Originally Posted by
phylosocopter
google coffin corner
More pertinent here is flight with reduced static pitch stability. Basically, reducing static stability margin (flying with the CofG and centre of pressure closer together) means you need less downforce on the horizontal stab to counter the nose-down pitching moment. This means lower trim drag AND less lift required from the rest of the airframe to counteract the horizontal stab downforce. Less drag = less fuel burn = more efficient. Less induced drag from needing less lift to counter the horizontal stab = less fuel burn = more efficient.
The design challenge is to minimise cruise trim forces on the horizontal stab whilst maintaining JUST enough static stability margin. Works fine until you stick two new ruddy great big engines (with more wetted surface area) forward of the normal centre of pressure...