Official FAR Craft Repository

[Van Disaster]

Reducing sweep will increase span which will help marginally, but then you'll mess up some other area of the envelope. As was said, less mass or more wing, basically.

[Nansuchao]

To all the FAR experts here around. I use FAR from KSP 0.25 more or less and built  many many SSTOs with it. However, this time I decided to go not big, not just huge but INSANELY MASSIVE!!!!

This beast down there, has the ability to deliver big and heavy payloads almost anywhere in the Kerbol System, thanks to the KSPI-E engines and the B9 parts. However, I'm unable to land it safely. At low speed, with LF/OX depleted, the nose want to go down and it's very difficult to bring it up. I know the design is not so sane, but I falled in love with it.

Do you have any suggestion on how to improve it?

 

Edited October 25, 2017 by Nansuchao

[Surefoot]

Land it vertically, on its tail ?

[Nansuchao]

3 hours ago, Surefoot said:

Land it vertically, on its tail ?

It has not the TWR, sadly. And it would be terrible to control it with such massive wings...

[capi3101]

Okay...let me make sure I've got this right - your plane is nosing downward when it's low on fuel? I ask because the stability numbers you've got in that screenie would suggest nose up behavior.

Typically, a plane that noses downward uncontrollably - "lawn dart" behavior, as keptin puts it - is a sign that the center of mass is too far forward of the center of pressure. I'm guessing you've got a huge swing forward in the position of the CoM as your fuel drains. You could confirm if this is the case with the RCS Build Aid mod; among other things, it adds a "dry center of mass" marker that will show you where your center of mass will be when you're out of gas. Not sure if the mod for 0.25 is around anywhere, unfortunately. Next best thing is to manually drain your tanks in the VAB and check to see what that does to your stability figures.

Without changing anything else about the design, about the only thing I could suggest is to increase the deflection on your pitching control surfaces, increasing the area of your pitch surfaces with parts from a mod like Procedural Wings, and/or add more pitching control surfaces (especially if you can do so on that upper wing pylon you've got going on there).

[Nansuchao]

30 minutes ago, capi3101 said:

Okay...let me make sure I've got this right - your plane is nosing downward when it's low on fuel? I ask because the stability numbers you've got in that screenie would suggest nose up behavior.

Typically, a plane that noses downward uncontrollably - "lawn dart" behavior, as keptin puts it - is a sign that the center of mass is too far forward of the center of pressure. I'm guessing you've got a huge swing forward in the position of the CoM as your fuel drains. You could confirm if this is the case with the RCS Build Aid mod; among other things, it adds a "dry center of mass" marker that will show you where your center of mass will be when you're out of gas. Not sure if the mod for 0.25 is around anywhere, unfortunately. Next best thing is to manually drain your tanks in the VAB and check to see what that does to your stability figures.

Without changing anything else about the design, about the only thing I could suggest is to increase the deflection on your pitching control surfaces, increasing the area of your pitch surfaces with parts from a mod like Procedural Wings, and/or add more pitching control surfaces (especially if you can do so on that upper wing pylon you've got going on there).

I built it so the CoM stay in the same place with all the fuel or without fuel. All the wings comes from B9 Procedural Wings, with the biggest surface controls I could make. It flies well only with the engines thrusting, probably for the vectoring they provide. The fact is that, for the first time, I have no idea on how to improve the wing design to make it more stable.

I’m on 1.2.2 by the way.

[capi3101]

Okay...let me try grasping at straws then. You've got 4,360 square meters of wing area. KER is reporting the craft has a mass of four tonnes. I'ma gonna guess that's bogus as all hell - what's the real mass? Better still, since your issues are coming up at landing, what's the dry mass of the craft? And at what speed are you attempting to land as a rule?

[kcs123]

17 hours ago, Nansuchao said:

Do you have any suggestion on how to improve it?

1. More wing surface area or less overall craft weight if you want low landing speeds.
2. Use, flaps/slats on landing.
3. Put some canrads near nose to help you with pitching issues, that one alone might be enough for proper landing

Example of similar "insane" craft design, but with more wing surface and much less engines, also last made for 1.2.2.:

 

 

[Nansuchao]

1 hour ago, capi3101 said:

Okay...let me try grasping at straws then. You've got 4,360 square meters of wing area. KER is reporting the craft has a mass of four tonnes. I'ma gonna guess that's bogus as all hell - what's the real mass? Better still, since your issues are coming up at landing, what's the dry mass of the craft? And at what speed are you attempting to land as a rule?

