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

[phoenix_ca]

Before writing about the tools in FAR, I am first adding some information about general concepts in aircraft design, so that everyone has a reference for terms and concepts that are dealt with in FAR. I just finished up my bit on static stability (still trying to decide what all to do with dynamic stability - it's important, but very technical). If anyone wants to read it and rate/hate for style, readability, clarity, etc., that would be awesome!

Awwwwwwwesome.

[Read have Read]

Things I hope to see on the Wiki:

- Anhedral/Dihedral: What is it, when to use it, how to use it, why does it work (done), how much is too much, examples.

- Area Rule: what is it and should we care?

- Mach Tuck: what to keep in mind when building and flying.

- Sideslips ands stalls: these are bad, but when can they be used for good? How to avoid them.

- Dutch Roll and other unwanted tendencies: why do they happen and how to avoid them

- Clean building practices for low drag

- Using FAR in the editor: what to look at and change every time (Mach/AoA, flap position) what to ignore, what can be useful, and what is the rest for?

- Using FAR in flight: Q is pressure, Cd is drag; what to keep an eye on, what the rest mean. How to tinker with the assists

- Gravity turns with FAR: best practices

- Aerobraking: best practices

- Unusual or unconventional designs and what can be learned from them (body lift)

- best speed for take off and landing.

- Building SSTO's and hypersonic vehicles

- Wings: aspect ratio, cord, sweep, shapes, flaps/spoilers.

- Controls: canards, tailplanes, stabalators, elevators.

Edited May 21, 2014 by Read have Read

[DaMichel]

@DresCroffgrin: Nice. But you have a loooooooong way to go if you keep doing it this thoroughly.

I think the the wiki should refer to this awesome thread for a general overview of aircraft design

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Wikipedia has quite a lot of information as well. Take this for example: http://en.wikipedia.org/wiki/Wing_configuration Very cool for inspiration!

On a different topic: I've been reading comments that FAR restricts your creativity and forces you to split your crafts in multiple launches and so on. So i was wondering how hard it would be to launch something ... creative ... well, creative relative to my play style. 4 failed launches: Over speeding and things breaking off, engines overheating and blowing up, collisions during separation, starting the gravity turn too early. This time i made it despite a badly designed craft and horrible piloting ...

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[DresCroffgrin]

@DresCroffgrin: Nice. But you have a loooooooong way to go if you keep doing it this thoroughly.

I think the the wiki should refer to this awesome thread for a general overview of aircraft design

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Wikipedia has quite a lot of information as well. Take this for example: http://en.wikipedia.org/wiki/Wing_configuration Very cool for inspiration!

<Lots of Awesome Pictures>

Indeed I do, although I'm only planning on documenting how to use and interpret the simulation tools in FAR (with general comments on how to remedy common problems). Those links look wonderful! The first one really needs to be linked to in the wiki, and the second one gave me a great read (I didn't know people have experimented with wings that fold into each other).

@People reading this thread who know aircraft design well: Does someone want to start writing up some general comments on design on the wiki? Or maybe tackle some of the things on Read have Read's list:

Things I hope to see on the Wiki:

- Anhedral/Dihedral: What is it, when to use it, how to use it, why does it work (done), how much is too much, examples.

- Area Rule: what is it and should we care?

- Mach Tuck: what to keep in mind when building and flying.

- Sideslips ands stalls: these are bad, but when can they be used for good? How to avoid them.

- Dutch Roll and other unwanted tendencies: why do they happen and how to avoid them

- Clean building practices for low drag

- Using FAR in the editor: what to look at and change every time (Mach/AoA, flap position) what to ignore, what can be useful, and what is the rest for?

- Using FAR in flight: Q is pressure, Cd is drag; what to keep an eye on, what the rest mean. How to tinker with the assists

- Gravity turns with FAR: best practices

- Aerobraking: best practices

- Unusual or unconventional designs and what can be learned from them (body lift)

- best speed for take off and landing.

- Building SSTO's and hypersonic vehicles

- Wings: aspect ratio, cord, sweep, shapes, flaps/spoilers.

- Controls: canards, tailplanes, stabalators, elevators.

EDIT: Does anyone know what "Pitch Setting" on the static analysis tab means? As far as I can tell, it affects the trim point and shifts the graphs around, but I don't know what it is representing (haven't run across that term in my studies, or a quick Google search ).

Edited May 21, 2014 by DresCroffgrin

[softweir]

I must admit that yes, I'm also slightly puzzled as to why Ferram wants to teach aerodynamics to each of us one by one, instead of doing it once and for all with a manual

But I can copy here something he wrote me on Reddit one day, which may not be a manual but is still VERY useful.

