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

[ferram4]

But L/D ratio has absolutely nothing to do with the pitch damping of an aircraft.  The only thing that matters is the lift slope of the wings and tail (how much lift they make per unit AoA) and how far they are from the CoM (so the moment arm they have to work with).  We're describing short-period motion here, drag can be completely neglected in the analysis of nearly pure pitching here and the L/D ratio does not matter.  The frequency and damping ratio of a plane will be independent of however L/D varies at full pitch input because the a plane in short-period motion is very much like a pendulum; the frequency is based on the distribution of mass and the static stability, the damping ratio based on the distribution of lift forward and back and the static stability.  The actual magnitude of the L/D ratio... just doesn't matter in short-period dynamics.

[tetryds]

@kcs123 Well, that is not really the case, the L/D curve does not affect the pitch moment balance, the inclination of the Cm curve does.
Your max pitch input shouldn't necessarily be earlier than the stall point if you are using a joystick, several maneuvers require controlled stall, and achieving that is way more complicated than tweaking some control surfaces.
Also, having your Cm zero earlier than the stall point alone does not really mean you are stall-free at all, remember that this is a static analysis, when you are pitching up, especially with a keyboard, the craft will pick pitch momentum.
This is probably what caused you to mistakenly assume that it has anything to do with the optimal L/D points, you empirically realized that if you go further than that the aircraft has a higher chance of stalling and thought that it was the case for every craft.
But what actually happened was that you did not have enough damping to hold back your momentum, the craft AoA went too high and you stalled.

@Kitspace First of all, start by normal propeller airplanes, and make sure your control surfaces do what they were meant to do, and that the airplane is stable (Cm curve going down at the very least)
Then, simply launch it, take off, and pitch up.
Your aircraft is going to perform that Stall-return-stall wobble, stop pitching, right click the elevators, decrease their deflection by 2 degrees and try again.
Do that until the airplane stops wobbling, then increase it slightly until the point where it pitches as much as it can before that happening.
Then test this at different speeds to make sure you are stable at most of them, or at least at the speeds you are willing to fly at.
Note down that deflection value, go on the SPH and set it, save craft, profit

Now how to design a more agile airplane is a whole different story.

[Kitspace]

First of all thank you for your responses!

I am perfectly happy and comfortable with designing nice and stable planes and the thing is that now I want to try and design a more agile plane. But the problem that I am having is that it feels like the plane itself is agile enough but the velocity vector does not want to follow the direction of the plane nose really. Instead it like almost wants the nose of the plane to follow the prograde marker and just keep flying the direction it was flying previously as soon as the controls are released.

It is also worth mentioning that a joystick mod is used so the inputs are reasonably smooth.

I have checked the static analisys graphs on all my planes and the thing is it does not show any signs of stalling or even any sudden changes in the pitching moment line gradient at quite severe angles of attack that you would never fly at. Certainly higher than what is being reached in flight.

I have a feeling though that the plane actually gets thrown beyond the angle of neutral pitching moment and there it starts oscillating.

And another question on top of that.

Why do flaps and slats made out of control surface parts turn out to be counter productive most of the time?

How to make them actually increase your lift coefficient without introducing a lot of stalling that was not there before?

Edited February 25, 2016 by Kitspace

[ratchet freak]

12 hours ago, Kitspace said:

Instead it like almost wants the nose of the plane to follow the prograde marker and just keep flying the direction it was flying previously as soon as the controls are released.

That's what a stable plane should do.

 

Don't turn by yawing. If you want to turn you should bank in the direction you want to go and then pitch up. This way some of the lift is directed sideways and will shift your velocity vector.

[eberkain]

Ok, so I'm new to the mod, but I was under the impression from the OP that this would remove drag from parts that are inside a fairing.  Just trying to understand how its supposed to be working.  Is it that the indicators still show up, but are not doing anything?  They sure seem to be. 

Edited February 26, 2016 by eberkain

[Svm420]

2 hours ago, eberkain said:

Ok, so I'm new to the mod, but I was under the impression from the OP that this would remove drag from parts that are inside a fairing.  Just trying to understand how its supposed to be working.  Is it that the indicators still show up, but are not doing anything?  They sure seem to be. 

You have a giant oversize fairing causing obscene drag look fine to me  

[eberkain]

45 minutes ago, Svm420 said:

You have a giant oversize fairing causing obscene drag look fine to me  

I will do some better pics when i get home, im pretty sure all the red drag indicators are from parts under the fairing, not the fairing its self.

[Kitspace]

Why would you think that I am trying to turn by yawing the nose of the plane around? I am mainly talking about vertical maneuvering not horizontal anyways.

I probably should have made it more clear in the first place that I have real life experience with piloting airplanes. And neither is that my first plane design in the game. I am just trying to move to something more advanced now.

[eberkain]

5 hours ago, Svm420 said:

You have a giant oversize fairing causing obscene drag look fine to me  

Let me start by saying, I've not been having problems, nor am I saying there is something wrong with the mod.  The rocket will in fact launch fine, I just noticed some excessive amount of red markers with the aero forces overlay turned on one launch and I am trying to investigate what is going on, I want to understand how it is supposed to work.  In the pics I am intentionally forcing the rocket to flip to see what is happening.  I took the same rocket from before and launched it a couple more times, there is definitely something weird going on for me.  

