I watched a flight mechanics lecture that mentioned that some combat aircraft like the Hornet has a 'mode' where the on-board computer simultaneously and automatically adjusts all the control surfaces to make the plane fly with Statically neutral stability which to my understanding means that the lift moment is constantly aligned with the plane's center of gravity.

The result is that the pilot is able to pull back on the controls and pitch up and release the input and have the plane continue to fly in a new direction without any tendency to diverge or return to equilibrium (the trim position).

Is this true and is this stability only possible with a constant re-balance of the control surfaces of the plane?


I know this is old, but for the people who are still looking for information regarding this subject, this explanation is not complete and partially inaccurate.

First, there are 2 types of stability. Static stability and dynamic stability. It is true that stability comes from the current situation attitude (angle of attack), but it has different effects for different aircraft. And it is possible to have positive static stability but with dynamic instability. Case in point, 727's deep stall where it's attitude effectively locks the tail in a 'wind shadow.' It causes oscillating moments that don't exist outside of this flight profile.

Static stability has to do with the short period moments of the aircraft. An aircraft with positive static stability is said to have a forces acting to try and return the aircraft to trimmed flight at its trimmed alpha. A neutrally static aircraft will desire to hold the current alpha even after disturbed. An aircraft with relaxed static or unstable static stability will continue to increase its alpha in the direction opposite of the flight path. In other words, neutral static stability will not remain at 2 degrees of pitch attitude (theta) once the stick is released. Instead, it will maintain the pitch rate (theta dot) that it had when the stick was released. Static stability will never have a description of oscillating moments, instead it will describe what happens right here, right now if a momentary force acts upon the aircraft.

Dynamic stability is the stability of the aircraft in the long term. It describes what happens after a certain amount of time. A dynamically stable aircraft will have no or little oscillating moments and will dampen oscillations over time. It will follow a trend that is predictable. An aircraft with neutral dynamic stability will maintain its oscillations until acted upon, and then after, continue whatever oscillations remain. But an aircraft with negative dynamic stability will continue to have diverging oscillations, increasing in amplitude from its original orientation.

The following 9 conditions describe, by example, what will occur with static and dynamic stability are both taken into consideration:

  • Positive static stability with positive dynamic stability: Aircraft will immediately attempt to return to its trim condition and will have oscillations that dampen out as the forces return to equilibrium.

  • Neutral static stability with positive dynamic stability: Aircraft will hold its current alpha and its pitch rate will remain constant with regard to its current alpha and the oscillations from the disturbance will slowly dampen out and return to equillibrium.

  • Negative static stability with positive dynamic stability (bit of a misnomer, explained below): Aircraft will continue to increase alpha and its pitch rate will increase. The misnomer is that there will be no oscillations because there are no forces acting to return the aircraft to its previous state in the long term. So this technically can't exist. Airspeed will continue to decrease and drag will continue to increase until it is more of a brick than a plane. This is undesirable in fighter aircraft as if it enters a spin, any oscillations that could help exit the spin will quickly dampen out and recovery will be nearly impossible. (with regards to the lateral axis only)

  • Positive static stability with neutral dynamic stability: Aircraft will want to return to normal flight, its AoA will decrease, and its pitch rate will reverse. But the aircraft will continue this oscillation indefinitely if no corrective measures are taken.

  • Neutral static stability with neutral dynamic stability: Aircraft will want to continue to maintain it's current angle of attack and any disturbances and oscillations for correcting forces will continue to oscillate as they did before. It have epitrochoidal motion.

  • Negative static stability with neutral dynamic stability (yes, this exists, and is what is common in some fighter aircraft): Without corrective inputs in a timely manner, this will have an oscillating period and will also continue to diverge from normal flight. This can be a good thing when trying to recover a fighter from a spin as the oscillations will continue and the aircraft can be "rocked out." The F-16's deep stall behavior is very much described like this, and can be described as a falling leaf. I have a video on this, from a simulator called DCS World, here. In fact, in the video, you can also see that the longitudinal axis also suffers from neutral dynamic stability in this flight envelope. The switch I press at 1 min and 45 seconds overrides the computers maximum control surface deflection window and I can then "rock the plane out" by increasing the amplitude of the oscillations until the airflow over the control surfaces allows the computer to recover from the deep stall.

