User boarding procedures causes a strange tiedown effect

[wlbragg]

From Jimis Hol on the FlightGear developers mailing list https://sourceforge.net/p/flightgear/codetickets/2741/

Untying wings and tail, occasionally, cause plane to drop down instead of raising up. It is a minor bug, for my opinion, but starts to be annoying once someone notice it. I fly C172, i save the state of craft when i quit and when i start a flight i always get out as walker to do my preflight checklist. I always secure my plane and tie it down in ground, so i find it tied down and secured when i start the game. I randomly peek passengers and baggage. I open doors and let them in by editing the 'fuel and payload' window. During the load the plane drops, a little, naturally toward ground because of the added weight. The bug is that ropes that tie the plane towards the ground prevent plane to drop down as much it wants. (That seems against physics because ropes can not transfer force that points inward to them. They cannot be pushed, they can only be pulled.) So, when i untie the plane i have the strange behavior. Left wing drops down when i untie it, right wing do the same when i untie it and tail drops in a much more degree when it is untied. That happens only when one had loaded weight while the plane was secured. If one loads the plane, then ties it and then unloads it, the normal behavior of ropes come back. For every rope he unties he sees the plane's frame to raise up. Tie and untie the plane without changing its weight between, does not make the plane move down or up in obvious matter. If it moves seems to move in the right direction. Load while plane is unsecured, tie it and then untying has the right or neglectable effect. But load it while it is secured and you will notice that ropes prevent it from dropping down pushing it up, contrary to what is expected. They do not pull it towards the ground.

P.S. I play release/2020.3 Thank you.

I think this will require someone to look into the physics of the tiedown code and see if we can account for this type of user procedures.