Flight-critical control
RB25S architecture, receivers, control servos and required aircraft-allocated sensors remain electrically independent from propulsion and non-critical accessories.
L-4 Restoration · Chapter 08
Electronics,
Power & Telemetry
The governing rule is simple: a propulsion, accessory, receiver, wiring or battery failure must not automatically remove control of the aircraft. The current architecture centers on FrSky X20R / ETHOS, an RB25S and two independent 2S LiFe control-power paths — while the final receiver combination, stabilization details, antenna locations and exact channel map remain open until the installed system is proven.
The Central Reason
Keep flying long enough to land safely.
A large model can survive the loss of propulsion if the pilot still has the receiver, servos and control surfaces. It cannot survive the loss of control merely because an ESC, accessory, connector, battery or single RF path failed.
The goal is not to make failure impossible. It is to keep one failure from becoming the loss of the whole airplane.
Design question:
If this item fails, does the pilot still have control?
If the answer is uncertain, the architecture is not finished.
Power Domains
Contain faults before they spread.
Flight-critical control, propulsion and non-critical accessories do not deserve equal authority over whether the aircraft remains controllable.
Propulsion
Motor, ESC, propulsion batteries and propulsion-specific instrumentation form their own domain. Their loss may end powered flight, but must not end control.
Accessories
Lighting, cameras and other confirmed non-critical loads are isolated so an accessory fault cannot pull down the control system. Tow-release safety classification remains a separate unresolved project decision.
Separate domains also make troubleshooting honest: a voltage drop, reset or warning can be traced to a defined branch rather than disappear inside undocumented adapter chains and shared leads.
Independent Receiver Power
Two paths — not merely twice the capacity.
One larger receiver battery would still be one battery, one lead, one connector and one complete failure path. Two independent 2S LiFe packs feeding separate RB25S inputs are intended to let the surviving path carry the flight-control load after the loss of the other.
Separate path
Separate pack, lead, connector and RB25S input with its own identity and condition record.
Separate path
The second path must be able to carry a realistic control load by itself, not only at idle on the bench.
Test the loss
Redundancy remains a claim until each input is deliberately removed while servos are moving and warnings are observed.
Why 2S LiFe?
Stable receiver power with a practical operating range.
LiFe receiver packs suit the selected flight-control architecture and repeated field use when properly charged, monitored and maintained. Chemistry does not remove the need for inspection.
Pack age and service history
Measured capacity
Loaded voltage
Internal resistance
Lead and connector condition
Charge returned after flight
A battery can show acceptable resting voltage and still have poor capacity, a weak connector or excessive internal resistance. Loaded testing proves the path.
Known Foundation vs Open Implementation
The architecture is real. The final receiver system is not being guessed.
Shared FrSky research explains why the X20R / Tandem ecosystem won, but the L-4 project still controls the aircraft-specific receiver, antenna, stabilization, power and channel implementation.
Current: FrSky X20R / ETHOS
Current architecture: RB25S
Current control power: two independent 2S LiFe paths
Open: final receiver combination
Open: antenna geometry
Open: stabilization implementation
RB25S Integration
Visible, cool and removable.
The RB25S is the center of the current architecture; its final physical mount will be developed with the cockpit installation.
Service access
Removal without dismantling unrelated cockpit systems or rewiring the aircraft.
Connector visibility
Power inputs, receiver links and servo outputs remain inspectable rather than buried beneath finished trim.
Cooling and strain relief
Clearance, support and cable routing are designed before the cockpit makes access difficult.
Equipment Packaging
Designing Around the Real Hardware
Catalog dimensions identify the box. An installation must also make room for everything that lets the box work, remain inspectable and come back out.
Measure the owned components
The current bench session covered the RB25S, TD R18, TD R6, GPS ADV, PAS70 ADV, GForce 2800 mAh 2S LiFe packs, Castle Phoenix Edge HV160, Savox SH-0255MG, tow-release hardware and FrSky Xact BLS5401H.
Measure the installation envelope
Mounting ears, plugs, antenna and wire exits, heavy leads, bend radius, strain relief, cooling space and hand access can require more room than the component body.
