# Are there any airplanes with the Centre of Gravity behind the Centre of Pressure? [duplicate]

We are used to seeing a forward CG in all airplanes, from a small Cessna to a large Boeing.

I wonder whether any airplane with CG behind wing CP exists in production? Is it even possible? If so, what about its stability?

EDIT: I mean wings CP, because it is the most common definition of the airplane CP. Usually for civil aircrafts the wings produce lift, and the tailplane produces downforce to counteract wings rotation moment about CG. These aerofoils (wings and a tailplane) have 2 separate CPs and two forces in the opposite directions. I ask about the wings CP in normal wing AoA ranges (excluding aerobatics, critical AoAs etc.)

In steady flight both points are exactly at the same lengthwise location. The pilot is able to shift the center of pressure somewhat by moving the elevator. Therefore, every aircraft that pitches up will become one where the center of gravity is behind the center of pressure.

But I guess you did not request this answer but a different one. That, however, would require a different question: Are there any airplanes with the Centre of Gravity behind the Aerodynamic Centre? Please read here about what those different centres are.

Most aircraft are indeed built such that the center of gravity is ahead of the aerodynamic center. This allows the pilot to let go of the stick momentarily and gives him valuable force feedback on the stick when the aircraft's speed or load factor changes. And it is required for civilian aircraft so they can be certified.

However, placing the center of gravity further aft has become a de facto standard for military combat aircraft with supersonic capabilities. Please read here why this helps. To summarize:

• Agility is enhanced
• Supersonic trim drag is greatly reduced
• Smaller wing and tail surfaces are needed for the same take-off and landing performance.

The first aircraft which was designed with the center of gravity behind the aerodynamic center was the Wright Flyer I. When designers found out that aircraft could be made naturally stable, they placed the center of gravity more forward, and only with fly-by-wire controls did they return to the rear center of gravity location. The first design to do so on purpose was the F-16, and today every fly-by-wire combat aircraft with supersonic capabilities follows the same path.

• +1'd but I feel your last paragraph disagrees with your source, which states that longitudinal stability was well understood by the time of the Wrights and that "[...] they experimented successfully with an aft tail. They knew that the configuration could easily be made stable. [...] the canard configuration, the most distinctive feature of the Wright aircraft, was not based on sound technical grounds of stability. It was rather a matter of control in pitch, especially under extreme conditions." From your answer I got the idea that rear tail stability was developed later. And ty for the read – AEhere supports Monica Feb 19 '19 at 12:52
• @AEhere: The first aircraft designers like Penaud and Lilienthal copied birds. Maybe they understood stability already; from what I read of the Wrights they clearly did not. They noticed the restlessness in pitch and decided to move the cg aft. That this helped at all was caused by the increase in pitch inertia which lowered the eigenfrequency and made the aircraft easier to control. I have read a book from 1911 where the author clearly was clueless about pitch stability. My impression really is that it took until around 1915 for pitch stability to be widely understood correctly. – Peter Kämpf Feb 19 '19 at 17:59
• @avtomaton: The wing has an CP, and so has the whole aircraft, including the tail. The latter one is the one I am talking about, and it should be obvious that it must be at the same datum as the CG for stationary flight. – Peter Kämpf Feb 20 '19 at 7:25
• @avtomaton: Then the CP of a wing should also not make sense to you. In all cases it is the point where all vertical pressure components can be summed up. You cannot look at the parts separately and add them. They interfere with each other, so they must be looked at in combination. If you think of a CP as something only a wing can have, consider this a chance to improve your understanding. – Peter Kämpf Feb 20 '19 at 21:00
• @avtomaton This is highschool physics, the airplane needs to be in equilibrium, so the combined forces on all the bodies (wings, tail, fuselage, etc) need to balance out. The step from the individual wing CP to the CP of the whole airplane is seldom taken in the books because it is assumed to be a trivial conclusion, and most of the time it is easier to forget the fuselage exists and just add up weight + wing lift + elevator lift. – AEhere supports Monica Feb 21 '19 at 8:08

The reason for a CG forward of the CP is that this configuration is generally longitudinally stable, similar to an arrow or dart. However, highly maneuverable aircraft, such as military aircraft, might have a CG behind the CP which makes the aircraft longitudinally unstable but more controllable. Complex computer controls will maintain controllability of the aircraft even though it is unstable.

https://en.wikipedia.org/wiki/Longitudinal_static_stability

checkout static margin and the diagrams in the above link.

• I understand the theory, but I asked about some examples... I understand that in theory that kind of airplane would demonstrate high instability, but I wonder whether they are really manufactured and used. – avtomaton Feb 16 '19 at 23:24
• sorry, I've been looking for a while and cannot find any specifics, besides some balancing examples in army manuals. My guess would be that supermaneuverable aircraft such as the f-22 have a CG envelope that goes behind the CP. – FurEter Feb 17 '19 at 1:29