A Terrain Follow performance model defines the flight characteristics of the aircraft while performing a Terrain Following procedure. It functions similar to basic cruise, but applies only during terrain following procedures and adds MaxPitchAngle and TerrainWindow properties. The MaxPitchAngle limits flight path angle between peaks and valleys that is flown as a safety parameter (avoid nose-low when flying near the ground) and smoothing function. The TerrainWindow is a “look ahead” parameter that controls how terrain is discretized when generating the profile; larger values give smooth flight paths.
The following table describes the parameters that comprise a Terrain Follow performance model. To access the Terrain Follow model,
- Open aircraft properties.
- Right click on in the performance models window.
- Select new model type.
- Select Terrain Follow.
- Click Add.
| Parameter | Description |
|---|---|
| Airspeed Type | The airspeed reference of the performance model - true airspeed (TAS True Airspeed: the speed that the aircraft is moving relative to the airmass that it is flying in.), calibrated airspeed (CAS Calibrated Airspeed: the speed reported by the airspeed indicator, corrected for position and instrument error.), equivalent airspeed (EAS Equivalent Airspeed: the airspeed at sea level (International Standard Atmosphere) at which the dynamic pressure is equal to the dynamic pressure at the aircraft's current true airspeed and altitude.), or Mach The ratio of the aircraft's speed and the speed of sound at the aircraft's altitude, with local atmospheric conditions. number. |
| Use Aero/Propulsion Fuel Flow | Select this check box to use the fuel flow calculated by the aerodynamics and propulsion strategies defined in the active acceleration performance model; if the acceleration performance model is not set to calculate fuel flow, then fuel flow will not be calculated for this performance model, either. |
| Scale Fuel Flow by Non Std Density |
Select this check box to scale the fuel flow based on the aircraft's actual altitude. If the airspeed type is TAS, then fuel flow will decrease as the altitude of the aircraft increases. If this option is disabled, the fuel flow will be calculated at the Default Cruise Altitude (if available), or at sea level. |
| Minimum | The minimum cruising airspeed and fuel flow. |
| Max Endurance | The cruising airspeed and fuel flow that will provided the maximum flying time possible for the aircraft. |
| Maximum | The maximum cruising airspeed and fuel flow. |
| Max Range | The cruising airspeed and fuel flow that will provided the maximum flight range possible for the aircraft. |
| Max Performance | A custom performance airspeed and fuel flow that you can use to model specific flight conditions. The values must be between the Minimum and Maximum parameters. |
| Max Pitch Angle | The maximum pitch angle (both positive and negative) the aircraft will be allowed to use; higher values result in a rougher ride. |
| Terrain Window | The time interval over which terrain points are sampled; smaller values result in a rougher ride and are more resource-intensive with respect to computation. |
| Max Possible Load Factor (no turn) | The maximum load factor - during straight and level flight - that the aircraft can bear. |
When defining a Terrain Follow model, it is important to understand the relationship between the Max Pitch Angle and the Terrain Window. If you specify a large value for the pitch angle and a small value for the terrain window, the airplane will not fly realistically (especially over rough terrain). Human pilots have an aversion to flying with high pitch angles close to the ground. They also mentally integrate or smooth the visual cues they get to smooth out their control inputs. This smoothing can be modeled via a time constant. The default time window of 3 seconds and pitch angle of 20 degrees is a good place to start. When using very high resolution terrain, avoid the use of small Terrain Window values, as this will increase computer resource consumption and lead to very unrealistic - perhaps even fatal to the pilot - flight.