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Performance

The rotor blade maths is the expensive part. If a big aircraft costs too much, try the Mi-8's settings first.

The main settings

  • Elements: 4 with Gauss quadrature is plenty for main rotors, 3–4 for tails.
  • Performance Profile: 1 and 2 currently use the same cadence and keep the rotor elements you set. Either one holds the rotor aerodynamics across drivetrain substeps and solves the airframe once per physics callback. 0 solves both every substep.
  • Topology Rotor Aero Callback Interval: 2 solves the rotors every second physics step (Mi-8).
  • Solve Rotors Every Substep: on is the most accurate and the most expensive (Huey).
  • Idle Rotor LOD (Flight component, on by default): when the aircraft's sitting on the ground, not moving, with nobody touching the controls for a second, the rotor aerodynamics run at half rate. Any input or movement puts it back to full rate on the next physics step. Engines, drivetrain and ground contact always run at full rate.

Aerodynamic step size

Max Aero Azimuth Step Deg (on the rotor unit) limits how far a rotor turns in one aerodynamic pass, and a rotor solve runs as many passes as it needs. Leave it on 0 (automatic) and it picks the step from the rotor itself:

  • Articulated main rotor, three or more blades, Multi Rate Flap on (the default): one aerodynamic pass every 1/30 s, with the blade flap and lead-lag stepped in substeps of 15° at most in between (more on that below).
  • Articulated main rotor, Multi Rate Flap off: 32°. Each pass puts the blades where they'll be at the middle of the step, so the flap phase holds and pure cyclic tilts the disc straight.
  • Teetering main rotor: 10°.
  • Any other rotor whose blades flap (tail rotors): 110°, and tail rotors also keep each pass to 1/120 s. The Mi-8's tail kicked the drivetrain at 3 passes of 75° each (11 ms per pass), but 4 passes at 8.3 ms flew clean. A blade flaps about once per revolution, and past about 110° per pass its aerodynamic damping shows up too late and the blades flap toward their stops. The AH-6's tail at 215° flapped about 8° and its thrust swung between roughly -500 and +1,750 N with the pedals still, which you could feel as jitter.
  • A rotor with no flap freedom: half a revolution, and main rotors also keep one blade spacing.

A positive number is a manual step in degrees and overrides the automatic one. -1 runs a single pass per solve (the Huey uses that). Manual values are really just a backup in case the automatic step misbehaves on an aircraft.

Multi Rate Flap

Multi Rate Flap (rotor unit, on by default) is for articulated rotors with three or more blades. The blade aerodynamics run once per pass, then each blade's flap hinge forcing gets carried around the disc. The blades sample it at their evenly spaced positions, and each blade reads it at its own position with its own live flap damping, in substeps of 15° at most.

That gets the flap phase about as right as that many full aerodynamic passes would, for roughly the cost of one. In a flap-only check against a fine reference, the disc tilt came out within about 4 % with no sideways error, where 32° passes were up to about 2° off sideways.

Inflow, vortex ring and stalled section memory still only advance once per aerodynamic pass, which is why an automatic main rotor keeps one pass per 1/30 s. Turn it off to go back to one flap step per aerodynamic pass (with Max Aero Azimuth Step Deg at 0 that's the 32° rule). It doesn't do anything on teetering, two-bladed or rigid rotors.

Trading quality for speed

Rotor Solver Step Scale (Flight Topology, Solver cadence) trades quality for performance on every rotor left on the automatic step.

  • 1 is the physics-based step.
  • 1.5 means 50 % longer steps and fewer passes. It's faster, but the flap phase and blade passage loads get coarser. On a tail rotor, anything past 110° risks the flapping problem described above. The topology log line for each rotor prints its step and warns you if it goes past that.
  • Below 1 is finer and slower.

With Multi Rate Flap on, it scales both the 1/30 s aerodynamic pass and the 15° flap substep.