Hmm, well maybe it's because of the three glasses of riesling I just had, or maybe it's because I just find it hard to believe anyone called chornedsnorkack
but I still have problems with this.
However, maybe we stumbled across something interesting. Think of the aircraft on the ground and with the bird (or hovercraft or helicopter) being suspended inside the aircraft from a cable which is hanging from a totally separate crane outside the aircraft. If the crane lowers the bird onto its seat again then the aircraft will weigh more. But if it picks it up then clearly it will weigh just the weight of the aircraft.
But, you say, and other people in the thread say, that if the bird is flying (at least in an enclosed cabin) then the downwash from the bird will make the aircraft weigh more. So that seems to go back to the original nice and simple question that started the thread - aerodynamic lift is a different force from the tension in the cable attached to the crane. So it must be the downwash that results in lift.
But suppose the downwash is insufficiently strong to reach the floor of the cabin (or it's blown horizontally by a fan for example) then is there no lift anymore?
Don't feel obliged to answer this - perhaps you have a busier life than mine over there in Estonia.