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

[ferram4]

The flight GUI should load whenever you enter the flight scene using a controllable vessel, though currently remotetech control parts can cause some issues with that. The editor GUI will load whenever you pick your first part for a new craft.

[Imagine]

I've been lurking this thread for awhile, figured I'd join in. First of all, hats off to Ferram for making a pretty sweet aerodynamics mod! I appreciate how it finally makes my rockets fly like rockets, and my bricks fly like bricks.

Lately, however, I've been having trouble landing an SSTO spaceplane. Here's a shot of my unmanned spaceplane coming in. The wings seemed to generate no lift coming in, and control surfaces exert only tiny amounts of control. It also appears that the drag coefficient blows up. Coming in pointed straight down, the atmosphere slows it down enough to land (in separate pieces) on its nose.

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Any ideas?

[Hodo]

I've been lurking this thread for awhile, figured I'd join in. First of all, hats off to Ferram for making a pretty sweet aerodynamics mod! I appreciate how it finally makes my rockets fly like rockets, and my bricks fly like bricks.

Lately, however, I've been having trouble landing an SSTO spaceplane. Here's a shot of my unmanned spaceplane coming in. The wings seemed to generate no lift coming in, and control surfaces exert only tiny amounts of control. It also appears that the drag coefficient blows up. Coming in pointed straight down, the atmosphere slows it down enough to land (in separate pieces) on its nose.

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Any ideas?

Where is your center of lift (CoL), in relation to your center of mass (CoM)?

If your CoL is to far behind your CoM your craft will be nose heavy and tend to "lawn dart" nose down no matter what your control surfaces are trying to do. I generally don't use canards anymore as a control surface, they tend to stall out before any other part of the wing, and are not that great in supersonic flight. So I use them as flaps for take off and landings and that's it.

Can you post a pic of the craft in the VAH with the CoL, CoM and CoT icons on?

[ferram4]

@Imagine: Can you post a copy of your output_log.txt? Also, what other mods are you using? The way the drag coefficient blows up looks like a bug, but I don't know (off the top of my head) what could be causing it. If you have any spoilers on your spaceplane, try removing them and see what happens.

[ozligia]

Hi all.

First of all, Big thanks ferram4 for this awesome mod.

The picture below shows my test model. When i tested this model it turned out that the plane's tail takes off earlier than the nose. But the plane's Center of Lift is slightly ahead of Center of Mass. Can anybody explain why this happens? I know how to handle this, but it's more important for me to understand 'the right design' basics.

http://imgur.com/nxwKfzU

PS Sorry for Google translate

[Autochton]

Ozligia, you want the center of lift to be behind the center of mass. Otherwise, it won't fly right, it will try to turn itself around to get the center of mass in front. Try moving the wings backwards, or putting heavy things further forward in the plane.

[BubbaWilkins]

Hi all.

First of all, Big thanks ferram4 for this awesome mod.

The picture below shows my test model. When i tested this model it turned out that the plane's tail takes off earlier than the nose. But the plane's Center of Lift is slightly ahead of Center of Mass. Can anybody explain why this happens? I know how to handle this, but it's more important for me to understand 'the right design' basics.

http://imgur.com/nxwKfzU

PS Sorry for Google translate

Your control surfaces appear to be angled downward. They are also in a position just behind your center of mass which would force the tail in the air first. Add some tail-wings with control surfaces and that should shift the Center of Lift back some. You might need to shift your main wings back even further, but you basically never want your Center of Lift ahead of your Center of Mass.

[ozligia]

Your control surfaces appear to be angled downward. They are also in a position just behind your center of mass which would force the tail in the air first. Add some tail-wings with control surfaces and that should shift the Center of Lift back some. You might need to shift your main wings back even further, but you basically never want your Center of Lift ahead of your Center of Mass.

Thanks for your answers. But, probably, i have not stated my goal properly.

Yes, control surfaces are angled downward and center of lift is in front of center of mass, because they are set as flaps and are in the 3rd position (fully lowered)

Tail-wings will definitely solve the problem, I've already tested it. But the goal of this model is to understand how FAR's aerodynamic works.

