SSTO spins horizontally uncontrollably.

[Edax]

I've been using this SSTO mothership to get my parasite spaceplane into orbit, but recently, they've lost the ability to make any kind of turns without spinning out of control and forcing an emergency landing. This used to be a stable SSTO until the ever-heavier payload attached to the front forced me to add more wings at the back. It was only recently this instability happened, and I don't know if it's a design flaw involving aerodynamics, game mechanics, or the new 1.0.3 areo model involved, or the fact that the MK1 LF fuel tanks are now way heavier, but if anyone can give me advice, I'd appreciate it.

Edited July 1, 2015 by Edax

[O-Doc]

Larger vertical stabilizers required at the back. I'd also not use angled tailfins, or at the very least, make them roll only control surfaces.

[A_name]

Larger vertical stabilizers required at the back. I'd also not use angled tailfins, or at the very least, make them roll only control surfaces.

I second more vertical stabilizers, and also make sure all control surfaces not on the horizontal stabilizers have yaw disabled. Other than that, make sure everything is absolutely symmetric.

[Edax]

Larger vertical stabilizers required at the back. I'd also not use angled tailfins, or at the very least, make them roll only control surfaces.

I suppose I should mention that I used to have 4 tail fins on this Spaceplane, but I removed 2 of them because they were unable to correct an uncontrolled spin, so I viewed them as dead weight (though removing them may have made the problem worse, I just don't know. Back when it had 4, it was capable of making minor turns before spinning uncontrollably.) It was my experience that putting on vertical stabilizers too far from the center would cause all the turns to roll the plane (though I had yaw enabled, so that could have been the problem).

So that answer is more vertical stabilizers with yaw disabled?

[A_name]

So that answer is more vertical stabilizers with yaw disabled?

The other way around. Your vertical stabilizers should only control yaw, and your elevons should only control pitch and roll.

[Edax]

The other way around. Your vertical stabilizers should only control yaw, and your elevons should only control pitch and roll.

Ah, okay.

[Edax]

The other way around. Your vertical stabilizers should only control yaw, and your elevons should only control pitch and roll.

The Spaneplane is now more stable and capable of small left-right inputs, but it still cartwheels horizontally. The primary problem I found was the forward T-800 fuel tanks, when I removed them, became more stable. I add 4 more tail fins and a Elevon 1 on the wing (and have them only control yaw, the horizontal Elevons now only control pitch and roll, now when the plane cartwheels, it's mostly recoverable, but I still don't understand why the plane keeps spinning horizontally from medium left-right rudder inputs. FYI it mostly spins out of control when mostly empty of fuel, and payload detached. When heavy, the controls are sluggish and difficult to keep steady unless I'm moving straight or pitching.

Edited June 30, 2015 by Edax

[LordFjord]

That sounds like an issue with shifting center of mass - center of lift relation.

Try RCS Build Aid.

It can show you your vessels center of mass and center of lift markers when out of fuel (and lots of other useful features to balance RCS and whatnot).

Or, if you don't want to use mods, show us a pic with the CoM, CoL markers enabled, maybe once with(without the payload).

The issue is most certainly that the craft is not balanced when you you eject the payload - and as you burn fuel it can get unbalanced as well.

In order to balance your vessels, try this:

- balance the payload on its own, make sure the CoM is a bit ahead of the CoL and that it stays there, even when fuel is low.

- balance the carrier ship on its own

- try to find an attachment point where both combined are balanced

It's not easy what you are attempting there

To the cartwheel issue: try to move the tail fins as far back as possible. When they are too close to your CoM, they are not helpful at all.

At what speed/altitude does this happen? Is it maybe an assymetric flame-out issue?

[O-Doc]

.

The Spaneplane is now more stable and capable of small left-right inputs, but it still cartwheels horizontally.

That's because your center of drag is above your center of thrust so when you fly out of thicker atmosphere your vertical control surfaces lose grip on the air to the point where you flip. You definitely need more control surfaces and probably an angled wing or two underneath the craft to balance out your drag profile. The other option is angled tail fins that are rotated inwards to help lift up the back end of your craft.

[Edax]

That sounds like an issue with shifting center of mass - center of lift relation.

Try RCS Build Aid.

It can show you your vessels center of mass and center of lift markers when out of fuel (and lots of other useful features to balance RCS and whatnot).

Or, if you don't want to use mods, show us a pic with the CoM, CoL markers enabled, maybe once with(without the payload).

The issue is most certainly that the craft is not balanced when you you eject the payload - and as you burn fuel it can get unbalanced as well.

In order to balance your vessels, try this:

- balance the payload on its own, make sure the CoM is a bit ahead of the CoL and that it stays there, even when fuel is low.

