Hey just a small Update! I got it to work now. I used Unrotate Vector. I found this method after scrolling through the comments somewhere. This is what my code looks like now and it seems to do what I want.
FVector V = GetVelocity();
FRotator R = GetActorRotation();
V = R.UnrotateVector(V);
float XVel = V.X;
FVector PlaneUp = GetActorUpVector();
PlaneMesh->AddForce((PlaneUp)*(XVel));
I don’t know if there is a better way of doing it but this seems to work for now :). Again thank’s for your help!