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

[baldamundo]

Any idea what this means?

 FAR Error: Aerodynamic force = NaN AC Loc = 1793.92113338764 AoA = 0

MAC = 1.68847206709385 B_2 = 1.52083446043253 sweepAngle = 0

MidChordSweep = 40.09 MidChordSweepSideways = 0.0937839486629087

 at CanardController

(Filename: C:/buildslave/unity/build/artifacts/StandalonePlayerGenerated/UnityEngineDebug.cpp Line: 56)

Had a whole string of the same errors in-flight, for my canards as well as my procedural wings.

Not obvious that it was having any in-game effect though...at least, I assume my going into a nosedive at the end was because I've designed my plane badly, not a bug...

EDIT: Full log here https://www.dropbox.com/s/835evce0q6mh0jz/far%20error.txt?dl=0

Edited October 18, 2015 by baldamundo

[damerell]

I'll also note that it looks like you didn't install FAR correctly at all, because the FAR icon that should be in the little toolbar is pure white, which only happens when someone fails to install it correctly.

I wonder if the check which produces this can be triggered by ATM smooshing the icons (which has happened, including to some icons which I think the most recent ATM should not smoosh).

I am not asking with any intention whatsoever of another round of win64/CKAN/FAR core wars, but because I've taken an interest sometimes in these ATM-produced issues.

[birdspider]

Hello, I can't seem to find doc on the various slider controls in the wing context menu. (Pitch,.., AoA%,) 1) are those from FAR ?, 2) can someone point me to a wiki/doc where those are explained ?

[Van Disaster]

Hello, I can't seem to find doc on the various slider controls in the wing context menu. (Pitch,.., AoA%,) 1) are those from FAR ?, 2) can someone point me to a wiki/doc where those are explained ?

They're inputs for the control surface; the craft's actual control settings are in the bottom left of the flight window, so if the pitch pointer down there is at 100% and you've set 50% pitch on a surface, it will move 50% of it's deflection if there's no other input to it. Mixing inputs works quite well for gentle maneuvering, but I've found it's not such a good idea ( other than AoA ) if you're building a more extreme craft. AoA is useful to feed into a surface to stop it stalling. There's other uses for AoA but that is one of the most obvious.

[tetryds]

Hello, I can't seem to find doc on the various slider controls in the wing context menu. (Pitch,.., AoA%,) 1) are those from FAR ?, 2) can someone point me to a wiki/doc where those are explained ?

In resume, it's a finer control of the deflections, while stock is *use for pitch or not*, FAR allows for deep fine tuning and complex controk surface deflection combinations.

You will also want to limit yoir controls to avoid stall or disassembly.

Be careful to not put something on -100% when trying to disable a certain function, 0% is at the center.

[Aerospace]

Hi everyone,

I apologize if this problem has already been discussed, 1122 pages are a lot to read...

I'm having a problem during reentry, my vessel flips (goes nose first) even if it is stable.

Do you have an idea why this happens?

Thank you!

[Svm420]

Hi everyone,

I apologize if this problem has already been discussed, 1122 pages are a lot to read...

I'm having a problem during reentry, my vessel flips (goes nose first) even if it is stable.

Do you have an idea why this happens?

Thank you!

http://i.imgur.com/Jlkfrfj.png

Your craft is too long. with the aerodynamic center is above your mass. The blue ball want to be below the yellow ball relative to the direction of travel.There are are several options as solutions. If you want to understand why I recommend you try and learn to use the stability analysis tool FAR offers they will tell you everything you could want to know without wasting Kerbal lives or dollars. Knowledge and understanding is always worth the time

Edited October 19, 2015 by Svm420

[blowfish]

Ignore the CoL indicator. It is meaningless in FAR. But yes, a vessel like that is likely unstable in that configuration.

[tetryds]

@Smv420 & Aerospace:

That is actually a stable position (Col wise), because he goes bottom first, but you are right when you say that he should watch the derivatives and ignore the col indicator.

Make a static sweep at mach 7 from -30 to 30 degrees, the craft is stable if directly at the airstream but probably goes unstable with a small angle.