I tried deploying the fairing before launch, launching without any fairing at all.  

no drag markers coming from the payload.  

I launched again with the fairing still on, yep, there are an ass load of markers coming out of the fairing.  

I have a fresh install of the mod, have not changed any settings or anything.   Is there something I did not do that is required to setup the mod correctly?  Is this behavior intended?

[T-Pilot395]

I think it's because of the way you're setting up your fairing and it's fully intended.

You see, your payload appears smaller in diameter than the rest of the rocket. Hence it's more streamlined and doesn't produce as much drag. Your fairing however is obscenely bigger than the rest of the rocket, so when faced with oncoming air resistance it's going to want to rotate to face the airflow engine-first. It's the reason why darts in real life are bigger at the back.

A more reasonable test would be to try hiding your payload in a fairing of the same size.

[ferram4]

First, yes, the arrows are wrong.  Parts that have no drag on them have excessively large drag arrows... for some reason.  I'll have to look at what causes that.

Second, it's not just drag.  Very large fairings will produce much more body lift than the rest of the rocket, which at low AoA is proportional to the cross-sectional area, and take a look at that fairing: its diameter is twice that of the rest of the rocket, which means that it's gonna be making, unit-length-for-unit-length, 4x the body lift.  All way ahead of the CoM.

The payload is actually probably a lot draggier than the fairing, but because it has less body lift, it's more stable.  Draggier things being more stable isn't that rare; reentry vehicles are far more stable than bullets are, but bullets are much lower drag.  Redesign the fairing to fit a lot tighter around the payload (your payload is a smaller diameter than your rocket, why the hell is the fairing a larger diameter than the rocket?!) and everything will be fine.

[StarStreak2109]

Hi there,

first of all, I do not know whether this is an issue of FAR, but having read that there were some issues like this for others as well, here it comes. Last night I was returning a craft from Minmus. It came down to a 156 km periapsis. Then I dropped it further to 35 km for the final reentry. Reentry speed was about what you expect from a LKO reentry. But the craft would slow down in the atmosphere at all, instead it kept accelerating somewhat, until it blew apart from aerodynamic stresses (killing the two crew in the purpose). I never reentered with this craft before, so it may well be some other problem, but I notice some weird entries in the log file, so I figured I ask here before...

Log file: https://www.dropbox.com/s/5ub7la28y77yrn1/KSP.log?dl=0

Craft file: https://www.dropbox.com/s/mov9bjuwmx45rmi/Daedalus%20MPCV.craft?dl=0

Thanks for any help in advance!

Sebastian

[ferram4]

Okay, the only FAR error I can see in there is one where somehow the GUI can't find the vessel that the part is attached to... very strange.

There's a lot of stuff actually happening in the log, I need you to tell me everything that you did with that vessel (and I mean everything, switching your view between crafts is a thing, loading it up is a thing, going back to the space center and returning is a thing, etc.) so that I have some hope of getting actual reliable reproduction steps.  While I'm thankful for the log, I also need you to tell me what mods you're using on the craft, and if possible, I need you to reproduce the issue with as few extra mods as possible.  Until then, I unfortunately really can't do much besides guess at what's going on and try random code changes to fix it (and probably introduce much worse bugs in the process).

[StarStreak2109]

Hi Ferram,

thanks for the answer. First of all, here is my (somewhat extensive) modlist: https://www.dropbox.com/s/tqn2zoksra9ivzx/mods.txt?dl=0

Not included, since I didn't load it via CKAN is Aerojet Kerbodyne.

The craft in question at the time of the incident uses the following mods:

• Near Future Spacecraft (Mk. 3-9 command pod); and
• SDHI Service Module System (for the SM); and
• Aerojet Kerbodyne (RCS thrusters).

What did I do:

1. The ship was docked to a spacestation around Minmus. I EVA'd two Kerbals over two it.
2. I undocked and plotted the return trajectory back to Kerbin (always focused on this ship).
3. I circularized in LKO and then...
4. ...burned for atmospheric entry.
5. Boom.

No switching vessels was involved nor any quickloading. Only thing I did was occasionally switching between IVA and external view.

Before making a separate install with only the involved mods, I'll try to reproduce the error in the install as-is.

[StarStreak2109]

Me again... Trying to reproduce the error, everything worked just fine. So, just a freak error? Anyhow, Here's the log file of my attempt at recreating the error along with a video of what I did...

The log file (flight begins at line 59,690): https://www.dropbox.com/s/55z187tmaz831bs/KSP_2.log?dl=0

The video:

Best regards and thanks for looking into this,

Sebastian

[eberkain]

On 2/27/2016 at 1:48 AM, ferram4 said:

First, yes, the arrows are wrong.  Parts that have no drag on them have excessively large drag arrows... for some reason.  I'll have to look at what causes that.

