Add CoM and CoA indicators to debug aero display

[Boris-Barboris]

9 hours ago, Snark said:

That's an interesting point.  The question is, in practice, what would that look like.  Would it be confusing noise?  Or would it be a useful utility to see what's going on?  Hard to say without actually trying it.  Hmm... makes me kinda tempted to write a mod and then see how it actually plays out.

Imagine a marker, that is usually right on CoM (level flight) with vector pointing to zenith and a little back and is shifting forward and backward and changing magnitude when you provide input. Direction of shift depends on current AoA, input sign and craft design. For experienced eye it won't be confusing, but it also won't contain anything useful in itself. For a newbie, well, length of the discussion in this topic could be a sign of problems with it.

7 hours ago, Snark said:

So, yeah.  Some issues there.

Another one. CoP behind CoM? Yes. Rotating back towards zero AoA? Yes. Zero-pitch stable? No. Equilibrium AoA is -90. And imagine the same banana-like craft on zero AoA: winglets push nose down and tail up. CoP is not even defined in that case, because all forces are balanced, but the torque is there, and it's strong. That's the point when CoP shows it's inability to describe behaviour. Because it's some magical integral characteristic, which doesn't even exist, and posesses questionable ability to replace individual forces in describing craft rotation dynamics. It's not even helping with stability: what's the benefit of seing it ahead of CoM right before flipping while observing the same behaviour situationaly on stable planes. Maybe, in some cases, it can give something, but even describing it's essence to someone just trying to fly is quite a hell.

7 hours ago, Snark said:

But an explicitly drawn (CoM) marker would be lots nicer. 

True.

 

8 hours ago, FullMetalMachinist said:

Yeah, he says that in the OP. 

No, read the OP:

14 hours ago, Snark said:

This would be the vector sum of all the current aerodynamic forces on the craft:  lift + drag.

That's CoP (with a couple of "if"s). Aerodynamics center we have already - it's CoL in SPH.

Edited January 22, 2016 by Boris-Barboris
typos

[Boris-Barboris]

@Plusck Those examples don't have a property to magically shift CoP instantly when they go out of equilibrium. That's a privilege of CoP of the aircraft )

54 minutes ago, Plusck said:

If the CoP or aerodynamic centre is ahead of the CoM then you have a problem

Again, with "if"s. Will you agree, that there are situations (caused by player input, and AoA is not 0), when perfectly stable craft's CoP is ahead of CoM?

Edited January 22, 2016 by Boris-Barboris

[FullMetalMachinist]

11 hours ago, Boris-Barboris said:

No, read the OP

I did, and right next to the bit you quoted, he calls it a center of aerodynamics marker (in bold, nonetheless). But perhaps we're getting caught up in semantics. I'll admit that I'm not educated enough to explicitly state the small differences between CoP and CoA. However, I would contend that the CoL marker in the SPH is not center of aerodynamics. It only shows lift from wings and control surfaces. A blank sphere would not have a CoL marker in the SPH, but would definitely experience aerodynamic forces as it flew through the air. 

Edited January 22, 2016 by FullMetalMachinist

[Boris-Barboris]

1 hour ago, FullMetalMachinist said:

It only shows lift from wings and control surfaces.

It shows lift from all parts with modules implementing ILiftProvider. That means, it's encompassing body lift too, despite common belief.

1 hour ago, FullMetalMachinist said:

A blank sphere would not have a CoL marker in the SPH, but would definitely experience aerodynamic forces as it flew through the air. 

It would experience drag and drag only (both in stock and FAR), and CoL has no relation to drag.

1 hour ago, FullMetalMachinist said:

I did, and right next to the bit you quoted, he calls it a center of aerodynamics marker (in bold, nonetheless).

Long story short, what he calls CoA, is usually called as "Center of Pressure". And we are absolutely fine with that, we both know what we mean. There is another widely used term - "Aerodynamic center". It has very little to do with CoA in Snark's meaning and CoP in mine, and I assure you, that we already possess it in the form of CoL in the SPH.

Edited January 22, 2016 by Boris-Barboris

[FullMetalMachinist]

2 hours ago, Boris-Barboris said:

That means, it's encompassing body lift too, despite common belief.

I just checked in the game, and yes this indeed does happen. I stand corrected. It took re-reading the whole thread, but I think I am finally starting to understand the difference between CoL and CoP, and how that all relates to aero stability. Thank you for patiently and clearly describing your points. 

And I have to say, now I do agree with your opinion that either a CoP or CoA marker in flight may not be as helpful or informative as we might hope. 

