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Pitch the craft up in the hangar & check the numbers again with takeoff speeds - could be a few things happening; CoM shift rearwards relative to aero centre as it pitches ( if your wing centre is below CoM that is common ), or excess body lift from the nose for example. Also, your CoL will be further forwards at slow speed.

Edited by Van Disaster
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Tried pitching up the aircraft, CoP surely moves forwards relative to CoM as expected. At sea level, take off speed and 15 degrees AoA, pitching moment (Mw) turns positive. So does Mq (pitching moment not damping out) and Xu ("If Xu is incorrect, you have summoned the Kraken and should file a bug report." - Ferram).

I don't know what to do about the Xu derivative. Also I don't understand why the pitching moment turns positive. (especially since static analysis suggests it's negative up to 30 deg AoA.) Any apparent design flaws? Horsontal stabilzer too far up?

I'm a bit suspicious of the nosecone. I swapped it out for another one and moved the main wing back, which yield a pretty stable aircraft, too stable at supersonic speeds IMO.

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The image with the red derivatives is showing a stall, or at minimum rubbish numbers - AoA is out of bounds of the simulation so it's showing 0 - so I'd not worry about that too much.

For something to try - remove the pitch control surfaces, balance the craft and put them back on again. If I'm having real trouble I do that, otherwise I generally guesstimate Mw based on the area of my pitch controls. At the moment for subsonic BDA combat craft that's usually somewhere around -0.1 at the default settings for the analysis panel.

Edited by Van Disaster
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Thank you. I'll try that.

Btw, as we are waiting for Adjustable Landing Gear to be updated, do you guys have any particular tricks for area ruling with the bulky stock gear? I always tend to put on the landing gear last, ruining everything. :)

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2 hours ago, ThorBeorn said:

Thank you. I'll try that.

Btw, as we are waiting for Adjustable Landing Gear to be updated, do you guys have any particular tricks for area ruling with the bulky stock gear? I always tend to put on the landing gear last, ruining everything. :)

Don't put it on last :P but really, nothing much - sticking proc nosecones on helps sometimes - you can stick them directly to the gear if you use octagonal struts & offset after, the absolute rotation offset mode is ultra-handy to straighten everything - or building better shaped enclosures with proc parts but the few times it's mattered ( contests when I can't use ALG for instance ) I've just tried to use the stock gear enclosures to fill area ruling holes.

Edited by Van Disaster
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On 5/1/2016 at 4:17 PM, ThorBeorn said:

I thought I'd turn to this thread for some help with a plane I built. I've been using FAR for years and I've run into a problem where my plane is unstable in pitch, wanting to backflip/frontflip as soon as it has any positive/negative AoA after taking off. We've all seen this before. However I don't understand why, all derivatives suggests it's stable as does the static analysis. Moving the main wing back helps ofcourse, but I shouldn't have to right? Any ideas?

I think I remember once reading a comment that if the blue ball has an arrow to it then go reinstall FAR.

I agree with you that the derivatives does not reveal the pitch instability you experience. Does it happen equally strong (or stronger) if you rotate nose up later down the runway and at all throttle settings?

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8 minutes ago, Rodhern said:

I think I remember once reading a comment that if the blue ball has an arrow to it then go reinstall FAR.

That is a good point I keep forgetting - it doesn't necessarily mean "reinstall FAR", but at least one part isn't being configured for FAR. Odd given how FAR picks up practically everything, but occasionally parts fall through.

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You're right I also remember Ferram saying that, but I think he reintroduced the arrow due to people requesting it. Could be wrong though. Are you saying that your blue balls are lacking said lift vector? :P

I've been installing mods manually for years, it shouldn't be that. And I'm not using many mod parts on this craft, perhaps it's the nose cone from SXT. But yeah, the numbers suggest almost neutral pitch stability yet it's obvious they are positive.

Regarding take off speed, it becomes less unstable the faster it goes and becomes statically stable above mach ~1.2. Engine thrust doesn't have any noticable effect, it's vector is slightly above CoM even I reckon.

 

Edit: I just started up a fresh game with only FAR installed. No lift vector on CoL here. Interesting...

Edit2: Turns out it's the cockpit from Quiztech causing the issue. I guess I'll report this to Ferram or Quiznos, if it's not due to Quiztech not being updated since 1.05.

