Unstable Planes

[Wizard Kerbal]

I made an F15 replica, and made sure the CoL was Behind the CoM, even though they were close to support super-maneuverability. When I turned though, it began to flat spin. I drained the fuel in the SPH, and still the CoL was behind The CoM. Help

[Snark]

9 minutes ago, The wizard of me said:

I made an F15 replica, and made sure the CoL was Behind the CoM, even though they were close to support super-maneuverability. When I turned though, it began to flat spin. I drained the fuel in the SPH, and still the CoL was behind The CoM. Help

Could you give us a screenshot?  Ideally in the SPH with the CoM display visible.

Note that having the CoL behind the CoM does not mean it's necessarily stable.  For example, if you've got the CoM close to the back of the vehicle, it'll probably be unstable regardless of where the CoL is.

Seeing the craft would make it a lot easier to provide specific advice. 

[Wizard Kerbal]

Bout this question, I redesigned my F15 and it fixed it’s predecessor’s problems.

And yes, the problem was a very far back CoL.

Jesus I went full out with the pitch and it turns around in like .5 seconds but then it spins out

Edited January 31, 2021 by The wizard of me
Turning resuslts

[FleshJeb]

Reduce the authority of your control surfaces, and fly it with caps lock on.

[Guest]

Yeah pictures would help. You should read the CoL/CoM "rule" as "if your CoL is in front of your CoM, your plane will definitely be unstable, but the converse is not necessarily true." 

The problem is with the centre of pressure, which changes depending on your flight attitude and other characteristics. Specifically, canard designs and designs with lifting parts -- such as Mk2 parts -- close to the nose are significantly less stable than you'd expect just by looking at the CoM/CoL. A very "traditional" design with a pointy nose, narrow wing near the middle, and hefty tail OTOH will behave pretty much like you'd expect.

I make a lot of canard designs, and I just tune them. If they're unstable, I move the CoL back or the CoM forward until they're stable. I also often bind the canard's deployment angle to the F/B axis, this way I can trim it very precisely: if it's designed to be safely stable (nose-heavy) at 0 degrees deployment, it'll likely be almost neutral at fairly low speeds at 5 degrees deployment. So for take-off and landing I set the canards to 5 degrees, and for cruise they're at 0-1. 

It is possible to design almost aerodynamically neutral, yet stable planes this way, which are really enjoyable to fly.

Edited January 31, 2021 by Guest