Official FAR Craft Repository

[capi3101]

I imagine it would be a challenge but with the right mods, it shouldnt be impossible. Would be a fun challenge.

I totally agree.

[woot]

So I have a little jet fighter that I really enjoy flying -- it pulls 30+ Gs regularly without issue (although it flies apart on demand if I roll during one of those turns). The problem is that it doesn't handle as precisely as I'd like. It suffers from a bit of roll instability, particularly when combined with pitch, and just in general isn't quite as responsive as I'd like

FYI, the horizontal stabilizers pitch at 12, the elevons are non-functional, the ailerons roll at 20, and the vertical stabilizers yaw at 9.

Anybody have any advice? If this were your craft how would you fix it up? I tend to keep it subsonic and between 0 and 3km altitude, mostly looping around KSC and shooting missiles (inaccurately) at some tanks.

As a secondary issue, if anyone knows why the transonic area ruling analysis in FAR isn't oriented correctly it'd be nice to get that fixed.

I can post the craft file on request, although it requires several mods.

[capi3101]

I imagine the cockpit and missiles aren't being voxelized properly by FAR; it'd probably explain most of the issues with the craft outright. Which mod are those parts from?

Let's start with that - which mods are you using?

And let's go ahead and run a U simulation with initial value of 5, and then a ß simuation also with an initial value of 5, using the 1km/Mach 1 derivatives. Post screenies of both graphs.

Myself, I'd probably lengthen the fuselage a bit, but I'll defer offering that as advice for the time being.

Edited July 10, 2015 by capi3101

[Hodo]

So I have a little jet fighter that I really enjoy flying -- it pulls 30+ Gs regularly without issue (although it flies apart on demand if I roll during one of those turns). The problem is that it doesn't handle as precisely as I'd like. It suffers from a bit of roll instability, particularly when combined with pitch, and just in general isn't quite as responsive as I'd like

http://i.imgur.com/jbwrmY1.jpg

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

http://i.imgur.com/pJ06caY.jpg

FYI, the horizontal stabilizers pitch at 12, the elevons are non-functional, the ailerons roll at 20, and the vertical stabilizers yaw at 9.

Anybody have any advice? If this were your craft how would you fix it up? I tend to keep it subsonic and between 0 and 3km altitude, mostly looping around KSC and shooting missiles (inaccurately) at some tanks.

As a secondary issue, if anyone knows why the transonic area ruling analysis in FAR isn't oriented correctly it'd be nice to get that fixed.

I can post the craft file on request, although it requires several mods.

Extend the fuselage length, right now your aircraft is short and fat, which is not good for a fighter or an aircraft at all. Basically you want a longer aircraft, not necessarily thin but longer sleeker look.

Like this SSTO Fighter for example, notice how it is smooth and longer.

[woot]

I imagine the cockpit and missiles aren't being voxelized properly by FAR; it'd probably explain most of the issues with the craft outright. Which mod are those parts from?

Let's start with that - which mods are you using?

And let's go ahead and run a U simulation with initial value of 5, and then a ß simuation also with an initial value of 5, using the 1km/Mach 1 derivatives. Post screenies of both graphs.

Myself, I'd probably lengthen the fuselage a bit, but I'll defer offering that as advice for the time being.

The cockpit is quiztech aero, the wings are b9 procedural, the missiles and whatnot are obviously BDArmory. It also uses adjustable landing gear and has an ejection module from Vanguard Tech. I've used this cockpit before and it seems to voxelize properly:

Here are the stability derivative simulations (let me know if I did these wrong; I don't know how to interpret these ones)

[blowfish]

The cockpit is quiztech aero, the wings are b9 procedural, the missiles and whatnot are obviously BDArmory. It also uses adjustable landing gear and has an ejection module from Vanguard Tech. I've used this cockpit before and it seems to voxelize properly:

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

Here are the stability derivative simulations (let me know if I did these wrong; I don't know how to interpret these ones)

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

http://i.imgur.com/ITt8ECu.jpg

Check inside the cockpit to make sure it's filled with voxels. I know there were issues with the QuizTechAero parts previously.

[Hodo]

The cockpit is quiztech aero, the wings are b9 procedural, the missiles and whatnot are obviously BDArmory. It also uses adjustable landing gear and has an ejection module from Vanguard Tech. I've used this cockpit before and it seems to voxelize properly:

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

Here are the stability derivative simulations (let me know if I did these wrong; I don't know how to interpret these ones)

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

http://i.imgur.com/ITt8ECu.jpg

The problem is your craft is short and stubby, like I said before. You have no length to the craft so the air is hitting the nose and basically folding around the aircraft itself leaving nothing but turbulence for the plane to fly through.

[Azimech]

I love soviet style WW2 bombers.

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[woot]

I figured out that the proximal set of AMRAAMs were causing the simulation issues. I also added 1m to the fuselage length:

Is that long enough? I'm trying to find the length that maximizes stability but doesn't sacrifice any maneuverability.

[capi3101]

Yeah, that should do the trick. You've got a pretty low overall wave-drag area and good non-oscillatory dynamic stability; how does it handle with the increased fuselage length?

