How can a plane be cheaply owned by multiple people?

I've been in a couple of partnership syndicates, including one that owned 3 Cessna 180s on floats, that was set up as a limited liability corporation with 15 equity members as shareholders, and more or less operated like a private flying club.

However, the sweet spot for a single airplane partnership is no more than 2-3 owners. Any more, and the potential interpersonal issues and the fact that you are creating a "flying club" are not worth the minor additional dilution of fixed costs going from 3 owners to 4 or more.

A lot of partnerships go sour, and the problem becomes how to sell an individual share in an airplane that the other partners may want to keep, and who don't want to buy you out. You have to find a buyer who is willing, and the buyer has to be approved by the other two partners. That can be tough.

It's best done with a written co-ownership agreement that lays out everything in writing to minimize disputes, such as how and who does maintenance, hourly rate to be charged, booking policy, and various rules. Of course you need to have full in-motion hull insurance to protect everybody's equity. Even a two-partner operation should really be run like a mini flying club to keep it going smoothly.

It's also better for the 3 owners to have the maintenance done by a 3rd party, with the partners just paying out equal shares of cash for whatever comes up. If one of the partners does significant work on the plane, it can create problems with uncompensated work leading to resentment, plus you get disagreements between the partners on what or how things should be done (this happened to a friend that was in an RV-6 threesome and did all the maint - he was secretly relieved when another partner wrote it off when he flipped it (unhurt) and he was cashed out on the hull insurance). Better to just give it to a mechanic and share the bill.

It can work quite well, because most owners only fly a few dozen hours a year, not enough to justify bearing all the fixed costs, and splitting ownership 2 or 3 ways has little practical impact on availability, but be careful. Some friends own 1/5th shares in a pristine Aeronca Champ, and it seems to work ok, and the cost dilution makes it pretty affordable, with each owner only having about 4K in equity, a thousand dollar annual split 5 ways, and a 65 horse engine that only burns 4 GPH.

what you describe could be called a flying club or shared ownership. These sorts of arrangements are common enough that you should be able to find a template for setting up one which contains the details of how maintenance and overhead are covered and how the club's finances must be recorded and reported.

The cost effectiveness and fairness are determined by how money flows into the partnership and how it flows out. I would suggest you look into the Experimental Airplane Association's documentation on airplane ownership.