Unstable spaceplane

[locustgate]

So it's been a while since I've played KSP and I seem to have forgotten how to make a ssto. My current plane seems to be rather unstable, it likes to wobble and try to flip over.

Edit: Tried moving tailfins out, no significant change.

EDIT 2: It seems to become stable when it hits 500ish m/s

Edited June 24, 2015 by locustgate

[*Aqua*]

Try to move the center of mass a bit further to the front / move the center of lift a bit more to the back. It will cause the plane fly more like a dart (= more stable). You can also lock the gimbal on some of the engines. This will decrease control authority causing SAS to oversteer less (= more stable).

If the plane wobbles use more struts. Struting the outer engines to the monopropellant tank should eliminate most of the wobble.

Also remove the radial intakes. They cause too much drag compared to the amount of intakeAtm they give. Air hogging doesn't give benefit anymore.

[TokiTech]

Center of weight will always need to be infront of center of lift to prevent place flips out itself. From screenshot it also looks like alot weight put to the back and barely any lift/stabilizer on front.

It is easy to take off this plane with only 4 rapiers, balance weight and engines count ratio to get optimal TWR.

[Nefrums]

I think it looks like you have to low lift compared to mass, try adding more wings.

[Juicy]

Try emptying the fuel tanks while in the hangar, starting from the front, and see how (badly!) it affects the position of your Center of Mass. You need a lot of counterweight in front of the Center of Lift to compensate for the mass of your engines, but if fuel is a part of that counterweight, you'll run into trouble.

Solution: try positioning your fueltanks very close to, if possible right on top of, your Center of Lift. Do the mass balancing with different parts.

You could even take a full fuel tank and disable it to act as counterweight to your engines as well. That way, you'll always lug around a tank full of unused fuel, so it's not really elegant, but it does add stability.

[Daveroski]

An easier solution might be to remove most of those radial air intakes.

You no longer need to max out the air intakes.

These intakes are adding mass to the rear of your ship removing them will move mass forward somewhat.

They also produce rather lot of drag.

If you feel that a few are needed for the middle engines mount them further forward. Four should be plenty.

[Red Iron Crown]

An easier solution might be to remove most of those radial air intakes.

You no longer need to max out the air intakes.

These intakes are adding mass to the rear of your ship removing them will move mass forward somewhat.

They also produce rather lot of drag.

If you feel that a few are needed for the middle engines mount them further forward. Four should be plenty.

Won't mounting draggy intakes further forward make the plane less stable?

[NathanKell]

Correct.

[Daveroski]

Won't mounting draggy intakes further forward make the plane less stable?

It certainly wouldn't be as unstable as it is right now.

Edited June 24, 2015 by Daveroski

[ajburges]

What's the takeoff weight? That is an awful lot of engines for a short Mk2 fuselage. I suspect you could go supersonic and orbital with much less. Less engine mass would shift CoM forward. You would also have less drag as you lose the catamarans.

If it is unstable subsonic but stable supersonic, I suspect the CoP is ahead of the CoL. Drag will play a comparatively larger part in the transonic region. It looks like all of your intakes are ahead of the CoM. This is especially troubling because those structural intakes are surprisingly draggy.

[Daveroski]

What's the takeoff weight? That is an awful lot of engines for a short Mk2 fuselage. I suspect you could go supersonic and orbital with much less. Less engine mass would shift CoM forward. You would also have less drag as you lose the catamarans.

If it is unstable subsonic but stable supersonic, I suspect the CoP is ahead of the CoL. Drag will play a comparatively larger part in the transonic region. It looks like all of your intakes are ahead of the CoM. This is especially troubling because those structural intakes are surprisingly draggy.

Well thanks for your well intentioned opinion.

Perhaps you might read the rest of the thread.

I made a copy of the unstable craft that the OP was having problems with. Not a perfect copy as I later saw several differences. howver the design is basically the same.

I was demonstrating that he could get rid of most of his radial air vents and actually move a few forward (Draggy as they are) and still have a much more stable craft.

As you can see, Bill had no problems with it.

[Good_Cat]

I'd bring the centre of mass a little further forward, they are close now with a little fuel use the CoM will shift behind the CoL and it'll flip, switch the Docking Port and Monopropellant tank around that should shift the weight around enough.

And bin the radial intakes not really worth it.

[Oafman]

That design is crying out for some canards!

But mainly you need the CoM further forward. Try sliding the tanks and engines attached to the main fuselage forwards, which will bring your CoM and CoL closer to the centre of the plane. Having all those rapiers in a neat line does look good but tends to result in too much mass right at the back of the plane.

[ajburges]

Well thanks for your well intentioned opinion.

Perhaps you might read the rest of the thread.

I made a copy of the unstable craft that the OP was having problems with. Not a perfect copy as I later saw several differences. howver the design is basically the same.

I was demonstrating that he could get rid of most of his radial air vents and actually move a few forward (Draggy as they are) and still have a much more stable craft.

As you can see, Bill had no problems with it.

You might want to chill out there buddy. Flight stability is affected by several factors. Just because you found one way to improve it does not mean it's the only way or even the definitive way. I don't know why you need to attack the observations of others.

OP made an observation of his craft that you don't generally see in threads about airframe instability. My comments at drag was to explore why he saw subsonic instability when he had supersonic stability. You have yet to provide a plauseable alternative. You merely show a similar craft with a more stable flight profile all around.

Where in the thread was either of my two points mentioned? As I see it, I was the first to question the engine array choice and the first to try and explain the velocity component observed. I also tried my observations to the effect they have on his crafts stability. You accuse me of not reading the thread, but I would invite you to understand the content of my one post.

[Daveroski]

You might want to chill out there buddy. Flight stability is affected by several factors. Just because you found one way to improve it does not mean it's the only way or even the definitive way. I don't know why you need to attack the observations of others.

OP made an observation of his craft that you don't generally see in threads about airframe instability. My comments at drag was to explore why he saw subsonic instability when he had supersonic stability. You have yet to provide a plauseable alternative. You merely show a similar craft with a more stable flight profile all around.

Where in the thread was either of my two points mentioned? As I see it, I was the first to question the engine array choice and the first to try and explain the velocity component observed. I also tried my observations to the effect they have on his crafts stability. You accuse me of not reading the thread, but I would invite you to understand the content of my one post.

Chill out? I'm sorry, did my post seem to contain elements of rage? Oh that won't do. I really must watch my temper.

[Red Iron Crown]

Easy there, folks, no need to get upset or make this discussion personal. Please discuss the OP's topic and try to be helpful rather than argumentative.