Protect the service path
Packaging must account for trays, fasteners and the route used to inspect or remove each item without dismantling unrelated systems.
Evidence state: these measurements and packaging requirements inform the design. They do not yet prove a final installed layout. The RB25S remains the current architecture; an RB30 is not part of this design.
Receiver Diversity
Redundancy must not share the same weak spot.
Two receivers mounted together with antennas bundled in the same orientation would share shielding, polarization and local-impact vulnerabilities. Diversity is physical as well as electronic.
Useful separation
Receiver and active antenna locations are separated where practical and remain independently inspectable.
Polarization
Antenna orientation is chosen to reduce shared nulls rather than to look symmetrical.
Real environment
Final placement is tested with batteries, structure, motor/ESC wiring and other conductive or shielding elements in their actual locations.
Receiver Selection
Selection follows installed-system testing.
Compatible FrSky receivers are evaluated for link redundancy, antenna diversity, telemetry integration, physical separation, channel needs, firmware compatibility, service access and any approved stabilization function.
Compatible protocol and firmware
Independent antenna placement
Required channel capacity
Demonstrated redundancy behavior
Telemetry and logging
Service access
Optional Stabilization
Assistance must never conceal a mechanical problem.
Any eventual stabilization must be treated as assistance, not as compensation for a warped wing, incorrect CG, unequal geometry, binding linkage, poor trim or structural flex. The exact implementation and mode structure remain open.
Correct sensor orientation
Correct correction direction
No oscillation
Recorded gain limits
Clear switch logic
Immediate stabilization-off option
Verified failsafe
Servo Power and Distribution
Test the complete control path under realistic load.
Each control surface receives a deliberate output, connector, wire, servo, linkage and mechanical record. Several servos are moved simultaneously while bus voltage and connector temperature are observed.
One surface, one record
Servo identity, output, direction, center, travel, current, linkage and replacement information remain traceable.
Mechanical first
Electronic matching does not substitute for free hinges, correct horn geometry, rigid mounts and direct load paths.
No hidden overload
Simultaneous movement reveals voltage drop, high current, heat, flex or weak connections that an unloaded bench test can miss.
Purpose-Made Wiring
Every extra connection is another possible fault.
Deliberate cable lengths reduce adapter chains, unsupported connectors, excess resistance and unidentified junctions. Protection must keep wiring inspectable rather than sealing it inside an unknowable harness.
Routing standard
Avoid sharp edges, moving controls, heat, battery restraint and high-current propulsion conductors.
Protection standard
Use grommets, clamps, strain relief, abrasion protection, labels and service loops where the installation needs them.
Documentation standard
Origin, destination, connector, polarity, wire size, protection and test result remain recorded.
Wing-Root Connections
Repeatable assembly without hidden partial engagement.
Wing-root connectors should be keyed, labeled, strain-relieved and located so engagement can be seen. Left and right arrangements are documented independently and no individual wire carries the pulling load during assembly.
Visible engagement
Positive orientation
Left/right identification
No loose adapter chain
Protected service loop
Connector retention
Recorded pinout
Post-assembly continuity check
Why Telemetry?
Expose a developing problem before it becomes an accident.
Telemetry is not installed to decorate the transmitter screen. It is selected to answer operational questions: Is the control link healthy? Are both receiver-power paths present? Is the bus stable under load? Is energy use normal? Are temperatures rising? Is an airspeed reading believable?
In flight: actionable warnings while there is time to respond
Postflight: diagnose brief dips, link events, temperature peaks and dropouts
Validation: compare cooling, battery, range and failover behavior to design assumptions
Owned FrSky Sensor Capability
Purchased does not mean allocated to this airplane.
The broader FrSky record proves ownership of sensor capability. L-4 allocation, physical installation, calibration and operational use still require aircraft-specific evidence.
GPS ADV
Owned position, altitude, distance, track and ground-speed capability. L-4 allocation/install status remains under review.
PAS70 ADV
Owned pitot-airspeed capability. Any L-4 installation must still prove probe location, tubing integrity, zero, leaks, calibration and believable field readings.
