Power groups, engines and shafts¶
A power group is the engines, the gearbox and the shafts coming off it. Most aircraft only need one.
Power Group (RFM_PowerGroupConfig)
| Field | What it is |
|---|---|
| Id | A unique name, like drivetrain.main |
| System | The engines, gearbox and shafts (below) |
| Governor Anticipator Rotor Id | The rotor whose collective the engine governor watches. Use the main rotor. |
| Sources Running At Spawn | On means it spawns with the engines running. Leave it off for normal play. |
System (RFM_MultiEngineSystemConfig)
| Field | What it is |
|---|---|
| Sources | One entry per engine (below) |
| Output Branches | One entry per shaft (below) |
| Reference Output Branch Name | The main rotor shaft, like shaft.main |
| Governed Rotor RPM | Main rotor RPM at 100 %. Has to match the main rotor's Nominal RPM and each engine's Governed RPM. |
| Combiner Inertia Kg M2 | Inertia of the gearbox itself, see the inertia table below |
| Common Accessory Torque Nm | Drag from stuff the gearbox drives (hydraulic pumps, generators) at 100 % RPM. The Huey uses 120 N·m. |
| Supervisor | Twin engine speed matching and load sharing. Leave it on (the default) for twins. The Huey turns it off since it only has one engine. |
| Lump Rigid Output Branches | See the inertia table below |
| Rotor Side Inertia Kg M2 | Only used when there are no output branches. Ignore it otherwise. |
Source (one engine, RFM_PowerSourceSlotConfig)
| Field | What it is |
|---|---|
| Name | A unique name, like engine.left |
| Engine Config | The turbine's data, see Engine config |
| Load Share Weight | Equal numbers means an equal share of the load |
| Normal Power Limit Fraction | Normal max as a fraction of the engine's rated power (for example max continuous ÷ takeoff) |
| Contingency Power Limit Fraction | Emergency (one engine out) max as a fraction of rated power |
| Auto Contingency When Sole Running Source | On means if the other engine fails, this one gets the contingency limit automatically |
| Input Gear Speed Ratio, Physical Source Inertia Kg M2 | Leave these at 1 and 0 |
Output branch (one shaft, RFM_PowerOutputBranchConfig)
| Field | What it is |
|---|---|
| Name | A unique name, like shaft.tail. Rotor units point at this. |
| Role | 1 = main lift rotor shaft, 2 = tail rotor shaft, 0 = other |
| Speed Ratio | This shaft's RPM ÷ the main shaft's RPM. For the tail it's tail rotor RPM ÷ main rotor RPM. |
| Inertia Kg M2 | The spinning mass on this shaft, see below |
| Parent Branch Index | −1 means it comes straight off the main gearbox (the normal case) |
Where does the rotor's spinning mass go?¶
A heavy rotor takes a while to spin up and keeps spinning when the power goes away. That's what makes autorotation work. RFM needs that rotational inertia in exactly one place, and there are two ways to lay it out:
| Layout | Main rotor inertia goes in | Tail rotor inertia goes in | Gearbox inertia goes in | Lump Rigid Output Branches |
|---|---|---|---|---|
| Separate shafts (recommended, Mi-8) | shaft.main Inertia |
shaft.tail Inertia |
Combiner Inertia Kg M2 | off |
| Lumped (UH-1) | Combiner Inertia Kg M2 (everything, referred to the main shaft) | tiny (0.0001) | included in Combiner | on |
Only use the lumped layout if you want Drivetrain Torsion (the shaft "wind-up" wobble between engine and rotor). It only works with one engine and lumped outputs. For everything else, use separate shafts.
Also set the rotor unit's Shaft Reaction Inertia Kg M2. That's how hard the fuselage twists when the rotor speeds up or slows down:
- 0 works it out from the blade mass (needs Blade Mass set on the rotor config).
- A positive number uses that value directly (the Huey uses 3103).
- −1 turns it off (the Huey's tail rotor).
To estimate a rotor's inertia: blades × one blade's mass × radius² ÷ 3. So a 2 blade rotor with a 7.3 m radius and 100 kg blades is 2 × 100 × 7.3² ÷ 3 ≈ 3,550 kg·m².
Don't have the blade mass? Use 0.0135 × max takeoff mass (kg) × radius². It comes out within 1 % on both templates (Huey: 3,110 against the real 3,103, Mi-8: 19,880 against 19,942).
Check: the [RFM] reference power system line prints the total driven inertia. If your rotor windmills like crazy in light wind with the engine off, this number is almost always too small.