Dynamic vs Static Stability

[phxpilot]

Could someone please provide the answer along with an explanation for the following question? It seems like it could be both a) and b).

Identify the type stability if the aircraft attitude tends to move farther from its original position after the controls have been neutralized.

a) Negative static stability.
b) Negative dynamic stability.
c) Positive static stability.

[A-FLOOR]

It's a). Negative dynamic stability implies a divergent oscillatory motion once perturbed (vibration with increasing amplitude) which brings the vehicle back past it's intended position, and then past it with increasing rates in each period. With true negative static stability the vehicle never returns to its intended position. It can be both so your train of thought is probably correct, but a) is the only one implied in the question and required for the described outcome. b) could also be true at the same time but is optional.

[keith williams]

If an aircraft is statically stable it will tend to return to its original attitude following a disturbance.

If it then osciliates with decreasing amplitude it is both statically stable and dynamically stable.

If it oscillates with increasing amplitude it is statically stable but dynamically unstable.

In this question the aircraft does not tend to return to its initial attune, so it is statically unstable.

Because there are no oscillations, the concept of dynamic stability is meaningless in this case.

[john_tullamarine]

Rather than thinking of tendencies to do this and that (yes, I know, that's how pilot training has addressed it for donkey's years .. but it is done, generally, in an inappropriate way, unfortunately) .. a better (and conceptually simpler) way of looking at (longitudinal) static stability (usually abbreviated LSS) is to consider stick forces .. which is how the thing is measured.

Starting from a trimmed speed (and without any retrimming), static stability requires that,

(a) to fly at a lower speed (doesn't matter how that state is achieved), and maintain that lower speed, requires a pull force on the stick

(b) to fly at a higher speed, and maintain that higher speed, requires a push force on the stick

The magnitude of the force as the speed delta varies (ie the slope of the speed against force curve) is a measure of the magnitude of the stability.

If the force gradient gets too shallow (static stability gets too low) the aircraft becomes too twitchy to fly reasonably and at risk of structural damage due to pilot mishandling .. hence one of the reasons for CG envelope limits.

If the force gradient reverses (ie pull becomes push and push becomes pull) the aircraft piloting workload becomes extremely high (the technique to fly it is quite different to normal flying) and, as a good bet, only folks with a flight test background are likely to be able to recognise and use the relevant techniques within the short timeframe available to get/keep things under control (typically after liftoff with a misloaded aircraft .. ie excessive aft CG)

If the aircraft is dynamically unstable, either you have a means of stepping out or you, very likely, die. Certainly, unless your aircraft has some fancy electronics in the control system, it is not flyable by a Mk. 1 pilot.

For those interested, some words of wisdom can be found at AC 23-8C (https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23-8C.pdf) p78 and subs

[JABBARA]

Static stability implies only tendency of the system. In another word we can estimate what will happen if we disturb the system. This means we know what kind of "static stability" that system has. Once the equilibrium of the system is disturbed then we can talk about the dynamic stability of system. Dynamic stability describes how the magnitude of drifting from the equilibrium state changes by time.

So , IMHO, answer should be (a).

[gums]

Some great info here, and the explanation by J.T. is really good.

Except for the Viper, my jets were very stable in the static mode, and prolly average in the dynamic mode. If you let go of the stick the sucker tried to get back to the "trimmed" AoA.

In the Viper it tried to get back to the trimmed gee ( unlike the 'bus, we could trim to anything from minus "x" to plus "x"), but the FBW control laws ruled, and not the inherent "relaxed" static stability. The interesting thing was once we had departed and had an AoA above 30 deg, the thing exhibited its natural aero characteristics because the computers only thwarted yaw and completely cut out our rudder pedal inputs. Turned out it was very dynamically stable, and ditto for static stability. So a manual control surface command via the electrons plus the inherent pitch authority once at the extreme AoA allowed us to "rock" the beast outta the deep stall. We went thru this long ago on AF447, huh?

I never flew a big plane, but I would have liked a strong aero tendency to get back to the trimmed AoA ( not speed or attitude in the FBW genre). I think the nasal radiator folks here that landed on a boat in the middle of the ocean will agree that strong pitch stability helps a lot. The other thing would be great directional stability, especially dynamic ( thinking about dutch roll).

[vilas]

gums
I would have liked a strong aero tendency to get back to the trimmed AoA ( not speed or attitude in the FBW genre)
Not exactly. The Airbus FBW does neither. Stick free it maintains 1g.

[gums]

@vilas

I know the pseudo one gee story. It is corrected for attitude, and is not pure gee as my Viper.

Mine trimmed to what you had set, even negative gee. In fact, I had students trim full back and let go. Sucked would do a pretty loop and hit the AoA limiter, ride it over the top, of then get back to the trimmed gee - about 3.5 gee.

[Winnerhofer]

http://www.pprune.org/tech-log/555473-stability-wars-why-did-relaxed-lose.html