Funny all my other planes not using this cockpit have been behaving quite predictably during high AoA according to my previous experience.

Edited by ThorBeorn
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That part will be using stock drag/lift, so it's not going to be particularily predictable :P generally sorting parts out for FAR is pretty simple, just compare the cockpit cfg with another 3rd-party one like one of Baha's & knock up a modulemanager patch to sort it.

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19 hours ago, Rodhern said:

I think I remember once reading a comment that if the blue ball has an arrow to it then go reinstall FAR.

I have no arrow on that blue ball and I have reinstalled FAR 2 times. Should I bee worried?

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Hi all! I'm brand new to FAR, but after lots of reading/experimenting I've arrived at a pretty successful design. She's stable all the way to trans-orbital velocities (did I make that word up?), but I'm having some issues with sideslip at 120-130m/s, which makes takeoff and landing pretty precarious. I'm wondering if there's anything I can do to fix that?

vnix1ri.jpg

Edited by SpiralOut
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Usually, wide body parts like this, it's more stable to have 2 yaw stabiliser instead of 1 like you can see on the f-22 for instanc, and it sometimes also increases you l/d if you angle it a little.

Also if you use an all moving yaw its going to be more unstable that a classic control surface

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18 hours ago, SpiralOut said:

Hi all! I'm brand new to FAR, but after lots of reading/experimenting I've arrived at a pretty successful design. She's stable all the way to trans-orbital velocities (did I make that word up?), but I'm having some issues with sideslip at 120-130m/s, which makes takeoff and landing pretty precarious. I'm wondering if there's anything I can do to fix that?

It's common for people new to FAR to not use enough yaw stabilization. It's not simply a case of throwing a big fin on the plane - sometimes you don't need a big fin! but some basics:

* Control force ( moment, to be a little more accurate ) is just basic lever physics, so there's two parts: the arm, which is the distance of the surface from CoM, and the force which is proportional to the size of it * airspeed.
* Lots of things count here: overall lift usually doesn't act straight through CoM, so a proportion of it is trying to rotate the craft around CoM. In this case we're talking about yaw, so if the craft starts yawing then the body being at an angle to the airflow will cause a rotation force on each side of CoM. Usually there's more force in front of CoM than behind, so we need a fin - in your case CoM is quite a long way aft and the distance from CoM to your fin is not so great, so you will need a bigger fin for both reasons.
* All-moving surfaces are not as naturally stable as fixed ones.
* The red derivatives do matter - Lr is usually fixed by fiddling with dihedral... BUT: your panel is showing your craft being stable at 27 degrees AoA, which is almost out of bounds of the simulation! at nearly 120m/s that is a really large AoA. I'd recommend adding some more wing before you do too much more fiddling.

You don't really need quite that much incidence for a spaceplane, you might find it ends up pitched down a bit at very high speed & you'll have negative body lift at that point. For spaceplanes usually my main wing's angle of incidence is zero & it cruises naturally slightly pitched up, engines & all. On the other hand I take the view that spaceplanes are only vaguely aircraft.
 

Edited by Van Disaster
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  • 2 weeks later...

Missing those landing gear too. I wasn't being able to play KSP for a long time. Decent number of my favorite mods are still not updated for 1.1.2.
I was inspired by the look of Gripen. I didn't created exact replica, but interesting thing that I noticed on Gripen is lack of tail elevators. It uses elevators on wings and canards in fronts. Engine and vertical tail surface is much further back.

I tried something similar and it turned out pretty well. Please note that it is rough example because I didn't have much time to fine tune everything. Don't know when I will be able to grab some free time for KSP, so I deceided to upload craft files regardless, in this unfinished state.

There was demand for examples of flap usage and stable but maneuverable crafts, so there it is my attempt to create such.

Here are some screenshots from test flights. Video might be more interesting, but I can't create one properly. You can download craft files and try for yourself, though. Mod required is B9 Aerospace, B9 PW, FAR , KJR and dependencies. Everything else is stock.

