J-20 Fighter stable or unstable canard airframe?

[AeroGav]

Canard-delta airframes aren't new.    The late 60s / early 70s brought us the SAAB Viggen, 

but it had a CoL behind it's CoM and therefore did not need FBW (fly by wire) technology to render it controllable.

However, most current fighter aircraft are designed with CoL ahead of CoM (inherently unstable).     They are completely dependent on FBW assistance - this includes tailed designs like the F-22 Raptor, as well as canards like the Eurofighter Typhoon -

But what of the dark horse, China's J-20?  It's  hard to find actual information about this aircraft rather than just endless trash-talking and trolling on the internet - yes we know China makes a lot of cheap and substandard stuff.   Yes we know they're new to this game and however it looks on paper it probably will not be as good of a weapons system as the Raptor in reality.  

What interests me is the airframe.    I suspect this might not be a "relaxed stability" or "inherently unstable" design.   Look at the canards in this takeoff video ,  they angle up to raise the nose

... and they appear to remain angled up after the aircraft rotates.  They are having to maintain an up force to maintain the positive AoA.   On an unstable design,  the higher the AoA, the more the canards must angle downward, to suppress the nose's tendency to rise ever higher.  Of course, it is somewhat complicated by the fact it also has elevons and thrust vectoring,  we can't really see what these surfaces are doing from this low quality footage.

[Boris-Barboris]

1 hour ago, AeroGav said:

most current fighter aircraft are designed with CoL ahead of CoM (inherently unstable)

There is nothing to gain from doing so, therefore I doubt that is the case. "inherently unstable" sounds like a buzzword.

[KSK]

14 minutes ago, Boris-Barboris said:

There is nothing to gain from doing so, therefore I doubt that is the case. "inherently unstable" sounds like a buzzword.

I know very little about aerodynamics but this quote from the relevant Wikipedia page is consistent with my limited understanding. The fine details I'll leave to other, more qualified, folks on this forum.

Intentional instability

The latest generation of fighter aircraft often employ design elements which reduce stability to increase maneuverability. Greater stability leads to lesser control surface authority, therefore a less stable design will have a faster response to control inputs. This is highly sought after in fighter aircraft design."

For completeness, the full section reads as follows but comes with two 'citation needed' flags.

The latest generation of fighter aircraft often employ design elements which reduce stability to increase maneuverability. Greater stability leads to lesser control surface authority, therefore a less stable design will have a faster response to control inputs. This is highly sought after in fighter aircraft design. The BAE Harrier GR7/GR9 employs a significant and obvious anhedral angle to its wings, reducing the inherent lateral stability of the wings mounted high on the fuselage.

A less stable aircraft requires smaller control deflections to initiate maneuvering; consequently drag and control surface imposed stresses will be reduced and aircraft responsiveness will be enhanced. Since these characteristics will typically make control by the pilot difficult or impossible, an artificial stability will typically be imposed using computers, servos, and sensors as parts of a fly by wire control system.

 

[Boris-Barboris]

2 minutes ago, KSK said:

which reduce stability

 

3 minutes ago, KSK said:

hich reduce stability

 

3 minutes ago, KSK said:

A less stable aircraft requires smaller control deflections to initiate maneuvering

 

note the difference

2 hours ago, AeroGav said:

unstable

 

When you cross the point where you become statically unstable, there is nothing more to gain. Statically neutral airframe has the best L/D among it's slightly altered towards stable\unstable configuration siblings.

[AeroGav]

Incidentally, I don't know if that thumbnail from the video is real or faked (not heard about this being a carrier airplane before) , but in that landing shot it looks like the canards are fairly close to maxed out just holding a moderate AoA.   For a carrier approach that's about perfect, your flaps are lifting, the canards are lifting, the leading edge is drooped, but the whole airplane is not at such an AoA that it'll be flying in the back of the lift drag curve (hard to control speed) or likely to tailstrike.        For maximum instantaneous turn rate , post stall maneuvering, it seems less ideal, but like i say could well be fake shot.

[KSK]

33 minutes ago, Boris-Barboris said:

note the difference

When you cross the point where you become statically unstable, there is nothing more to gain. Statically neutral airframe has the best L/D among it's slightly altered towards stable\unstable configuration siblings.

Difference duly noted.

I suppose it might depend how much you cared about L/D vs. maneuverability? Genuine question - would you always care about optimizing L/D, or are there aircraft types (like fighters) where maneuverability is going to be more important than raw speed?

