Why is my spaceplane stuck at 90º of prograde when entering atmosphere ? (using FAR)

[m1sz]

So this is something that really annoys me, because I dont know why its happening!

I have this SSTO, capable of going to orbit and back 4 times without refueling, it has COM before COL, and is that way even with empty tanks. In fact, when I reenter atmosphere, I use to get all the fuel in the front tank, so my plane goes straightforward. But it's not happening, it wants to turn and gets stuck at a very unlikely aerodynamic way, 90º of prograde!!. Also, I think it's suffering a really heave mach effect, I cant pitch above mach 1.

You can see everything in this video, it's a little boring, but its just 4mins long. No music atm, maybe ill add some youtube random music!

I would like to hear from you guys, if you know what is happening, and WHY!!!, I guess it has something to do with Ferram Aerospace Research, but im not sure...

Thanks in advance for your time!

(also, I hope it's in the right subforum!, was thinking about addon request and support, but people there were just asking for different stuff)

[Claw]

Well, I haven't been using FAR yet, but here's a few things that I thought of watching the video.

I believe you when you say the CoM is in front of the CoL. However...

- Your CoL might actually be in front of your CoM, even though it might look otherwise in the SPH.

-- Remove your landing gear and check the placement for all your fuel configs. The landing gear causes the SPH to lie about the CoM because landing gear is ignored in flight. (Unless FAR fixes this? Sorry, I don't know...)

-- I also say this because your chutes look pretty far forward, as if they are close to your CoM too. So if it's lying, it might be causing those problems too. Try moving your chutes back too.

- If you've built in very little margin on your CoM/CoL spacing, then it can still cause stability issues such as flat stalling.

-- Try building in a little more margin and see if that helps.

- If your CoL is low compared to your CoM, that can also cause instability even if it's behind your CoM.

-- Add some dihedral, a V tail, anything like that and see if it's the problem.

- I also don't know anything about the FAR drag model, but if for some reason the nose is much draggier than the tail, it might be causing your plane to flip at high speeds. I haven't seen this problem in Stock, but the drag model is different.

EDIT:

I just watched the video again, and one more thing to think about. It looks like you have a lot of control surfaces on the back. If FAR is a good modeler of super/hypersonics, then that might also be your problem, especially if your CoM/CoL margin is small (although your SAS trim looks okay).

Anyway, when you go supersonic, the Center of Lift shifts back. With such large control surfaces, you might actually be changing the location of the CoL when you try to activate at supersonic speeds. Once it goes unstable, you don't have enough control authority to get it back. You could try deactivating some of the surfaces via tweakables and see if that works.

Edited January 27, 2014 by Claw

[m1sz]

Thank you a lot Claw, I'll check that out moving the COM, and will post a picture of the plane soon showing COM and COL, thanks again!

[Claw]

You're welcome and good luck.

I don't use FAR so I don't know if any of that will fix it, but hopefully it will help you stumble on the answer.

[Starwhip]

FAR: Biggest pain in the [()()()] for airplanes. You have to build them like real airplanes or they WILL NOT FLY. If you made it in stock and then transferred to FAR, you're gonna have problems. But otherwise, your CoM should be inline with your CoT, and your CoL behind and above the CoM.

[UH60guy]

Here's my take on it. Note, I don't know anything about FAR, but I've studied aerodynamic engineering and I hear FAR is a decent model.

Your plane looks wider than (or just about as wide as) it is long. That's a big no-no in supersonics. That's why it's trying to align with the flowstream along its long axis (the wings).

If your brain can handle it (it's tough) google the Whitcomb Area Rule. Boiled down, it says in essence the ideal shape for a supersonic body is a long cylinder thingy with pointy ends on each side. Now, that's a model of the cross sectional area of the plane. So, to keep the cross sectional area constant across the body where the wings stick out, real world supersonic aircraft have Coke bottle shapes (the fuselage gets a smaller cross section to make up for the cross section of the wings).

Bottom line, go for long and skinny, not short and wide.

Also, any excessive control inputs you make at those speeds won't help much! You don't have a lot of maneuverability in KSP until you get subsonic.

Edit: it could also be incompatibility between FAR and other mods you are using. Again, I haven't used FAR or your cockpit, so who knows.

Edit again: Now that I think about it the Area Rule is about low supersonics, you are in the hypersonic range there. I never got to that point in my studies, so this is talking out of my lower extremeties, but I'd agree that you may have too many control surfaces. Any small deflection is going to have big implications. And, depending if FAR treats them differently from the wings, think of it as a large "something" about the same size as your wings which the model is treating in a different manner.

Edited January 27, 2014 by UH60guy

[m1sz]

Thank you all

First of all, I though about it being wider than longer, but the plane is not pointed sideway, following the wings, it gets 90º of prograde, and tryes to stay that way, having the most larger surface pointing prograde. It's hard to explain.

WHen i have the time, ill test some changes and post them back, thanks again for your time

EDIT: cockpit is just the stock one, the only change is the interior

[Claw]

FAR: Biggest pain in the [()()()] for airplanes. You have to build them like real airplanes or they WILL NOT FLY. If you made it in stock and then transferred to FAR, you're gonna have problems. But otherwise, your CoM should be inline with your CoT, and your CoL behind and above the CoM.

While it is certainly one of the predominant factors, the CoM/CoT and CoM/CoL relationship isn't the only thing that affects stability.

Here's my take on it. Note, I don't know anything about FAR, but I've studied aerodynamic engineering and I hear FAR is a decent model.

Your plane looks wider than (or just about as wide as) it is long. That's a big no-no in supersonics.

I think it's a big no-no because of drag. Although abnormal drag might be the issue since I hear FAR is a pretty good model, as I suggested with drag at the front end.

That's why it's trying to align with the flowstream along its long axis (the wings).

I would think if it's a long axis problem, his plane would go out of control in yaw and stay sideways instead of in a deep stall.

If your brain can handle it (it's tough) google the Whitcomb Area Rule. Boiled down, it says in essence the ideal shape for a supersonic body is a long cylinder thingy with pointy ends on each side. Now, that's a model of the cross sectional area of the plane. So, to keep the cross sectional area constant across the body where the wings stick out, real world supersonic aircraft have Coke bottle shapes (the fuselage gets a smaller cross section to make up for the cross section of the wings).

Bottom line, go for long and skinny, not short and wide.

Edit again: Now that I think about it the Area Rule is about low supersonics, you are in the hypersonic range there. I never got to that point in my studies, so this is talking out of my lower extremeties, but I'd agree that you may have too many control surfaces. Any small deflection is going to have big implications. And, depending if FAR treats them differently from the wings, think of it as a large "something" about the same size as your wings which the model is treating in a different manner.

The coke-bottle/area rule is for transonic drag reduction. I agree with you that seems typically easier to design long/skinny, but I've seen a lot of FAR designs that do not hold to that.