Edited January 22, 2016 by FullMetalMachinist
Spelling

[Enceos]

CoM and CoL in flight for the stock aero debug menu is actually such a cool idea.

[Stoney3K]

CoL also doesn't show body lift or lift from actuated control surfaces. So even if you have something that may look stable in the SPH, it may get unstable with higher AoA due to body lift.

Center of drag is actually a useful measure for rocket builders for the "Why does my rocket flip" question. It may not be the most accurate way of showing aerodynamics for you FAR gurus, but it's a ballpark figure.

Another one. CoP behind CoM? Yes. Rotating back towards zero AoA? Yes. Zero-pitch stable? No. Equilibrium AoA is -90. And imagine the same banana-like craft on zero AoA: winglets push nose down and tail up. CoP is not even defined in that case, because all forces are balanced, but the torque is there, and it's strong.

In that case the CoP definitely is defined. It is a vector which has zero magnitude, centered at CoM, but it does have a torque component along the pitch axis.

If you showed that in-game as a quaternion, not as a linear vector, you would have no trouble displaying that, ie. having a circular arrow that indicates torque along the correct axis.

Separating the CoP in a CoL (which we basically already have) and a center of drag would be more useful since it allows you to determine the drag behavior on rockets and SSTOs.

[numerobis]

The CoL marker has all the purported problems of the CoP marker, yet it's useful. So I don't understand the opposition to a CoP marker.

It handles the problem of surprising readouts at zero angle of attack by simulating a small positive AoA: notice how wings show lift even at precisely zero angle of attack in the SPH and VAB.

As fuel drains, CoM shifts. That's part of the education of a new plane designer: is your plane stable as the tanks drain?

As angle of attack varies, CoL shifts. That's also part of the education, usually with respect to canards: rotate the root so you can see the CoL change, make sure it's ahead of CoM in all cases.

[Boris-Barboris]

14 minutes ago, numerobis said:

yet it's useful

Could you name one use case?

[numerobis]

Is that a serious question?

The use case for the CoL marker is helping people design stable KSP airplanes. The CoL marker is very often used for that purpose, as evidenced by this search.

That it's an incomplete aid is not a great argument against -- it just has to be 20% better than nothing.

[Boris-Barboris]

15 minutes ago, numerobis said:

The use case for the CoL

I'm asking about CoP, sorry.

Edited March 23, 2016 by Boris-Barboris

[numerobis]

Basically the same. The markers when you display both CoP and CoM indicate where the torque arm is.

Is your rocket flipping? Well, probably when you get off prograde you've got torque that pushes you further away from prograde -- i.e. CoP is ahead of CoM and gets worse with angle of attack. Maybe it's something else, but most newbies could see that, understand the problem, and learn.

[ferram4]

@numerobis: You're correct, as angle of attack changes, the CoP shifts.  If you're treating CoL as just the lift component that goes into the CoP, it's completely and utterly useless since adding a constant torque somewhere will change the location of the CoP/CoL despite it having no effect on the stability of the craft.  Otherwise you could argue that deflecting control surfaces, since it will shift the CoP/CoL will make craft unstable, but that's just not how it works.

Any aerodynamic indicator needs to be applied at the location where the change in aerodynamic forces with AoA is applied, because aero stability is entirely based on how aerodynamic forces change, not their specific magnitude this second.  Incidentally, that's what FAR modifies the CoL to be (a more traditional and useful Aerodynamic Center) and it works out great; I actually haven't bothered designing a plane with the other FAR graphs in awhile.

Now stock models the CoL using the lift-portion-of-CoP method, so you're probably wondering why it's even useful right now.  That's because (working only with pitch here for simplicity) if there is no aerodynamic torque at zero-lift, the CoP and Aerodynamic Center are at the same location.  The problem occurs once you stop looking at rockets and planes with symmetrical wings with no angle of incidence on them and things get really weird.  Especially since the CoP can shift outside the craft and then, well, it's pretty obviously useless.  But that's correct for the CoP and that's why it's never used in stability analysis in real life, only as something of a theoretic curiosity.  Aerodynamic center is the way to go.

[Boris-Barboris]

5 minutes ago, numerobis said:

The markers when you display both CoP and CoM indicate where the torque arm is.

That's what the CoP "opposition" is trying to convey here for two pages: they don't. Like, really, you need to comply to a list of stupid inconvenient conditions to make them do it. And then you would be left with lever arm of some force, with no relation to stability traits.

You may want to show newbies a torque in flight? Well, they already do see torque, because they see their craft rotating. Another indicator for something so obvious is redundant.

I believe the best you can do is add CoM and CoL (corrected to show body lift) to debug menu.