[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[AdmiralTigerclaw]

The Blackbird is quite a tempermental beast, yes. Irrecoverable pitch-up if you exceeded 18 degrees AoA, and almost assured engine flameout if you exceed 10 degrees AoA above 20,000 feet. SOP for anything other than a minor hiccup would be ejection.

[theflyingfish]

SOP for anything other than a minor hiccup would be ejection.

minor hiccup

ejection

Pilot: *hiccup*

Copilot: "Frank, you okay?"

Pilot: "Yeah, just had a minor hiccup."

Copilot: "Oh, I hate it when that happens."

Later...

Pilot: *burp*

Copilot: "EJECT EJECT EJECT."

I'm sorry, I couldn't help it.

[sarbian]

Well, I do not really care for MechJeb, but if that means easier interoperation of FAR with things like Kerbal Engineer and the Realism mods I am all for it.

According to sarbian, the changes involve interfaces to FAR's code that permit MJ (and presumably other mods) to query FAR about the vessel without actually affecting anything.

Just passing by to reply to that. The change allow for other mods to query FAR part for different info inflight. The one I have working now is control surface torque, but more will/should come.

BUT this code won't come to MJ fast since I need to add some code changes (hooks) into MJ so we don't have to maintain a separate MJ+FAR dll.

If you want to talk with me more about FAR support in MJ come to the MJ thread, no need to fill this one

[asmi]

First, according to the laws of physics, the point of application of a force can be moved to anywhere along the line of the force without any change in the effect. The indicator chooses the point that is closest to the center of the vessel.

It's not unless you live in a parallel universe with different laws of physics Hint - the moment arm for the force (which is a definition of torque) depends on the point of application

Second, the indicator is actually representing the derivative of the force and torque, not the plain value. Specifically, the arrow represents the derivative of the force, and the position is chosen so that this change in the force, if applied there, would cause the right change in torque.

What derivatives of force and torque are you talking about? Torque is another word for force (specifically, it's component that is perpendicular to the line connecting center of mass with the point of application) applied to a certain moment arm (t = F * d)

When the linear force change becomes very low, the lever arm to produce the necessary torque grows a lot.

And why exactly there is a need to produce certain torque? The point of aerodynamics is to calculate the force and it's point of application - everything else will be handled by PhysX. So I would like that exact point and force to be displayed.

[Snjo]

It's not unless you live in a parallel universe with different laws of physics Hint - the moment arm for the force (which is a definition of torque) depends on the point of application

It's important to be clear about what you mean by "the line of the force".

In the first example there, the thrust are NOT in the line of the force, they are in the line of the lever. In the second, they ARE in line with the direction of the force.

The second example is an interpretation of the pendulum fallacy just for kicks. An engine at the front of the plane should not pull the plane out of a spin as if the engine were pivoting freely towards the direction you wish you were going, it will pull you in the direction you are at that split second, and the in a different direction a little later.

Not saying I've got everything right, just make sure which phenomenon you are describing.

edit: As we know, in a rear wheel drive car, putting the pedal down in a slide is worse than it would be in a front wheel car, so it's not that cut and dry. But then again the front wheels can steer, so that changes everything.

Edited November 11, 2013 by Snjo

[a.g.]

Hint - the moment arm for the force (which is a definition of torque) depends on the point of application

Hint: T = (R + c*F) x F = R x F + c * (F x F) = R x F for any scalar c.

What derivatives of force and torque are you talking about?

The first-order derivative of force and torque by angle of attack.

And why exactly there is a need to produce certain torque?

Because by definion the aerodynamic center is more or less the location where the derivative of the pitching moment by angle of attack is zero. Now, I'm very shaky on all of the specifically aerodynamic stuff, but I interpreted that as the location of basically the torque-free resultant location, only operating on the derivatives instead of the forces. Note here that generally the derivative of the torque of a number of forces is the same as the 'torque' of the derivatives of forces, provided the points of application are constant; the analogy is also correct if locations change, but you are going to use finite-difference approximation for everything anyway.

P.S. What I see likely happening in your screenshot, is that the total force only slightly increases to the right with AoA increase; however, its center of application moves relative to the center of mass to produce notable change in torque. This is equivalent to an equivalent increase in force applied way outside of the bounds of the craft. I expect if you tilt the craft more, the direction and location of the arrow actually flips around.