Reading up on this (which has been interesting so thanks for starting me off on that!), I've found some articles which suggest that some designs do put the centre of mass behind the neutral point but I'm really just skirting the rabbit hole of personal ignorance on this one.  I could certainly throw more citations at the thread but I'd just be quoting them without any real feel for how accurate (or otherwise) they are.

Happy to wait for the better-informed to weigh in here!

Edited October 9, 2017 by KSK

[Boris-Barboris]

6 minutes ago, KSK said:

I suppose it might depend how much you cared about L/D vs. maneuverability. Reading up on this (which has been interesting so thanks for starting me off on that!), I've found some articles which suggest that some designs do cross that point

The only reason I can think of is trans\supersonic shenanigans, in case designers have to optimise the regime (maybe to compensate for mach tuck, idk) so hard they are forced to make the design unstable on other regimes, but I have a feeling this problem was solved without making such drastic sacrifices.

[p1t1o]

I can tell you that the Viggen was not designed with relaxed stability, it doesnt look it but the Viggen is an aircraft of 1960's vintage, it was incredibly modern in its time, and I believe still in service to some extent. The canards are there to provide short take-off/landing performance by providing extra lift to assist with rotation/flare (delta wings are prone to having poor control moment in pitch, especially at low speed).

The trailing moveable surfaces on the canards are not control surfaces but flaps.

It is not a fighter jet but specifically designed for low level interdiction (strike behind enemy lines), manouverability not really an issue, but it was fast (supersonic at sea level is considered fast even today). It essentially has an afterburning airliner engine squeezed in the back.

**edit**

Oh ok, retired in 2005, but still!

**edit#2**

50 minutes ago, KSK said:

I suppose it might depend how much you cared about L/D vs. maneuverability? Genuine question - would you always care about optimizing L/D, or are there aircraft types (like fighters) where maneuverability is going to be more important than raw speed?

If you word it differently, the question gains a different character - ask "Would you ever design an aircraft for pure speed?" 

The answer is: Today? Rarely.

Look at designs like the F-104 starfighter. A design based almost entirely on speed, it is an almost perfect high-speed airframe.

Unfortunately almost every single other characteristic was poor, it was only really just capable of taking off and landing.

So no, you dont often build aircraft of any type (civilian, military, whatever) with only one design characteristic in mind any more.

Especially since there is a pseudo-hard limit to the top speed of modern jet aircraft, about Mach 2.2, and modern engines can push you there quite easily. Any faster requires stupidly more energy and you soon hit thermal limits or you need a fundamentally different type of aircraft.

 

**edit#3**

@AeroGav The J-20 is almost certainly relaxed-stability. What you are seeing with the canards is a high-lift configuration. Though the airframe is unstable, during landing/takeoff lift is maximised, so in conjunction with the other control surfaces, which are also operating in max-lift mode the whole airframe is still unstable but the canards are still producing lift.

What you are probably not seeing is that the rear elevators are pitched down (or "less pitched up") more than you would expect to maintain the attitude. It might not be visible to the naked eye, but it is likely. 

These modern canard designs are sometimes called "triplanes" - elevons, wings and canard, and all control surfaces (and some have extra ones in weird places) work together as a homogenous whole to fly the aircraft.

Edited October 9, 2017 by p1t1o

[AeroGav]

38 minutes ago, p1t1o said:

It is not a fighter jet but specifically designed for low level interdiction (strike behind enemy lines), manouverability not really an issue, but it was fast (supersonic at sea level is considered fast even today). It essentially has an afterburning airliner engine squeezed in the back.

Maneuverability depends on what kind of "maneuverability".  It probably wasn't capable of  particularly high angles of attack,   if DCS world is correct, the engine betrays its airliner heritage by suffering compressor stalls  at high AoA.

At more moderate AoA and higher mach numbers, perhaps in a medium altitude transonic sustained turn, it would give its contemporaries, such as a Mig 21,  F-104 Starfighter ,  F-4 Phantom  a pretty hard time.     Again though ,  with the turbofan engine (contemporaries all used pure turbojets) you get  good cruise economy and massive power boost in afterburner, but especially bad fuel consumption in burner.      Overall a mixed bag.

1 hour ago, KSK said:

I suppose it might depend how much you cared about L/D vs. maneuverability? Genuine question - would you always care about optimizing L/D, or are there aircraft types (like fighters) where maneuverability is going to be more important than raw speed?

