Space plane flips over

[FalloutBe]

Hello

 

I want to build a space plane but it keeps flipping over!

When I try to take off on the runway, it likes to go left or right, and it's really difficult to keep straight.

When I do get off into the air, after a few seconds it's nose flips up and the plane starts flying backwards. When I let it fall down to kerbin with the engines disabled, it also flies backwards?!

I don't understand how as the center of lift is behind the center of mass.

I also tried to make sure the fuel firstly starts to drain at the back, and gets pumped forewards, but it doesn't help.

Any ideas?
 

Edited August 20, 2016 by FalloutBe

[ForScience6686]

A couple things I notice.  Your landing gear are at an odd angle.  They should be pointing straight down.  You have such little wing area.  I would also guess you have to much elevator.  I would get rid of the horizontal stabilizers at the rear.  Make sure your control surfaces are set properly as well.  As a space plane, I think you'll have issues on re entry with that cockpit and nose.  I could be wrong but it looks like the v tail has incidence pushing the nose down.

[Aegolius13]

2 hours ago, ForScience6686 said:

Your landing gear are at an odd angle.  They should be pointing straight down.  

To make sure your landing gear point straight down, select the Rotate gizmo and toggle to "absolute" mode.  Use the F key on PC (not sure how you do it on console).  

Also agree your wing area looks low.  The regular delta wing might be a good main wing for a plane that size.  

If you do want to use a control surface in front of the center of mass, make sure you click it and set it to "inverted".  Otherwise it will go the wrong way when you try to manuever, which might contribute to instability.  But honestly I'd swap that one out for a larger static wing, and use the rear fins for pitch and roll.  

One other thing - do you not have any airbreathing engines?  I kind of doubt that thing would make it to orbit just on Darts.  One of the main reasons the SSTO concept is viable for a spaceplane is that airbreathing engines are about 10 times as efficient as rockets.  (The other big one is that lift helps negate gravity drag).  

 

[foamyesque]

1 hour ago, Aegolius13 said:

If you do want to use a control surface in front of the center of mass, make sure you click it and set it to "inverted".  Otherwise it will go the wrong way when you try to manuever, which might contribute to instability. 

This is unnecessary. The "invert" setting applies only to the "deployed" mode; pitch and yaw controls will automatically adapt.

 

4 hours ago, ForScience6686 said:

I would also guess you have to much elevator.

If anything, they have too little. Those are delta-deluxes, which have almost no control surface area compared to their size.

 

5 hours ago, FalloutBe said:

When I try to take off on the runway, it likes to go left or right, and it's really difficult to keep straight.

This is because you've got angled landing gear and because your back end is trying to lift off, and because your CoL is behind your gear, it acts as a lever, jamming your nose gear into the ground and removing traction from the rear gear. The combination of those destroys your stability on the ground, just like having a CoL ahead of your CoM does in the air.

[Snark]

13 hours ago, FalloutBe said:

I don't understand how as the center of lift is behind the center of mass.

It's because the position of the center of lift isn't all that important.  Having it behind the center of mass is generally necessary, yes, but it's not sufficient.  It's a very common misconception that "an aircraft will be stable if its center of lift is behind the center of mass."  What's important is that the center of drag be behind the center of mass.

The problem here is that your center of mass is near the back of the craft, with that long draggy fuselage sticking out in front, and no good way to stabilize it.

If you can move the CoM a lot farther forward, that will help you.

I'd also suggest getting rid of that double slanted tail and have a single vertical stabilizer in the middle.  Slanted-tail and twin-tail designs can have problems; a single vertical stabilizer tends to work best.  Be sure to disable roll authority on the vertical stabilizer, that will make it more effective.

Agree with the other posters that you need to straighten your landing gear, and add more wing area.

[FalloutBe]

Thanks a lot everyone!

All the replies helped a lot, and I've now built multiple planes which are easy to control!

This post also helped me a lot

and this chart was the reason for me to pick the aerospike engines. I believed they were my best choise in terms of IPS but it turns out this chart did not include airbreathing engines!

So yeah, those are even better on Kerbin.

Edited August 22, 2016 by FalloutBe