Setting up idle speed is a four-step process:

  1. Setting the minimum “smooth” running speed.
  2. Getting the internal friction right.
  3. Setting the right amount of throttle to overcome the internal friction.
  4. Setting the fuel flow for that idle throttle

The “minimum engine running speed” setting in Plane Maker does NOT set the idle speed itself, but the minimum engine RPM that results in a smooth operation. This speed is lower than what we want the engine to actually idle at. If the RPM falls below the minimum running speed we will experience “stumbling” that manifests in RPM and torque fluctuations. If RPM drops far enough below that minimum speed, combustion itself will be affected and unable to sustain engine operation at all. The engine will come to a halt. The strength of the stumbling effect at lower-than-minimum RPM is determined by the number of cylinders that is set in Plane Maker. A four cylinder engine will exhibit more stumbling than a 6 cylinder engine or a radial engine with even more cylinders.

For a typical flat-four engine, the target idle will be around 600-700 RPM, while the minimum running RPM should be set closer to 500. This provides a good margin to keep the engine running under load from the alternator and at higher altitudes and hotter days where the engine makes less power. If the RPM drops below that setting of 500, expect erratic engine behavior.

The idle throttle setting serves one purpose, and that is overcoming the engine’s internal friction. The idle speed of the engine is determined by the equilibrium of the torque produced by the engine at idle throttle setting and the torque consumed by the engine at idle. That torque that needs to be overcome is the engine’s own internal friction, plus the small torque consumed by the propeller rotating at slow speed, plus any additional loads from the generator the engine needs to turn.

The engine’s internal friction is controlled in Plane Maker with the “engine friction” ratio. X-Plane’s guess at 1.0 works for a big-bore high compression direct-drive engine such as the IO-550 found on a Cirrus or Columbia. Other engines with different compression ratios or gearing will likely require different values here.

A good way to find the right internal friction, if the propeller is set up correctly, is to turn the engine off in flight (by cutting the mixture) and slowing the aircraft down to just above stall speed. The engine should almost stop turning as the airplane approaches stall speed, but rotate practically unaffected at greater air speeds (note that once fully stopped, a disproportionately high airspeed is needed to get the prop turning again).

The suggested process is to tune the sim/aircraft2/engine/engine_friction_ratio dataref in the simulator, take note of the value that produces the desired result, and then set it to the acf permanently in Plane Maker.

Once the internal friction is set, the airplane should be placed at a sea-level airport in standard atmosphere (15C OAT, 1013hPA QNH) and the electrical system loaded up with all electrical loads that are reasonably expected to be on on the ground ready for take-off, that is avionics bus powered, navigation and landing lights on, etc.

Then the idle throttle adjustment (sim/aircraft2/engine/high_idle_ratio) should be made so that the engine runs at a minimum smooth idle RPM with the throttle all the way out.

This will likely be too low to run the generator or alternator enough to charge the battery. That is not a bug, but corresponds to behavior found on many real piston aircraft. Basically all real piston engines require a positive throttle input to run at a high enough RPM to sustain generator load to charge the battery along with other electrical loads.

It is important to let the engine run for a minute or longer, as it might take some time to find the equilibrium between drive torque (generated by the engine) and drag torque (consumed by the engine itself, the propeller, and the generator).

The idle should be tweaked in the simulator with the sim/aircraft2/engine/high_idle_ratio dataref, and the desired value then set permanently in Plane Maker.
Once the engine runs stable at a low RPM in this config, it is up to the user (i.e. pilot, not aircraft developer) to run the throttle up enough to sustain battery charging.

Finally, the desired fuel flow at this setting can be adjusted with the dataref sim/aircraft/overflow/ff_rat_idle_PRP in the simulator and permanently saved to the acf in Plane Maker on the SFC (specific fuel consumption) tab.

Note that setting the engine up like this corresponds to a sea-level setup as would be performed on most aircraft.

Taking this aircraft to a high elevation airport such as Leadville, CO, will result in the engine stumbling or maybe even stopping at idle in a very short period of time. That is not a bug, but corresponds to how a real aircraft would behave at high density altitudes. It is absolutely necessary to lean the mixture and increase the throttle to sustain a good idle at high density altitudes, both in the real world and in X-Plane. In practice, the pilot will add throttle to keep the engine running, then slowly pull out the mixture control and watch the tachometer, to lean the engine to peak RPM, then reduce the throttle again to the desired RPM as dictated by electrical needs.