In a 9G manoeuvre (turn, loop or some combination thereof) the wings are producing 9 times as much lift as the aircraft's weight.   Which means even with a 1:1 thrust weight ratio (which is tough to do, not all fighters have that) and a 9:1 lift drag ratio (VERY hard to do in an airframe optimised for a very wide speed range , supersonic, or at high angle of attack) you're going to have a hard time not loosing kinetic energy as you manoeuvre.      

53 minutes ago, p1t1o said:

Especially since there is a pseudo-hard limit to the top speed of modern jet aircraft, about Mach 2.2, and modern engines can push you there quite easily. Any faster requires stupidly more energy and you soon hit thermal limits or you need a fundamentally different type of aircraft.

 

Also, there is a school of thought that afterburners make you extremely visible to everyone that wants to shoot you down,   even super cruising supersonic flight causes frictional heating that makes the plane visible in IR.   

[p1t1o]

17 minutes ago, AeroGav said:

Maneuverability depends on what kind of "maneuverability".  It probably wasn't capable of  particularly high angles of attack,   if DCS world is correct, the engine betrays its airliner heritage by suffering compressor stalls  at high AoA.

At more moderate AoA and higher mach numbers, perhaps in a medium altitude transonic sustained turn, it would give its contemporaries, such as a Mig 21,  F-104 Starfighter ,  F-4 Phantom  a pretty hard time.     Again though ,  with the turbofan engine (contemporaries all used pure turbojets) you get  good cruise economy and massive power boost in afterburner, but especially bad fuel consumption in burner.      Overall a mixed bag.

Oh, I didnt say it wasnt manouverable  

But in service, you would hardly ever find a Viggen in medium altitude transonic flight, and a Viggen in a dogfight is living on borrowed time (available AA load was very rudimentary, and dogfighting with your AG weapons still on board would be suicide). It is built to perform best down in the weeds

Having said that, an air-superiority variant was eventually produced, showing that it at least had some real ability.

Apparently the Viggen DCS module has some issues with perfecting the engine/performance envelope sim, but yeah, as far as sims go, DCS is the best of the best.

Edited October 9, 2017 by p1t1o

[YNM]

IMO the key is that it's a canard. It's a fin, like rockets. Just that it's up front. You want to angle up ? Push the front up, deflect air down.

Not sure about aerodynamics. Exteriorly similar to SAAB Gripen.

Comparison :

Apparently the logic is different, very evidently on Gripen the canard is used minimally.

Edited October 9, 2017 by YNM

[AeroGav]

1 hour ago, YNM said:

Apparently the logic is different, very evidently on Gripen the canard is used minimally.

If the Gripen has relaxed stability, or to put it another way, has CoM and CoL very close together, it will not need to keep angling the canard up to maintain a high angle of attack, only to start the nose moving upward in the first place.  You see that on takeoff, then the canard keeps a very minimal angle.

compare with Viggen,  you can see a  lot of canard deflection to maintain the nose angle after takeoff, it's CoL is far aft and is trying to push the nose back down again.

For the opposite, anyone who's ever built a spaceplane has probably seen this - coming back from orbit, SAS is holding a nose-up re-entry attitude, but due to CG shifting to the rear when empty (heavy engines at the back), it is unstable.    You will see that SAS is actually trying its hardest to push the nose down and stop it rising further, because the airplane is unstable and suffering a positive feedback loop.    When the atmosphere gets thick enough to overwhelm RCS, and the controls max out on nose down input, the plane flips out of control.

Edited October 9, 2017 by AeroGav

[YNM]

31 minutes ago, AeroGav said:

If the Gripen has relaxed stability, or to put it another way, has CoM and CoL very close together, it will not need to keep angling the canard up to maintain a high angle of attack, only to start the nose moving upward in the first place.  You see that on takeoff, then the canard keeps a very minimal angle.

compare with Viggen,  you can see a  lot of canard deflection to maintain the nose angle after takeoff, it's CoL is far aft and is trying to push the nose back down again.

For the opposite, anyone who's ever built a spaceplane has probably seen this - coming back from orbit, SAS is holding a nose-up re-entry attitude, but due to CG shifting to the rear when empty (heavy engines at the back), it is unstable.    You will see that SAS is actually trying its hardest to push the nose down and stop it rising further, because the airplane is unstable and suffering a positive feedback loop.    When the atmosphere gets thick enough to overwhelm RCS, and the controls max out on nose down input, the plane flips out of control.