- balance the carrier ship on its own

- try to find an attachment point where both combined are balanced

It's not easy what you are attempting there

To the cartwheel issue: try to move the tail fins as far back as possible. When they are too close to your CoM, they are not helpful at all.

At what speed/altitude does this happen? Is it maybe an assymetric flame-out issue?

.

That's because your center of drag is above your center of thrust so when you fly out of thicker atmosphere your vertical control surfaces lose grip on the air to the point where you flip. You definitely need more control surfaces and probably an angled wing or two underneath the craft to balance out your drag profile. The other option is angled tail fins that are rotated inwards to help lift up the back end of your craft.

I don't think it's asymmetric flame-out issue, you'd hear a flameout noise and the engines would spark. I have a ram intake for every turbojet engine. Maybe the air is entering unevenly?

The cartwheeling happens pretty much at any low to middle altitude. I usually run an airworthiness test with the payload detached, and the fuel in the back drained so that the center of mass is slightly ahead of the center of lift (this also simulates reentry where I pump all the fuel forward). It's during these tests that I keep cartwheeling. I suppose I can move the tail fins further back, but because there's a slope on the surface area of the plane, would having the tail fins at a 20 degree angle would cause stability issues?

Would it be better if I posted a video of the cartwheeling?

Edited June 30, 2015 by Edax

[TheXRuler]

Also maybe a thing to think about. Most planes (I think all of them but I will not state that here since someone will make a point of proving me wrong ) barely use the rudder. You do not turn a plane using the rudder. If you want to turn left you roll the plane a little (or a lot, depending on how hard you inted to turn) to the left and then pull up gently. This is where the rudder is used, to prevent the plane from sideslipping toward the left too far since rolling to the left will make your nose slide to the lower left side. You can test how much your plane sideslips by switching off ASAS and rolling to one side, then watch what happens.

With the amount of vertical stabilizers you have sideslip should be very managable via control input. Trying to steer with your rudder is generally a very bad idea (exceptions are eg. final approach to the landing strip at low velocities or IRL countering crosswinds). I would recommend taking a finished craft file (doesen't matter whether it's one of the example crafts or from the spacecraft exchange here on the forum) of something that looks similar to a jet fighter craft and getting some practice with that. You will quickly notice that on most craft the rudder has very little authority when trying to steer, however depending on how well they are built you might be able to pull six or seven Gs when rolling ~75° and then pulling "up" hard.

Also angling your tail fins should not be a problem as long as you do not rotate them relative to your airflow. What I'm trying to say is tipping them forward our backward, left or right should not be a problem. However rotating them left or right will dramatically increase drag and might do funky things with your flight profile.

A video would definetly help, as that would maybe let us see whether it is your input or something about the way the plane is built that is making it flatspin.

Edited June 30, 2015 by TheXRuler

[Majorjim!]

I suppose I should mention that I used to have 4 tail fins on this Spaceplane, but I removed 2 of them because they were unable to correct an uncontrolled spin, so I viewed them as dead weight (though removing them may have made the problem worse, I just don't know. Back when it had 4, it was capable of making minor turns before spinning uncontrollably.) It was my experience that putting on vertical stabilizers too far from the center would cause all the turns to roll the plane (though I had yaw enabled, so that could have been the problem).

So that answer is more vertical stabilizers with yaw disabled?

In flight check that all engines are thrusting the same. There is an unbalanced thrust bug. Also check that all tanks are draining evenly.

[Edax]

That sounds like an issue with shifting center of mass - center of lift relation.

Try RCS Build Aid.

It can show you your vessels center of mass and center of lift markers when out of fuel (and lots of other useful features to balance RCS and whatnot).

Or, if you don't want to use mods, show us a pic with the CoM, CoL markers enabled, maybe once with(without the payload).

The issue is most certainly that the craft is not balanced when you you eject the payload - and as you burn fuel it can get unbalanced as well.

In order to balance your vessels, try this:

- balance the payload on its own, make sure the CoM is a bit ahead of the CoL and that it stays there, even when fuel is low.

- balance the carrier ship on its own

- try to find an attachment point where both combined are balanced

It's not easy what you are attempting there

To the cartwheel issue: try to move the tail fins as far back as possible. When they are too close to your CoM, they are not helpful at all.

At what speed/altitude does this happen? Is it maybe an assymetric flame-out issue?

I've noticed that the tail wings are close to the CoM when drained of fuel, even though their already at the back of the plane. Is that the sole cause of the flatspining? Maybe I could stick the tail wings on rear struts?

Edited July 1, 2015 by Edax

[Edax]

Yep, sticking some tail fins behind my Spaceplane has done wonders for control. I'm now no longer have to fly a deathtrap to get my ships into space.