[Hodo]

I figured out that the proximal set of AMRAAMs were causing the simulation issues. I also added 1m to the fuselage length:

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

Is that long enough? I'm trying to find the length that maximizes stability but doesn't sacrifice any maneuverability.

Looks MUCH better.

And adding length does not always sacrifice maneuverability. That is a factor of mass, CoL vs CoM position and atmospheric density at that altitude and speed.

[FourGreenFields]

And adding length does not always sacrifice maneuverability. That is a factor of mass, CoL vs CoM position and atmospheric density at that altitude and speed.

Adding length usually adds mass too though.

And mass distribution also effects maneuverability to a smaller degree, which is usually also influenced by length.

Edited July 11, 2015 by FourGreenFields

[VentZer0]

Ported over my SU27 design to 1.0.4 with the latest FAR.

I got some stability and AOA issues. My craft gets airborne beyond 120m/s which is far to fast for my liking. It also makes landings ... interessting. At level flight I have to pitch up 15° at 400km/h to maintain altitude.

Is there anyway to increase lift without changing my design?

Please watch the video for more details

[tetryds]

@VentZero, your post has been moved from the FAR mod thread.

Please keep craft sharing and design discussion to this topic to not cluster the mod topic.

[FourGreenFields]

Ported over my SU27 design to 1.0.4 with the latest FAR.

I got some stability and AOA issues. My craft gets airborne beyond 120m/s which is far to fast for my liking. It also makes landings ... interessting. At level flight I have to pitch up 15° at 400km/h to maintain altitude.

Is there anyway to increase lift without changing my design?

Please watch the video for more details

Watching the video is a bit difficult with a 5kB/s connection.

So just some stuff that may, or may not help:

- Reduce fuel load

- Add bigger flaps, or (if possible without causing stalls) increase flap deflection

- Add (bigger) slats (= deflectable leading edge)

- Maybe reducing wing mass/strength is also an option, but that can ofcourse easily lead to problems for a design like that.

Without screenshots I can't give more tips atm though.

Edited July 11, 2015 by FourGreenFields

[VentZer0]

Watching the video is a bit difficult with a 5kB/s connection.

Ouch. Analog modem, they still exist?

I tried to compress most of the screenshots

http://imgur.com/a/fMXkV

The craft has a TWR > 1 at 27tons, which is a realistic weight for the SU27.

Note that the monopropellant tanks are there to balance the weight of the craft.

I used B9 procedural parts and editted some of them to fit my needs, resulting in a liftless procedural structure (basically the part that holds the craft together) and the wings which I made to specs of the SU27 considering MAC, semispan, taper ratio and so on. The engines are also custom, they're supposed to mimik the Saturn AL31FM which is one of the engine variants of the AL31 family that the SU27 uses (145kN each, 1.57tons, vectorthrust of 15°).

The stabilty issue I have is shown in the video, hard to see in screenshots. Basically I turn hard at about 650km/h the AOA reaches > 70° and then the plane flips from ~90° banked left or right to 180° upside down

Edited July 11, 2015 by VentZer0

[Hodo]

Ouch. Analog modem, they still exist?

I tried to compress most of the screenshots

http://imgur.com/a/fMXkV

The craft has a TWR > 1 at 27tons, which is a realistic weight for the SU27.

Note that the monopropellant tanks are there to balance the weight of the craft.

I used B9 procedural parts and editted some of them to fit my needs, resulting in a liftless procedural structure (basically the part that holds the craft together) and the wings which I made to specs of the SU27 considering MAC, semispan, taper ratio and so on. The engines are also custom, they're supposed to mimik the Saturn AL31FM which is one of the engine variants of the AL31 family that the SU27 uses (145kN each, 1.57tons, vectorthrust of 15°).

The stabilty issue I have is shown in the video, hard to see in screenshots. Basically I turn hard at about 650km/h the AOA reaches > 70° and then the plane flips from ~90° banked left or right to 180° upside down

One answer I can give you from the video was the snap roll stall that the craft went into at one point in the video. That is because your craft lost lift over one wing before the other, caused the craft to spin. This is a common type of stall in fighters. The fix for it is to actually limit the AoA of the craft in some situations, mostly pilot skill. Real aircraft use automatic slats to help maintain stability in high AoA maneuvers, which isn't possible yet in KSP.

[kcs123]

Nothing unusual for such harsh maneuver.

While you have tried to replicate real life plane you have most probably missed some of details that can change flight behaviour a lot. For examle, how far is control surfaces on tail from main wing ?

Even few cm could make a big difference in behaviour, translated into game it could be just pixel away in part placement.

Your plane suffering from sideslip/yaw issues, that is obvious, based on provided graph data. Try to move vertical tail surfaces slightly further back and maybe slightly bigger that will not going to be much different from real plane if your goal is to replicate plane as much accurate as possible.

Other thing that you can try is to replace two vertical surfaces with just one, move it further back, since you already have needed tail section for that and make that surfaces bigger. Although it will no longer be replica of real life plane.