Mass is broken in VAB but works in flight. This thing weight 5.000 tons and doesn’t fit in the runway, I have to take off from the grass close to the Runway. I would like to land as slow as possible, but as soon as I try to loose speed to land it nose down and explode.

 

8 minutes ago, kcs123 said:

1. More wing surface area or less overall craft weight if you want low landing speeds.
2. Use, flaps/slats on landing.
3. Put some canrads near nose to help you with pitching issues, that one alone might be enough for proper landing

Example of similar "insane" craft design, but with more wing surface and much less engines, also last made for 1.2.2.:

 

 

Beautiful SSTO, but still “stock” size. I upscaled the S2W fuselage to 10m. For it’s fuselage, the craft it’s not so heavy, since it has just enough LFO to reach LKO, then the main fuel is Hydrogen that is pretty light. I’m trying to add some big winglets on the front.

[capi3101]

46 minutes ago, Nansuchao said:

Mass is broken in VAB but works in flight. This thing weight 5.000 tons and doesn’t fit in the runway, I have to take off from the grass close to the Runway. I would like to land as slow as possible, but as soon as I try to loose speed to land it nose down and explode.

Okay. 5,000 tonnes and 4,360 square meters of wing area gives you a wing loading of 1.147 tonnes per square meter. That's a very high number; usually I make my designs so that they have at most 0.7 tonnes per square meter; more often, I shoot for 0.5 tonnes per square meter (comparable to the Concorde or Space Shuttle orbiter). High wing loading is going to give you better high speed performance, but it also gives you high takeoff and landing speeds; with a wing loading that high, it wouldn't surprise me if your stall speed is above 200 m/s. It may be what I'm seeing in your stability screenie there isn't inherent pitch up tendencies, it may be that FAR can't calculate reasonable stability figures because Mach 0.35 at zero meters altitude isn't producing enough lift. Try bumping the mach speed up and see what happens.

@kcs123's advice is sound in this case, regardless.

Edited October 26, 2017 by capi3101

[tetryds]

@Nansuchao that X wing gives me shivers.

It is awful, super awful, there is only a very specific biplane configuration that can theoretically fly well at supersonic speeds, and it's certainly not that one.

The first step is changing that.

Another detail is that big wings are useless at high speeds, most of the (efficient) lift is provided by the hull itself, like the skylon.

In this case you may not have such a big runway to land something with a wing that small, but since this is KSP I would even recommend using some mod with retractable thrusters to help you break and takeoff. With that you will also approach the runway at a much lower angle of attack, which will help on holding it together while breaking.

There are a lot of engines on that, are you really sure you need that many? I know its big but a small force for a longer time should do the job more efficiently.

Well, that is mostly it for this one.

 

If I was to make that as efficient as possible I would stick it to a booster "crane" stage that goes to orbit with it then goes back to land on kerbin, leaving it on orbit, will allow for much higher payloads at LKO.

Edited October 26, 2017 by tetryds

[Nansuchao]

@capi3101 @kcs123 @tetryds

Thanks to all of you. I realized that wasting more time on this spaceplane would be useless. 

The wing design is awful, I know, but very fascinating. The problem was also that the entire thing was too huge, it was also killing my frame rate. 

I built in a few minutes a much smaller version with similar wings, and it flies like a charm.

All began with the idea to use a wing shape close to the one in Star Wars and a beautiful orbital engines that sadly is 10 meter in size.

The New version has a different engines and is slower in orbital maneuvers, but it flies.

Thanks to all of you for your help.

[Van Disaster]

On 26/10/2017 at 2:22 PM, capi3101 said:

Okay...let me make sure I've got this right - your plane is nosing downward when it's low on fuel? I ask because the stability numbers you've got in that screenie would suggest nose up behavior.

Small note - the simulation is out of bounds, so the numbers are garbage anyway ( AoA >0 is the tell ). I think there's something more than just high wing loading going on there.

[capi3101]

31 minutes ago, Van Disaster said:

Small note - the simulation is out of bounds, so the numbers are garbage anyway ( AoA >0 is the tell ). I think there's something more than just high wing loading going on there.

I figured as much. The KER mass figure in the original screenie caught my attention, though, so I figured I'd check on it since I was out of any other ideas.

