Someone recently posted about how an upgrade to a heavier prop would result in an unfavorable CG.
Someone else suggested (temporary) ballast, such as oil or gear, as far aft as possible.
I'm always interested in W&B problems / solutions,
so I decided to work out how I would address this issue on my own airplane.
(1953 C180, 1652# with 34.63"CG)

My first impression was that to balance out 11# more prop at station -40",
adding 11 pounds ballast at +40" should do the trick. Wrong.
You need to add it at the same distance aft of the aircraft's CG as the extra weight up front is forward of the CG.
So to counterbalance 11# added at station -40, add 11# at station 109 (at the aft end of the standard baggage compartment).
That works out to 1674# and 34.63" CG-- perfect.

If you want to keep the weight to a minimum, you can add less ballast farther aft--
adding 8# at 139" (aft end of my extended baggage area) does the trick:
1671# and 34.64" CG.

If weight isn't a concerm you can always add more weight farther forward.
"A case of oil" is a pretty common solution, let's say it weighs 22 pounds.
Adding that at station +71.5", you come up with 1685# and 34.63" CG.
33 pounds heavier than where you started, but the CG is right on the money.

Adding weight with a longer arm might work out from a static balance point of view, but what about the dynamic balance problem which would surface should the aircraft (already not certified for acro) gets into an unintentional spin?

Quoting from MTV on another thread related to W&B:

"The question piqued my curiosity, so I checked the Type Certificate for the 180. All models have some verbiage like this in the TC:

(a) "This airplane must be operated as a Normal Category airplane in compliance with operating limitations stated in the form of placards, markings, and manuals. No acrobatic maneuvers, including spins, approved. "

It seems (without doing the physics) that adding weight to the rear of the aircraft would make this warning even more valid.

When we did the weight and balance on the Bearhawk we did it as per the book with the airplane level(tailwheel lifted) and the tail weighed 75 lbs. We then lowered the tail and weighed it again and it weighed 150 lbs. The fellow did the weighing with some scales they use for Nascar so he showed us how much the cg moved rearward. Do not remember the amounts but we were quite surprised. It made us aware of how much the cg moves rearward in the flare for landing.

PapernScissors wrote:It seems (without doing the physics) that adding weight to the rear of the aircraft would make this warning even more valid.

If you’ve added weight to the nose, you may need to add to the tail (or aft of the cg) as necessary to keep the resulting cg within limits. The resulting cg is the important part of this equation - not necessarily how much weight is where.

It’s a matter of moments - forces (applied by control surfaces) and lever arms (their distance from the cg).

175 magnum wrote:When we did the weight and balance on the Bearhawk we did it as per the book with the airplane level(tailwheel lifted) and the tail weighed 75 lbs. We then lowered the tail and weighed it again and it weighed 150 lbs. The fellow did the weighing with some scales they use for Nascar so he showed us how much the cg moved rearward. Do not remember the amounts but we were quite surprised. It made us aware of how much the cg moves rearward in the flare for landing.

This can be a deceiving exercise - remember that the cg has a vertical component as well. When you tilt the aircraft (level flight vs three point) you don’t change it’s lateral cg location(not accounting for fuel slosh), you’re just seeing the gravity vector move as it acts through the cg’s three dimensional point in space.

EDIT: corrected an autocorrect error.

PapernScissors wrote:.....I checked the Type Certificate for the 180. All models have some verbiage like this in the TC:
(a) "This airplane must be operated as a Normal Category airplane in compliance with operating limitations stated in the form of placards, markings, and manuals. No acrobatic maneuvers, including spins, approved. "
It seems (without doing the physics) that adding weight to the rear of the aircraft would make this warning even more valid.

Yes, that's probably true, but it should not be unsafe as long as the loaded CG is kept within the design limitations.
Manufacturers usually do considerable flight testing when developing these limitations, which I imagine are pretty conservative--
that way they protect themselves from pilots who might tend to push the envelope.

The alternative to compensating for CG changes is to just live with them.
In this case, a heavier-by-eleven-pounds propeller, I didn't do the math to see if it would cause the loaded CG to be out of the approved CG envelope and therefore unsafe. But even if it didn't, the change in CG might adversely effect the handling characteristics.

Hodrod, You beat me to it. Just as you said, Cessna spent lots of time flight testing to establish the cg envelope. Stay within the envelope and life will be good. A few more pounds on the nose is not often a problem. Usually the problem is at the other end of the teter totter with loading progressively moving the cg back aft of the limit. Usually the only time you end up with a forward of limits cg is with two big guys in the front seats with full fuel on heavy forward cg amphibs. That is usually solved with usable ballast, survival gear, adult beverages, dead moose etc. As for operation with the cg at or forward of the aft design limit, its done all the time with large aircraft in revenue service. It is more fuel efficient due to reduced tail downforce.

The change in cg really does change the handling especially in slow flight/landing mode. The forward cg can have you run out of trim on landing. On my 172 I carry my survival bag (50) in the extended cargo bay and it changes the flight characteristics completely and for the better.

There are limits to what "just stay in the envelope" mean in terms of spin recovery. But a TC moment arm and mass envelope is all that is usually necessary for safety.

In a spin, the mass distribution does indeed make a large difference in spin recovery technique and feasibility. The dynamics depend on the distance^2 of a mass from the center of mass (CG), not just the distance. It is a very different ballgame. More mass distribution away from the CG means flatter spins and slower recovery, for example. Same total mass, different distribution, different dynamic effects. Think of an ice skater in a spin, or a hinged rotorcraft blade.

This factor never really makes it to the general pilot population because the cert specs are actually very conservative, and to my knowledge there are no documented TC's out there where the published static loading envelopes do not already take into consideration potential dynamic loading characteristics (things like full wing tip tanks, for example).