Well, so far things look such, but don't forget that these planes have lots of controls to play with. Gripen, for instance, still have thrust vectoring and some more control surface on the back. The same goes to J-20. It's hard to judge things when you have lots of variables.

[p1t1o]

To be clear, with relaxed stability, it is much more complex than "canard goes up, nose goes up".

Without stability, those canards will be making hundreds of deflections per second in both directions, in order to maintain a stable attitude, and if you look closely at airshow footage at an aircraft with these features, you might see th e nose pitch up with an appropriate canard deflection and mid-pitch up the canards will suddenly deflect sharply down, this is to arrest the nose-upwards motion which would continue to accelerate otherwise, possibly overloading the airframe (almost certainly in fact).

(Normally, the AoA of the tailplane would auto-arrest the pitch-up, that would be the definition of stability)

So its not surprising that the canards might not behave exactly the same in every takeoff/landing. At any particular moment in time, even knowing the aircraft attitude and control input, it would be very hard to predict where the canard should be, because it constantly needs to stabilise the aircraft from departing controlled flight.

Again, the canards on the Viggen are not control surfaces but lift devices. Viggen is stable. It is more of a "double wing" than a "wing-canard".

 

Edited October 10, 2017 by p1t1o

[renhanxue]

The Viggen's canards have flaps (which incidentally cannot be directly controlled by the pilot, they have three possible fixed settings that are configured by the ground crew depending on the weapons load to keep trim reasonable in flight, and otherwise only deploy together with the landing gear), but are - as p1t1o correctly points out - not control surfaces. Instead they serve both as vortex generators that increase lift on the main wing, and to improve local stability at certain angles of attack. The concept is known as a close-coupled canard wing and it was (as far as I know) invented at Saab in the early 1960's (at the very least they filed for a patent on it). If you want to know how it works, there's a lot of interesting details to be found in one of the few really in-depth texts about the Viggen in English: a NASA technical memo that is a translation of a text by the chief aerodynamic engineer of the Viggen project, Krister Karling. It's actually a fairly accessible text even though it goes into a lot of detail, since it starts from a quite basic level, giving you a quick crash course in aerodynamics, and works itself up (or down, depending on how you look at it) from there.

The Gripen's and Rafale's canards both exploit the close-coupling effect to improve low speed/high AoA characteristics (basically, to get a lower landing speed), the Gripen because it was designed for short-field operations and the Rafale because there is a naval variant for carrier operations. The Typhoon on the other hand has far smaller canards that only really serve as control surfaces - they were intended to increase maneuverability to a ridiculous degree in combination with thrust vectoring, but the Typhoon's thrust vectoring engine was cancelled and so it maintains its position as king of expensive boondoggles and bizarre developmental dead ends.

Edited October 13, 2017 by renhanxue

[renhanxue]

On 10/9/2017 at 4:19 PM, AeroGav said:

Maneuverability depends on what kind of "maneuverability".  It probably wasn't capable of  particularly high angles of attack,   if DCS world is correct, the engine betrays its airliner heritage by suffering compressor stalls  at high AoA.

The TF30's in the F-14 (which entered service at roughly the same time as the Viggen) suffered from the exact same problem, possibly to an even greater extent, but unlike the F-14 the first Viggen version was a strike aircraft, not a fighter, and in practice didn't care as much. On the air superiority version which came along ten years later, Volvo Aero added an extra compressor stage to the engine and much reduced the risk of compressor stalls at high AoA. Still, though, the Viggen is still a gigantic, vaguely triangular, flying barndoor, and while the air superiority variant actually had a legitimately good instantaneous turn rate for its time, if you had to make more than a full circle, well, you might as well call it a day.

[mikegarrison]

On 10/9/2017 at 4:57 AM, Boris-Barboris said:

There is nothing to gain from doing so, therefore I doubt that is the case. "inherently unstable" sounds like a buzzword.

That's actually completely wrong. Modern fighters are designed to be unstable because it makes them more maneuverable. Statically stable airplanes have to fight their own stability in order to change directions, while unstable airplanes have no such problem.

The trick is to add the necessary stability into the avionics instead of building it into the aerodynamics.

