Help - I'm Stumped on why this Aircraft is Unstable

[Doctor_E]

I've been designing this SSTO to go to Minmus for the past few days, and I've encountered a problem that I just can't seem to solve. Even with the center of lift behind the center of mass, the plane flips over upon liftoff like the COm is in front of the COL. Does anyone know what's going on? I've tried everything and it doesn't seem to work.

 

Craft File: https://www.dropbox.com/s/vt720udwauxqzkg/Minmus Aircraft Mrk_ 2.craft?dl=0

Mods needed: OPT Space Plane Parts

Edited April 9, 2017 by Doctor_E

[MiffedStarfish]

I was always better at designing VTOLs, but unless you want do do loop the loops on your orbital ascent, I don't think you need the COL quite THAT close to the COM.  Also from what I can see, over half of the control surfaces are in front of the COM, which can't be good. I would remove the front canards and disable some of the control surfaces in front of the centre of mass. 

If someone comes along who knows how to build planes, ignore this. 

 

[FancyMouse]

Looks a textbook CoL in front of CoM situation

(note that stock CoL indicator in SPH isn't your true CoL due to lack of body lifts and AoA consideration)

[WanderingKid]

I'm not sure about OPT Space Plane Parts, but I know the COL/COM indicators can lie even with some stock bits.  If it's not registering lift off the pieces in the front of the plane, your COL is WAY up front.

I'd start with dragging those forward swept wings WAY back and seeing what happens.

[Spricigo]

Basically your CoM it's so far behind that your plane want to fly tail first.

Also canard... When you pitch the center of drag moves forward,  another reason to the plane wanting to fly tail first. 

Finally,  this red sphere is the CoM  for the ship empty.  Which means that after some fuel burned the CoL will be in front of the CoM

[Doctor_E]

Thanks for all the responses. I guess it was a case of the COM and COL being incorrectly shown. I had to drag the wings back only a little bit farther than I had *sigh*.

[kcs123]

You are using RCS build aid that shows dry mass way too behind CoM. After liftoff, as soon as you spend a bit of fuel CoM is behind CoL making craft unstable. Also, with stock aero, it might not always show CoL properly due to body lift and stuff others have mentioned.

[AeroGav]

The stock aero model treats parts in one of two ways - as wings, or as fuselage pieces with drag cube.

The SPH / VAB blue indicator only takes account of parts which classify themselves as wings in the config file.

Even though other parts make lift and drag too.

Get the mod called CorrectCoL,   it  makes the blue indicator take account of every part on your ship, you will find the CoL was further forward than you thought it was.

 

Also, as others have pointed out, the Red indicator shows that your CoM is going to move aft when the fuel tanks are empty, making the situation worse.  Your small adjustment will might make it possible to launch but on return from orbit it's going to spin out.

To fix this , you've either got to move the CoL much further aft , or rearrange stuff  with more dry mass (engines) moved forward and more fuel tanks aft, so the CoM doesn't shift so much.

The first solution is easier I suppose, but makes the craft less efficient in flight.   You will need a  lot of torque to keep the nose up.  This is not as bad if you are lifting the nose with canards up front, but if you're keeping the nose up by pushing DOWN from elevons at the back, you're fighting against the wings.   Also because your CoM is so close to the back,  elevons at the back are going to need more force to achieve the same nose up torque .   I normally set elevons to roll control only and canards to pitch control only for that reason.

 

[Aegolius13]

Others have touched on this, but when your CoM is very far towards the rear, even moving lift back may not work too well to provide stability.  Whenever possible, I try to avoid having all of my engines at the very back, since they're so dang heavy. For example, you could try moving those Rapier nacelles forward.  Unfortunately that would probably mess with the aesthetics of your ship, which looks really nice as is. 

[Spricigo]

55 minutes ago, Aegolius13 said:

Others have touched on this, but when your CoM is very far towards the rear, even moving lift back may not work too well to provide stability. 

A bit further explanation : 

Tail heavy plane s are unstable,  each time it deviate from the airflow it wants to keep going. And since most of the wings/control surfaces are close of CoM it's difficult to apply the restaurative force necessary. 

Adding offence to injury: even with enough control authority to point in the right direction, the plane don't want to stay there so constant correction is needed.