I know, that:

- we have no lift at all with zero angle of attack, only increasing drag. It is realistic and rather expected.

- for the plane to be stable, the center of lift should be behind center of mass (but that does not mean it will fly)

- to increase lift should use flaps. Lift is proportional to flaps deflection, but deflection should not exceed stall angle.

But, i can't understand why when the center of lift is ahead of center of mass, the tail is in the air first. As i understand, in that case we should have a clockwise torque (and this is really what i need to increase angle of attack and take off, gain speed and fully retract the flaps). With the center of mass in front of center of lift we should have counter clockwise torque.

Now i have only 1 explanation - when using flaps lift force is applied to control surfaces themselves (not to the point where blue vector is drawn) so that actual center of lift is slightly (or even more) shifted back. As a result the actual center of lift appears to be behind center of mass and causes tail in the air first.

Or am i missing the meaning of center of lift arrow?

In fact there is another question. As I've noticed lowered flaps push center of lift forward to the nose of the plane. In many cases it was true, but on some designs it was more complicated. Position 1 pushed center of lift forward to the nose, however remaining 2 pushed center of lift back to the tail (sometimes even further than with flaps fully retracted). In all cases the main wings surfaces were not angled.

[ferram4]

@ozligia: The difference is the simple difference between the slope and y-intercept of a line. You're concerned with the pitching moment (yellow line) of the aircraft, which is (for the most part) a simple linear equation.

y = C_mα*x + C_m0

y = pitching moment

x = angle of attack

C_mα = pitching moment slope

C_m0 = zero angle of attack offset

Now, the location of the CoL relative to the CoM is related to the slope of the line, C_mα:

(CoL location - CoM location) ∠C_mα

This controls the stability of the plane, and it needs to be negative for the plane to be stable; this corresponds to the plane trying to stay at some given angle of attack, which is controlled by the next section, the y-intercept, C_m0. That value is a little more complicated, but negative lift behind the CoM increases C_m0, positive lift in front of the CoM increases C_m0, positive wing camber (which you have in your design) decreases C_m0, pitch-up inputs increase C_m0. You can read it off of where the yellow line crosses the vertical axis, which for your design is negative; this means that your plane is going to try to pitch downwards if it is at zero angle of attack. Ideally, C_m0 is positive, which causes the plane to naturally hold a small positive angle of attack without any control inputs; if C_m0 is 0, then the plane is essentially just a lawn dart until a pitch-up input is added to keep in on course.

The long story short is that your plane wants to start pitching downwards due to the camber on the only lifting surface; the plane's pitch instability causes it to be even less stable. Adding a small tail at the back with a negative angle of attack should take care of all your problems.

[Van Disaster]

The CoM-before-CoL-always issue is more complicated than "basically never", especially with extremely fast planes and the assists FAR has. Something of an advanced topic though.

A good rule of thumb would be that you need a certain length of lifting surface to be longditudinally stable as well; whether that's one long delta wing or a wing and a tail or canard, or tandem wings or whatever other crazy combination you want to try. Pure mechanics would also say that control surfaces further away from the centres are more effective. If you think of the lift centres of lifting surfaces as ... trestles, or wedges, or something equally thin you rest things on, you can see that resting your plane on two points is considerably more stable than one single one. I don't think I'd try flying anything that didn't have a longditudinal surface "length" of at least half the plane's length, and a lot would also depend on the mass distribution. Having engines at the rear is putting a lot of mass out on the end of a pivoting arm, going back to mechanics again.

[ozligia]

ferram4, Van Disaster: Thanks, now i start to understand what is going on.

[theflyingfish]

@ferram:I'm sorry. I feel bad for making all these requests...

But one thing that as always irked me is the high connection strength that all parts in KSP have. It takes one of the limiting factors of aircraft design (maximum g-force) and blows it wide open. If I build a craft correctly, I can reach 20 g using only stock, more if I use pWings (because of how it overestimates connection force to allow for Error.) But in order to decrease the breaking strength, you also decrease the connection stiffness, right? (Exacerbating spaghetti craft and the "always rolls left" bugs.)

