Flipping and mass

[Jestersage]

During reentry, my plane can flip upward if the angle is bigger than 30. Now I do not want to reduce the wings near the front (which makes take off difficult), so I want to adjust the weight, such as fuel distribution or consumption priority. The question then is: How much mass-distribution actually contribute to flipping?

[Echo__3]

Try to design your craft so that the center of mass does not change or perhaps shifts a little forward as fuel is used.  You can also add control surfaces like airbrakes or deploy your vertical control surfaces so that more drag is generated at the rear of the craft.

[18Watt]

If you have fuel left during re-entry, transfer some of it to the forward tanks to increase stability.   This will have the side effect of making the plane ‘nose-heavy’.  So, after re-entry, once the plane has slowed down you might need to shift the weight towards the rear again.

[AHHans]

18 hours ago, Jestersage said:

How much mass-distribution actually contribute to flipping?

A lot! Your plane wants to flip if the center of mass (CoM) is behind the center of pressure (CoP) - the point where the aerodynamic forces (in game: lift and drag) apply.

If a plane that is stable in normal flight wants to flip during reentry then this can be due to two causes: the CoM moving backwards and the CoP moving forwards.
The CoM moving backwards can happen if you have most of your fuel in front of the fully fueled CoM, i.e. the empty CoM is behind the fully fueled CoM. This is fairly easy to see - and then correct for - in the editor.
The CoP moving forwards can happen because at low speeds (e.g. subsonic speeds) the lift is the dominant aerodynamic force and at hypersonic speeds the drag is the dominant force. In spaceplanes that have the (heavy) engines at the back and the comparatively light crew cabins etc. at the front, you tend to put the wings (and thus lift) fairly far back to have it fly well at low speeds. But this often leads to having a good portion of the drag in front of the CoM and thus make it flip happy at hypersonic speeds.

Edited June 1, 2021 by AHHans
fixed typo

[herbal space program]

On 5/31/2021 at 2:59 PM, Jestersage said:

During reentry, my plane can flip upward if the angle is bigger than 30. Now I do not want to reduce the wings near the front (which makes take off difficult), so I want to adjust the weight, such as fuel distribution or consumption priority. The question then is: How much mass-distribution actually contribute to flipping?

It contributes massively to flipping, just like @AHHans said above.  If your CoL ever gets significantly in front of your CoM,  then you are toast! What you want to do to fix it is while you are getting far enough down into the atmo that your plane is starting to feel aerodynamic forces, set your SAS to hold your desired attitude and then select your fore-most and aft-most tanks using alt-RMB. Then take a look at your pitch indicator on the lower left of the screen. If it shows that your SAS is constantly applying downward pitch, then start transferring fuel from the aft tank to the fore one, stopping right as the needle reaches the neutral position. You may need to make further adjustments of this nature as the air gets thicker, but if you use that pitch needle during SAS hold as your guide, you should be able to avoid serious problems. For my part, I always take pains when designing my craft to try to have the CoL just behind the CoM both with all the tanks full and all the tanks empty. Using wet wings mounted towards the rear can help you a lot in doing this. Trying to mount at least some of your engines more forward on the wings rather than at the back helps too.

Edited June 2, 2021 by herbal space program

[herbal space program]

On 5/31/2021 at 2:59 PM, Jestersage said:

 Now I do not want to reduce the wings near the front (which makes take off difficult), so I want to adjust the weight, such as fuel distribution or consumption priority.

Just noticed this statement. If you need to put a lot of wing area in the front to take off, that means you are too nose-heavy when fully fueled.  I usually have nearly all the main wing area towards the back, with maybe a couple of canards mounted near the front. Where you put your wheels matters a lot too.  You want your rear wheels to be behind the CoM but relatively close to it, so that they bear the bulk of the weight but will still represent a good pivot for your craft to pitch up without applying too much force. Your nose wheel OTOH should be as far forward from the CoM as you can get it, so that it bears as little of the plane's weight as possible.