OK. What, however, are "good" or "bad" value for these plots? Is "big" good? Is "negative" bad? THAT'S the info we need for each and every plot, individually. I am sure a person can work some of it out, but not with any certainty and not without an excess of trial and error that a well-thought-out tutorial could avoid.

[Tiron]

OK. What, however, are "good" or "bad" value for these plots? Is "big" good? Is "negative" bad? THAT'S the info we need for each and every plot, individually. I am sure a person can work some of it out, but not with any certainty and not without an excess of trial and error that a well-thought-out tutorial could avoid.

Actually if you hover over it, it tells you. Up to a point anyway. Just...not so much on the 'how to fix it' front.

[Taverius]

Does anyone know what "Pitch Setting" on the static analysis tab means? As far as I can tell, it affects the trim point and shifts the graphs around, but I don't know what it is representing (haven't run across that term in my studies, or a quick Google search ).

Pretty sure its so you can set the pitch control input used during the simulation, ie how much, if any, you're pulling/pushing on the stick.

[Tiron]

()@% ()@*%)( @()%() @%()

I figured it out.

It's not anything with the actual layout of the plane, but purely how I built the inner wings, to get the shape I wanted.

At the time, pwings couldn't handle a shape like that(without looking REALLY stupid anyway), so I built it out of a bunch of wing pieces. Five of TVPP's NTBI wings, to be exact. Four 2x1 wings, turned sideways, plus a 2.5x2 tailfin, also turned sideways.

Something about the 2x1 wing panels being turned sideways induces a HUGE amount of N-Beta. And that's it, so far as I can tell. Time to try Pwings again I guess...sigh. This is going to be a LOT of work.

[Pyroyoshi]

Hey Ferram, I am having a strange problem and I don't know if it is a bug or something isn't right on my plane that is causing an issue. When I try to run any of the analysis tools in the editor all I get is a grid field with no lines being generated. If I run one of your craft files it works fine though. I have uploaded an imgur album with a screenshot of both so you can see what I mean. Also the plane picture does really well till I hit about 600m/s then acts very weird rolling and pitching with the slightest control input.

http://imgur.com/a/npX63

Also, I don't know if has any effect but the wings and control surfaces are procedural dynamics.

Edited May 23, 2014 by Pyroyoshi

[KerikBalm]

As camlost and phoenix_ca discussed, this is how lifting line theory models the effect of tip vortices.

The model does correctly capture the advantage that high-aspect-ratio wings have: In a high-aspect-ratio wing, a smaller percentage of your wing area is close enough to the tip to be penalized.

Ahh, I misunderstood the description of what FAR was doing then. So the way FAR works, high aspect ratio wings are indeed better (as they should be)?

I was imagining this

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(lets say each of those "\" is a swept wing, and "|" is the fusalage) would count all the wign peices as "root" peices

and this:

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|_\

|__\

|___\

(Ignore the "_"s they're just for spacing)

Would be counted as 1 root, and 3 "tip" peices, or maybe 1 root, 2 normal, and 1 tip, and would perform worse.

When in fact the first one would be counted as 4 tip peices, and the 2nd one would be counted as 3 root and 1 tip peice if I understand FARs rules correctly?

[camlost]

Ahh, I misunderstood the description of what FAR was doing then. So the way FAR works, high aspect ratio wings are indeed better (as they should be)?

I was imagining this

|\

|\

|\

|\

(lets say each of those "\" is a swept wing, and "|" is the fusalage) would count all the wign peices as "root" peices

and this:

|\

|_\

|__\

|___\

(Ignore the "_"s they're just for spacing)

Would be counted as 1 root, and 3 "tip" peices, or maybe 1 root, 2 normal, and 1 tip, and would perform worse.

When in fact the first one would be counted as 4 tip peices, and the 2nd one would be counted as 3 root and 1 tip peice if I understand FARs rules correctly?

When people talk about forms of wings they assume the same reference area. On top of that, high AR has lower induced drag, thus better gilde ratio. Low AR has better high-AoA capability and less wave drag. That's why low-speed airplanes usually have high AR wings, while modern fighters have low-AR wings.

[KerikBalm]

yes, I know the benefits of high AR wings, I just misinterpreted the desription or wha FAR does. My initial interpretation of the FAR description was that it was actually penalized high AR wings

[thorfinn]

I tried to add some stuff to the wiki (not Ferram's coefficient rules of thumb yet). I hope it can be useful, but I expect more knowledegable people to revise it.

[Volt]

I have a little question regarding neutral stability, particularly for aircraft that would be flown by many, many little computery bits IRL, like the F-16 and the Typhoon. Might it be possible to increase the normal stability of an aircraft which is neutrally stable in its normal configuration by using large flaps behind the centre of lift to move it back? This would certainly make flight easier without compromising on maneuverability. Additionally, could you use an airbrake under the rear of the fuselage to achieve something similar during approaches? I'm thinking of an airbrake whose root is nearest the nose, which swings open to maybe 20-30 degrees maximum, generating some deflection lift as well as drag and moving the Cl back.

