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

[SilverWolf]

Hmm.  I'm not using AJE, Actually, that's an engine I just welded together and threw on there for part count.  If I understand you correctly, you're talking about Intake area, which I think I manually calculated for this engine after I threw it into 3Ds Max...  Let me take a look at something, and I'll get back to you in a second.

Edit:  I measured it at an area of 0.573 m².  Is that enough to trip your code up?

Edited August 4, 2016 by Verran

[Boris-Barboris]

59 minutes ago, ferram4 said:

Well, to start off with, the CoP moving off the wing is a thing that happens.  When you're looking to put a force center somewhere that allows all pitching moments to be handled by only lift and drag forces, if the lift or drag forces are very small they need a large moment arm to cancel the pitching moments.  Resulting in the CoP being placed somewhere completely off the vehicle.  Which is why the CoP is absolutely terrible for any discussions of stability and why FAR uses the aerodynamic center instead because at least by allowing a constant moment you get a force center that is somewhere reasonable.

del.

Will give a proper proof tomorrow, but i'm sure result will be the same, CoP of wing is inside it's mesh.

59 minutes ago, ferram4 said:

I'll accept PRs to add band-aids, but I'm not going to spend lots of time tweaking, adjusting and bugfixing a system I'm planning to rip out.

Ok, np, I guess I'll just fix it locally for my tastes.

Edited August 4, 2016 by Boris-Barboris

[ferram4]

@Verran: Yes, that is certainly more than enough to trip it, because (for some unfathomable reason) the areas in-game are scaled down by a factor of 100.  The circular intake part you welded on has, by default, an intake area of 0.006 (uh... dekameters^2?).  So... that explains everything then.   Welcome to KSP, where sometimes things don't make sense.

@Boris-Barboris: So... now redo this analysis, acknowledging that cambered wings can have a substantial non-zero pitching moment even when the lift force is zero and the drag force is small.  And the CoP must be placed so that the minuscule lift and drag forces have sufficient moment arm to balance the large, constant pitching moment.  Similar effects happen on wings in stall as well.

Actually, to get more into the maths, for an ideal, thin, 2d airfoil, the aerodynamic center (force location when allowing for a constant pitching moment) is always located at the quarter chord.  Further, assuming you know the magnitude of that zero-lift pitching moment coefficient, you can solve for the forward/back location of the CoP using: X_cp = 0.25*c*(1 + 4 * Cm/Cl), where Cm is pitching moment coeff, Cl is lift coeff, and c is the chord length.  Yeah, the CoP moves around a lot.

[SilverWolf]

Uh, hold on, so it (the game) thinks the intake area is 57.3 square meters?

Edited August 4, 2016 by Verran

[ferram4]

Yes.  Yes it does.  The multiplying is a hold-over from awhile ago when the intake areas were originally set and made no sense, and to keep compatibility with any mods that balanced off of that the base values were never changed.  So in order to keep things consistent FAR has to also account for that factor, so even though you've set it to 0.573 m, everything thinks that you actually have intakes the size of an aircraft carrier.

[SilverWolf]

Ha ha, wow.  Go, Go Squad for the Win...  My grandmother would beat me if she heard the obscenities that just emerged from the hole in my face.

Thanks for clearing that up.

Now, I have another question for you.  What are you using to account for raw surface lift, i.e. action reaction?

[ferram4]

Currently all the wing lit is handled simply as an approximate implementation of lifting line theory, with compressibility corrections and some 2d shock-expansion calculations to adjust lift slopes for handling higher Mach number behavior.  Calculate the lift based on angle of attack, apply it, include induced drag, that's pretty much it for wing lift.

[Boris-Barboris]

35 minutes ago, ferram4 said:

So... now redo this analysis, acknowledging that cambered wings can have a substantial non-zero pitching moment even when the lift force is zero and the drag force is small.  And the CoP must be placed so that the minuscule lift and drag forces have sufficient moment arm to balance the large, constant pitching moment.  Similar effects happen on wings in stall as well.

1. You are not forced to place CoP on wing chord.

2. We are talking about stall, Lift is not zero. So no, effect is not similar.

[SilverWolf]

Okay.  This is why I'm asking:

 

[blowfish]

@Verran Lifting bodies are unstable, that is a well-established fact.

[SilverWolf]

@blowfish My head's been stuck in the sand, for the last few years.  I'm experiencing FAR for the first time, tonight.  Being a well-established fact is all well and good, but when you say "Lifting bodies," the Mk.2 Fuselages are the only pieces I've seen lift code attached to.  So where's this coming from?  I didn't put it in there.