 

 

Edited by kcs123
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On ‎5‎/‎22‎/‎2016 at 6:43 PM, kcs123 said:

Missing those landing gear too. I wasn't being able to play KSP for a long time. Decent number of my favorite mods are still not updated for 1.1.2.
I was inspired by the look of Gripen. I didn't created exact replica, but interesting thing that I noticed on Gripen is lack of tail elevators. It uses elevators on wings and canards in fronts. Engine and vertical tail surface is much further back.

I tried something similar and it turned out pretty well. Please note that it is rough example because I didn't have much time to fine tune everything. Don't know when I will be able to grab some free time for KSP, so I deceided to upload craft files regardless, in this unfinished state.

There was demand for examples of flap usage and stable but maneuverable crafts, so there it is my attempt to create such.

Here are some screenshots from test flights. Video might be more interesting, but I can't create one properly. You can download craft files and try for yourself, though. Mod required is B9 Aerospace, B9 PW, FAR , KJR and dependencies. Everything else is stock.

 

 

On a delta wing design you really don't want flaps like you have.  All they would end up doing is pushing the nose down more than creating lift.   I found using leading edge slats with flaps helps far more on a tailless delta wing design.  

 

Lately I have been testing variable geometry wing systems with stock parts + IR.   So far I have found that a simple Mig-23"ish" design is capable of mach 2.3 at 1km alt and still be quite agile, 7g turns.  And capable of 80m/s landing speeds when wings are at their normal position.   I can use flaps normally on it and it flies like a dream at low speeds and quite well at high speeds.  Really happy with the design testing.  I will post pictures later when I am at home. 

 

 

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12 hours ago, Hodo said:

On a delta wing design you really don't want flaps like you have.  All they would end up doing is pushing the nose down more than creating lift.   I found using leading edge slats with flaps helps far more on a tailless delta wing design.

That is why this plane have canards too :). I never found flaps useless, regardless of wing shape, is it delta, straight or swept wing. You need to set other control surfaces properly to counterpart negative pitching down effect from flaps. I have used slats only on some heavy weight crafts where flaps were not enough because of overall craft weight, just to lower down pitching momentum, so craft can be easier to control.

I found pitching down effect from flaps quite usefull. Designs with slightly tilted down nose worked best for me on take off landing procedures, less likely that craft will bounce during takeoff or steer left and right without controle.

Here is small album that explain how I use flaps on my designs.

 

 

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4 hours ago, kcs123 said:

That is why this plane have canards too :). I never found flaps useless, regardless of wing shape, is it delta, straight or swept wing. You need to set other control surfaces properly to counterpart negative pitching down effect from flaps. I have used slats only on some heavy weight crafts where flaps were not enough because of overall craft weight, just to lower down pitching momentum, so craft can be easier to control.

I found pitching down effect from flaps quite usefull. Designs with slightly tilted down nose worked best for me on take off landing procedures, less likely that craft will bounce during takeoff or steer left and right without controle.

Here is small album that explain how I use flaps on my designs.

 

 

The thing is I think your wings have to much sweep for them and the overall design is short.  While it is a medium weight craft in my books, you have a good take off speed 120-140m/s.  That is pretty usual for most modern aircraft.   My larger SSTOs have a take off speed of 150-180m/s. 

JxF9aHI.jpg

 

bVYYUdR.jpg

 

The wing designs on most of my previous version aircraft followed this style of cranked arrow delta.  It worked quite well for what I needed it to do. 

 

 

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22 minutes ago, Hodo said:

The thing is I think your wings have to much sweep for them and the overall design is short.  While it is a medium weight craft in my books, you have a good take off speed 120-140m/s.  That is pretty usual for most modern aircraft.   My larger SSTOs have a take off speed of 150-180m/s.

Just realized that I actualy posted two similar crafts, one shown in hangar and other on runway. Pictures on runway were taken before ones in SPH and I loaded wrong craft.
Both crafts are similar in shape and size to show flaps usage, though. Craft in SPH have only one engine. Main problem with it is that runway is not long enough to reach take off speed. For take off purposes, I temporary use rocket mode, to accelerate fast enough for take off, switch back to air breathing mode once craft lift off.

Once in air, single engine is enough to push craft in orbit, however, in upper atmosphere, single engine does not have enough power in air breathing mode, you need to switch to rocket mode much sooner. Whole thing is just not effective enough for practical usage, that is a reason why I created similar craft with two engines.

