I am wondering which conditions are possible and what they mean:

  1. Aircraft is statically stable in pitch and there is a trim point (for a specific surface deflection)

--> So in this situation my aircraft will always return (in case of a disturbance) to the trimmed condition in "free flight mode" , right? d_cm/d_alpha < 0 --> stability criteria

  1. Aircraft is unstable

If I have an unstable aircraft, d_cm/d_alpha > 0 . But to my understanding, there should be still one point (one angle of attack) where it is stable , so a trimmed condition should be still possible. Because I should always be able to make something (like an aircraft) stable by using/moving the right control surfaces to a certain level so that there is no moment acting on the aircraft. This should be the concept of how combat fighters fly? Unstable in the whole but really agile with a fast control algorithm?

Can you confirm me this?

Thank you Lucas


2 Answers 2


From a physics standpoint, the term "unstable" means that, if a system is displaced from its equilibrium point, it will experience a net force or torque in the same direction as the displacement. The typical way of visualizing that is to imagine a ball on a dome. If you put the ball at the precise peak of the dome, it can balance there indefinitely. But, if the ball is moved even slightly, gravity will tend to pull the ball farther and farther from the peak, until it rolls off the dome entirely.

In other words, just because the ball isn't moving, doesn't mean that its in a stable position.

The same thing is true for an unstable airplane. Yes, it's possible to get it into a configuration where it's "balanced" (for lack of a better term), with no control input required to maintain straight-and-level flight. But that doesn't mean that its stable. Any disturbance -- a gust of wind, passengers moving around the cabin, fuel being burned, etc. -- will move the plane away from that oh-so-carefully-achieved equilibrium, whereupon aerodynamic forces will start to pull it even further, requiring the pilot (or, more likely, the flight computer) to actively correct.


The statically unstable aircraft has its Neutral Point forward of its Center of Gravity. Since the statically stable orientation requires the Neutral Point to be aft of the Center of Gravity, the statically stable trim point of the statically unstable aircraft is at some angle to the flow going backwards (if the NP is right on the C of G, there is no statically stable trim point).

In other words, the statically unstable airplane continuously wants to switch ends like a shopping cart being pushed backwards. It's easy to visualize if you take it to the extreme. Imagine the C of G of an airplane is right back in the tail and you send it through the air. It will immediately flip around so it's going backwards, like a dart you threw tail first.

So a fighter operating with its C of G right on its Neutral Point will have no stable trim point at any AOA forwards or backwards, and if the C of G is aft of the NP, its statically stable trim point will, theoretically at any rate, be somewhere going the wrong way.

In either case, you now have to use software and sensors to designate an artificially determined trim point at a desirable AOA, and use active flight control inputs, running transparently in the background and not noticeable by the pilot, to hold the body at that AOA as if that was a natural statically stable position.

It's as if you hired someone walking along behind you to hold your shopping cart straight for you as you pulled it along backwards, only allowing it to change direction as they saw you trying to turn it.

You could call that person your Artificial Stability Assistant. If that person lets go, you will have your hands full trying to keep the cart from spinning around. If the statically unstable fighter's multiple redundant artificial stability systems all fail, you'll simply be ejecting.


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