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

[Execute13]

How does FAR react to self-intersecting rockets? Does it treat each leading face separately?

For example, I use a generic set of fairings from Fairing Factory. Because of the different sizes of engine clusters, I often have to make the fairings partway up a serial rocket go into the tank above. Also, when clustering I will sometimes clip structural parts into the bottom of the tank to make the cluster look better. How would this affect the aerodynamics?

[foamyesque]

@foamyesque: That's a very cool way of packing that payload. Everyone who can't figure out how to launch their rovers, space station sections, etc. learn from this guy.

Downside is that you trade radial compactness for a very long fairing, which can cause wobble. As ever, life is compromises.

Oh, also, as far as mechanical assistance goes (ASAS, etc) I've found that a handy option is to use MechJeb's attitude adjustment window and increase Tf from its default of 0.1 to 0.25 or higher. It dramatically slows down MJ's command inputs, which isn't great in space but very, very useful in avoiding catastrophic aerodynamic loading in atmosphere flights.

[localSol]

could you please explain how a 15 may be different from a 5 or 50

[...] i do not fully understand how the [FAR Flight Assistance] numbers effect the flight like wing stabilization .05 vs .10 etc if it could be explained i would really appreciate that.

The way I got comfortable with FAR was to take note of when FAR says a control surface will stall at an angle. It tells you what angle for each control surface. They are all individual, each potentially different. It is safer to set your deflection to the value is warns you of, but testing the plane yourself is the best way to know what will happen. For some planes it doesn't seem to matter too much. But FAR gives you the flexibility of choosing what to do with each surface.

Flight assistance settings : you can get familiar with the value they start with by default, (They work well for most planes) and then if it doesn't seem to do much, try doubling the value and see how the plane responds at various speeds, all the way to the plane's theoretical maximum speed. If a very high setting causes the plane to go crazy immediately, then I find that increasing won't help - you can try halving it and seeing if that is more stable. While the assistance systems are pretty stable, they can sometimes kind of reveal a plane's natural instability if they start to freak out. You can try flying the plane without assistance to get an idea of how much correction it has to do, to get an idea of how reasonably controllable a plane is..I noticed that if I needed a very high wing leveler setting, I could imagine what part might be causing trouble especially if it is an unfamiliar (mod) one, or a strut was causing a constant rotating force on the whole plane (a ksp/unity uh..peculiarity, not FAR)

[Doomydoom]

The higher ups said they needed a business jet that "looked cool" and could "do all that 10g ****" but not blow up. The result:

[Weatherman159]

Hey there, it's me again. So having solved my previous problems, I immediately started creating new ones. Sorry for spamming the thread so much, but I haven't really found any good FAR tutorials around.

I'm building a shuttle-like craft (will be taking off vertically with an external fueltank and boosters) and landing on the runway, so I decided to test its' flight capabilities. It takes off reasonably well, but almost immediately starts to roll violently to the left. It's built completely symmetrical. Help?

I have more pictures if you need a different angle.

[Weatherman159]

Hey there, it's me again. So having solved my previous problems, I immediately started creating new ones. Sorry for spamming the thread so much, but I haven't really found any good FAR tutorials around.

I'm building a shuttle-like craft (will be taking off vertically with an external fueltank and boosters) and landing on the runway, so I decided to test its' flight capabilities. It takes off reasonably well, but almost immediately starts to roll violently to the left. It's built completely symmetrical. Help?

I have more pictures if you need a different angle.

Edit: I noticed the wings were flexing quite a bit when physics first kicks in after launch. Added a bunch of struts to the wings and fuselage, now flies mostly straight. It still wants to plummet nose down when not at full throttle but that is a minor issue.

[Autochton]

I think I may be having a similar issue. Using B9 parts and pWings, I've built a SABRE-using spaceplane - and I notice that the center of lift is yawed some 10-15 degrees or so to the right. Basically, it looks like I should expect it to yaw, and indeed it does exactly that, to the point that it's impossible to take off. All my derivatives have the right signs (and wasn't that just a hassle to get to happen), CoL is well behind CoM, CoT lines up as it ought. The plane is utterly, entirely symmetrical. What in blazes goes on? In fact I find it's happening to any plane at all with B9 parts, whether tiny or gigantic, and is independent of whether I use segmented wings or pWings. I'll experiment further with what is going on here. Any input is highly welcome.