[Boris-Barboris]

3 minutes ago, mikegarrison said:

Modern fighters are designed to be unstable because it makes them more maneuverable.

Define unstable and maneuverable please, so we can discuss things on common ground.

Edited October 14, 2017 by Boris-Barboris

[mikegarrison]

1 minute ago, Boris-Barboris said:

Define unstable and maneuverable please, so we can discuss things on common ground.

It's been forever since I had my last class on this. I doubt I could dredge up a legitimate definition of stability. Static stability is easy -- any perturbation tends to create a force opposing it. Dynamic stability is trickier. However, conceptually it's roughly equivalent to the concept of over-damped versus under-damped harmonic motion. The one is stable but slow, the other is much faster but unstable.

All my airplane design books are in the office, so if you want the textbook definitions, you'll have to wait until the weekend is over.

[Boris-Barboris]

1 minute ago, mikegarrison said:

Static stability is easy -- any perturbation tends to create a force opposing it

Let's go on with this one, it's good enough for static. In all my posts above, and in this post, when I say stable\unstable\neutral, I mean it's static flavor.

Now hear out my point: There is no reason to intentionally make airframe statically unstable for the sake of it. There may be airplanes out there that are st.unstable, but I do believe their designers would gladly make them neutral if they could. For example, I did read unofficial reports of people saying F-16 being st.unstable on subsonic, because they were forced to do it to keep supersonic performance in certain margin. If they could find the way to keep supersonic performance at bay while making subsonic phase neutral, the plane would perform generally better. Instability is a necessity, not a goal, because it brings larger risk with it and bears no reward in itself.

Unstable craft is not more maneuverable than a neutral one. It has worse sustained turn rate. It has the same or worse aiming time (let's say you fly with stable pitch 0 and need to have stable pitch 10 deg. as soon as possible). It's even more risky than a neutral one. The only thing it can actually do better is to flip as fast as possible, and the place for such maneuver in modern doctrines is hard to be seen. This maneuver tears the plane to pieces or kills the pilot on high speeds, and on low speeds TVC is, imo, much more important.

[p1t1o]

42 minutes ago, Boris-Barboris said:

Let's go on with this one, it's good enough for static. In all my posts above, and in this post, when I say stable\unstable\neutral, I mean it's static flavor.

Now hear out my point: There is no reason to intentionally make airframe statically unstable for the sake of it. There may be airplanes out there that are st.unstable, but I do believe their designers would gladly make them neutral if they could. For example, I did read unofficial reports of people saying F-16 being st.unstable on subsonic, because they were forced to do it to keep supersonic performance in certain margin. If they could find the way to keep supersonic performance at bay while making subsonic phase neutral, the plane would perform generally better. Instability is a necessity, not a goal, because it brings larger risk with it and bears no reward in itself.

Unstable craft is not more maneuverable than a neutral one. It has worse sustained turn rate. It has the same or worse aiming time (let's say you fly with stable pitch 0 and need to have stable pitch 10 deg. as soon as possible). It's even more risky than a neutral one. The only thing it can actually do better is to flip as fast as possible, and the place for such maneuver in modern doctrines is hard to be seen. This maneuver tears the plane to pieces or kills the pilot on high speeds, and on low speeds TVC is, imo, much more important.

Long story short, far from being simply a tool for fancy air-show aerobatics, instability gives you the option and ability to use an extended airspeed and AoA envelope.

[Boris-Barboris]

21 minutes ago, p1t1o said:

instability gives you the option and ability to use an extended airspeed and AoA envelope.

how so?

[p1t1o]

5 hours ago, Boris-Barboris said:

how so?

Without writing out paragraphs on, and pushing the limits of my knowledge of fluid dynamics and modern air combat, its hard to put into few words, but one stand out factor is that an unstable airframe can achieve higher AoA, with faster onset, with less drag and more importantly, significantly more control authority. Control responsiveness is increased across a wide area of the flight envelope due to the lack of a strong aerodynamic correcting moment. Not having to compensate for that correcting moment also reduces opportunities for drag and loss of lift. This brings many benefits, some of which are less obvious, such as increasing high-altitude supersonic manouverability or improving short-take-off/landing performance (ability to hold a more aggressive flare, or ability to rotate at a lower speed). It also is not only relevant to the pitch axis, instability in the roll axis brings greater roll responsiveness as well.