I recommend having more weight on the bottom, and/or more control, as well as making it shorter.

Try to have your COM as low as possible until the graphs show a decent angle where your craft is stable.

If electricity is not a problem, try making the pilot hold retrograde during reentry.

[baldamundo]

Sorry if I've overlooked it, but is there a guide somewhere to exactly what the FAR 'Flight Assistance' buttons do?

[Van Disaster]

Ignore the CoL indicator. It is meaningless in FAR. But yes, a vessel like that is likely unstable in that configuration.

Not completely, I do still use it as a baseline judgement ( in the SPH at least ) & it's effective enough for that.

Small point: I always save and reload craft as a final check because I've noticed the derivative panel can give different results ( I seem to remember this from a long time ago ). I can't duplicate it reliably, unfortunately.

[Aerospace]

Thank you for all your replies!!!

Following your advises, I tried to perform a reentry with SAS locked in retrograde mode and it went well, Jebediah survived...

Then I tried again a "free" reentry and I found, for example, that my vessel is unstable at Mach 5.75 @ 31 km of altitude, so I used FAR tools (I did the computation inside the SPH). Here the results:

Then, I used the FAR tools also with the well-known stable vessel, i.e. Pod + Heat shield and, surprisingly, they have similar results!!!!!

What do you think?

[ferram4]

Yes, you were testing the stability of the capsule facing forward, not backwards as in the standard reentry orientation. In that case, the mk1 pod w/ heatshield alone is stable, but the pod + science instruments underneath it is not.

I think it would have made much more sense to do the test in the VAB, where it's a lot easier to see which orientation is which, and that you should have actually made sure you were testing the thing you thought you were testing.

[tetryds]

@Aerospace: What ferram meant was: flip your craft (the part that is meant to reentry) upside down on the VAB and check it's stability.

Upside down because on the VAB FAR accounts for UP as forward, if you are falling bottom first, the bottom has to be pointing up.

You get used to it

[Van Disaster]

An aerodynamics query: a couple of us building BDA drones noticed this, at around 1.2M the Zu figure in the static derivatives goes positive, which is implying the lift force is downwards - but the tab is showing a state of stable forward flight. Pulling the wings off changes the steady state AoA completely so I don't believe it's showing the craft being ballistic during some strange region of transition to fully supersonic airflow, so ... what exactly *is* that state? it's in a specific velocity region of about 1.18M to 1.22M.

There's some specific wing shapes that don't exhibit that behaviour, but it was more about not understanding what the derivative is telling us.

Edited October 22, 2015 by Van Disaster

[ferram4]

No, that implies that increasing the velocity will decrease the amount of lift... which while very rare, is possible for some configurations. A situation where changing flow characteristics are enough to overpower the increase in dynamic pressure, or a situation where some portion of the vehicle that creates a downforce gets more of a boost than a portion that creates an upforce.

[MatterBeam]

Hi ferram!

I'm trying to build long-endurance planes for the moment, and I was wondering about the factors acting on the 'estimated range' figure.

For example, my craft, at 10km altitude, has a speed of 280m/s and a Mechjeb-calculated fuel duration of 54 minutes. I estimate my range to be about 907km under power, and another 5km or so while gliding.

However, the estimated range from the FAR flight data varies between 1000 and 3000km range, heavily dependent on the current angle of attack. Can you tell me more about this?

[Van Disaster]

No, that implies that increasing the velocity will decrease the amount of lift... which while very rare, is possible for some configurations. A situation where changing flow characteristics are enough to overpower the increase in dynamic pressure, or a situation where some portion of the vehicle that creates a downforce gets more of a boost than a portion that creates an upforce.

Right. I should probably have listened to myself when I noticed "u" was acceleration rather than velocity... and thinking about it the lift is just the craft mass ( providing it can actually lift it ) in that tab. Thanks for clearing it up.