Second, it's not just drag.  Very large fairings will produce much more body lift than the rest of the rocket, which at low AoA is proportional to the cross-sectional area, and take a look at that fairing: its diameter is twice that of the rest of the rocket, which means that it's gonna be making, unit-length-for-unit-length, 4x the body lift.  All way ahead of the CoM.

The payload is actually probably a lot draggier than the fairing, but because it has less body lift, it's more stable.  Draggier things being more stable isn't that rare; reentry vehicles are far more stable than bullets are, but bullets are much lower drag.  Redesign the fairing to fit a lot tighter around the payload (your payload is a smaller diameter than your rocket, why the hell is the fairing a larger diameter than the rocket?!) and everything will be fine.

Thanks for the reply.  I'm still learning how to build good designs, I can finally do a real gravity turn with FAR installed, and I love how it just utterly destroys my bad designs when they flip, lol.  Good to know that the drag arrows are not working as intended.  

[TeeGee]

Help.

No matter what I do I can't seem to get the CoL below the CoM. The fairing I have is stock and whenever I remove it the CoL goes WAAAYY below the CoM but when I put it on this happens.

Edited February 29, 2016 by TeeGee

[blowfish]

@TeeGee I'm not sure if the CoL indicator is displaying correctly or not, but you shouldn't need an actually stable rocket - thrust vectoring (and control surfaces in your case) should be plenty to keep it on target with SAS.  In fact, I've found stable rockets quite difficult to control at times because you're basically stuck pointing prograde.

[Lorunification]

Hey everyone,

Sorry, if this is a repetetive question or something, but I seem to have a problem with the current FAR release that I already ran into with some of the recent releases. I cant quite figure out what is wrong with my installation.

Here is the issue: The default starter rocket (Mk1 Capsule + RT-5 Flea + parachute) does not seem to generate any drag whatsoever during descend. The overlay does not show any drag indicators and the vessel slams into the ground at about 700m/s.

I think i already read about this issue somewhere, but can't remember how to solve it. Is this caused by an incompatibility with another mod? Any help is greatly appreciated!

Thank you in advance!

 

Edit: Seems to be the same issue already in discussion. But instead of reentering a "real" vessel, i have a completely new vessel with 3 parts.

Edited February 29, 2016 by Lorunification

[TeeGee]

4 hours ago, blowfish said:

@TeeGee I'm not sure if the CoL indicator is displaying correctly or not, but you shouldn't need an actually stable rocket - thrust vectoring (and control surfaces in your case) should be plenty to keep it on target with SAS.  In fact, I've found stable rockets quite difficult to control at times because you're basically stuck pointing prograde.

But when I try to pitch away from my prograde meter my rocket flips. All of the real world rockets have CoL behind CoM and I would like to build reliable rockets that are hard to lose control of if possible. 

[blowfish]

49 minutes ago, TeeGee said:

But when I try to pitch away from my prograde meter my rocket flips. All of the real world rockets have CoL behind CoM and I would like to build reliable rockets that are hard to lose control of if possible. 

Real-world rockets are actually unstable.  How far are you pitching away from prograde?  Even without fins, you're usually fine within a few degrees, with SAS enabled.

[Hodo]

57 minutes ago, TeeGee said:

But when I try to pitch away from my prograde meter my rocket flips. All of the real world rockets have CoL behind CoM and I would like to build reliable rockets that are hard to lose control of if possible. 

This is a common mistake.  You are most likely pushing the nose to far from the current flight path.   I generally start my "gravity" turn at 1km alt or 100m/s which ever happens first.  I go 5deg at first, then another 5deg about 10-20s later.   Then from there the rocket will usually work on its own from there.   By around 10-15km I am pitched over almost 60deg and accelerating like a scalded dog.

[TeeGee]

2 hours ago, Hodo said:

This is a common mistake.  You are most likely pushing the nose to far from the current flight path.   I generally start my "gravity" turn at 1km alt or 100m/s which ever happens first.  I go 5deg at first, then another 5deg about 10-20s later.   Then from there the rocket will usually work on its own from there.   By around 10-15km I am pitched over almost 60deg and accelerating like a scalded dog.

Well I've been playing with FAR and RO for a long time but I usually build rockets with CoP behind the CoM which usually isn't difficult in real sized rockets because of how tall they are BUT I'm currently trying out 0.64x Kerbin and my rockets generally are much shorter than my RO rockets due to the decreased delta v requirements to get into orbit. Engine gimbal in stockalike engines is NOT as good as real life engines so if you build an unstable rocket like mine, chances are more likely you'll lose control when you pitch too far away from the prograde vector. 

[Hodo]

Hmm, odd I haven't had this problem.  I may have to test out some more rockets in my game to see what all has changed.  I havent launched an actual rocket in almost 3 or 4 months.

[123nick]

the water doesent seem too slow down parts like it should i think drag is increased - when a pod was bouncing in an out through the water, i couldnt pitch up or down while in the water, it was like, fixed too facing retrograde. but it still seemed too maintain speed, sometimes even gaining speed being forced out of the water. i think i had this problem before, but dont know the fix. any idea?