PT100 temperature sensors
Owned temperature-sensing capability. Final heat-critical locations depend on actual propulsion and equipment packaging plus aircraft-specific allocation.
Correct evidence chain: owned sensor → aircraft allocation → installation → calibration/test → operational use.
Voice Warnings
A warning must still sound important.
Continuous narration can bury the one message that requires immediate action. Voice alerts should be limited and prioritized; routine information remains available on demand or at deliberately spaced intervals.
Highest priority
Control-link health, receiver-bus voltage and receiver-power/failover conditions.
Operational priority
Propulsion energy reserve and critical temperatures when those signals exist in the final aircraft.
Advisory information
Airspeed, GPS and routine values support judgment only after installation and calibration; they do not replace visual flying.
Flight Modes and Controls
Clear switch logic with no hidden surprises.
Whatever final mode structure is adopted, each switch position must be documented and checked with propulsion disabled before a material programming change is flown.
Switch identity
Rates and expo
Flap/elevator relationship if used
Stabilization state if used
Warnings enabled
Failsafe unchanged
Smooth transitions
Recorded software version
No mode may create an unexpected reversal, abrupt trim shift, hidden throttle behavior, inaccessible stabilization-off position or different failsafe outcome.
Bench-Test Program
Prove the architecture before the airplane is closed.
The system is validated progressively: individual components, integrated architecture, then the real installed aircraft with motor-running, orientation and interference effects present.
Individual components
Battery inputs, RB25S, candidate receivers, servos, sensors, switches and telemetry are tested separately.
Integrated architecture
Operate the complete control system under simultaneous servo load while deliberate failures are introduced and bus behavior is observed.
Installed aircraft
Range, orientation, interference, vibration, motor-running, wing assembly, cooling and service access are tested in the real installation.
Deliberate Failover
Redundancy is a claim until something is unplugged.
Each redundant element is disconnected one at a time while servos are moving and warnings are observed. Tests are repeated at realistic control load and, where safe, with the propulsion environment present.
Receiver Battery A
Receiver Battery B
Primary receiver path
Redundant receiver path
Selected telemetry sensor
Confirmed non-critical accessory branch
Propulsion command / power interaction
Range and Interference
Test the completed electromagnetic environment.
Range and orientation checks are repeated with the propulsion system operating through planned power levels while servos continue moving and telemetry remains active. Any link degradation, jitter, reset, voltage dip or telemetry dropout is investigated before progression.
Link quality
Receiver resets
Servo jitter
Telemetry dropout
Sensor instability
Voltage dips
Temperature rise
Orientation sensitivity
Documentation Package
The aircraft must remain understandable later.
A safe installation is also a documented installation. The record must explain what every connection does and what test evidence supports the released configuration.
Block Diagram
Power domains, hubs, receivers, servos, sensors and accessories.
Channel Map
Transmitter control through RB25S output to each device.
Connector Schedule
Type, pinout, polarity, label and location.
Wire-Routing Map
Physical paths, protection, supports and service loops.
Programming Record
Modes, mixes, rates, expo, gains, alerts and failsafe.
Test Record
Loads, failures, range, interference, telemetry and corrective actions.
Configuration Control
One understandable as-flown reference.
A safe setup can become unsafe after an undocumented receiver change, firmware update, reversed connector, moved battery, altered endpoint, gain change or new warning threshold. Material changes trigger the relevant tests again.
Transmitter file and firmware
Receiver and RB25S setup
Power batteries and connectors
Servos and sensors
Wiring and channel map
Modes, gains, alerts and failsafe
Weight, balance and test status
Completion Standard
Controllable after a realistic single failure.
The electronics architecture earns release only when a realistic single failure does not unexpectedly remove control and the pilot receives the warning needed to recognize the event.
Power: either receiver-power path can be removed under load
Link: selected receivers and antenna geometry demonstrate useful diversity
Load: simultaneous servo movement does not expose unstable bus behavior
Interference: motor-running tests do not produce unexplained link or sensor faults
Understanding: every wire, channel, connector and configuration state remains documented
Change means retest. The signed as-flown record belongs at the end of the evidence chain — not at the beginning of the design process.