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

[ferram4]

Yeah, I slipped up and didn't update the version number for the GUI. It should be fine.

[DaMichel]

@DaMichel: Okay, first, the Stability Derivatives are only truly valid around a specific velocity + Mach number + AoA condition, which means that the data isn't valid for the condition you're looking at. Besides that, the assumptions used in that analysis are violated by stalling anyway; they're intended to prevent stall. It looks like the main cause of Xw being wrong is that you have so many intakes; basically it indicates that it makes less drag at a slightly higher angle of attack than at the angle of attack it's currently at. That's the one deriv that does have a "correct" sign, but it's okay if it's wrong. Temperature is best found by flying a sounding rocket with a thermometer attached; take a look at the display, and that will tell you the temp at that altitude.

As for the static analysis, it's not Cm that you're really concerned about, it's the slope. And the sudden positive Cm slope when it stalls makes it stall more as it begins stalling; you want to have lower aspect ratio wings at the back of the plane, higher aspect ratio wings at the front to make it more stable during stalling.

Thanks, very useful info there, especially the part about stalling.

@DaMichel, just so I can complete the picture in my head with the explanation ferram gave (and so learn something in the process), does the craft flip at mach 0.9 without touching the controls, while flying level and straight? If you did a pull-up before the stall, then would be useful to analyze the graph with the "Pitch setting" to 1?

I was in pretty much level flight, maybe in a few degree shallow climb with like 3° AoA when the nose began to rise. So totally normal. I didn't pull up. Controls were in trim position. Tried to keep the attitude. So no big control inputs.

@Ferram Of course i tried to use the information about speed and altitude to make the analysis! You see, i plugged in the numbers from the screenshot and guesstimated the temperature. That should be valid for before the stall, shouldn't it?

Edit: I should have said almost valid, because while certainly not in level flight, i was still pretty close to it.

P.s. I had a quick look at the simulation window. Question: are the flight parameters absolute numbers or differences from level flight conditions? E.g. if i enter u=200 (m/s?) The plot goes out of the graph area. So that's weired.

Edited December 20, 2013 by DaMichel

[ferram4]

Those are differences from level conditions; I suppose I should try and work in some kind of delta symbol in there. It'll just give you an idea of what oscillations to expect for your plane if it's given a slight "kick" to one of those variables.

[DaMichel]

Those are differences from level conditions; I suppose I should try and work in some kind of delta symbol in there. It'll just give you an idea of what oscillations to expect for your plane if it's given a slight "kick" to one of those variables.

Yup

Another question: The simulation shows the solution for the linearized equations of motion, where the coefficients are the stability derivatives and these are constant, correct?

I made my plane more stable by moving the intakes further back and putting them symmetrical on opposite sides. It is actually quite stable up to mach 2. The pitch up is still there but better controllable.

I wonder if the pitching motion starts as stable oscillation which is so large that it leaves the realm of the linear stability analysis. Some sort of extreme short period oscillation. Does this make sense? It would explain why the derivatives are mostly good.

[Hyomoto]

I guess I was just lucky before. I first started being able to fly last patch, when I turned off the torque wheels in the command pod, and I amazingly never had this issue. I think it had to do with low speed versus lift, as once I got higher and faster, it seemed better at holding a heading but by that point I just used trim and flew normally. I'm hoping we'll get a tweakable for SAS parameters as well as default command pod behavior, it seems odd they missed torque-wheels.

[louiz347]

Just dropping to say I don't know how I played without this mod until today. Now KSP is much more complete, my shuttle can glide like the real one.

And also love how my big planes need a very big part of the runway to full stop. Sometimes maybe too much, I have to even abort land if I touch the runway too late, it's very fun!

[forsaken1111]

I have made some good progress designing usable planes recently but have been encountering a problem... when I try to use SAS to hold a heading the plane will 'bounce' continuously overcorrecting up and down over and over. It flies more or less straight but the bouncing motion is really annoying and can sometimes cause structural failure and really makes landing hard. Is there something I can tweak to fix this?

[jrandom]

I finally got .23 in a stable enough state to try out some planes last night. I'm getting some pretty serious pitching-up behavior at higher speeds which, from what I've read, is the correct behavior, no?

Looks like I'm going to have to learn how to build planes all over again! (This is a good thing. If I wasn't learning new things I'd be bored outta my mind.)

