Here is a guide I've put together for getting the best possible visuals/graphics in FlightGear. These settings run both my CPU and GPU at about ~30%
I am using:
CPU: Intel i7 4770k (stock clocks)
RAM: 16gb Corsair Vengeance DDR3 1600 MHz
GPU: Zotac Geforce GTX 1060 AMP! with 6gb of VRAM (stock clocks)
If you encounter lag on your system, start by turning down the features least important to you.
We will start with Launcher settings. Open the FlightGear launcher, go to the "Environment" tab and enable "Advanced Weather Modeling" and "Real-world weather".
Next, go to the "Settings" tab and set Renderer to "Atmospheric Light Scattering" and set Anti-aliasing to "4x".
Next will be our command line settings. From the "Settings" tab of the launcher, scroll all the way down to the box under "Additional Settings". This is your command line. Enter this into your command-line (NOTE: Set "--geometry=" to any resolution compatible with your monitor, also works with downsampling resolutions):
--enable-random-objects --enable-ai-traffic --geometry=3200x1800 --texture-filtering=16 --fog-nicest --enable-distance-attenuation --enable-clouds3d --shading-smooth --enable-specular-highlight --enable-real-weather-fetch --enable-ai-models --enable-horizon-effect
Now, we will do our in-game settings:
Launch FlightGear and if the menu bar is not showing, press F10.
Go to View>rendering options. Enable Animated jetways, Pylons and power lines, Detailed Roads and Railways, Random Scenery objects, Vegetation Shadows, Scenery objects, Custom settings (fine-tuning), Particles, Precipitation, 3D Clouds (move sliders for clouds all the way to right), and ensure Atmospheric Light Scattering (ALS) is enabled.
Next set Buildings to "OpenStreetMap Data", set vegetation to High Density and set Terrain Textures to "Region-Specific".
Click Shader options, move all sliders all the way to the right. Click "Close".
Next go to View>Adjust LOD Ranges and ensure it's set to "Distance (m)". Set Scenery Detailed to "3000", Scenery Rough to "18000" and Scenery Bare to "60000". Set AI/MP Aircraft to "20000" and AI/MP Interior to "100". Click Apply, click OK.
Next, go to Environment>Weather and ensure "Detailed Weather" is selected. Click "Advanced Settings". Enable Terrain effects, Terrain Pre-Sampling, Generate Thermals, Cloud Shadows, and Realistic Visibility.
Click "ok", Click Apply, Click OK.
NOTE: When launching FlightGear, if the weather doesn't seem right, go to Environment>Weather and just click "OK" to get the weather to reload.
Congratulations, enjoy your improved visuals!