Is there a workaround for this? Can we go plead with Harvester to increase the stiffness of part-part connections? I just miss the days when my aircraft would spontaneously disassemble if I took a hard turn above Mach 1...

*note: I rarely, if ever, use struts.

[AtilaElari]

This may have been asked before, if it was, I'm sorry for repeating.

I absolutely enjoy this mode and it is awesome. Besides one single thing - the G force created by the drag. So now whenever I drop from orbit into the atmosphere (with the angles and speeds rather close to those used IRL) the G force spike into the "red" zone in seconds and the ship pulverises. Not cute. Is it possible to do something with it, besides slowing down with engines before entering atmosphere (the thrill of re-entry is in the roar of a wind and red superheated air around the droppod!)? May be change some specific value in the plugin's code, if that is allowed by the developers?

[Cinocal]

I've never had any issues with FAR and re-entry, and I've tossed some large winged crafts into the atmosphere at 3000mps and they have not disintegrated. Only when I have something really wobbly and poorly built do I experience any drag induced structural failures.

[asmi]

This may have been asked before, if it was, I'm sorry for repeating.

I absolutely enjoy this mode and it is awesome. Besides one single thing - the G force created by the drag. So now whenever I drop from orbit into the atmosphere (with the angles and speeds rather close to those used IRL) the G force spike into the "red" zone in seconds and the ship pulverises. Not cute. Is it possible to do something with it, besides slowing down with engines before entering atmosphere (the thrill of re-entry is in the roar of a wind and red superheated air around the droppod!)? May be change some specific value in the plugin's code, if that is allowed by the developers?

Can you please elaborate on what do you think is realistic? Real-life spaceplanes (Buran and Space Shuttles) reentered with stalled-wing configuration which lead to the bulk of braking happening high in atmosphere where air is thin, so unless you enter apmosphere with insanely high velocity and angle it's simply impossible to break plane apart - at least I've never had that happening, and I had done some very agressive maneuvers like aerocapture on Duna/Kerbin/Eve straight off interplanetary trajectory.

[Van Disaster]

This may have been asked before, if it was, I'm sorry for repeating.

I absolutely enjoy this mode and it is awesome. Besides one single thing - the G force created by the drag. So now whenever I drop from orbit into the atmosphere (with the angles and speeds rather close to those used IRL) the G force spike into the "red" zone in seconds and the ship pulverises. Not cute. Is it possible to do something with it, besides slowing down with engines before entering atmosphere (the thrill of re-entry is in the roar of a wind and red superheated air around the droppod!)? May be change some specific value in the plugin's code, if that is allowed by the developers?

Heh, do I need to post my mach 216 aerocapture at Jool again? 9.8km/s and nothing falls off, it's routine now. The first few attempts resulted in the plane going out of control rather than falling apart.

Before you post your re-entry profile, there is a huge flaw in your thinking already - "angles and speeds rather close to those used IRL". Kerbin ( I presume you're talking about Kerbin ) has an atmosphere *way* more compressed than Earth's. I quite often glide all the way from my deorbit burn to the runway, so there's no need to use engines to slow down in atmosphere - but then again I burn for deorbit almost half an orbit back around.

[5parrowhawk]

Been having a problem with a canard design. It (or at least a previous version) seems pretty stable in most flight regimes; transonic is a bit unstable but nothing terrible. The main problem is loss of vertical control authority at low speeds (100-120 ms-2), even with full flaps. The craft adopts a nose-down attitude and basically lawn-darts.

I'm using two pairs of canards, with dihedral and anhedral respectively to hopefully deal with canard stalling issues, but no dice. Is FAR simulating spanwise airflow or something? Tried to add elevons right at the back, but that didn't help and only seemed to make it unstable on takeoff.

Any suggestions?

[AtilaElari]

I'v HyperEdited a few drop capsules into a 150km orbit and dropped it at different anles, and re-entry at steep angles resulted in death from overheating. Wich is weird sicne FAR doesn't add heat and I deleted Deadly Reentry.

Then again I may wery well be doing something wrong.