  • Positive static stability with negative dynamic stability: This is what you want when you want to make passenger planes into vomit planes. The corrective forces acting on the plane cause it to overshoot and the oscillations increase in amplitude. But the plane is still trying to right itself in the short term.

  • Neutral static stability with negative dynamic stability: At this point, I don't know if there is a point in describing the behavior because I think you might be able to get the gist. The plane will want to maintain its current alpha, but will then reverse, causing larger amplitudes of oscillation on each reversal. It might have 1 or 2 cycles before it completely diverges from controllable flight. Not the characteristics you really want to have in an aircraft.

  • Negative static stability with negative dynamic stability: Just pure chaos. Basically any orientation along the lateral axis will have a 'butterfly effect' response and unpredictable behavior. Forces put in to try and correct it will have an oscillatory response that will increase in amplitude on the next cycle, causing the need for a larger input to correct and the pattern repeats.

Currently studying flight dynamics as a hobbyist and with 2 years of engineering school. Current project is for model aircraft with a flight control computer to emulate an aircraft with negative static stability and neutral dynamic stability.

And, for references sake, so you can figure this out on your own in a sort of mental simulation:



First, definitions- The purpose of trimming to to free the pilot from having to exert a constant pressure on the controls i.e., when the aircraft has been trimmed, there is no need for the pilot (or autopilot) to give any control input (for a set of conditions like level flight or climb).

An aircraft which is (statically) neutrally stable tends to stay in its new attitude when it's disturbed. For example, if you pitch the nose up by, say $2^{\circ}$, and then immediately after that it stays at $2^{\circ}$ nose up.

As can be seen from above, you need not balance the control surfaces continuously while the aircraft has been trimmed. Stability has to do with the aircraft attitude and not control surface movement.

The aircraft (like F/A-18 Hornet) require constant operation of flight control surfaces (by computer) because they have relaxed static stability- they will oscillate and diverge from an attitude if left to themselves.

  • $\begingroup$ This is not as clear, to me, as it should be. So, a statically neutral stable aircraft, pitched up those 2degrees will stay at that new attitude, with no changes to the control surfaces, by either the pilot/autopilot or computer? $\endgroup$
    – CGCampbell
    Nov 1 '15 at 15:40
  • $\begingroup$ @CGCampbell That's correct. Positive static stability means it returns to the previous attitude, neutral static stability means it keeps the new attitude. This is an aerodynamic property. What aeroalias is saying is that the F/A 18 is NOT aerodynamically stable. It has a computer that automatically reacts to changes in order to make it possible for the pilot to control it. From what the op says, apparently the flight controls are programmed to mimic neutral stability. $\endgroup$
    – TomMcW
    Nov 1 '15 at 17:38
  • $\begingroup$ Maybe @rhinodriver will confirm if that's how the F/A 18 is programmed $\endgroup$
    – TomMcW
    Nov 1 '15 at 17:39
  • $\begingroup$ @TomMcW said it best: apparently the flight controls are programmed to mimic neutral stability. Well said! $\endgroup$
    – Ralph J
    Nov 2 '15 at 2:29

"pilot is able to pull back on the controls and pitch up and release the input and have the plane continue to fly in a new direction"

Thrust must be added to "continue to fly in the new direction" or else the aircraft will slow down and depart from straight line flight.

What happens next will depend on the directional stability of the aircraft. Once the aircraft begins to curve downward, the change in relative wind will increase AOA over the entire aircraft.

This means non-lifting parts of the aircraft, such as the fuselage, can contribute to the "weathervaning effect". As the nose drops, speed, and lift will increase.

So, with any computer-augmented control system, it might be a good idea to find out exactly what it does. Computer control inputs should be the same as human ones, but computers are tireless and many times faster.

A change in static stability from positive to nuetral could be accomplished in flight with a shift in center of gravity by dropping ordinance or transferring fuel, but it might be easier to keep it neutral all the time and have different levels of autopilot.

Just "adjusting control surfaces" generally makes the plane pitch, roll, or yaw.


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