[Hyomoto]

I have made some good progress designing usable planes recently but have been encountering a problem... when I try to use SAS to hold a heading the plane will 'bounce' continuously overcorrecting up and down over and over. It flies more or less straight but the bouncing motion is really annoying and can sometimes cause structural failure and really makes landing hard. Is there something I can tweak to fix this?

Yeah, this is the same problem I was having, but even without FAR my plane wasn't able to hold its nose up with SAS (no idea why). So either its a 0.23 problem, or a plane issue. You can use your trim, it will allow you to have precise control over your flight since holding a stable elevation is mostly what you were probably using SAS for anyways.

[ÄlveKatt]

Do you have a changelog hidden somewhere?

[ferram4]

Changelog is in the readme file, in the download.

[Hodo]

I have one small request.

Can we get a settings option for the flight data display when in game. I would love to cut down the amount of information it displays to just what I absolutely need.

[FlowerChild]

Click on the arrow on the side of the toolbar; that will open a dropdown menu that will allow you to unlock its position. Then move it all over the place.

Ferram, just as a small request, it might really help if the above was in the readme or elsewhere in the instructions for the mod. I just spent a good half hour fiddling with how to move the button as it was covering rather crucial interface elements, before finding this post.

It also really doesn't follow any existing interface convention I am familiar with, so it's not particularly intuitive

[Ratzap]

Ferram, just as a small request, it might really help if the above was in the readme or elsewhere in the instructions for the mod. I just spent a good half hour fiddling with how to move the button as it was covering rather crucial interface elements, before finding this post.

It also really doesn't follow any existing interface convention I am familiar with, so it's not particularly intuitive

Or you could have visited the thread for the toolbar API where the author has a video which shows how that is done.

http://forum.kerbalspaceprogram.com/threads/60863-0-23-0-Toolbar-1-2-1-Common-API-for-draggable-resizable-buttons-toolbar

[FlowerChild]

Or you could have visited the thread for the toolbar API where the author has a video which shows how that is done.

http://forum.kerbalspaceprogram.com/threads/60863-0-23-0-Toolbar-1-2-1-Common-API-for-draggable-resizable-buttons-toolbar

Fair enough, but since the toolbar API is integrated directly into the mod, that's not necessarily apparent either. Also, with KSP's tendency to miss interface clicks, I think I had even tried clicking the little arrow previously, and when it didn't work the first time, assumed that wasn't how the thing worked. I'm an extremely experienced game player, programmer, and designer, and again, it took me half an hour of fiddling, looking at config files, and searching the thread to finally figure out something that is rather trivial in the grand scheme of things just to start playing with FAR installed.

I mention it now, not because it benefits me as I already know how it works, but rather to benefit other players who will also likely be confused by it, and Ferram himself who will likely have to answer the question of how to do it many times in the future. When you've already been using something for awhile and have become habituated to it, things like this are not necessarily apparent anymore.

To paraphrase one of my favorite quotes by Doyle Brunson "things only seem obvious a few minutes after you figure them out"

Edited December 21, 2013 by FlowerChild

[Andon]

So... I'm looking for some advice.

I design planes that fly. Fly well, that's another question entirely. But one thing I've come across in every plane, regardless of how well it flies, is turning SAS on causes it to twitch roll, back and forth several times a second. Usually it ends up slowly rolling in one direction or another. Is this something that can be addressed?

Additionally, it seems like the nose of my planes always want to pitch down. I'm assuming this is a bit of a design problem, but when I turn on SAS, it only slows the down pitch. I leave it for a few seconds and I'm going down instead of up. The planes have more than enough control to make sure they point in the right direction, but SAS isn't working properly with it. Again, is this something that can be addressed?

[Tharios]

So... I'm looking for some advice.

I design planes that fly. Fly well, that's another question entirely. But one thing I've come across in every plane, regardless of how well it flies, is turning SAS on causes it to twitch roll, back and forth several times a second. Usually it ends up slowly rolling in one direction or another. Is this something that can be addressed?

Additionally, it seems like the nose of my planes always want to pitch down. I'm assuming this is a bit of a design problem, but when I turn on SAS, it only slows the down pitch. I leave it for a few seconds and I'm going down instead of up. The planes have more than enough control to make sure they point in the right direction, but SAS isn't working properly with it. Again, is this something that can be addressed?

I ran into similar problems.

Now that .23 is out, what I did was to put big flaps on the wings that control pitch only, with a single small flap on each wing to control roll. That eliminated a large part of my issues.

