How stable is your aircraft? It depends on what you're flying. Let's take a look at why that's the case.
Two Types Of Stability
Stability is the ability of an aircraft to correct for conditions that act on it, like turbulence or flight control inputs. For aircraft, there are two general types of stability: static and dynamic.
Most aircraft are built with stability in mind, but that's not always the case. Some aircraft, like training airplanes, are built to be very stable. But others, like fighter jets, tend to be very unstable, and can even be unflyable without the help of computer controlled fly-by-wire systems.
Static Stability
Let's start with static stability. Static stability is the initial tendency of an aircraft to return to its original position when it's disturbed.
There are three kinds of static stability:
- Positive
- Neutral
- Negative
Positive Static Stability
An aircraft that has positive static stability tends to return to its original attitude when it's disturbed. Let's say you're flying an aircraft, you hit some turbulence, and the nose pitches up. Immediately after that happens, the nose lowers and returns to its original attitude. That's an example positive static stability, and it's something you'd see flying an airplane like a Cessna 172.
Neutral static stability
An aircraft that has neutral static stability tends to stay in its new attitude when it's disturbed. For example, if you hit turbulence and your nose pitches up 5 degrees, and then immediately after that it stays at 5 degrees nose up, your airplane has neutral static stability.
Negative static stability
Finally, an aircraft that has negative static stability tends to continue moving away from its original attitude when it's disturbed. For example, if you hit turbulence and your nose pitches up, and then immediately continues pitching up, you're airplane has negative static stability. For most aircraft, this is a very undesirable thing.
Dynamic Stability
Now that you have static stability down, let's go over the really fun one: dynamic stability. Dynamic stability is how an airplane responds over time to a disturbance. And it's probably no surprise that there are three kinds of dynamic stability as well:
- Positive
- Neutral
- Negative
Positive Dynamic Stability
Aircraft with positive dynamic stability have oscillations that dampen out over time. The Cessna 172 is a great example. If your 172 is trimmed for level flight, and you pull back on the yoke and then let go, the nose will immediately start pitching down. Depending on how much you pitched up initially, the nose will pitch down slightly nose low, and then, over time, pitch nose up again, but less than your initial control input. Over time, the pitching will stop, and your 172 will be back to its original attitude.
Neutral dynamic stability
Aircraft with neutral dynamic stability have oscillations that never dampen out. As you can see in the diagram below, if you pitch up a trimmed, neutrally dynamic stable aircraft, it will pitch nose low, then nose high again, and the oscillations will continue, in theory, forever.
Negative dynamic stability
Aircraft with negative dynamic stability have oscillations that get worse over time. The diagram below pretty much sums it up. Over time, the pitch oscillations get more and more amplified.
Why Aren't All Aircraft Stable?
It really comes down to what your aircraft is built for. Stable aircraft, like Cessna and Piper training aircraft, are built to be statically and dynamically stable, making them easy to trim and fly 'hands off'.
However, jets like the F-16, are built to be unstable, making them highly maneuverable and easy to pitch, roll and yaw aggressively.
In a future post, we'll talk about the designs features that make those aircraft stable or unstable. Until then, enjoy the fact that your aircraft (most likely) doesn't have negative static stability.