Edited November 11, 2013 by a.g.

[asmi]

It's important to be clear about what you mean by "the line of the force".

In the first example there, the thrust are NOT in the line of the force, they are in the line of the lever. In the second, they ARE in line with the direction of the force.

This is what is really happening:

As you can see, moment arm is different since it depends on the point of application.

edit: As we know, in a rear wheel drive car, putting the pedal down in a slide is worse than it would be in a front wheel car, so it's not that cut and dry. But then again the front wheels can steer, so that changes everything.

Since I happen to own and drive real-wheel drive car I know a thing or two how it behaves

[a.g.]

This is what is really happening:

As you can see, moment arm is different since it depends on the point of application.

However you forgot sin(angle), so the effective arm is exactly the same.

[SZDarkhack]

This is what is really happening:

As you can see, moment arm is different since it depends on the point of application.

These two forces produce the exact same moment. You can either project the force perpendicular to the arm (which is what you did with the green arrow) or project the arm perpendicular to the force (which is the blue case). In both cases the projection factor is the sine of the angle between the arm and the force (as per the cross product). In particular, the dashed green arrow is sinθ times shorter than the green (or blue) arrow, and the 2nd moment arm is sinθ times longer than the 1st. These cancel each other out. Although, the demonstrated identity that (R + c*F) x F = R x F should have convinced you already.

a.g. is absolutely correct on this one. The only thing that matters for torque is the magnitude of the force and the distance of the *line* of the force from the pivot. Not the actual application point along this line.

[asmi]

However you forgot sin(angle), so the effective arm is exactly the same.

Yea, you're right on that one - sorry I was confused

But my point that CoL have to be somewhere inside the body still stands as right now it's very counterintuitive as to what exactly this indicator shows...

[a.g.]

It shows the change in both the total aerodynamic force and total torque if AoA is increasing, with the directions immediately obvious, and the proportion between force and torque implied from the lever arm to the center of mass. For planes with actual non-stalled wings this indicator should give the conventional Center of Lift location, which due to some aerodynamic laws related to wings doesn't move much and provides guidance for stability. When you get into nonlinear drag interactions, it does act weird sometimes, but then you don't exactly have any conventional lift there.

And before you complain about the use of derivatives: that's actually how the FAR CoL indicator always worked; the only thing that changed is that instead of hoping that the stock CoL code will add up the data correctly (which it doesn't), it now computes everything itself and feeds just the final result to the stock code.

[a.g.]

Another thing is that I downloaded the part from here, and the picture I see is quite different. Now, the change in the graphs is likely due to some fixes I didn't send to Ferram in time for the release. However, the CoM location & mass is also different for some reason.

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[asmi]

Another thing is that I downloaded the part from here, and the picture I see is quite different. Now, the change in the graphs is likely due to some fixes I didn't send to Ferram in time for the release. However, the CoM location & mass is also different for some reason.

Oh, I'm sorry - just found out that I use modified configs from here: http://forum.kerbalspaceprogram.com/threads/54327-Realism-Overhaul?p=768850&viewfull=1#post768850

This is "real" version of the capsule.

[Guest]

Yeah, BobCat's version is quite crazy. I heavily modified the Soyuz to remove such oddities. My version's got it's COM inside the capsule, behind the seats, about where the machinery (and thus most of it's mass) would be IRL.

[mossman]

Hi;

I go from 60-100 FPS to less than 10 as soon as I hit mach 1 when flying a 54-part rocket. Is this normal?

I won't be able to use this mod if this is the kind of performance I should be expecting (And I have a fairly powerful system...).

[ferram4]

@mossman: Are you using any other mods? Does the output_log have any errors in it? The only thing I can think of that would drop performance would be calculating the stock mach effect and reentry shaders, but FAR doesn't change any of that.

[minimuffin]

Nice mod Ferram! Must of been hard coding this. I don't use this as one of my mods, but as soon as a make a "Realism Mode" install of KSP I will install this along with many other mods like FASA and Modular Fuels.

[mossman]

@mossman: Are you using any other mods? Does the output_log have any errors in it? The only thing I can think of that would drop performance would be calculating the stock mach effect and reentry shaders, but FAR doesn't change any of that.

