In aircraft design, I’ve seen the misconception that if a little bit of stability is good, more must always be better. Makes sense, right?
Wrong! …well, mostly.
In aeronautical engineering, “stability” is basically a measure of how quickly and aggressively an aircraft tries to return to its equilibrium flight state after a disturbance.
If you’re flying along and a gust of wind pushes your nose up higher than your cruise condition, your longitudinal (i.e. pitch) stability determines how quickly the nose naturally sinks back down to where you had been flying.
A stable aircraft will have the nose drop back down to where it had been before the disturbance, while an unstable aircraft may not try to level itself—or worse, the nose may keep pitching up higher!
There are a lot of factors that go into this stability measurement, like:
- Center of gravity (CG) location
- Tail size
- Wing and tail position
But those are topics for other emails. For now, you just need to know that this stability is a specific measurement for a given aircraft flying with a set CG.
It feels very intuitive, then, that you’d want your aircraft to return to its original position very quickly after any disturbance, right? And you’d be right, up to a point.
Another thing that stability quantifies is how much of a moment (i.e. a force applied at a distance) is required to rotate the aircraft. If you have a very stable aircraft, then it will require a large moment to move its nose up and down.
If you’re imagining turbulent weather aloft, then maybe a highly-stable aircraft sounds appealing so you’re not bouncing around so much.
But sometimes you actually WANT your aircraft’s nose to move: you’re descending to land, or starting a climb. Or you’re rolling the aircraft to execute a turn. In these cases, you’d rather the aircraft not be as tough to maneuver as a cruise ship, so you’re not looking for a super-stable vehicle. You want your control surfaces to create a predictable response to a reasonable input.
This is the balancing act—and dare I say it, job security—of an aeronautical engineer.
- You want your aircraft to have enough stability to be a pleasure to fly, and to return to its equilibrium after any disturbances.
- But too much stability will make your aircraft harder to maneuver, require larger control surface inputs, and often make it fly less efficiently.
Honestly, this is part of the fun of aero design for me. You’re not always looking to maximize or minimize. A lot of the time, you’re seeking balance instead.
Fun fact: this is also why a lot of modern fighter aircraft are purposefully designed to be unstable. A marginally stable to unstable aircraft requires only the smallest deflection of a control surface to execute a quick roll or dive, which is what makes them so maneuverable. Modern computers assist in maintaining stability so the pilot doesn’t get overloaded with all the small adjustments that would be needed to stay flying straight the rest of the time.