Weight tuning

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Making a stable, lightweight, and complex vehicle is a difficult challenge in RoR. You will certainly run into this problem if you try to do a light airplane (any airplane less than about 20 tons).

The root problem is that strong beams attached to light nodes are physically unstable. In RoR physics, only nodes bear weight. To ensure a realistic distribution of weight, the node weights are computed as this:

• Compute the sum of the length of all beams
• From the global net weight, compute the linear density of beams (total mass/total length)
• For each node, set its weight as the sum of the weight of all beams attached to it (in fact, half the weight of the beams, to avoid double counting weight)
• Add extra load weight to the loaded nodes (the total load is divided by the number of nodes declared as loaded)
• Fix weight of ropes, hooks, and cameras to predefined "stable" values
• To ensure stability of all nodes, if a node is found to be below 50kg, its weigh is set to 50kg (this can happen if a node is only connected to very short beams)

At the end of the process, the final, total mass of the vehicle can be higher than anticipated, especially if the last rule applied to a lot of nodes. With this rule, a 100 node vehicle can never be below 5 tons. Note that wheel node weights are computed separately depending on the wheel weight settings.

To know if your vehicle has gained weight, enabled mass debugging in the Configurator and load the vehicle into RoR. Leave and look in RoR.log and check the last line "TOTAL VEHICLE MASS". This is the final total mass (including wheels).

If it is too much, you will need to lower the minimum node mass setting. You can do it with the " Minimass" command (at the beginning of the truck/boat/airplane file):

minimass
25.0

You can also lower the global mass, to reduce the mass of "heavy" nodes (those not concerned by the minimum mass rule).

When you lower minimass, you will encounter physics instability. The solution to instability is to lower the rigidity of beams, using the "set_beam_defaults" to soften them until they are stable with the new mass settings.

But when you lower the rigidity, parts of the vehicle will be too fragile. So use set_beam_defaults to adjust strength depending on the stress elements will bear. For example, landing gear beams will need stronger beams, and fuselage softer beams. You can also use loaded nodes to balance and/or stabilize specific unstable nodes.

So with the minimass and set_beam_defaults commands, with much guessing, trimming, and luck you can achieve lightweight airplanes.