[zzz]

message has been deleted

Edited June 10, 2013 by zzz

[ferram4]

@Doomydoom: Thank you for giving everyone an example of the type of wings that would never be used for commercial jets, mainly because they would have massive issues with aeroelastic divergence and flutter (which I hope to implement at some point). Divergence is kind of simple: lift twists the leading edge of the wing upwards, the wing makes more lift, leading to it twisting more; eventually it twists enough to break. Flutter is the same type of thing, but add transient motion; this is a decent video of it:

Basically, those forward wings should be ripped off.

@Autochton: Try struting the wings down; symmetry-created joints do not have equal strengths, so your plane may become subtly asymmetric.

Otherwise, I have found a minor wing interference bug that I've fixed. It shouldn't just affect B9 wings, but it might be causing some minor problems.

[camlost]

Questions:

1.The airfoils have no thickness or curvature, do they? Any thought on introducing that?

2.How was the lift from fuselage calculated?

3.Does the fuselage has interference on the wings?

[Dirt_Merchant]

Does this mean we might expect to see a hotfix tonight? Additionally, since I'm here I'll just go ahead and state, your mod is the reason I play ksp. Thank you very much for providing the community with this gem, I eagerly await future developments. One last thing I just recalled, could we perhaps sometime hope to see leading edge slats, spoilers, flaperons, spoilerons, slaterons, rudderons and the myriad other control surface variations? The use of these more advanced control surfaces would provide my space program with some extremely valuable tools. Ive had some success with the use of V tails, but the rolling tendency of rudder-vaders makes it a challenge, reminds me of flying an old m35 bonanza. Anyways, so stoked to have this mod!

[Doomydoom]

@Doomydoom: Thank you for giving everyone an example of the type of wings that would never be used for commercial jets, mainly because they would have massive issues with aeroelastic divergence and flutter (which I hope to implement at some point). Divergence is kind of simple: lift twists the leading edge of the wing upwards, the wing makes more lift, leading to it twisting more; eventually it twists enough to break. Flutter is the same type of thing, but add transient motion; this is a decent video of it:

Basically, those forward wings should be ripped off.

@Autochton: Try struting the wings down; symmetry-created joints do not have equal strengths, so your plane may become subtly asymmetric.

Otherwise, I have found a minor wing interference bug that I've fixed. It shouldn't just affect B9 wings, but it might be causing some minor problems.

But they look so cool and sci-fi-y! Why would the forward swept dealies I have on there be more likely to rip off than a normal wing? And why can't I ever make something that wouldn't explode in real life? What about this, how would this explode?

[ferram4]

@camlost:

1. Airfoil thickness is simulated to an extent. The subsonic lift model requires some amount of thickness to function properly, particularly with regard to stalling properties. The supersonic lift model has it built into the design, simply because wings can be more effective in supersonic flows if they have some thickness.
   Camber isn't accounted for for two reasons: first, the way KSP handles mirrored parts is not to actually "mirror" them; it's to flip it upside-down. This means that if I were to add any modeling of camber effects planes would spontaneously roll, which while amusing the first time, would get boring very, very fast. The second is that camber doesn't create lift at supersonic speeds, only drag and pitching moments. So for the vast majority of KSP vehicles (which I assume are designed for supersonic flight) camber is detrimental to performance, not beneficial.
   
2. Body lift is handled by an approximation based on Newton's Sine Squared Law, as used as an approximation for hypersonic aerodynamics. More refinements are planned, but there's a lot of data for me to go through and the lift of blunt bodies is a very complicated problem. It's actually a bit harder to deal with than wings due to the flow characteristics (large amounts of flow separation, large pressure changes, pressure drag dominating skin friction drag).
   
