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Comment by sandworm101 | Hacker News Reader

All high performance aircraft are, by definition, unstable. An airframe optimized for one flight regime will not be as suitable in another. An airliner like the 737 is optimized for cruise flight (30k feet, almost supersonic) and so isn't ideal during takeoff/landing. Making it good at both isn't an option. So we have systems to modify and adapt the cruise airframe for low-level flying such as this antistall system.

The only really stable commercial aircraft are, perhaps, the bushplanes. They are not fast and not fuel efficient over any real distance.

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> All high performance aircraft are, by definition, unstable.

This doesn't follow. Not even fighter jets were inherently unstable before modern control electronics. Using the electronics to make the flight characteristics better is not the same as the airframe being inherently unstable.

There were stable at altitude, but at low speeds those "fighter jets" became very twitchy. Delta or swept wings are a problem when low and slow. Their landing speeds were sometimes extreme and/or their angle of attack so high that visibility was an issue.

Fighter jets are also, in comparison to airliners, not very aerodynamically efficient. They can be fast but burn lots of fuel while doing so. They are like a motorcycle compared to a bus.

I don't much care for planes, but I found this 'short take off and landing' contest pretty impressive:

https://www.youtube.com/watch?v=2pw4OE7gM2M

Few if any non-military aircraft are aerodynamically unstable. No airliner is. Take any airliner, knock out all its control systems, and it will continue to fly.

>> Take any airliner, knock out all its control systems, and it will continue to fly.

Until they hit an AOA that puts the tail in the wind shadow of the wings. Then some will pancake. They can slip sideways into scary turns/dives and will not recover on their own. And they are delicate. It doesn't take too much force to twist them out of shape.

Aerodynamic stability doesn’t mean it’ll automatically recover from a stall. It means that if it’s flying in a steady state and you perturb it, it will tend to return to the original state.

In a deep stall you can’t recover from, it’s because the aircraft’s stability is greater than its control authority, not because it’s unstable.

It might be possible in the future to have commercial aircrafts with an airframe changing depending of the flight regime. See https://m.phys.org/news/2019-04-mit-nasa-kind-airplane-wing....

Well, they do. That's what flaps are. They change the shape of the wing to better optimize for slow flight. Swing-wings work but are hideously heavy/expensive. We will never see an airframe able to morph into constantly ideal shapes like a birds do.

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