[ferram4]

@Krakenfour: All that affects estimated range is fuel fraction (how much of the plane is fuel relative to not-fuel), lift-to-drag-ratio, velocity, and thrust-specific fuel consumption. It assumes steady flight, so minimal acceleration, and that you will maintain the same L/D and velocity over the entire flight, and because of the decrease in plane mass, you'll be able to gain altitude and have less drag to deal with as you fly. End result is that it's similar (in a way) to a dV calculation; a tonne of fuel burned on an empty plane gets you further than a tonne of fuel burned on a heavy, fuel-laden plane.

@Van Disaster: No, no, NO. U is velocity. In terms of derivatives, Xu is the rate of change of X with respect to u. It doesn't give a damn about acceleration at all. It doesn't care how u changes, it just says that (under the assumptions taken for calculating the derivatives) that if u changes "so much", X will change Xu * "so much", as "so much" approaches 0.

...this is apparently what I get for displaying anything involving calculus.

[cantab]

Feature request: Allow control surfaces to respond to trim only, direct inputs only, or both. The most obvious application is to copy the way some airliners move the whole tail for trim while also having separate elevators. I'm not sure how feasible it would be though.

[Van Disaster]

@Van Disaster: No, no, NO. U is velocity. In terms of derivatives, Xu is the rate of change of X with respect to u. It doesn't give a damn about acceleration at all. It doesn't care how u changes, it just says that (under the assumptions taken for calculating the derivatives) that if u changes "so much", X will change Xu * "so much", as "so much" approaches 0.

...this is apparently what I get for displaying anything involving calculus.

Hehe - well I understand the principles of calc ( it's very hard *not* to when you work with pseudo-realtime code ), I just get tied up by the language of it sometimes. My maths teacher doing advanced calc was worse than rubbish & managed to not only make me fail to understand what he was teaching, but undo the basics I understood just fine but yep, ok, I see what Zu is saying completely now.

Now to work out *why*. I don't suppose there's some hidden feature that'd help there?

A feature request *I'd* like would be to integrate the Dynamic Deflection mod ( or at least the idea ) on a per-input basis per control surface, so for instance certain surfaces would act as pitch controls only within a certain range of dynamic pressure & speed. Additionally the flight assistant PID settings saved per craft or another per-craft damper PID for each axis, so it's possible to set them up to damp out any control bounce automatically. Dynamic Deflection modifies the surface deflection range based on a graph keyed to dynamic pressure vs max deflection %, with points input by the user, rather like an enhanced DPCR augment per control.

Edited October 23, 2015 by Van Disaster

[baldamundo]

How do people determine the appropriate attitude for spaceplane re-entry in FAR? Do I just want to fly in at approximately my critical angle of attack?

[Van Disaster]

How do people determine the appropriate attitude for spaceplane re-entry in FAR? Do I just want to fly in at approximately my critical angle of attack?

From 80-90km orbit, retro burn until 20km AP and hold 2-3 degrees pitch up until I'm slow enough that I think I can fly rather than fall. I've done it that way forever, it still seems to work fine provided the craft either has enough authority or has a high pressure area at the back.

[Bookstore44]

So I was trying to make a stuntplane that can tailspin (no success so far), and I found an oddity. Stalled wings seem to strongly pitch my plane down.

I made this rocket, with wings in front of the center of mass and at a 45 degree angle. The yellow Cm line looks positive at negative angle of attacks (when the wings aren't stalled), but then dips right when the Cl does.

The rocket is a stayputnik, small inline reaction wheel, fl-t800 fuel tank, lv-t30 liquid fuel engine. Two wingconnector type B's are placed in front of the center of mass, and then rotated by 45 degrees.

I launch it. The small inline reaction wheel is pitching it up. The body lift of the fuel tank is pitching it up. The drag on the stalled wings is pitching it up. The lift on the stalled wings is pitching it up. And at first, when the rocket isn't going that fast, it pitches up as expected:

But immediately after that picture was taken, it reverses direction and pitches down, fighting all those forces.

and then it oscillates, rolls a bit, and completely loses control as you'd expect.

Are stalled wings supposed to pitch down like that?

[The Optimist]

It seems that this mod causes random bits of my ship to explode when in atmosphere, even if I'm travelling at 10 m/s