J-20 Fighter stable or unstable canard airframe?

[p1t1o]

On 10/14/2017 at 8:16 PM, renhanxue said:

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.)

From what you are saying, yes, perhaps I should not be disagreeing with @Boris-Barboris so hard. I was also confusing some aspects of unstable design with overall modern design principles.

Either way, to answer (in bold) some of your points Boris:

On 10/14/2017 at 3:09 PM, Boris-Barboris said:

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

Fully fly-by-wire prevents this, otherwise would be unflyable. (but I am talking vs a stable aircraft here)

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

To be clear, less drag under certain circumstances. Less severe control inputs required to maintain AoA. (This one works as it is vs stability)

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

To be honest, this is probably a little off the mark as it is to do with operating at high AoA again - my thinking here was more related to the high-alpha performance of the airframe than its stability - illustrating that degree of stability is not a single design choice, it must be in concert with the design of the entire airframe. No point in being able to reach high AoA if your airframe cannot operate in that regime. So an unstable airframe has better control authority at high AoA because it must have. Not the best way to make the point to make in hindsight, my bad 

However one example that demonstrates this, which you did not address, was with short takeoff/landing. Delta wings, for example, are prone to low elevator control moments due to the elevator surfaces being quite far forward when compared to traditional tailplane planforms. This increases rotation speed when taking off, with an unstable (or as we now know we are talking about, also neutrally stable) airframe, pitchup requires far less control input, decreasing takeoff run. 

 

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.

This is probably where it become obvious that we are arguing in slightly different directions. I think your definition of "neutral stability" falls under my broad definition of "instability" (as compared to "stability").

 

Neutral airframe does all of this better.

So why, in your opinion, does anyone design an aircraft differently? There must be a good reason, unless you are prepared to go for something esoteric like corruption or incompetence? I would imagine that there are advantages of magnitude (as well as disadvantages but all design choices are a tradeoff of some sort) with an "unstable" as opposed to a "neutral" design.

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

Well I mean, you'd still have to read them

To conclude, Im glad it looks like we were mostly on the same page I think, but some good points of discussion anyways!

Edited October 16, 2017 by p1t1o

[Boris-Barboris]

59 minutes ago, p1t1o said:

However one example that demonstrates this, which you did not address, was with short takeoff/landing. Delta wings, for example, are prone to low elevator control moments due to the elevator surfaces being quite far forward when compared to traditional tailplane planforms.

Problem of delta wing itself.

On different designs: i'm pretty sure the balancing itself is the same for all modern fighters - as close to neutral as possible, but preferably stable. Every big player is already experienced enough to build reliable control systems. Wing plan however is closely related to tradition, existing industry and tactical requirements, imo.

Yeah, looks like we just speak in different terms.

[mikegarrison]

On 10/16/2017 at 4:40 AM, Boris-Barboris said:

Yeah, looks like we just speak in different terms.

Thinking it over, that's quite possible.

The thing is, nobody wants to fly in an airplane that is ultimately unstable. Very early airplanes were like this, and they killed a lot of their pilots because the pilot had to *constantly* keep ahead of the airplane. If the pilot ever relaxed or stopped paying attention to the airplane, it could go completely divergent very quickly.

So a lot of learning and science went into making airplanes that wouldn't do that.

I promised some quotes. This is taken from Raymer's Aircraft Design: "The basic concept of stability is that a stable aircraft, when disturbed, tends to return by itself to its original state." Static stability means that, when disturbed, the forces on the airplane will push in the restoring direction. Dynamic stability means that there is some damping involved, so that the restoring forces don't overshoot. (As I said, this is analogous to the basic physics concept of harmonic motion.)

Maneuvering means things like turn rate, climb rate, etc. Handling quality means that even when making maneuvers, the plane responds linearly to the pilot's input. In modern fighters, this extends to things like post-stall handling and decoupling potential and kinetic energy in maneuvers (e.g. being able to change speed without changing altitude or turn without losing speed, etc.).

But the disconnect comes from boundaries in the concept. Back in the day, the way to make a stable and good-handling airplane was limited to making sure that the restoring forces were automatically generated without any intelligence in the control loop. This limited configurations and had the effect of always having the restoring forces fighting against the maneuvering forces. But with the addition of computers, now there was more freedom in the physical design of the airplane because the computer could insert itself into the control loop to keep the airplane stable. So yes, a stable airplane is always desired, but the question is how that stability is created.