Edited October 27, 2017 by capi3101

[Guest]

Ok, so ive been trying to make an F-15 like plane, but It isnt manuverable enough. Im using AJE, and the empty TWR is a little over 1. Bigger wings (decreasing wing loading) helps, but it still isnt manuveralbe enough. Ill post a .craft later. Im using my phone RN. Any tips?

Edited October 27, 2017 by Guest

[capi3101]

6 minutes ago, dundun92 said:

Ok, so ive been trying to make an F-15 like plane, but It isnt manuverable enough. Im using AJE, and the empty TWR is a little over 1. Bigger wings (decreasing wing loading) helps, but it still isnt manuveralbe enough. Ill post a .craft later. Im using my phone RN. Any tips?

If you're looking for maneuverability, you need to destabilize the plane - CoM at or slightly behind and slightly above the CoL. An anhedral, low main wing will largely do the trick. You want to be careful doing this - too much and the plane won't be controllable. A fly-by-wire mod such as Atmosphere Autopilot will help you keep control.

[Guest]

5 minutes ago, capi3101 said:

If you're looking for maneuverability, you need to destabilize the plane - CoM at or slightly behind and slightly above the CoL. An anhedral, low main wing will largely do the trick. You want to be careful doing this - too much and the plane won't be controllable. A fly-by-wire mod such as Atmosphere Autopilot will help you keep control.

BDArmory AI wont cooperate with AAutopilot. The Cg and Cp are as close as I can get them with out the plane pulling excessive alpha, nosing down, excessive alpha etc. There isnt thrust vectoring either.

Edited October 27, 2017 by Guest

[capi3101]

1 hour ago, dundun92 said:

BDArmory AI wont cooperate with AAutopilot. <snip />

That right there is a true shame...and here I was considering adding BDArmory to my install just to liven things up a little bit......

Well, out of curiosity, what is your current wing loading, and what 's your current aspect ratio?

[Surefoot]

If you want to do a F-15 that works you have to pay attention to all the aerodynamic details (wing surface ratio, thickness, anhedral, sweep, etc.), all proper control surfaces sized like the real one, and the CoM should be really near the CoL almost on top of it. Fighter jets are very twitchy by nature. Just look at the KU-57 example i posted on previous page (page 47) here, it flies very well but i had to go into a lot of details to be close to the original.

If i look at that blueprint:

I can see that the wings have a specific shape, slight anhedral, are top mounted, and pitch ailerons are quite offset to the rear possibly for increasing their authority.  The wing root starts with a weird oblong shape that's on the side of each intake, i suppose that's for area ruling (which is also a thing in FAR so we have to pay attention to it), i also notice the "thin waist" and on photos of the real thing the rounded shapes for each engine pod. Notice each control surface size and shape, and the big fixed twin vertical tails. Taper is also taken into account by FAR somehow and it's noticeable on that blueprint.

The F-15 has no leading edge slat (notice the difference between control surfaces and leading edge of each wing) so it should be a relatively straightforward design to do in KSP. Intake pods cannot be done exactly like on the F-15 but we can do something close.

(edit) slapped something together very quickly:

Yep, it works It's not as unstable or twitchy as my KU-57 but flies still very nice.

Of course there's no double cockpit length wise in KSP so the front is a bit shorter here.

I think i got the system to make that F-15 apart from the intakes i'm not happy with what i did there. I also could elongate the cockpit by adding a fuselage section behind it. But that was a quick job and the CoM is really well where it is now.

So yeah, it flies, it rolls pretty well and has enough pitch authority to destroy itself from G forces and incoming airflow. Also it's a stable design, so SAS is enough to fly it.

Edited October 28, 2017 by Surefoot

[Hodo]

On 10/27/2017 at 7:08 PM, Surefoot said:

If you want to do a F-15 that works you have to pay attention to all the aerodynamic details (wing surface ratio, thickness, anhedral, sweep, etc.), all proper control surfaces sized like the real one, and the CoM should be really near the CoL almost on top of it. Fighter jets are very twitchy by nature. Just look at the KU-57 example i posted on previous page (page 47) here, it flies very well but i had to go into a lot of details to be close to the original.

If i look at that blueprint:

I can see that the wings have a specific shape, slight anhedral, are top mounted, and pitch ailerons are quite offset to the rear possibly for increasing their authority.  The wing root starts with a weird oblong shape that's on the side of each intake, i suppose that's for area ruling (which is also a thing in FAR so we have to pay attention to it), i also notice the "thin waist" and on photos of the real thing the rounded shapes for each engine pod. Notice each control surface size and shape, and the big fixed twin vertical tails. Taper is also taken into account by FAR somehow and it's noticeable on that blueprint.

