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. Oct 11, 2016 at 20:44
• Oct 11, 2016 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? Oct 11, 2016 at 16:34
• "aft" is a fancy word for "towards the back of the aircraft". Dictionary Oct 11, 2016 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? Oct 11, 2016 at 16:36
• @DevanshRathi you may check out this related question. Oct 11, 2016 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?) Oct 11, 2016 at 16:41

Consider an arrow. Its center of pressure is behind the middle of the arrow due to the fletching (feathers) at its back (aft) end, but its center of gravity is close to the middle. If it isn't pointing directly into the path it is taking, the center of pressure, being behind the center of gravity, pushes in the direction to restore it to pointing where it is going. Were it the other way around, the center of pressure would cause it to diverge from that direction, ultimately turning around. That's why arrows are shot with the feathers in the back, not the front.

Note that the vertical surfaces on an airplane (i.e. the vertical stabilizer and rudder) are on the back of the airplane for the same reason.

If the center of mass is behind the center of lift, then the tail wings must produce an upward lift.

This lift decreases when the plane pitches up.

This decrease in force makes the plane pitch up more which is unstable equilibrium