In a conventional airplane, if you remove the tail the airplane will no longer be stable. In the same way, a computer-stabilized airplane will no longer be stable if you remove the computer. Both airplanes are "stable" by design (since the design includes the computer), but the computer can compensate for configurations that, without the computer, would not be stable. And it is the freedom to use these new configurations that allows the airplane to be more maneuverable.

When the phrase "inherently stable" was used early in this thread, it referred to the condition where the computer was not included as part of the stability loop. For modern airplanes, however, this is rapidly becoming an obsolete concept.

Edited October 19, 2017 by mikegarrison

[Boris-Barboris]

4 hours ago, mikegarrison said:

Static stability means that, when disturbed, the forces on the airplane will push in the restoring direction. Dynamic stability means that there is some damping involved, so that the restoring forces don't overshoot

Not really. Static stability is stability of the airframe on rails, pivoted through it's center of mass. It's easily evaluated in the wind tube. In linear models it's responsible for so-called "short-period mode" wobble. It's a property of a small part of the flight model. Useful and important, but partial.

Dynamic one is property of the whole model, when it's actually imitating disturbance from equilibrium during flight. Something something "phugoid mode" for linear models, yada yada, wich appears when actual 2\3D motion and lift comes into play. There's nothing wrong with the overshoot.

 

I don't see the point of including pilot or control system in the plane concept and discussing it's stability. Of course it's stable, it wouldn't fly otherwise. From the very beginning we discussed airframes.

[mikegarrison]

2 hours ago, Boris-Barboris said:

Not really. Static stability is stability of the airframe on rails, pivoted through it's center of mass.

Hey, I quoted my source, from an actual textbook written by a known expert and everything. It's the same definition you actually agreed to earlier in the conversation, and now you are disputing it?

https://en.wikipedia.org/wiki/Daniel_Raymer

Edited October 19, 2017 by mikegarrison

[mikegarrison]

2 hours ago, Boris-Barboris said:

I don't see the point of including pilot or control system in the plane concept and discussing it's stability.

Well, you may not see the point of discussing it, but there are airplanes out there flying right now that are not stable unless their computers are active.

[Boris-Barboris]

13 minutes ago, mikegarrison said:

Hey, I quoted my source, from an actual textbook written by a known expert and everything. It's the same definition you actually agreed to earlier in the conversation, and now you are disputing it?

You never quoted his definition of static or dynamic stability, only his "concept of stability". You then proceeded to expand on concepts of static and dynamic stability, and I didn't like your interpretation of dynamic one, I think you missed the point, that's why I expanded both conceps further.

There is not a single definition in this thread.

Just now, mikegarrison said:

Well, you may not see the point of discussing it, but there are airplanes out there flying right now that are not stable unless their computers are active.

Read the whole line please.

[mikegarrison]

Just now, Boris-Barboris said:

You never quoted his definition of static or dynamic stability, only his "concept of stability". You then proceeded to expand on concepts of static and dynamic stability, and I didn't like your interpretation of dynamic one, I think you missed the point, that's why I expanded both conceps further.

There is not a single definition in this thread.

I actually did quote him on static and dynamic stability. I just didn't put the quote marks around it. The definition of handling quality also came directly from his book.

I did not miss the point on dynamic stability. I did, perhaps oversimplify it. Yes, overshoot can actually be stable, as long as it decreases with time. But if the overshoot increases with time, it is unstable (even if the static stability condition is met). As I alluded to, it is the same concept as harmonic motion.

https://en.wikipedia.org/wiki/Complex_harmonic_motion#/media/File:Damped_oscillations.gif

The dynamically unstable condition is when the "underdamped" case is actually negatively damped, so that even a small oscillation actually grows with time rather than decreasing.

[Boris-Barboris]

31 minutes ago, mikegarrison said:

I did not miss the point on dynamic stability. I did, perhaps oversimplify it. Yes, overshoot can actually be stable, as long as it decreases with time.

Now we:
a). agree
b). both see, that mr.Raymer's interpretation is either oversimplified, or thorn out of context without expansion on his meaning of "overshoot".

Edited October 19, 2017 by Boris-Barboris