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

[Pyromaniacal]

I ran the flight again, and this time managed to get it into orbit. It's sitting at a 100x115km orbit, but I ran a few tests on reentry into Kerbin and found the glider doesn't like to keep a steady flight path in the thick Kerbin atmosphere. Would Duna be any different, or should I scrap this particular project?

Yes, you were correct in assuming the CoL and CoM were for the glider only. It took some shenanigans, but I did get the CoL behind the CoM, if only just barely.

[luckyhendrix]

Thx for the advices , I redesigned the wings and now it is stable enough to be flyable ! You just have to be carefull not to take too high AOA or it'll flip around, but FAR AOA limiter works quite well in that regard.

[PetWolverine]

This mod is essential. I don't think I would want to play without it.

I've done some experimenting with spaceplanes in the last few days and developed a plane that can reach Mach 5.3 at (if I remember right) about 25 km before I switch the Sabres to rocket mode and punch into orbit. It feels fairly stable at all speeds, as long as it's going in a straight line - I can turn on SAS and not get any wobble, at which point I can basically walk away. I got it that way largely by tuning the numbers in FAR CAS, but there's one that I can't seem to get right: Ixy should be near zero, but mine is consistently a large negative number.

From the tooltip, it sounds like this means that when the plane starts rolling, it should become easier to pitch, and vice versa, which matches my experience in flying it: It's certainly not impossible to turn, and I've even managed to turn it all the way around at about Mach 2 while keeping a fairly steady altitude (I overshot the Space Center). But turning is a constant battle with pitch.

Here's the bird:

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Ixy is -99.12 kg * m^2. I've tried moving the center of mass around, moving more mass closer to the center of mass, adding more vertical stabilizers, changing the span of the tail to move those vertical stabilizers, changing the sizes of the wing tips and the stabilizers, but none of it seemed to help. I'm wondering if the problem is something more fundamental to the design, like the way the wings curve upward, or if there's some tweak I just haven't thought of yet. Any suggestions?

Edit: I found this article on inertial coupling that ferram4 linked to a ways back (waaay back, around page 60-ish). So I guess, roughly, having the wings curve way up like that makes it so that the axis that the plane wants to roll around is pitched down compared to the axis that I want it to roll around; so rolling away from center makes the nose droop, but rolling back to center should raise it up again. Seems like a fix would not be trivial, although moving the center of mass forward might decrease the effect (while also making it turn even more slowly, but then I'm not expecting this thing to ever be super-maneuverable).

Edited August 20, 2013 by PetWolverine

[ferram4]

With a design like that (which seems to have too much wing to start with) part of the problem might just be that the you've got a lot of wing far from the CoM; that will make rolling difficult to start with. Then there are probably some issues with the CoL being too far behind the CoM, which would make pitching more difficult. Then there's the fact that wings with that much dihedral will cause the plane to roll back and forth a bunch any time you try to turn it; you'd be better off having the wings mostly level and adding a larger vertical tail.

[Sparker]

Thank you very much for that mod. It brings very many things I was dreaming of about planes design in KSP.

[PetWolverine]

@ferram4, thanks for the suggestions. It sounds like at the design stage when I kept adding more and more wing, I should perhaps have stopped and thought about what I was doing. The CoL is indeed a ways behind the CoM, but pitch authority isn't really an issue except between about Mach 1 and 1.5, which I tend to just blast through on my way to higher speeds anyway. Rolling back and forth, on the other hand, combined with roll-induced pitching, is a bit of a problem. Maybe my next design will be less outlandish.

[ferram4]

Ok, so version 0.9.6 is out, which fixes some bugs in the drag code, provides some extra support for procedural fairings and some other minor changes.

The planned guide will have to wait for a bit, since I've realized just how much of a task that will be and I want to do it right. Essentially, it'll have to be "Aerodynamics for Dummies" in addition to documenting FAR's changes for it to be truly useful.

[nhnifong]

I'm bug hunting, because I managed to get a craft on a kerbin escape trajectory, while actually having an orbital velocity of about 1800. That shouldn't be possible. I don't want to point fingers without any evidence, but I think it was caused by FAR. I've removed the plugin and have not seen the issue again since.

[ferram4]

FAR doesn't change anything in the orbit code, which is where this bug would have occurred. The only things I can think of that would cause this would be gravity hack or something like hyperedit. FAR doesn't touch the orbit projection code at all, so you're probably talking to the wrong guy. Check out the other mods that you've been running and see if any of those caused it.

[nhnifong]

Ahh, I think I was fooling around with hack gravity. It must have stayed on after revert or something.

[Van Disaster]

Ok, so version 0.9.6 is out, which fixes some bugs in the drag code, provides some extra support for procedural fairings and some other minor changes.

Thanks Ferram; anything fixed that is likely to cause a redesign? ( like guppy-sized planes going back to flying bricks, etc )

The planned guide will have to wait for a bit, since I've realized just how much of a task that will be and I want to do it right. Essentially, it'll have to be "Aerodynamics for Dummies" in addition to documenting FAR's changes for it to be truly useful.

Well, you could just summarize basics & throw in references for further reading. I'm most eagerly waiting for this to find out just how much FAR actually does, I think.