[blowfish]

2 minutes ago, Verran said:

@blowfish My head's been stuck in the sand, for the last few years.  I'm experiencing FAR for the first time, tonight.  Being a well-established fact is all well and good, but when you say "Lifting bodies," the Mk.2 Fuselages are the only pieces I've seen lift code attached to.  So where's this coming from?  I didn't put it in there.

Okay, let's back up a step.  Most shapes, including cylinders, cones, and cubes, will generate lift at positive angle of attack.  The bottom surface will deflect air downward, and the airstream will tend to stick to the top surface as well forcing the air down and by extension the object up.

With the exception of wings (which are treated differently), FAR models body lift for all parts.  The Mk2 parts have a lifting module in stock, but FAR removes it and calculates body lift 100% based on the shape.

[SilverWolf]

Okay, you're just stating that the fuselage lift processing is unstable, and has been for some time.  Got it.

Thanks for your time guys.  Let me know if you need help with anything.

[tetryds]

@Verran blowfish forgot a very important thing.

FAR actually uses the external shape of the craft to calculate body lift and drag, so if you make something like a fairing out of structural panels it is going to behave as it should, for example.

[Max.10.07]

I have a question, does FAR actually simulate a difference in air pressure and then lift from that change? Like when a typical lets say 747 is in flight, the air above the wing moves faster, reducing pressure, and making a pressure difference, allowing the low pressure at the bottom to push it up.

[blu3wolf]

3 hours ago, Max.10.07 said:

I have a question, does FAR actually simulate a difference in air pressure and then lift from that change? Like when a typical lets say 747 is in flight, the air above the wing moves faster, reducing pressure, and making a pressure difference, allowing the low pressure at the bottom to push it up.

well, thats not actually the way it works on a typical 747. Several generations of flight instructors have been making that mistake in instruction...

[softweir]

2 hours ago, blu3wolf said:

well, thats not actually the way it works on a typical 747. Several generations of flight instructors have been making that mistake in instruction...

Indeed. Wind-tunnel tests using pulsed smoke and sparse diffusions of particulates show that the air moves only fractionally faster over the top of the wing than over the bottom, and the difference is too small to explain the lift gained.

[LordOfMinecraft99]

You may not remember my issue because it was a while ago, but here is my log. (BTW my issue was that with FAR and R.O installed the wings dont give any lift.

Link:
https://drive.google.com/file/d/0B-Rnwo9xqjx0aWduU1VWelpFbFE/view?usp=sharing

[cantab]

8 hours ago, Max.10.07 said:

I have a question, does FAR actually simulate a difference in air pressure and then lift from that change? Like when a typical lets say 747 is in flight, the air above the wing moves faster, reducing pressure, and making a pressure difference, allowing the low pressure at the bottom to push it up.

To my understanding, not really no. FAR does not attempt to simulate actual air flow, that would be way too much work for the computer. Instead it just models it using simpler equations that basically say "If the wing is at angle alpha doing speed v at height a then it will generate lift force F." There's some complication to handle interaction with nearby wings, but it's basically a parametric model not a computational fluid dynamics one.

I think. Ferram might correct me.

If I'm right, then that would imply that FAR doesn't model wake turbulence behind aircraft, unless Ferram specifically put that in. Whereas CFD would produce that effect inherently. There's little *need* for it in KSP though because not many people do formation flying.

Edited August 4, 2016 by cantab

[Gryphon]

Dear @ferram4,

Did you ever expect that by releasing FAR, you'd end up teaching classes in the forum thread?

[Van Disaster]

No wake turbulence, there's no air to make turbulent just density. Wake turbulence would be a hell of a thing to model, that's a pretty fine-grained effect given you somehow now have to keep track of the entire atmosphere...

RE airflow and lift, don't forget air moves spanwise also.

[Elmetian]

Hi guys,

I'm once again finding that the stock SAS is just atrocious for more exotic designs. I recently rebuilt a B2-esque flying wing in 1.1.3, and for some reason it's even less stable now than the original was in 1.0.5 (see pic in the spoiler)

Spoiler

 

The stock SAS really really doesn't like using the split brake rudders. Without a vertical stabiliser it's necessary to have the SAS using the brake rudders to correct any changes in yaw. I know these brake rudders work because I can manually fly the wing, but this requires making corrections every second just to keep it level and heading in one direction, even with the SAS helping out. The SAS does use them, but no where near 100% deflection meaning there's a lot of side slip.