Both crafts are still WIP, I need to learn again optimal re-entry procedure. It is much different from KSP 1.0.x. I was able to return both crafts to runway, but with a lot of trouble in re-entry. Figured out "Kerbal" method of re-entry, to put craft in uncontrolable spin around 50 km in atmosphere. Atmosphere is thin enough to be safe from ripping off wings apart, Q is arond 1-2 kPa, but craft in uncontolable spin was heated more equaly, preventing cockpit to explode. Once craft is slowed down enough, around 15km, I put spin under control and fly to runway as usual.

I never needed any kind of additional thermal control parts, now I need to add those on craft too, to be able to have more reliable method of re-entry.

I experiment with different design aproach, short and wide crafts, swept vs various delta designs, with and without elevators on tail, with and without canards, etc.
Don't know if you are noticed, but that craft on first picture use elevators on main wings, without small horisontal wing on tail, but with canards to help in pitching.
Drawback is that it need small power from engine to have more controls with engine gimbals. Whole point is to create craft capable to reach orbit with least amount of force, trough sleek craft design, rather than brute force power from engines.

Oh, I also upoaded bunch of new small aircrafts powered by jet engines. All of them with decent maneuverability, some of them can survive 15-18g turns at sea level. Just recently I have installed EVE and scaterrer, so I need to take new screenshots from test flight, for better entertainment when you watch all those crashes :)

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Seems rather easier to build spaceplanes recently, not quite sure why. Been knocking out prototypes & finding I need way less engine than before... I don't think the B9 engines changed since 1.0 at least.

27209806102_554268f2c8_b.jpg

~13t to 80km it appears - adding more fuel doesn't help, so maybe I'll try a third engine. Figures are with a full load. On the other hand...

27209805462_4a4ff102d7_b.jpg

That's already lifting double & hardly costs any more. Will be interesting to see what HL-sized craft can do these days, I haven't built one since 0.90.

27236797291_2ae43e0e74_b.jpg

And that was actually first - ~10t to ... well, wherever 7k dV takes you from LKO. Either needs all engines in rocket mode or assistant SRBs to get to speed by the end of the runway, but once it's going it's just fine. RATO launches are as spectacular as ever, that's one reason to do it that way :P

27307174315_3fc0b13905_b.jpg
26700028024_298b478d21_b.jpg

I suspect I will fairly quickly give it larger nuclear rockets.

They all need painting & decorating & little fittings like lights & antennae & lifesupport when I decide on one, but surprisingly quickly tweaked so far.

27031955400_235ff7a121_b.jpg

For re-entry I just make sure I'm coming in fairly shallow, pitch up 5-10 deg & pop airbrakes ( helps if you've got unequally sized/deflected airbrakes at that point - setting an upwards deflecting set as flaps is also handy ), and just adjust pitch to keep heating as high as the craft can stand. Not quite got my re-entry points worked out yet, I keep overshooting, but it all seems manageable.

Edited by Van Disaster
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@kcs123 Honestly the easiest re-entry profile is the simplest.  I use the shuttle re-entry profile.  I try to keep the nose up to about 15-20deg AoA.  And a rate of decent of around  100-200m/s at max.   By the time I hit 30km I am usually doing mach 6.0 or less.  And 20km my goal is less than mach 3.5.   

 

This is easy with most of my craft with airbrakes or spoilers in the design.  I usually have one on a smaller aircraft, and two on larger aircraft and on my heavies they get 3 pair of air brakes. 

EDghYST.jpg 

But this monster also returned with its cargo inside of it.

42TIcjj.jpg

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47 minutes ago, Hodo said:

I try to keep the nose up to about 15-20deg AoA.  And a rate of decent of around  100-200m/s at max.   By the time I hit 30km I am usually doing mach 6.0 or less.  And 20km my goal is less than mach 3.5. 

That method worked well for me too in KSP 1.0.5. But in KSP 1.1.2. things are a lot different. Craft properly slow down. at 30 km I'm at 5 mach or less. Problem is that cockpit still overheat too much. It need additional radiators near cockpit to keep it cool. Even slight mistake in re-entry profile lead to ka-boom. I want larger error margines, as I'm not always precise in piloting procedure.

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