As an aircraft designer you want to understand the properties of your design. Perhaps you want to compare it to other aircraft, not to mention get solid answers to a variety of what-if questions, such as:

• What happens to trim drag if you re-size the horizontal tail or change your wing’s aspect ratio?
• How much wing twist should you incorporate to promote favorable stall characteristics while not impacting cruise drag too much?
• How much torsion, shear, and bending loads will the wings have to react? How much stick force will the pilot experience when deflecting the ailerons at a give flight condition?
• What is the minimum required elevator deflection at forward CG in the landing configuration?
• What do your airplane’s Phugoid or Dutch Roll modes look like?
• Are they satisfactorily damped?

There are hundreds of other questions that all require hard-to-get answers. Until now.