Learn which aircraft.cfg and AIR-file settings change FSX aircraft performance and cruise speed, with safe editing, testing and realism checks.
To change an aircraft’s performance in FSX or FSX: Steam Edition, back up its aircraft folder, then adjust engine-power and aerodynamic-drag values in aircraft.cfg in small increments. Do not merely change cruise_speed: it is a reference value, while actual cruise speed comes from thrust, drag, propeller data and the AIR file.
Why doesn’t changing cruise_speed make the aircraft faster?
Changing cruise_speed under [Reference Speeds] alone does not alter the aircraft’s physical performance. It is primarily a reference and planning value; it does not supply more engine power or reduce aerodynamic drag.
In steady level flight, speed settles where available thrust equals drag. Weight, altitude, temperature, mixture, propeller settings and engine condition also matter. Published cruise figures are normally true airspeed, so compare KTAS with KTAS, not the indicated airspeed shown on the panel at altitude.
The related max_indicated_speed and max_mach entries define reference or operating limits. Raising them does not make the aircraft accelerate faster, and it can hide an unrealistic or unsafe flight model.
Which FSX performance setting should I change?
Choose the setting according to what is wrong with the aircraft rather than changing several values together.
| Observed problem | First setting to examine | Effect and caveat |
|---|---|---|
| Climb and cruise are both too slow | power_scalar under [piston_engine] or [turboprop_engine], or thrust_scalar under [jet_engine] | Raises available power or thrust where that scalar is present. It also changes acceleration and climb, and may not produce realistic fuel consumption. |
| Climb is close to correct but cruise or top speed is low | parasite_drag_scalar under [flight_tuning] | Lowering it reduces profile drag. It also affects deceleration, descent, glide performance and range. |
| Climb or low-speed drag is wrong at high angle of attack | induced_drag_scalar under [flight_tuning] | Changes induced drag most noticeably when producing substantial lift. It is not the first choice for correcting high-speed cruise alone. |
| Propeller RPM, blade response or efficiency is wrong | Propeller and engine records in the .air file | Requires deeper flight-model work; a simple power increase may conceal the actual fault. |
| Only the displayed or planned cruise figure is wrong | cruise_speed under [Reference Speeds] | Corrects the reference figure without meaningfully changing flyable performance. |
Avoid using cruise_lift_scalar merely to chase a speed figure. It changes the aircraft’s lift and trim relationship, which can disturb approach behaviour and the rest of the envelope.
How do I edit and test aircraft.cfg safely?
The safest method is to establish a repeatable baseline, alter one parameter and test the same flight condition again.
- Locate the active aircraft folder. The files are normally beneath
SimObjects\Airplanes, but the installation root differs between boxed FSX and Steam Edition. Our guide to finding the active FSX aircraft folder covers the common locations. - Back up the complete aircraft folder. At minimum, preserve untouched copies of
aircraft.cfgand every referenced.airfile. Common configuration sections often affect every livery in that folder. - Identify the correct AIR file. The
sim=entry in the selected aircraft’s[fltsim.x]block names its AIR file without the extension. Do not assume the first AIR file in the folder is the one being used. - Record a baseline. Use the same weight, centre of gravity, altitude, weather, power setting and aircraft configuration. Record true airspeed, indicated airspeed, climb rate, engine output and fuel flow after the aircraft has stabilised.
- Make one small adjustment. Scalar values commonly start at
1.0. Change them by roughly two to five per cent per test; for example,parasite_drag_scalar=0.98represents two per cent less parasite drag, not a guaranteed two per cent increase in speed. Use a full stop as the decimal separator. - Save without changing the filename. Keep
aircraft.cfgas plain text and make sure the editor has not createdaircraft.cfg.txt. Reload the aircraft; restart FSX if the result does not update. - Retest the whole envelope. Check take-off distance, climb, cruise at several altitudes, descent, approach, stall behaviour, fuel flow and range. A change that fixes one cruise condition can damage several others.
For performance errors that vary with speed or altitude, edit the underlying AIR-file curves rather than stacking large scalars. The AIR file is binary and needs a compatible editor; our detailed tutorial on FSX flight-dynamics and AIR-file parameters explains the relevant aerodynamic and engine data.
Why isn’t my FSX performance edit working?
Most failed FSX performance edits come from changing the wrong file, not reloading the aircraft or testing under inconsistent conditions.
- Wrong installation or aircraft copy: boxed FSX, Steam Edition and additional simulator libraries may contain similarly named folders.
- Protected installation folder: Windows permissions may prevent the real file from being replaced or may redirect the edit. Edit a copied file outside the protected folder, then copy it back with the required permission.
- Hidden file extension: a file saved as
aircraft.cfg.txtis ignored by FSX. - Aircraft not reloaded: reselect the aircraft after saving. If the old result remains, close and restart the simulator.
- Custom add-on logic: complex aircraft may calculate thrust, engine limits or drag through gauges and custom code. Configuration scalars can have limited effects or break system logic, and an add-on repair or update may overwrite them.
- IAS and TAS confused: indicated airspeed decreases relative to true airspeed as altitude increases. Match the airspeed type, altitude, power and atmospheric conditions used by the published performance figure.
The cleanest tuning order is to match engine output and climb first, then drag and cruise, followed by fuel consumption and low-speed checks. If fixing one test point breaks the others, restore the backup and correct the underlying AIR-file curve instead of applying a larger scalar.