What is the difference between a forward, aft, high and low center of gravity?

Question

I would like to know the difference between when the center of gravity (CoG) is forward and aft? And high and low?

I already know that the lower the CoG, the greater the static lateral stability. A forward CoG is generally more stable than an aft CoG. So does this mean that a low CoG is the same as an aft CoG?

Note: If the effects of an AFT CG is not the same as Low CG, then what's the effect of each one of them or what's the difference between them?, and most importantly when is the CG is AFT or Forward?, and when is the CG Low or High?

Answer 1

I'm not sure I'm understanding what you're trying to get at, but let me try to answer the sentence:

most importantly when is the CG is AFT or Forward?, and when is the CG Low or High?

Whether the CG is aft or forward refers to its position along the longitudinal axis of the airplane, that axis that runs from the aircraft nose to the tail. Usually if people refer to just "the CG" they're typically referring to the longitudinal position of the CG as that is generally the most critical. Boeing refers to the longitudinal axis measurement as the BA, balance arm, although the longitudinal CG is usually spoken of in terms of the percent of the mean aerodynamic chord.

Let's say you have a freighter with a single 10,000 lb pallet in it. If that pallet is in the forward most position capable of handling it, the longitudinal CG would be considerably more forward than if the pallet were in the aft most position. Very possibly in either instance the respective forward and aft CG limits would be exceeded. If a loadmaster said only, "I've got a forward CG," he might mean that the load is forward of the forward limit, or he might simply mean that the load is within the limit but more forward than he likes.

Now let's say that we split the 10,000 lb pallet into two 5,000 lb pallets, and we put one of them in that most forward position on the main deck and the other in the same longitudinal position on the lower deck. That wouldn't change the longitudinal CG because both pallets are at the same longitudinal position. However, it would change the vertical CG as measured along the vertical axis. Loadmasters on 747s don't have to worry about the vertical CG because there are no defined limits for it in 747 weight and balance manuals. Loadmasters on 767s, however, do as there are defined limits. If a 767 loadmaster says he has a high CG, he can mean that the maindeck load is too great given how much is in the lower deck. He also might mean that the vertical CG of the maindeck load itself is greater than 42" above the maindeck floor. Boeing refers to the vertical axis measurement as the WL, water line.

Though you didn't ask about it, there is a third axis, the lateral axis, extending from wing tip to wing tip so to speak. Boeing refers to the position along this axis as the BBL, body buttock line, and there are typically limits as to how far from the aircraft centerline the lateral CG can be.

Answer 2

Basically your question is a combination of two aspects that were already discussed in quite a lot of detail:

Stability is already explained pretty well in What does it mean for a plane to be aerodynamically stable?. The summary is that forward centre of gravity makes the aircraft more stable, but slightly less efficient, while aft centre of gravity makes it less stable.

The vertical position is then discussed in How do the Airbus Beluga's wing/control surfaces deal with its high center of gravity? In short, vertical position of centre of gravity is mostly irrelevant. The lift in straight and level flight acts on a line passing through centre of gravity and banks with the plane, so it can't create any moment. Only the lateral force in a side-slip can, leading to slightly lower yaw-roll coupling with high CoG, but one does not normally rely on yaw-roll coupling, so it is not very interesting.

Answer 3

The vertical position of the CG on an airplane and its effect on stability is going to depend largely on the position of the wings. A high mounted wing with a low CG on the fuselage will create a very stable aircraft about its longitudinal axis; a high CG on a low mounted wing will create dynamic instability about this axis. Many aircraft which are designed for maneuverability e.g. aerobatic aircraft, military fighters, etc. have a shoulder mounted wing where the center of lift lies very close to the center of gravity along the vertical axis creating neutral roll stability.

CGs in the forward range of their approved envelop develop good lateral stability but at the expense of more induced drag as the tailplane or canards are required to produce more lift to counteract the torque the CG creates about the center of lift. Aft CGs reduce induced drag but lead to lateral instability and can be quite dangerous in a stall or spin due to the diminished effectiveness of the tailplane.