Prop Department

The Shift to Digital Jet Control Scrapping a sketchy lawn mower cable for a fully automated, drive-by-wire system isn't just about aesthetics; it's about not getting airborne by accident. The Prop Department team took a Soviet-era Aero L-29 Delfin jet engine and shoved it into a Japanese mini truck, but the real magic happens in the silicon. By moving to a 100% drive-by-wire setup, we replace mechanical failure points with precise electronic timing. This setup allows for automated start sequences where the computer handles the heavy lifting, ensuring the engine doesn't melt itself into a puddle of aluminum during ignition. Hardware Stack and Prerequisites Before you start dragging elements onto a logic board, you need the right hardware. This build centers on the Unitronics Unistream PLC, an industrial-grade Programmable Logic Controller with an integrated HMI (Human Machine Interface) touchscreen. Unlike a hobbyist microcontroller, this unit is designed for high-interference environments and provides robust I/O for managing fuel pumps, ignition coils, and high-torque ClearPath Servos. **Prerequisites:** * Familiarity with DC electrical systems and relay logic. * Basic understanding of 3D scanning and CAD for custom mounting. * Access to PLC software (like UniLogic) for ladder logic implementation. Decoding Ladder Logic and Global Variables Programming a PLC is a departure from standard C++ or Python. We use **Ladder Logic**, a graphical programming language that mimics electrical circuit diagrams. Instead of lines of text, you use "rungs" to define logic. If a switch is closed (True), power flows to the next element. ```ladder |--[ Start Button ]--[ Engine RPM < 500 ]----( Fuel Pump 1 )--| | | |--[ Fuel Pump 1 ]---[ T1 (5s Delay) ]-------( Ignition Coil )--| ``` In this environment, **Global Variables** are the connective tissue. They allow different sub-programs (ladders) to share data, such as engine temperature or throttle position. A critical discovery during this build was that you cannot declare the same global variable in multiple ladders without causing a logic conflict. This error essentially "bricks" the automation until the redundancy is removed, as the processor receives conflicting instructions on which state the variable should occupy. Precision Linkage and Bench Testing The physical interface involves mapping digital values to the ClearPath Servos that physically move the jet’s throttle valves. Using Technic software, we calibrate the "homing sequence"—ensuring the motor knows exactly where zero is. * **Digital Limits:** We set software ceilings to prevent the servo from over-extending and snapping the custom 3D-printed linkages. * **Simulation Mode:** The Unitronics HMI allows for a demo simulation, where you can watch the virtual pumps and valves toggle before actually dumping fuel into a turbine. Safety Protocols and Practical Gotchas No matter how good your code is, you need a mechanical fail-safe. We implemented a spring-loaded mechanical fuel shut-off valve operated by a physical T-handle. If the PLC freezes or a global variable causes a runaway throttle, pulling that cable is the only way to sleep at night. Common pitfalls include backwards wiring on relay banks and forgetting to enable "auto-fuel" bits in the logic. These small oversight errors are the difference between a successful 45-gallon burn and a silent, frustrating engine bay. Always label your pin outs in a master document; you will thank yourself when you're troubleshooting at midnight.