The F-15 has no leading edge slat (notice the difference between control surfaces and leading edge of each wing) so it should be a relatively straightforward design to do in KSP. Intake pods cannot be done exactly like on the F-15 but we can do something close.

(edit) slapped something together very quickly:

Yep, it works It's not as unstable or twitchy as my KU-57 but flies still very nice.

Of course there's no double cockpit length wise in KSP so the front is a bit shorter here.

I think i got the system to make that F-15 apart from the intakes i'm not happy with what i did there. I also could elongate the cockpit by adding a fuselage section behind it. But that was a quick job and the CoM is really well where it is now.

So yeah, it flies, it rolls pretty well and has enough pitch authority to destroy itself from G forces and incoming airflow. Also it's a stable design, so SAS is enough to fly it.

You have too much wing surface area compared to the F-15.  It is actually REALLY close to the MiG-25 Foxbat.

 

[Surefoot]

I used the diagrams i posted above, which i believe represent the real F-15. Thing is the MK2 body is a bit wider, otherwise i tried to respect proportions, also you might be induced into thinking that because of the stubbier nose (nothing i can do there, i have to do with the MK2 parts i have). Like i said, it's quickly slapped together, not up to the level of detail of my KU-57. It still flies rather well and has fighter-like agility.

Edited November 5, 2017 by Surefoot

[Schwarz]

Can someone please explain to me what factors influence Mach 1 wave drag area? 
I can understand that it is kept low by preventing stuff from abruptly sticking out, thinner wings, etc., thus smooth airflow.
But how it actually works is pretty difficult for me to get my head around.
Like this:


Wave drag area is 0.712m^2.
But if i stick on a targeting pod:

What just happened there?
Besides that, I've put extra stuff on the aircraft, and yet the max cross-section area reduced, which is really counter-intuitive.
https://i.imgur.com/MEb6I3M.jpg

[tetryds]

@Schwarz it is not the cross section area, but the cross section delta (variation).

Turn on the green and yellow curves.

You will want to keep the green curve as smooth as possible, keeping the yellow curve close to zero.

Abrupt cross section variations generate a lot of transonic drag. But once you are far past the transonic regime the streamline and cross section itself will start to matter more.

So, for transonic regimes having that targeting pod may improve performance, but it will make it worse for higher mach numbers.

Edited November 6, 2017 by tetryds

[Guest]

On 10/27/2017 at 5:08 PM, capi3101 said:

That right there is a true shame...and here I was considering adding BDArmory to my install just to liven things up a little bit......

Well, out of curiosity, what is your current wing loading, and what 's your current aspect ratio?

Dynamic deflection (fortunately) will cooperate with BDA though.

[Mine_Turtle]

@capi3101, as you have suggested I have moved this discussion over to this topic.

This is a question that probably belongs over on the Official FAR Craft Repository. 

The optimal tailform for your plane is going to depend entirely on what you're going to be using it for. My own self, I tend to stick to a standard flying tailplane design (i.e. fuselage mounted but with a full-motion horizontal stabilizer) to fight Mach tuck; pretty much all the craft in my Auk spaceplane series have that setup. The exception - the Auk V - is a tailless delta, and its pitch characteristics suck.

So, what kind of plane are you wanting to build?

Whenever I am designing one of my SSTO(heavy lifters in the range 100-1000 tons), once I am done with initial fuselage form, fuel tanks and rough engine sizes I switch over to wing configuration. Dihedral delta wing, that fits into 5 mach shock cone, seems like a no brainer as it provides as much lifting surface as possible, while minimizing drag at transsonic speeds and increasing plane stability. For example, a couple of my latest designs,:

https://i.imgur.com/kbhtJvZ - Do not mind red numbers, I fixed them by raising wing position.

https://imgur.com/pikHVuu

However, when it comes to designing vertical and horizontal stabilizers I do not have any algorithm to choose, which one fits best. I still tweak their size to have stable aircraft, but when it comes to choosing tail configuration the only guideline I follow is that my plane looks good. In the first picture I went for T-tail and this resulted in a very stable plane overall. Second craft received two vertical angled fins on engine nacelles(similar to blackbird configuration) and it kind of worked too. 

 

So, I am looking for guideline to chose the optimal tail configuration, depending on craft geometry, CoM and CoL position and so on.