The stock SAS also has trouble using the elevators. Because the engines are above the wing, they tend to apply a rotational force that pitches the nose down. I've angled the engines so that the thrust is as close to being through the CoM as possible, but it still requires some correction. Using the alt+S/W trim controls does help, but this tends to make it even more unstable in the lateral axis.

What I really need is a stiffer SAS that doesn't slip. Currently the SAS will turn off momentarily to allow player input, but it also does this sometimes without any appreciable reason. I know it is possible to make the SAS more rigid with a PID tuner, but I can't seem to get FAR's working properly. I've also tried Pilot Assistant before anyone suggests it, and although I had some success with it a year ago, I found the latest version to be too complicated. It also doesn't help that there isn't a manual available anywhere as far as I can tell. I just want to make this easier to fly, I don't need an autopilot that allows me to adjust heading and pitch and a billion other variables...

Please can someone explain for me, step-by-step, how I set up the PID tuner in FAR to force the SAS to hold the aircraft steady and make greater use of the control surfaces? When I've used PID tuners previously changing Ki helped to make the SAS stiffer but this doesn't seem to be the case now. Do I need to engage the stock SAS, or simply click FAR's flight assistance toggles, or both? What am I doing wrong? (as you can probably tell I'm getting a bit frustrated...)

Edited August 5, 2016 by Elmetian

[AccidentalDisassembly]

3 hours ago, Elmetian said:

Hi guys,

I'm once again finding that the stock SAS is just atrocious for more exotic designs. I recently rebuilt a B2-esque flying wing in 1.1.3, and for some reason it's even less stable now than the original was in 1.0.5 (see pic in the spoiler)

  Reveal hidden contents

 

The stock SAS really really doesn't like using the split brake rudders. Without a vertical stabiliser it's necessary to have the SAS using the brake rudders to correct any changes in yaw. I know these brake rudders work because I can manually fly the wing, but this requires making corrections every second just to keep it level and heading in one direction, even with the SAS helping out. The SAS does use them, but no where near 100% deflection meaning there's a lot of side slip.

The stock SAS also has trouble using the elevators. Because the engines are above the wing, they tend to apply a rotational force that pitches the nose down. I've angled the engines so that the thrust is as close to being through the CoM as possible, but it still requires some correction. Using the alt+S/W trim controls does help, but this tends to make it even more unstable in the lateral axis.

What I really need is a stiffer SAS that doesn't slip. Currently the SAS will turn off momentarily to allow player input, but it also does this sometimes without any appreciable reason. I know it is possible to make the SAS more rigid with a PID tuner, but I can't seem to get FAR's working properly. I've also tried Pilot Assistant before anyone suggests it, and although I had some success with it a year ago, I found the latest version to be too complicated. It also doesn't help that there isn't a manual available anywhere as far as I can tell. I just want to make this easier to fly, I don't need an autopilot that allows me to adjust heading and pitch and a billion other variables...

Please can someone explain for me, step-by-step, how I set up the PID tuner in FAR to force the SAS to hold the aircraft steady and make greater use of the control surfaces? When I've used PID tuners previously changing Ki helped to make the SAS stiffer but this doesn't seem to be the case now. Do I need to engage the stock SAS, or simply click FAR's flight assistance toggles, or both? What am I doing wrong? (as you can probably tell I'm getting a bit frustrated...)

Give Atmosphere Autopilot a try first IMO, it might do what you want - or not.

[LordOfMinecraft99]

No offense or anything, but can someone please help with my issue? I posted logs on this very page.

[Vladokapuh]

On 4.08.2016 at 2:52 AM, ferram4 said:

@Vladokapuh: Well, FAR doesn't affect landing gear at all, so that means the only change is that you're hitting that ground harder with FAR or that you're getting a really high AoA after hitting the ground to get you back off the ground again.  There really aren't any other options.

Hmm, good to know this isnt intended.
I actually land more gently with FAR, so i dont know where the bouncing comes from, removing FAR fixes it, but the game just doesnt feel right without this mod.
I noticed that there seems to be tiny physics jerk when i land, and the suspension seems to reset for a second when i touchdown, and that just catapults my planes up.

I guess my install is just messed up, gonna troubleshoot it or just reinstall.
Thank you for response, and have a nice day!