If you want to know exactly why and how it influences those and other factors, you are at the point where you are going to have to start reading research papers. There is no "one big thing about instability" which makes it a silver bullet for all aircraft, instability doesnt work just on its own, there is no single factor or coefficient that defines "maneuverability". Without an airframe capable of flying within such an extended envelope, its not much use.

So you cannot, of course, just "add instability" to any airframe and get an increase in "maneuverability", you have to design it from the ground up, and yes, as in the F-16 there may be factors other than agility that dictate your aerodynamics.

[Boris-Barboris]

3 minutes ago, p1t1o said:

can achieve higher AoA

You can perfectly make a flip-happy stable airframe.

4 minutes ago, p1t1o said:

with faster onset

Faster to reach certain AoA? Yes. To reach and not exceed it? No, not really.

5 minutes ago, p1t1o said:

with less drag

Why would unstable airframe be less draggy than a stable one?

6 minutes ago, p1t1o said:

significantly more control authority

Why would making an airframe unstable increase authority of it's control surfaces?

 

11 minutes ago, p1t1o said:

Control responsiveness is increased across a wide area of the flight envelope due to the lack of a strong aerodynamic correcting moment. Not having to compensate for that correcting moment also reduces opportunities for drag and loss of lift.

Yes, but if you make that strong correcting moment very weak, you make abovementioned "advantages" of unstable design moot, while still being stable. Just because the craft is hard or impossible to fly manually doesn't mean it's unstable. If your airframe takes 1000 years to return back to prograde after small AoA perturbation, it's still stable.

16 minutes ago, p1t1o said:

increasing high-altitude supersonic manouverability

That's the only point I can stand besides. And it always implies, that the designers just coudn't find a stable way. I do not state that there was a stable way for each of those cases (let's assume F-16 is actually statically unstable when subsonic, though the sources are mostly unofficial), but I also doubt anyone can prove that there wasn't.

20 minutes ago, p1t1o said:

improving short-take-off/landing performance (ability to hold a more aggressive flare, or ability to rotate at a lower speed)

Neutral airframe does all of this better.

21 minutes ago, p1t1o said:

If you want to know exactly why and how it influences those and other factors, you are at the point where you are going to have to start reading research papers.

Not really, I just need to find someone who will point me to the exact research paper.

[renhanxue]

5 hours ago, p1t1o said:

Without writing out paragraphs on, and pushing the limits of my knowledge of fluid dynamics and modern air combat, its hard to put into few words, but one stand out factor is that an unstable airframe can achieve higher AoA, with faster onset, with less drag and more importantly, significantly more control authority. Control responsiveness is increased across a wide area of the flight envelope due to the lack of a strong aerodynamic correcting moment. Not having to compensate for that correcting moment also reduces opportunities for drag and loss of lift. This brings many benefits, some of which are less obvious, such as increasing high-altitude supersonic manouverability or improving short-take-off/landing performance (ability to hold a more aggressive flare, or ability to rotate at a lower speed). It also is not only relevant to the pitch axis, instability in the roll axis brings greater roll responsiveness as well.

If you want to know exactly why and how it influences those and other factors, you are at the point where you are going to have to start reading research papers. There is no "one big thing about instability" which makes it a silver bullet for all aircraft, instability doesnt work just on its own, there is no single factor or coefficient that defines "maneuverability". Without an airframe capable of flying within such an extended envelope, its not much use.

So you cannot, of course, just "add instability" to any airframe and get an increase in "maneuverability", you have to design it from the ground up, and yes, as in the F-16 there may be factors other than agility that dictate your aerodynamics.

You're kinda missing the point Boris-Barboris is trying to make, here. If the aircraft is statically unstable at a given set of conditions, then any perturbation (such as control surface input or turbulence) will lead to a self-reinforcing pitching, yawing or rolling moment - that is, a positive feedback loop. Pitching up will cause a moment that works to pitch up further, for example. That is what Boris-Barboris means when he says that such aircraft are good at flipping over very fast. To fly such an aircraft in a controlled way, all such moments must immediately be counteracted by counter-moments, so instead of fighting the correcting moment you're fighting the tendency to flip over. If the aircraft is statically neutral on the other hand, a perturbation does not lead to a correcting moment, but neither does it lead to reinforcement of the perturbation. What you are describing is essentially that, and Boris-Barboris wants you to use the correct terminology for it. (I think. Sorry if I misunderstood anything.)

Edited October 14, 2017 by renhanxue