies of motion. The Standard ISO-2631 (unfortunately restricted access) states that 0.2 Hz oscillations are especially nauseating.

Given this background, you will have to identify a maneuver which results in the biggest motion conflict possible and ideally features a lot of motion in the 0.2 Hz range. Because there are currently only very unsufficient models of this motion conflict (which is an ongoing research topic, one model is described here for example) it is imposible to classify which flight maneuvers are best suited for this task (or at least with good accuracy). I would propose motions with unexpected centrifugal forces, and unexpected orientation turns. Centrifugal forces screw up the correct sensing of Earth's G-Vector by the vestibular system which then is interpreted (by a lot of people) as a unexpected motion. Because of a lack of accurate models it is not possible to foresee which maneuver is the most nauseating, but I would propose a stall turn (or hammerhead) as it ticks all these boxes.

However when going back to the ISO-2631 I can speak from personal experience that the absolute fastest way to make people airsick is by flying a vertical standing sinusoid flight-path with a frequency of 0.2 Hz. This is an incredibly fast way of making people airsick.

One note given your sentence:

Likewise, a 1G barrel roll is much less disorienting than extended inverted flight.

Motion sickness research suggests that this is not true, simply because the human motion sickness mechanism is centered around oscillation in some sort. Extended inverted flight is a static condition while a 1G barrel roll is a motion with a distinct frequency spectrum, which would be more adapt in provoquing motion sickness...

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  • $\begingroup$ Positive to zero gee cycling, even far from a 5 second period, will do it reliably to almost everyone, even with experience, if continued long enough. They don't call it the "Vomit Comet" for no reason... $\endgroup$
    – Zeiss Ikon
    Jun 29, 2021 at 14:11
  • $\begingroup$ @U_flow, great answer. I updated my question to clarify that I was referring to a single perfectly-coordinated barrel roll-- much like an amusement park ride-- , as opposed to the discomfort, disorientation, and discombobulation stemming from a long (minutes) period of inverted flight. $\endgroup$
    – Kenn Sebesta
    Jun 29, 2021 at 14:50
  • $\begingroup$ This is consistent with my experience. Specifically, it is the downward leg of the flight path where the vomiting occurs. $\endgroup$
    – Tyler Durden
    Jun 29, 2021 at 15:17
  • $\begingroup$ Would you please describe a vertical standing sinusoidal flight-path? Would that be the path taken by an aircraft pitching up and down in a sinusoidal fashion? If so, what does "standing" mean? $\endgroup$
    – rclocher3
    Jun 30, 2021 at 23:14
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    $\begingroup$ @rclocher3 the flight path I describe is an aircraft pitching up and down in a sinusoidal fashion. With standing I meant to describe "vertical" therefore in an up-and-down fashion, but "pitching motion" describes it more clearly. $\endgroup$
    – U_flow
    Jul 1, 2021 at 10:53
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The answer by @U_flow is very good. But, I want to add a little to it. The NASA KC-135 was referred to as the "vomit comet" for a good reason. NASA flight surgeons would insist that everyone keep an air sickness bag visibly sticking out of an easy access pocket. They also offered everyone prescription motion sickness medication (Scope-Dex). I know of at least one experienced pilot and astronaut who proceeded to fill up a bag while flying on the "K-bird". That aircraft was flying parabolas, but there is little outside visibility from inside most of the aircraft.
Meanwhile my experience of flying inverted has been less disorienting than other aerobatics.
I have also seen motion sickness, as well as spatial disorientation (which are related), when in instrument flying conditions. Flying in the clouds causes a loss of visual reference and can lead to a sensory conflict.
As you might expect experience matters. People can get used to different things. Part of that is probably due to the expectation of what will occur. You can learn to expect "which way is up" and the correct direction to glance to find the horizon. For an experienced pilot, who is not flying, just keeping a light tough on the controls provides additional feedback about what the other pilot, and therefore what the aircraft, is actually doing. That can be very helpful.
The worst one is probably one that you have never done before, with some light turbulence added, without a clear visible horizon.

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