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

[Wanderfound]

How? You say 'not too excessive wing area' - how do I figure out whether I have too much wing area? Or too little, for that matter?

By FAR analysis tools and test flying. If you're going to get these pitch-up problems, you'll have bad pitch derivative numbers and a misplaced Cm line. Just the AoA sweep that you posted alone is enough to say that it was going to go heavily unstable at moderate AoA; you can get away with a hint of Cm inflection if you've got sufficient pitch authority, but once it starts shooting up like that it's game over.

Have you had a poke at the "How to use FAR graphs" part of the Kerbodyne build tutorial?

"Too much" vs "too little" is always tricky. There does seem to be a fair tendency for KSP builders to err on the side of "too much", though. Too little wing means high AoA, high takeoff speed, slow turning. Too much wing is excess weight, excess drag, excess fragility and possible aerodynamic complications.

As a general rule, the more conventional the airframe, the easier it is to get the aero right. There's a reason they built them that way. The benefit of a delta is that it's very good at maximising lift while minimising drag; the tradeoff is that deltas have crappy stability. You didn't see a lot of successful deltas in the pre fly-by-wire days.

You can make most things work with enough fiddling, but when in doubt, use the design clichés.

It also helps to think about the problem in terms of basic forces. If it's pitching up, there's too much lift at the nose and not enough at the tail. So, either remove lift from the front, add lift to the back, or extend the nose or tail to alter the position of the lift relative to CoM (thereby increasing or reducing the leverage).

Similarly, if it's yawing, you need more tailfin drag at the back. Roll instability calls for wingtip extensions or dihedral.

Edited November 23, 2014 by Wanderfound

[Iskierka]

As another alternative, try very narrow deltas. Concorde is a good example - with its very low aspect ratio, it retains a very high control margin against the CoM, despite only having one surface. (Plus, the single surface and smooth shaping give it very good L/D, at least for such a low AR) Last Concorde replica I made was one of the most stable aircraft I've made in KSP - granted, it wasn't space-rated due to the strange vertical CoM/thrust offset, and it probably couldn't hold high AoA on aerodynamics alone, but for simply a plane, it's a pretty good design.

[steve_v]

Hodo, that last pic you posted appears to feature some kind of runway lighting or approach guidance - may I inquire as where you got it?

If it's what I think it is... I've been wanting that forever.

[ferram4]

Something to keep in mind is that the idea behind the CoL / CoP / Aerodynamic Center idea is that of linear aerodynamics; forces change approximately linearly with angle of attack, and that can be used to work back a position where all those forces are applied. When the aerodynamics are highly non-linear, this leads to situations where the CoL / CoP / Aerodynamic Center shifts heavily with angle of attack, and definitely cannot be expected to stay in the same position for all angles of attack. This is especially true for hypersonic flight; in subsonic and supersonic flight, Lift ∠AoA. However, in hypersonic flight (read, as M -> ∞), Lift ∠cos(AoA) * sin²(AoA), which is highly nonlinear and is the reason for the large changes in stability with increasing AoA for most of these designs. This is why checking the slope of the Cm line (it should be negative) is a better solution than just relying on the indicator.

[Jovus]

Thanks, guys. For what it's worth, I know about the tools pretty well (knowing for example that a positive sloping CoM is bad, which is why I posted it first) because I've been listening to you knowledgeable folks whenever you give anyone advice about airframes. What I've been struggling with is how to apply said advice; e.g. I know that positive slope on your CoM is bad, but not how to design to fix it.

That said, you've given some pretty good advice in these last posts, so thanks!

[Hodo]

Hodo, that last pic you posted appears to feature some kind of runway lighting or approach guidance - may I inquire as where you got it?

If it's what I think it is... I've been wanting that forever.

Its from the Kerbinside plugin. Adds lots of stuff including other launch sites and buildings around KSC.

[ferram4]

Some of you may have noticed that FAR includes many fewer example crafts than it used to as a consequence of the SP+ part integration. Since I've always considered example crafts to be good things to have so that people can reverse-engineer them, I've built a few real-life replicas with stock parts that people can play with in the next FAR release:

Guess which is which.

[SpaceJuice]

I love this mod! I use it to disintegrate my rockets.

Edited November 23, 2014 by Spacejuice
I didn't space the i and use

[Hodo]

Kind of funny Farrem I just did a pic like that for my new fleet of SSTO combat craft.

Oddly enough they are all FAR craft, and work quite well.

[Cairan]

Some of you may have noticed that FAR includes many fewer example crafts than it used to as a consequence of the SP+ part integration. Since I've always considered example crafts to be good things to have so that people can reverse-engineer them, I've built a few real-life replicas with stock parts that people can play with in the next FAR release:

http://i.imgur.com/g2NKbLs.png

Guess which is which.

From left to right:

MiG-21

F-18 Hornet

F-4 Phantom II

MiG-19

Edited November 23, 2014 by Cairan

[Van Disaster]

Hodo, that last pic you posted appears to feature some kind of runway lighting or approach guidance - may I inquire as where you got it?

If it's what I think it is... I've been wanting that forever.

The old PAPI mod still works, and there is a popup HUD tool with various glide path indicators on it also.

