# Is the differential between the forward and aft CG limits indicative of the overall stability of an airplane?

Studying up on the Cessna 162 Skycatcher POH to take my checkride in soon and I noticed the differential between the maximum and minimum CG's is a mere 2.5 inches from one to another.

I got curious and looked up the CG limits for the two other planes I train in, the CSA/CZ SportCruiser and the Cessna 172, and saw that the differences between their forward/aft CG limits are 6" and 12", respectively.

In terms of aerodynamic stability, I would indeed rank the Skycatcher (2.5" CGfwd/CGaft differential) as the least stable and the Skyhawk (12") as the most stable. Is there any connection between forward/aft maximum CG differentials and the amount of aerodynamic stability in an aircraft?

Edit: Still reviewing the responses, having been sent to the encyclopedia several times to study up on new terminology, but I just want to emphasize that the SportCruiser (6" diff) is dramatically more stable in the air than a Skycatcher (2" diff). Both of these aircraft have the same max gross takeoff weights, 30 foot wingspans and 23 foot lengths. So perhaps the Cessna 172 could be ignored for the purposes of addressing the question.

• Very interesting question. Might be worth normalizing the intra-CG limit differential to something else too, like the plane’s overall length or the chord length. (You’d expect a smaller plane to have a smaller differential than a bigger plane for the same stability characteristics.) Aug 25, 2016 at 0:34
• @Peter, true. Engineers usually express the CG range (as well as the static stability margin, i.e. the distance from CG to the aerodynamic centre, and several other characteristics) in terms of Mean Aerodynamic Chord. So the range may be specified as, say, 15..25% MAC. But MAC is a theoretical value, so a fixed station and physical units are often more convenient for pilots and service personnel.
– Zeus
Aug 25, 2016 at 15:28

## 2 Answers

Normally, the aft limit is governed by static stability, while the forward limit is governed by the elevator control capacity (but sometimes, dynamic stability: excessive static stability may cause oscillations).

So, a narrow CG range would rather suggest that the elevator and/or trim capacity is not sufficient to cope with the more forward CG.

However, in practice there may be other reasons for restricting the range. For example, it may simply be impossible to load the aircraft (without exceeding the station load limits) in such a way as to put CG out of the defined range. I think C152 is close to that. Or, say, the stall or spin characteristics are unsatisfactory, so the aft limit may be restricted further than what normal stability dictates.

Needless to say, artificial stability augmentation (including FBW) may broaden the CG range, primarily by affording a more relaxed aft limit.

• Expanding on the reasons is the expected utility of the aircraft. The 162 is a small trainer with 2 side-by-side seats and limited baggage space. It's just not possible to move the CG very far. OTOH a Cessna 206 or 210 with three rows of seats needs to be able to accommodate a much larger variation in loading options -- from 2 fat guys in the front to a family of six with baggage. Aug 25, 2016 at 12:49
• You may enhance your answer by adding drawing and references to illustrate and support your answer; and to provide further reading. Aug 25, 2016 at 12:59
• Wouldn't one expect the aircraft's static stability to govern the location of the middle of the allowable CG range, with both the forward and aft limits being defined by the aircraft's maximum pitch control authority? Jul 5, 2018 at 22:20
• @Sean, no. The middle is not really any sort of equilibrium or the optimum. The farther aft the CG is, the less statically stable the aircraft is. To control a statically unstable aircraft, one needs extremely fast responsive controls (normally driven by computers), but not necessarily 'strong' ones. Sure, at some extreme you'll hit the limit again, but this will not be due to insufficient moment produced; rather, because hitting the travel limit denies you the opportunity to make further dynamic corrections, which are vital to compensate instability. This applies only to really unstable a/c.
– Zeus
Jul 6, 2018 at 0:31

Aerodynamic static stability is controlled by the distance between the center of gravity and the neutral point of the aircraft. A limited c.g. range has nothing to do with stability per se. Every aircraft gets more stable, and more heavy on the controls, when the center of gravity is moved forward.

Two-seaters with little payload, as LSA tend to be, need less c.g. variation than four-seaters which should be flyable with the rear seats both occupied and unoccupied without adding ballast. I would guess that Cessna restricted c.g. travel even more when they cured the Skycatcher of its flat spin tendencies.

For the rest of the effects please read Zeus' answer. I see no need to replicate its content here.