3. Wings detect fuselage interference in the same way they detect wing interference, so the effect is there.
   

@Dirt_Merchant: No hotfix, since I've changed how cargo bays are handled and I want to make sure that is working right before I release it. As for most of the control surfaces you ask for, the physics are there for them, though the code doesn't allow for spoilers for roll control (I'll admit I don't quite understand the design reasoning behind choosing spoilers over ailerons, unless it's easier to maintain or build). You can actually set up a control surface as a flap, attach it to the front of a wing and it will act as a leading-edge slat.

I'll admit I don't particularly like the use of control surfaces for multiple roles, mostly because then you can get control inputs working a cross-purposes.

@Doomydoom: The reason forward swept wings are more likely to rip off is because of the way the wing bends. If the wing is unswept, bending is independent of torsion; it doesn't affect the higher angle of attack at the wingtips that cause the wing to tear itself apart. If the wing is swept forward, it exacerbates the problem; when the wing bends the wingtips end up at higher angles of attack, which means more torsion and the wing is more likely to tear itself apart. With a swept back wing the wing bends in such a way that if counteracts the problem; bending reduces the angle of attack and fights off torsion.

A properly swept-back wing can actually ward off all problems of static aeroelastic divergence, only having to worry about flutter. Of course, this is assuming the material is isotropic (acts the same in all directions). Anisotropic materials (read: composites) can be used to counter this. This is why swept forward wings had to wait until composites came along and for most purposes there's no reason to go with forward swept over backward swept wings.

The design you've posted should actually be good; the only problem that I can see are limited to the typical supersonic aircraft problems: with very low aspect ratio wings and most of the mass in long, thin tube your vehicle will be prone to inertial coupling. Now, this doesn't mean that it will spontaneously explode, it just means that you'll have to be cognizant of the fact that yawing motion might cause it to pitch up like crazy. The other problem is that it wouldn't follow Richard Whitcomb's area rule which (in the real world) would determine how much transonic drag it would run into. However, FAR doesn't simulate that (and I don't want to) due to the fact that it would make things very, very difficult to do in KSP. Of course, you asked me to tear it apart, so I'm nitpicking now.

Edited June 11, 2013 by ferram4
Answer Doomydoom

[Dirt_Merchant]

@ferram4: Thanks for the quick reply, I have experimented with reversing flaps to function as slats, which in turn prompted me to pose the question about control surface variations. As far as spoilers go, or more accurately spoilerons, they are less complicated than are ailerons though this benefit isn't usually required on small aircraft, but is very apparent on larger, heavier craft that see much higher forces on the control surfaces during deflection. For full details regarding their use, I'd need to do some additional research. As for their use in KSP, I'm not really sure where I'd use them, but given the option, I'm sure I'd come up with some goal requiring their use. While you are correct regarding the increased potential of adverse effects of multi-role control surfaces, especially when misused, the additional depth and flexibility provided to your mod by this addition would be incredible. Surely you can see the benefit of the use of flapperons, or rudderons in certain situations. Anyway, as a third generation pilot, and having grown up in aviation I am very much impressed with your work to date, and look forward to continuing development. If there is anything I might do to assist in the development of this plugin, don't hesitate to ask.

[Van Disaster]

You can simulate the divergence issue by holding something flat ( like a knife ) under the tap; point it up and tilt it slightly, you can feel it wanting to tilt further ( and if it's a really floppy plastic thing it might even bend ). Point it down and tilt it, and you can feel it attempting to straighten up again. It's worth noting there have been some successful forward swept testbeds, it's a bit more sane now we consistently design unstable aircraft anyway. One of my oldest FAR designs has a bat-wing shape which has a forward sweep component, that seems pretty happy ( several orders of magnitude less forward than the above design though! ).

One thing I'd like is FAR to remember the orientation of my "flaps" - I've attempted to vertically mount some as stability airbrakes for re-entry, but they'll revert to attempting to be symmetrical which unfortunately means one will retract forwards... which is the same issue as not being able to use them as spoilers, FAR is in this case being a bit too clever. Can we not just have them work like brakes & obey negative angles?

I remember you saying some time ago that mounting control surfaces on leading edges was a bad idea, so I'm glad that works now. Will have to revisit STOL, I think. Is there enough body lift code to attempt lifting bodies yet?