No obvious errors. I do have a lot of other mods, but the rocket I just tested it with is using only stock parts. I'll try uninstalling everything and see if it changes. Thank you for the reply.

[Pyromaniacal]

Hi guys, I'm kinda new to the spaceplane building thing, and have been building standard jets with the career mode, stock, B9, and FAR. I assembled one, and after a few versions, I keep having the plane want to nose down. I've adjusted the trim, but it's still rather annoying. Are there any suggestions that can help me get experienced with this sort of thing?

I've also noticed that at lower altitudes, whether rockets or aircraft, that if I go too quickly, the SAS likes to vibrate the control surfaces. Is this normal? Is this the game or FAR telling me to slow the hell down? I think it's the latter, but I'd like to make sure.

Thanks for the help!

[ferram4]

@mossman: There really shouldn't be any sudden increase in calculations at Mach 1; if anything, the calculations get more intense at relatively high mach numbers below that, so if the increase in lag happened when you passed Mach 1 then it's probably something else and you're just attributing it to the wrong thing. If it ends up being FAR, post your specs, since most other people don't have this issue.

@Pyromaniacal: There are a few possibilities for nose-down problems:

1. Your CoL is too far behind the CoM, so your control surfaces aren't effective enough.
   
2. You're flying too high without enough velocity, so your control surfaces lose effectiveness and you're being dragged back down by gravity.
   
3. Your CoT is above the CoM. Keep in mind that the indicator in the editor is based off of max thrust, which you won't get with jets in flight.
   
4. Your fuel is somehow draining faster from the back than from the front and the CoM shifts far forward in flight.

As for the control surface twitching, that is a somewhat common thing, even for some stock builds. It's more noticeable with FAR since control surfaces take time do deflect instead of being insta-deflect like in stock; since stock SAS doesn't anticipate any kind of control delay, it can overcompensate with those surfaces.

[nosscire]

Sorry if this has already been requested before but the thread is 195 pages long...

In any case, while I absolutely love the aerodynamics part of this plugin, I dislike the huge UI it has. I'm sure it's sometimes useful when building and flying space planes, but I haven't gotten very far into that yet. I am still building allot of rockets though.

I know it starts minimized, but the button tends to pop up in the middle of the sceen each time I move to a different area (building to platform or reverse). I would want to request one out of two solutions:

1. Ability to dock the button so that I don't have to drag it off the middle of the screen after minimizing. Similar to how the Chatter plugin button is docked to the bottom of the screen.

2. Have the UI hidden unless a part is added, similar to Engineer or MechJeb

The first it the most important, the second would be good for when I just don't need it at all.

Thanks!

[mossman]

So I installed the Kerbal Joint Reinforcement plugin without uninstalling anything else -

And the problem seems to have gone away.

Strange.

[Pyromaniacal]

Arrighty, thanks for the tips! I will try to implement the changes!

[theflyingfish]

Sorry if this has already been requested before but the thread is 195 pages long...

You can change how many posts are displayed on your page at once... To me, this thread is 49 pages long.

You don't have to hit the "next page" button as many times. Saves me a bunch of time when reading threads.

[AdmiralTigerclaw]

Hey Ferram, I need to ask a quick question on some aerodynamics here.

I've got a high-altitude supersonic turbojet vehicle.

It's got dihedral properties and is stable in all modes of flight except one...

The symptoms it shows is that it remains stable in flight up to a speed of mach 2.3 - 2.5 and an altitude of 16 km. Around this point, the vehicle begins to buffet around on the roll axis violently and seems more related to altitude than velocity. Around 18 km, the buffeting suddenly turns extremely violent and it gets kicked right out of its stable flight envelope almost every time (rolls over, sideslips, leaves a stable AoA and goes into a falling leaf spin)... Once that occurs, well, you know how that ends.

I'm unsure what the symptoms are telling me about the vehicle, so I'm not certain what design changes I need to make. I theorize it has something to do with supersonic shock boundaries, but that doesn't explain to me why altitude seems to have more effect than velocity.

On that end, if FAR is modeling supersonic shock boundaries for vehicles, it would be nice to have a graphical representation of these shockwaves in the build system. Something that overlays semi-transparent shock cones over the vehicle or something when you check the box and run a static check. That way people can determine what parts are effected by sonic cones and take the measures needed.