In an aircraft share be prepared to lose everything you put in. Anything you get back at the end is a bonus. These are not sound investments like bricks and mortar. They should retain their value but don't be surprised if they don't.
In the worst case, if you can't afford the monthlies the goup will sell your share, take the amount owing off then give you the difference, its a bit like repossession. They may simply give your share to someone willing to pay the monthly bills. Some planes are £1000 per share but £200 per month. In this case you will happily walk rather than spend 10 months waiting for a sale. There are syndicates that give non voting shares away just to get the monthly subs in. These can work out well, if the plane becomes US for a while or you don't fancy it any more just walk.
Bear in mind the problem may not be of your making. What happens if your £20,000 aircraft requires a wingspar mod at a cost of £25,000. Or someone dings the prop , shockloads the engine and the insurance won't pay - cost £20,000. If the aircraft is a group of 10 and you all paid £2,000 per share, will you pay another £2,000 to keep the same thing you had before, or will you vote to sell it for scrap and take £300 each, then invest your £2300 in something else.
On the bright side this rarely happens. Shares in established groups pretty much change hands regularly at the standard rate. Accidental damage is usually covered by insurance and everyone is happy.PA28s and 152s change hands fast because there are lots of new ppls looking for something to fly. Try selling a half share in a Hunter and you may be waiting a little longer.
Good Luck