As for your pitch problem, perhaps your CoL is too far behind your CoM for the speeds you're flying at...and/or not quite enough lift in general? Also, I find slightly upward-canted wings help with stability, and a pair of outward-canted tail fins with small flaps with pitch and roll turned off (so they affect only yaw) helps a lot.

Now...my only problem is supersonic stability...which I think is more an altitude/speed problem than design. I may just be going too fast, too low.

[jrandom]

Also, I find slightly upward-canted wings help with stability, and a pair of outward-canted tail fins with small flaps with pitch and roll turned off (so they affect only yaw) helps a lot.

"Upward-canted" means wingtips higher than root? Or angle of attack is tilted upwards at the front? And then... what the heck is "outward-canted"?

[softweir]

Yes, "upward-canted" does mean wingtips higher than roots.

"Outward-canted" means tail-fin-tips further out than roots.

[Tharios]

"Upward-canted" means wingtips higher than root? Or angle of attack is tilted upwards at the front? And then... what the heck is "outward-canted"?

sorry...yes, tips higher than root. And for the tail, it means the tips of the tails are angled away from the centerline of the craft. But the facing of both tails and wings are still inline with the direction of flight.

[ferram4]

@Flowerchild: I guess I'll put some info into the readme about it. I don't expect that it will answer people's questions though, since you'd be amazed how few people read the readmes. Or actually follow installation instructions.

@Andon: The best way to deal with SAS twitches is to get reduce control authority on that axis. You don't want me to address SAS' behavior, since I would basically just remove it entirely or limit it to very slight damping inputs.

If your plane always tries to pitch down, even at full pitch up, that means that it's too stable and you need to move the wings forward a bit.

The thing to keep in mind about SAS is that it's designed to figure out what it needs to do after being activated without knowing anything about the vehicle it's on beforehand. But if the vehicle is rotating too much it will simply try to damp the motion rather than go crazy trying to hold a heading. It sounds like you're getting it to do the latter, which means that your plane needs to be designed with more of a zero-control input pitch-up tendency; angle the tail down a bit, angle the canards up a bit, put the engines below the CoM.

[SnappingTurtle]

Since .23 I've been having trouble with my planes getting seemingly random but massive kicks in the roll axis under certain conditions (no SAS but FAR wing leveler on). I have some red numbers in the stability derivatives (I put in the estimated temperature and mach number where I was having these problems) but I don't know how to fix these. Before .23 I never paid attention to the stability derivatives and stuff just kind of worked so I don't know what's up.

http://i.imgur.com/OnV6iuF.jpg

P.S.

FAR could do with its own subforum for these kinds of questions.

[jrandom]

I'm discovering that with this latest version of FAR, I do not know how to build planes any more. Any tips would be greatly appreciated.

1. No matter what I do, the aircraft wants to pitch up a lot at higher speeds. My center of lift is behind my center of mass. (As it noticeably so, not right on top of it.)
   
2. Regardless of wing layout (straight, swept, attached at the horizontal sides of the plane, attached at the top of the plane, canted up, flat, whatever), I get these sudden violent roll movements above 400m/s. it looks like some invisible baseball player is swatting at the tips of my primary wings.
   

I've mainly been doing swept-wing configurations, so I tried a wide, rectangular model and get the same pitch-up and violent-roll-twitches behavior, both with and without SAS/FAR flight assistance.

Edit: I see SnappingTurtle is seeing similar behavior with the roll axis.

Edited December 21, 2013 by jrandom

[FlowerChild]

@Flowerchild: I guess I'll put some info into the readme about it. I don't expect that it will answer people's questions though, since you'd be amazed how few people read the readmes. Or actually follow installation instructions.

Hehe...actually, as an experienced modder, no, I wouldn't be at all surprised by how little people read

Despite that, it's not something for which I have a better idea to suggest. Unfortunately, the interface method itself is a tad obtuse and without some kind of formal explanation won't necessarily be intuitively figured out by the player.

[ferram4]

@jrandom: The pitch-up tendency is due to the main wings being put high combined with the large intake on top. Basically, there's a lot of draggy stuff above the CoM for that plane.

I've noticed the roll kicks too. I think it's something to do with the new supersonic shock solution, but I haven't tracked down the exact source yet.

@FlowerChild: Yell at blizzy78 then. GUI design is not my forte. I get the concerns, but I don't know how to make it clearer to be honest.