[VentZer0]

One answer I can give you from the video was the snap roll stall that the craft went into at one point in the video. That is because your craft lost lift over one wing before the other, caused the craft to spin. This is a common type of stall in fighters. The fix for it is to actually limit the AoA of the craft in some situations, mostly pilot skill. Real aircraft use automatic slats to help maintain stability in high AoA maneuvers, which isn't possible yet in KSP.

Makes sense, >70° AOA is not desireable at all, at least in a turn. My problem with AOA limiter is the way to tune the PID so that there's no rocking anymore. Any tips? I tried the Ziegler-Nichols Method to tune the PID but that didn't get the desired effect yet.

How could I increase overall lift of the plane, so that I can takeoff and land at lower speeds without tailstrikes due to high AOA?

[Hodo]

Makes sense, >70° AOA is not desireable at all, at least in a turn. My problem with AOA limiter is the way to tune the PID so that there's no rocking anymore. Any tips? I tried the Ziegler-Nichols Method to tune the PID but that didn't get the desired effect yet.

How could I increase overall lift of the plane, so that I can takeoff and land at lower speeds without tailstrikes due to high AOA?

If I were at home I would post my exact PID tuner settings I use for almost everything. And I dont have that rocking anymore on any aircraft unless it is at its edge of flight envelope.

I know the real Flanker has leading edge slats that auto deploy on low leading edge pressure situations. This helps it maintain control in high AoA situations, there is also an audible alarm that goes off in the cockpit that warns the pilot of an impending stall.

I would also suggest running and using Kerbal Joint Reinforcement, it will help for structural strength without over abundance of struts.

As for the take-off and landing speeds, the trick I use is leading edge slats and flap use. Again the real Flanker uses both for take off and landing.

Your take off speed should be around 140-180kts depending on load of craft. I use knots for my increments of speed it is easier to find that information.

Landing speed should be about 120-140kts again depending on the current load on the craft.

Almost all of my craft take off and land at those speeds except my Mig-21 SSTO, it has a 200-230kts take-off and a 180-200kts landing speed, much like the real Mig-21.

Edited July 11, 2015 by Hodo

[FourGreenFields]

Ouch. Analog modem, they still exist?

Internet stick, and if I pass a certain ammount of download volume it puts a limit on my connection.

Pictures usually aren't a problem, just takes a while to load. But vids... not a chance.

Anyway, I know it's annoying with Adjustable Landing Gear, but calculating stuff with raised gear is usually a good idea (except for calculating performance for landing approach, etc. ofcourse).

The stability issues do sound and look alot like an asymmetric stall -> snap roll (wouldn't call it spin unless it keeps spiraling down, instead of instantly recovering). Thick wings allow you to pull higher AoAs, but would prob skyrocket the wave drag area on your design. Otherwise, what Hodo said (it kinda is possible to do automatic "slats" though, just need to set the leading edge to negative AoA deflection).

However, do you actually pull 70° AoA? Or do you pull till less, and then the nose suddenly lifts? If so, your tail stalls first, loses lift, and now that your tail wings aren't countering anymore, your wings will pull the nose up. Possible solutions would be thicker tail wings, or tail wing slats. Or not pulling that tight.

With B9PW it shouldn't be a problem to add slats/make the slats bigger. Same for flaps.

Set the leading edge control surfaces (if I'm not mistaken your leading edge is already a control surface) to either spoilers, or flaps, then modify the custom action groups so you can control them. They should deflect up at 0° AoA, and then deflect towards the airflow (-AoA deflection, I think you already use that) so they don't cause a stall. That should increase your lift a bit. Don't expect too much though.

EDIT: You could ofcourse also rotate the slats, instead of letting them deflect as flaps/spoilers.

Edited July 11, 2015 by FourGreenFields

[woot]

Yeah, that should do the trick. You've got a pretty low overall wave-drag area and good non-oscillatory dynamic stability; how does it handle with the increased fuselage length?

Well it seems a lot worse. I get stall oscillations in pitch, it seems generally sluggish, and I haven't been able to pull even 20Gs. And since I removed most of the armaments, it's actually lighter than it was before, so it seems that the extra fuselage length is likely to blame. Any other ideas?

[Hodo]

Well it seems a lot worse. I get stall oscillations in pitch, it seems generally sluggish, and I haven't been able to pull even 20Gs. And since I removed most of the armaments, it's actually lighter than it was before, so it seems that the extra fuselage length is likely to blame. Any other ideas?

You know there isn't a fighter on Earth that pulls 20Gs. That is a LOT of G forces.

What is your control pitch set at? Is it at the default 20deg? If so there is one of your problems right there. The craft is "overcontrolling".

[woot]

You know there isn't a fighter on Earth that pulls 20Gs. That is a LOT of G forces.

What is your control pitch set at? Is it at the default 20deg? If so there is one of your problems right there. The craft is "overcontrolling".

I'm aware that any real-life pilot would be dead, but that doesn't seem relevant. As mentioned above, the horizontal stabilizers have 12degree RoM.

[Hodo]

I'm aware that any real-life pilot would be dead, but that doesn't seem relevant. As mentioned above, the horizontal stabilizers have 12degree RoM.

I am not talking about the pilot, I am talking about the airframe itself.

Anyway, 12deg. Ok are you flying with SAS turned on?