[MAKC]

In FerramAerospaceResearch.cfg for every entry: @angluarDrag = 0. Intentional?

[ferram4]

@Van Disaster: Yeah... that would be about 5 pages on its own. There's stuff to cover about how the aerodynamic center of wings starts at the quarter chord, then shifts forward near Mach 0.9 before shifting back to ~0.4 chord for supersonic flight. There's the way drag of unused attachnodes facing the airflow have their drag increase from 1 to 1.86 as Mach number heads to infinity and how attachnodes facing away from the airflow start at a low value, peak near Mach 1 and then fall away towards 0 as Mach number increases. Honestly, that'll be about a 3rd of the guide, covering what FAR changes (in qualitative terms so I don't have to update the guide every update).

@MAKC: Nope. Fixed now.

[PetWolverine]

@ferram4: So, I think I figured out why my Ixy was so high with that plane: Heavy use of procedural wings. I'm not sure if these mods are meant to work nicely together yet, so you can consider this either a bug report or a data point when you get around to making them work, as appropriate. But actually I'm not sure the behavior is correct with B9 wings either, since I don't really know anything about this stuff.

So, as I understand it, nonzero products of inertia result from uneven mass distribution around the relevant axes, so that when you try and rotate the plane around some axis, it wants to rotate around another axis as well, or really, around some line that has a component along both axes. Maybe this isn't quite right, but what I'm fairly certain of is that two wings of very similar shape should have very similar behaviors.

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I took these screenshots with 0.9.5.5, but tried it again after updating to the latest, and saw the same behavior. I did all this with Editor Extensions's "vertical snap" option as well as angle snap, to make sure the wings were exactly centered in all cases. (Maybe Editor Tools is part of the problem? I haven't tried without, but maybe somebody else can check this.)

I don't see any reason why a fuel tank with two perfectly matched wings should want to pitch when it rolls. On the other hand, I'm not sure how the single B9 wing doesn't have some inertial coupling - maybe the game puts its center of mass at its center, rather than toward the back as it should be. I'm also not sure I'm understanding any of this correctly, but when I took a plane (not the same one I showed before) that had been very unstable with pWings and rebuilt it with B9 wings while keeping the same shape as much as possible, the result had an Ixy much closer to zero and was much easier to control (and I haven't even added struts yet, so it looks sort of like an ornithopter when it lifts off).

[Sparker]

Yesterday I was testing how my spaceplane behaved during re-entry and I made one interesting thing. I entered the atmosphere at low angle, I was pitching a little, about 4-6 degree, and managed to change my orbital trajectory and get out of atmosphere without any crucial speed losses. Would it be possible to make the same thing with shuttle or buran(or any other spaceplane)?

[Torminator]

I had a curiosity: Would it be doable for a total newb to edit the .cfg for B9's brakes so that FAR will see them as configurable control surfaces? Sorry if this has been covered, but this thread is massive.

I don't even know if it would be possible, but ideally I'd like to be able to use the B9 brakes to help control a craft, mostly because I think it would look cool. (this is why most of my planes don't fly. They do make quite nice-looking lawn darts, though.)

I will say that I finally managed to make an SSTO in FAR, with 2 B9 wide body cargo bays, and actually had enough fuel left to make it to the Mun. I was quite happy.

[ferram4]

@PetWolverine: That difference is likely due to the fact that the CoM of the procedural wings is probably shifted backwards while the CoM of the B9 wings is not. Also, you should make sure to add struts between your wings if you want to properly test a vehicle, since the change in wing geometry can be enough to greatly affect the performance of the aircraft.

@Sparker: It is possible, however you need to understand that the space shuttle and buran were designed to reenter at ~35-40 degrees angle of attack, which means that they made a lot more drag than lift. It's likely that your spaceplane made much more lift than drag and so was able to lift out of the atmosphere.

If you want to try a proper reentry, try pitching the spaceplane up so that the prograde marker is at the very bottom of the navball; if you have any fuel or monopropellant left, move it around to balance the plane; use RCS to control angle of attack and fuel transfer to keep the plane stable, but not so stable that you can't maintain the high angle. Once you've slowed down a lot, pitch it down to a lower AoA and head for the runway.

@Torminator: It could be done, in theory, but it's possible that the way that the part is oriented it won't work properly. You could try copying over the changes to the smallCtrlSurf part (find it in ferramaerospaceresearch.cfg in the FerramAerospaceResearch folder) to see if that changes things.

If your planes make good lawn darts then they are probably too stable; try moving the CoL forward and see if that helps.

[BubbaWilkins]

Ok, so version 0.9.6 is out, which fixes some bugs in the drag code, provides some extra support for procedural fairings and some other minor changes.

The planned guide will have to wait for a bit, since I've realized just how much of a task that will be and I want to do it right. Essentially, it'll have to be "Aerodynamics for Dummies" in addition to documenting FAR's changes for it to be truly useful.

Ferram:

I'm afraid that whatever you did to the drag code just busted any prop driven craft I have to about 1/4 speed.

