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I’m not a pilot but know a thing or two about flight dynamics. As you know there are currently quite a few aircraft under development designed for low-and-slow and (eventually) autonomous flight. These are often referred to as air taxis, eVTOL planes et cetera. Their engineering may require a lot of new thinking but perhaps not. This relies in part on the answer to the question whether they’re designed to be optionally autonomous or exclusively so. My question pertains to aircraft that are NOT to be controlled by humans or only in the case of an emergency, but even then only indirectly through an advanced fly-by-wire system, one probably more advanced than those in modern fighter jets which would be impossible to fly without FBW.

The autonomous air taxis will be more affected by atmospheric turbulence than commercial high-and-fast jetliners and turboprops, and they’ll have to dynamically compensate for this to ensure a smooth and safe ride. To achieve this requires continuous adaptation relying mostly on gyroscope measurements coupled to small to moderate steering actions under autonomous control. Responsiveness is key here and that’s what computers are good at.

So let’s say we’re designing an airplane that’s to be autonomously controlled at all time (obviously with sufficient redundancy). Might for example a plane with a high moment of inertia provide a smoother and at least as safe ride as a “properly” designed airframe designed for manual control? Or do you think the old rules still apply: low(-ish) rotational inertia provides superior handling and passenger experience.

My thinking is that a flight computer that’s programmed to stay within rationally determined limits (to prevent spins et cetera) can fly a plane not just safely but smoother as well. This is akin to how very large ocean vessels are hardly affected by waves and winds compared to small ships. The small vessel is likely to have superior handling characteristics under optimal conditions and yet you'd be much rather off sailing on the large one even if both ships are able to stay afloat in extreme weather.

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  • $\begingroup$ Questions asking "do you think..." are explicitly off topic as opinion based. $\endgroup$
    – FreeMan
    May 7, 2021 at 16:03
  • $\begingroup$ The B2 is an example of a touchy-to-control design (flying wing), being made stable by computer system control. It's not for passenger comfort, of course, but for maintaining flight stability while remaining in specific flight corridors, which an be very small, when on mission. $\endgroup$
    – atc_ceedee
    May 8, 2021 at 15:07

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The underlying assumption of your question seems to be that a higher control bandwidth always leads to better disturbance suppression. Since a higher control bandwidth is easier to achieve with low-inertia systems, aircraft with low inertia would have better disturbance suppression.

This assumption can work in theory, especially for SISO (single-input, single-output) systems. However, for aircraft, we are essentially dealing with MIMO systems. While we can achieve near-perfect disturbance suppression for one degree of freedom (e.g., zero variation in altitude), this will inevitably lead to large excursions of other degrees of freedom. For example, disturbance suppression in altitude will lead to large variations in pitch as we try and keep AoA constant with every gust. So, for a MIMO system like an aircraft, we have no choice but to formulate a cost function of what we consider a 'smooth' ride, and balance the control scheme across various degrees of freedom.

Since we are considering a balancing act, it then makes sense to tune the internal dynamics of the aircraft such that a smooth ride is naturally achieved. So that means: high inertia for degrees of freedom experienced by the passenger, and low inertia for degrees of freedom not felt by the passenger. In practice: larger fuselage inertia and smaller control surface inertia.

Unsurprisingly, this is how it works in practice: larger aircraft are generally more comfortable, whereas at the same time, degrees of freedom not felt by the passengers are exploited in modern aircraft for gust suppression. For example, wings are more and more dynamically decoupled from the fuselage by highly flexible composite wings. Ailerons are split in smaller sections for gust suppression, you may find videos of "dance of the ailerons" for modern aircraft.

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    $\begingroup$ I'm not sure if this answers the question because IMO the question isn't clear, but it's still informative. $\endgroup$
    – GdD
    May 7, 2021 at 12:27

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