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?


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

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