This craft for instance can't even get over 33m/s and take off now:

Similarly my smaller single engine K-51 (P-51) and Biplane derivative suffer severe speed hits. They can take off, but can't come close to their previous cruising speeds. Bear in mind that each of these was designed and tuned in your previous version. Also, a small request. Could you leave links to older versions archived in the first post? I didn't hang on to them myself and have no way to revert back.

Edited August 23, 2013 by BubbaWilkins

[ferram4]

Oh, I see what's happening here. FAR is accounting for the tapering of the prop engines including the props, which makes them appear very un-aerodynamic. I'll look into a fix. Until then, a link to a mediafire folder containing the most recent versions is currently on the front page.

[BubbaWilkins]

Oh, I see what's happening here. FAR is accounting for the tapering of the prop engines including the props, which makes them appear very un-aerodynamic. I'll look into a fix. Until then, a link to a mediafire folder containing the most recent versions is currently on the front page.

Appreciate it!

If that is the case, than this might also explain the over-the-top instability related to the use of helicopter rotors as well.

[PetWolverine]

That difference is likely due to the fact that the CoM of the procedural wings is probably shifted backwards while the CoM of the B9 wings is not. Also, you should make sure to add struts between your wings if you want to properly test a vehicle, since the change in wing geometry can be enough to greatly affect the performance of the aircraft.

This sounds like a testable hypothesis. To the lab! We should expect that if I move the wings forward and back, it should have a noticeable impact on the products of inertia (or at least Ixy, the one I'm interested in).

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That seventh image is telling. I don't know why I didn't try it before - I tried a single B9 wing, but not a p-wing - but notice that Ixy in that shot is roughly half what it is in all the previous p-wing shots. It certainly makes sense for it to be non-zero in this case; the "craft" is unbalanced. But adding another one on the other side should balance it, returning it to zero, and instead it doubles it. So it would seem that adding more p-wings always changes Ixy in the same direction, regardless of where the p-wings are.

Is it always the same amount? Well, no; in the eighth shot we see Ixy more than doubling when we go from 1 wing to 2. But when we add a third wing, bringing the craft back into balance along the x axis (but way out on the y axis), it is now almost exactly triple the value for one wing. Will it return to zero if we mirror each wing? No; even though it is now perfectly balanced along the y axis, and no more out of balance than before along the x axis, Ixy doubles again. You can see how this would add up to thoroughly destabilize a plane of any appreciable size.

I suspect that the y offset of the wing's mass is always being given the same value, regardless of its actual position. Of course, this could be a problem in the p-wing code rather than yours, but I think at this point the conclusion that it's a real bug is inescapable.

Also, I understand the necessity of struts; I didn't intend to leave them off, but saw my wings flopping as I was lifting off and thought hey, just roll with it and see how it flies. My point was that the B9 wings with horribly, uncontrollably variable geometry were more stable than the very rigid procedural wings of the previous iteration. This is of course just my perception, and if the numbers in the hangar are just for reference and aren't used in any in-flight calculations, then I would have to look for another explanation for the difference between these two planes. But there's still a mistake in those numbers.

Sorry for the wall of text. Anyway, don't get me wrong, I love this mod and appreciate the work you're doing. I'll just stick to B9 wings rather than procedural for the time being.

[Subcidal]

Has anybody made a Delta V chart with FAR in mind? Or just know the general amount of Delta V you save getting into orbit with this mod?

[jrandom]

Has anybody made a Delta V chart with FAR in mind? Or just know the general amount of Delta V you save getting into orbit with this mod?

I can generally get to orbit if I build my launch/orbit stage with 3500m/s delta-v. I usually have a decent margin left once I've circularized orbit at 80-90km.

[Subcidal]

With a probe core (Sputnik), two antennas (Remote tech =P) and a procedural fuel tanks with a engine on it in vanilla i needed a minimum of 4533 Dv with FAR i saved 721 Dv Ending up with an orbital Dv of around 3800 needed. for that specific craft.

I was using MJ's limit acceleration feature so i'm sure that effected it some. Not in the mood to do a thorough test right now, I'll see if anybody else comments with the Dv amount they aim for.

Edit : Just took another rocket I have and subtracted 720 from the 5410 Dv needed for a munar intercept. Ended up with 221 Dv to spare after the intercept burn, so 720 seems to be around the right amount as far as I can tell. I'm sure this isn't the same if you build a rocket using a pancake as your blueprints.

Edited August 23, 2013 by Subcidal

[Pyromaniacal]

Hello, me again. Because I use Procedural Fairings, I've been thinking about using this plugin. I mean, all they do with vanilla physics is add mass. I've been known to take a nerf for aesthetics, but why should I when I can make it a buff?

Before I fiddle with aerodynamics, however, I have some questions.

I am under the impression that I have to set each control surface in order for them to operate properly. Is this true? How difficult is it? Will I have to restart any flights I'm progress that use control surfaces?

I have a set of launchers that have been proven to get certain masses up to certain orbits. These lifters use asparagus staging. All have nosecones. Does FAR make this less effective? Will they still lift like they did? Should I instead pick up a pack like NovaPunch or KW Rocketry? Which do you all prefer?

Thank you for your time!