After deciding to put a laundry chute in our house the vexed question was whether to have the door/flap swinging either to one side or the other, hanging down or swinging up. Which ever way it was done the next question was how to keep it there so that it didn't get in the way, have someone walk into it or have it fall down on someone's head.
After much deliberation (which is the best part about making something, as all the many ways that come to mind can be critically examined) The best and simplest way was to use an over centre weight which would hold it securely both raised and lowered and be very quick and easy to use.
While the weight appears to be in the way when the flap is lowered, the only time the adjacent washing machine is used is when the flap is raised, so the weight is then completely out of the way.
The upstairs flap only needs the hinges setting far enough forward from the wall to give a large enough angle for the flap to stay back under it's own weight when
raised.