[DivisionByZero]

I've added the start of a description of how I use the FAR windows in making craft. It probably needs a lot of help, but the process has been (kinda) working for me developing/learning how to make craft.

https://github.com/ferram4/Ferram-Aerospace-Research/wiki

I wrote it up because some people seemed to be asking for "practical" advice for making crafts and using the stability windows. Something happened with some screenshots I was going to add, so I apparently have to retake them and add them in later. Now that I think of it, though, if there are specific image requests, that might help me understand what to add.

Cheers!

[MAKC]

Regarding leading edge slats.

My understanding of them is that they're supposed to decrease the stall speed of the wings at high alpha but it seems they do not work properly or atleast the editor simulation does not recognize their effect.

I tested their effect in the editor at take-off speeds by pitching the entire craft up to high alpha with slats at maximum deployment and observed two things:

1. Cl remained constant regardless of flap (slat) setting.
   
2. The AoA for level flight even increased slightly when they were deployed which to me would seem to defeat their purpose.
   

Besides these observations I also noticed that any control surfaces placed on the leading edge drastically moves the CoP backwards for some reason, whether they are flaps are not and regardless of their deployment. Is that how they are supposed to work or am I just doing it wrong?

Edited May 24, 2014 by MAKC

[ferram4]

Wow. I take a break from the forums for a bit and a wiki pops up. You guys are awesome.

I guess I'll get to work typing up a simple explanation of the assumptions behind the static analysis (I suppose I should change the name to include "linear" in the description so that it's completely clear) and where it will and won't work. I'll also look into writing something up on the design process of the Eclipse SSTO and its earlier BSZ prototypes to add to the wiki, since that was something I was planning on doing anyway. Only criticism is that the coordinate system diagram in the Definition of Terms and Symbols is terrible and confusing. Good as a stopgap, terrible in the long term.

I'll try getting some of that squared away, but I've got an job interview on Tuesday to prepare for, so that takes priority.

@Pyroyoshi: That means that something broke horribly in the simulation. I need a copy of the output_log.txt to trace back where the error is (hopefully). As for your flight troubles, they're probably unrelated; you've got no yaw stability on that craft, so at high speeds it's going to sideslip everywhere and cause massive problems; attach a larger vertical tail (much larger) and see if that helps.

@MAKC: That's correct for leading edge slats; they reduce Cl at a specific AoA very slightly, but should increase the maximum AoA the plane can reach before stalling. They're not there to increase lift, they're there to allow the wing reach a higher AoA before stalling, thus increasing maximum Cl by extending the linear portion of the Cl vs. AoA curve.

[Tic-Tac]

Wow, thanks for the wiki, I haven't played with this mod for about 5 months, and my old planes just don't work anymore, the helps there are... helpful

[MAKC]

@MAKC: That's correct for leading edge slats; they reduce Cl at a specific AoA very slightly, but should increase the maximum AoA the plane can reach before stalling. They're not there to increase lift, they're there to allow the wing reach a higher AoA before stalling, thus increasing maximum Cl by extending the linear portion of the Cl vs. AoA curve.

Now I get it.

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Did a quick test to confirm and they do indeed work. Thanks, man.

[DaMichel]

I always found slats to behave in odd ways - for my layman level of knowledge ofc.

On wings like that they move the CoL back instead of forward and not by a small amount.

[MAKC]

I always found slats to behave in odd ways - for my layman level of knowledge ofc.

http://i.imgur.com/LGgWLvU.jpg

On wings like that they move the CoL back instead of forward and not by a small amount.

Yes, I've also noticed strange behavior with swept/deltas.

[Volt]

Quick question. Are drag values like these: expected? Because this aircraft is ridiculously slippery. It has an amazing glide ratio, considering it's a jet bomber.

Sorry for the huge picture today.

Edited May 24, 2014 by Volt

[Tiron]

Wow. I take a break from the forums for a bit and a wiki pops up. You guys are awesome.

I agree with the last part of the statement, although I'm unsure how much the fact you weren't around had to do with it

Mostly me whining and Taverius pointing out there was an empty wiki that would be perfect for such things, and then awesome people jumping all over it.

[NathanKell]

Volt, you're not going very fast. Seems about right.

[Pds314]

WOW! The dynamic pressure changes really make a big difference. I got the Hypersonic Demon to mach 2+ at sea level and anything more than a slight tap caused it to lose wingtips, then wings, then disintigrate and blow up the cockpit in mid-flight.