Jovus - you have too little wing area when you find yourself flying at high AoA a lot - that is a high drag situation which means you won't be getting the most out of your propulsion. I've yet to find a really too-much situation because that just means I can add more payload however if it's a small craft then it might be hard to control because it's resistant to changes in direction in some way or it's slower than it should be without high AoA as a cause. People tend to err towards too little far than too much, though so I would not really worry about overdoing wing, especially for a larger/heavier craft.

Edited November 23, 2014 by Van Disaster

[ferram4]

@Cairan: Got the first one right, strikes on all the others. The final one I suppose is fair, without seeing the engines you miss its most distinctive feature.

[DaMichel]

Cool, Ferram4.

My guess:

2) http://en.wikipedia.org/wiki/Saab_Draken

3) Never seen this one ...

4) could be http://en.wikipedia.org/wiki/F-86 edit: http://en.wikipedia.org/wiki/English_Electric_Lightning

Edited November 23, 2014 by DaMichel

[HoneyFox]

The third one looks like F-101 Voodoo.

[Dr. Death]

Some of you may have noticed that FAR includes many fewer example crafts than it used to as a consequence of the SP+ part integration. Since I've always considered example crafts to be good things to have so that people can reverse-engineer them, I've built a few real-life replicas with stock parts that people can play with in the next FAR release:

http://i.imgur.com/g2NKbLs.png

Guess which is which.

the one on the left seems to be a MIG-21, or a french plane. The one on the middle-right its an F-4 phantom. Do i get my 64-bit support now?

[Hodo]

Some of you may have noticed that FAR includes many fewer example crafts than it used to as a consequence of the SP+ part integration. Since I've always considered example crafts to be good things to have so that people can reverse-engineer them, I've built a few real-life replicas with stock parts that people can play with in the next FAR release:

http://i.imgur.com/g2NKbLs.png

Guess which is which.

From left to right.

Mig-21, Drakken, F-101 Voodoo, F8 Crusader or a Mig-19 or Electric Lightning, Hard to tell on the last one due to the angle.

Edited November 23, 2014 by Hodo

[ferram4]

From left to right it's MiG-21, Saab 35 Draken, F-101 Voodoo (can be differentiated from the F-4 by the lack of dihedral wingtips, the change in sweep on the trailing edge, the T-tail, and the attempt at triangular wingroot intakes), and the English Electric Lightning (differentiated from the other options by the straight trailing edge near the tips of the wing and the tail; the vertically mounted engines and the height of the fuselage aren't quite clear).

The Lightning is fast, but I don't think i got the characteristics quite right, since it can be a little finicky. The MiG-21 is very fast for its size, but gets a little unstable at high AoA due to the strong dihedral effect from the swept wings. The Voodoo flies well, until you get it into a high AoA spin, in which case it's a deathtrap (which seems accurate, woo hoo!). The Draken is apparently completely awesome and only gets problems if you get it into a flat spin, in which case you are also doomed.

[ferram4]

Okay, new stuff, have fun; version 0.14.4 is out! Details in the changelog, as always.

[Hodo]

Okay, new stuff, have fun; version 0.14.4 is out! Details in the changelog, as always.

Awesome!

Ferram4, I think you do this because you are trying to punish yourself for a past life transgression.

Or you are just sick and twisted and are a perfectionist like me.

[Jovus]

Stability Deriv tab now takes entry in terms of planet, altitude and Mach Number, not density, temperature and Mach number

!!!

So lovely.

[Guest]

Nice update. How's the wing code overhaul going? I'm really excited for this.

[Beetlecat]

Auto-deflection? That's awesome!

[BudgetHedgehog]

Nice update. How's the wing code overhaul going? I'm really excited for this.

Indeed - the CSS dearly needs proper FAR compatability.

[Drew Kerman]

last revision of the day:

http://imgur.com/a/SW7mZ

Somehow I got the green line to curve way up. I think it was after I moved the main wings just a smidgen back to allow the air stair better access to the hatch. It wasn't the science pods, I re-analyzed with them off and got the same performance. Crazy - I now have so little drag I've had to do power-off approaches and install air brakes.

I've also gone and added little details, like nav/strobe/landing lights.

Just loaded this up in 0.25 the first time with latest FAR (yes, the one that dropped today) and my cockpit is tearing of around mach 0.3. I checked the cockpit part config and the stack node size was 0. I changed it to 3, saw the ball get bigger in the SPH. No effect, still rips off at mach 0.3. Fuselage node looks like a 2 (it's scaled with TweakScale, so don't know for sure - I downgraded to 1.45). Node issue was my best guess, not sure what else to look at :/ This never happened under 0.24.2 with prev FAR versions.

Kinda funny watching the headless ship take flight as the cockpit rolls down the runway tho, lol

[ferram4]

@Dragon01: Currently working on an algorithm to find the union of polygons in the same plane. Then comes combining polygons in different planes, but that do end up intersecting. Then comes figuring out how to model that in subsonic and supersonic flow. It's getting there though.

@Gaiiden: Well, nothing changed regarding aerostructural failure or forces applied in the recent update, so I don't know what's causing the issue. This is why you update quickly, because I have no idea where this issue might be coming from, since FAR 0.14.1 is old now.