[foamyesque]

Note to self:

Remember to turn off Wing Leveler when doing a vertical takeoff.

[grom]

What is going on with my CoL here?

[Subcidal]

Just noticed something. The Squad parts included in FAR and the parts that the module manager .cfg changes are slightly different. the FAR squad parts only adds FAR modules to the MK1-2 pod and MK 1 pod. Where as the modulemanager.cfg for FAR adds it to both those capsules and the MK1 - MK3 pods.

Does it matter at all if command pods have the FAR module in their .cfg? Because I don't really see why it would seeing how it still works fine on the MK1-MK3 pods even though there is no FAR module enabled on them. And I don't see a difference with it enabled on the MK1-3 pods by using the modulemanager.cfg either. Just wondering as I was about to go through and add the FAR module onto all of my command pods, But this has confused me.

Edit :: Also, Does the little FAR window you get inflight not apply to parts that aren't located in /Gamedata/? (I.E the ones loaded in the old /parts/ folder structure) And does it affect their aerodynamics or is it just the lack of that window?

Edited June 12, 2013 by Subcidal

[taniwha]

grom: the wind is coming from the door, so the CoL indicator "looks" a little confused, but don't worry too much about the direction of the arrows. The important thing is the location of the ball.

Note that for 3x symmetry, the ball will be off-center. This too is normal, and it indicates that the craft will be roll-stable with 2 wings up and one down (like a Y). Keep this in mind during your gravity turn if you use 3x symmetry .

[ferram4]

@grom: That is likely a minor error in the CoL indicator; it should be pointing straight out, properly modeling the direction that lift goes in... I'll check that out and see if I'm modelling something wrong.

@Subcidal: The special FAR modules are only really necessary for the MK1-2 and MK1 pods, since they need special aerodynamics specified due to their unique shapes. The other cockpits are handled perfectly fine as it is. The FAR GUI module is applied to all parts that have "ModuleCommand" defined for them, regardless of where they are placed.

[Autochton]

A bit of further investigation and experimentation as to my 'twisted CoL' issue has revealed a few interesting things: First of all, it seems to mainly happen with B9 parts, as mentioned, but pWings also have it. I'm putting together a pair of spaceplanes, using mainly B9 parts. However, the interesting part is this: If I use symmetry to attach paired parts, the CoL invariably twists. "Recalculate CoL" will occasionally shift it, but the twist remains. If I then separate the symmetry-attached parts by selecting a closer stack part to the root, (usually in my case a fuel tank somewhere near the middle of the central hull) and then reattach it, it's fine and straight! After this, also, "Recalculate CoL" resets the CoL to a straight position. But if I then fiddle with a symmetry part again, well, back to twisted it goes. Struts do not seem to have material effect, either. (Incidentally, this also greatly affects my stability derivatives: Xw refuses to be negative with a twisted CoL, while with a straight one it goes negative without fuss. Also, the state of CoL at save determines whether the plane will fly at all, or start dancing a jig halfway down the runway.)

It smells of there being a bug somewhere, but I couldn't really say whether it's with FAR's CoL calculation, or in the B9 and/or pWing parts.

Edited June 12, 2013 by Autochton

[grom]

@ferram4 expect I'm having it flip out on south/north rotation, so it's matching the CoL in VAB

[MR4Y]

I'm having a interesting and weird thing happening. I'm not sure it's a bug. I'll be posting a video as soon as possible.

[Van Disaster]

Not entirely sure where to put this, as it's similar to an issue I've had in the past I thought this would be the place to start.

Nothing inherently wrong with that, right? uses the P-wing with the B9 part - uses two pieces per wing in fact, to get round the shielding issue with the cargo bay.

What the issue is is the return of the high-rate "flutter" I've had before with reversed parts - doesn't even need to get airborne before it's starting to oscillate around the roll axis at a few Hz. I can get in the air thanks to large amounts of strutting and there are times it calms down, but eventually it'll just rip the wings off. I presume it's something to do with speed vs air density vs angle of attack, but that's just a guess based on observation.

[MR4Y]

Here's the video on the "strange bug"

http://www.youtube.com/watch?v=zScmu9AMhsA