Entering at the slope angles is fine, no arguments on that matter.

Edited October 2, 2013 by AtilaElari

[rhoark]

Someone who understands the wing parameters could help this be FAR-enabled for its release:

http://forum.kerbalspaceprogram.com/threads/40761-Kerbin-Mini-Shuttle

[Hodo]

Been having a problem with a canard design. It (or at least a previous version) seems pretty stable in most flight regimes; transonic is a bit unstable but nothing terrible. The main problem is loss of vertical control authority at low speeds (100-120 ms-2), even with full flaps. The craft adopts a nose-down attitude and basically lawn-darts.

I'm using two pairs of canards, with dihedral and anhedral respectively to hopefully deal with canard stalling issues, but no dice. Is FAR simulating spanwise airflow or something? Tried to add elevons right at the back, but that didn't help and only seemed to make it unstable on takeoff.

Any suggestions?

The only thing I can come up with, and I am no expert. Is the canards angle of attack (AoA) is off. I noticed in the picture that your CoL is tilted forward which is pushing the nose down.

[Van Disaster]

Lack of control authority at low speed suggests you just don't have enough longditudinal torque, which is either because you don't have enough effective control surface or it's not far enough from the centre of mass; add to that, if the large control surfaces on your outer wings are flaps then lowering them will pitch the nose down ( and the issue pointed out above ). Moving CoL forward might help some too.

FAR had some interaction between vertically stacked surfaces but I don't really remember what it is; it was explained when someone asked about biplanes months ago.

[ferram4]

@theflyingfish: You have no idea just how much I would love something like that. I believe there was a mod, KIS Connect (IIRC) that essentially removed the flexing between parts (so not exactly what we'd want, but close), but that was several KSP versions ago.

I believe (read: my personal speculation) the issue with this is the way that parts are connected, using a combination of a linear spring and a torsional spring; this results in equal and opposite forces being applied to each part when they are under load, but the differing masses of the parts leads to unequal acceleration between each part, causing extreme flexing between parts of very different masses. The problem is trying to find a different way of handling joints that is computationally cheap, removes the flexing problem, and conserves of momentum.

@AtilaElari: I gather you're taking a spaceplane in with virtually zero angle of attack (lowest drag situation), correct? Try repeating it with your spaceplane holding 30-40 degrees angle of attack throughout the entire thing like the real space shuttle did; if it can't achieve that, it needs a redesign.

From your second post, it sounds like you didn't actually delete Deadly Reentry properly.

@5parrowhawk: You need those canards to be further forward. My suggestion would be to put a liquid-fuel-only fuel tank in between the cockpit and the rest of the ship and use the extra length to put the canards further forward. Adding small control surfaces to the back of your wings (right by the jet engines) and setting those to add pitch authority should help a bit; angling them to produce a slight pitch-up tendency at no pitch input is also probably a good idea. There's also the possibility that the canards are interfering with each other / the wings and producing less lift than you expect in their current position, so you may not need two sets of canards if you make the plane longer.

[NathanKell]

For now you can use DRE's g-force damage. It causes other problems (and the limit is a bit too low for high-strength aircraft!) but it does keep you honest, sure enough!

[AtilaElari]

Ferram4, I was speaking about the drop pods of the rockets, not the spaceplanes.

And I think I'v figured out what I was doing wron - sicne Kerbin's atmosphere is thicker than on Earth, one needs to make deorbiting angles sloper so the craft would spend as much time as possible in the upper layer of the atmosphere. It makes percision drops rather complicated to predict, though.

[uselessengineer]

If I'm not mistaken the Drag force is calculated as follows: D = 0.5*rho*S*C_d*v^2.

The drag coefficient I will try to estimate from a test launch and the info given by the FERRAM flight info window. My question is about S. Where do you get the cross-section area from?

Any help on that would be highly appreciated

@ThaineFurrows: The surface area S is determined from the model geometry for each part; currently it isn't output anywhere. I'll see about adding an output for that somewhere.

Did this ever get addressed? I am trying to determine area on a re-entry vehicle in order to land at a specific place.