# Why should the centre of mass be ahead of the centre of pressure?

Why is the centre of mass located ahead of the centre of pressure? And what would happen if centre of mass would be too close or too far away from the centre of pressure?

• It isn't. The overall centre of pressure must coincide with centre of gravity to maintain attitude; otherwise the plane would rotate. – Jan Hudec Oct 11 '16 at 20:44
• – Peter Kämpf Oct 11 '16 at 22:46

On most airplanes, the horizontal stabilizer provides a downward lift. The center of mass is between the nose gear and the main gears.

If the center of mass is too aft:

• Stall (and spin) recovery will be difficult or even impossible
• Increases the chance of tail strike
• The plane will tip on its tail if the main gears are not moved aft

If center of mass is aft of center of pressure, then the horizontal stabilizer must provide positive (upward) lift; the tail structure must be made stronger to support the higher lift. The wings will be moved forward of the main gears to keep the aircraft balanced on the ground. Such designs are called Tandem wing. See also this question.

• Thanks for answering, but I dont really know what "aft" means, Could you clear that up? – Devansh Rathi Oct 11 '16 at 16:34
• "aft" is a fancy word for "towards the back of the aircraft". Dictionary – abelenky Oct 11 '16 at 16:35
• Oh, thanks! Sooo, what would happen if the centre of mass is ahead of the centre of pressure, but really really close to it. What are the problems that are faced when this happens? – Devansh Rathi Oct 11 '16 at 16:36
• @DevanshRathi you may check out this related question. – kevin Oct 11 '16 at 16:38
• Why should the tail assembly be stronger if the tail provides an equal amount of upwards lift vs. downwards lift? Also, this answer doesn't really address why stall/spin recovery will be more difficult (perhaps just link to a question like this?) – Sanchises Oct 11 '16 at 16:41