Function
The Lua script program is designed for Jeti remote controls. With this program, the remaining capacity of a propulsion system is calculated and displayed on the screen. Additionally, users can set alarms or retrieve the current status with a switch.
The “calca-Elec” and “calca-Gas” apps can work together with other apps like “Display” from Thorn or “dbdis” from Dieter. However, it should be noted that running the program on older transmitters may utilize resources and is recommended for standalone operation.
Background
To display the remaining capacity, sensors for electric propulsion systems or flow meters for combustion engines are available. However, these often require additional installation and can be expensive. For small aircraft, the extra weight and space required are also factors to consider. As a result, many pilots still rely on the simple “timer method” which can work if the flight style is consistent or the timer is set conservatively. However, using this method might lead to occasionally flying a battery completely empty or even requiring an emergency landing.
The Solution for Jeti Pilots
The “calca” apps calculate consumption based on the position of the throttle stick. The accuracy depends on the quality of inputs and typically matches that of dedicated sensors. An exception is gasoline engines, where flow rates are less known and can vary significantly with the aircraft’s orientation. Jet engines with a linear throttle curve are easier to set up, as the consumption values are often provided in the manual for idle and full throttle, which can be directly adopted.
Technical Details
The program defines the throttle characteristics of the propulsion system based on up to five measurement points. Potentially empty measurement points (defined as “-1” in the mask) are calculated by interpolating neighboring values. Subsequently, the curve is refined, and the throttle range is divided into 40 segments with interpolated values.
When the transmitter is turned on or the motor is running, the script calculates the consumed current or fuel amount every second based on the throttle position and adjusts the percentage representation of the remaining capacity. For electric propulsion systems, the voltage drop during flight is also taken into account. The definition is that a combustion engine has no consumption when the motor protection switch is off, while an electric propulsion system always has at least the consumption set at 0% throttle position, which typically corresponds to the receiver and servo idle current.
Accuracy
The accuracy was tested over a year, with fully charged batteries at home used to determine differences between the actual values and the target values (stored in an Excel table). The results showed a variation of about +/- 2% between identical batteries, which was better than expected. Typically, the pilot flies until reaching 25% remaining capacity, resulting in a cell voltage of about 3.75 volts at home. This allows the batteries to be stored without requiring immediate recharging. Differences may be slightly larger with different batteries.
Practical Use
For jet engines, settings can sometimes be directly taken from the manual. For electric models, the author initially flew with a temporary sensor to measure consumption more indirectly. Now, ground measurements with a clamp ammeter are sufficient.
For gasoline engines, obtaining precise consumption values can be challenging, and some experimentation might be necessary.
Challenges
There are potential pitfalls. Forgetting to adjust the throttle stick proportionally can result in a significant error. Some controllers re-calibrate at the start of each flight, causing the motor to produce almost full power at 50% throttle stick position, rendering the calca script unusable. Additionally, the script could potentially crash, although it is rare. In such cases, the Lua program is simply terminated without warning.
Safety
Lua programs run in a kind of “sandbox” and generally cannot cause harm to the transmitter. If they are faulty or consume too many resources, they are simply terminated. However, there can be issues with file access. Writing to a full or defective memory card, such as when trying to take a “print screen” during flight, can lead to freezing of the display and make it impossible to shut down the transmitter normally. However, the control of the model is unaffected, as it operates independently. Most file accesses occur at the end when shutting down the transmitter or switching models. In the development of the program, the transmitter occasionally crashed from such actions. In such situations, the transmitter battery can be reset, or a simpler method involving pressing and holding two buttons (e.g., “Power” and “Esc” on the DS16) for a few seconds can be used to recover.