cadetstimp
I'm New Here
Hello all
After completing, modifying, repairing and maintaining a number of experimentals, I've come up with a few observations that may or may not be useful. Just preferences and observations I've collected.
1) GPU port
Every aircraft should have one IMHO. As soon as you need to spend time updating software, databases, troubleshoot systems, configure....a need for more than battery power arises. A GPU port doesn't necessarily have to be an exterior door. Makes for an easier jump if necessary.
2) 28V vs 12V
More than once, I've come across an electrical installation that reaches maximum capacity and the owner wishes to add more... especially air conditioning. A problem arises because the charging system, wiring, breakers and components have all been embedded with 12VDC items. The avoidance of more expensive refit/rework usually ends up in a hybrid system with two alternators...something that makes me sad every time I see it. Design for a 28VDC system. Before integration/completion...it doesn't add that much more in cost.
2) Terminations - Prefab vs In aircraft
Personal belief .... I believe routing, combing, anchoring and terminating harnesses in the aircraft is a much better than prefab bench work that gets placed into the aircraft. Don't get me wrong - some things such as control sticks and component disconnects should be done on the bench. If the majority of the termination is done in the aircraft - a natural service loop and serviceability will occur with fewer unnecessary disconnects. Placement of components, harness and assembly are naturally done better. Think about it...how you figure out to terminate and assemble in the aircraft is going to be very similar to how disassembly and maintenance will occur.
3) Terminal strips
Too many times I've seen poor wire protection and poor bussing due to terminal strips. Terminal strips should not be used for bus distribution. Circuit Breakers should be located and grouped strategically. The bussing/distribution should occur via bus bars directly on the circuit breakers. Remember circuit protection is mainly to save the wire....not the components; to prevent wires and wire bundles from melting into a useless mess. Be mindful whenever the gauge of a wire changes protection should be there. Using remote terminal strips away from the breakers for bussing is dangerous if the smaller gauge wires are not protected. It is too easy to feed a larger gauge wire to a terminal strip, leaving the smaller gauge distribution wires unprotected. I suggest not using terminal strips at all, but if you do ...the gauge of the wires on each side of a junction should all be the same. Terminal strips also make a poor disconnect solution (i.e. panel or component disconnect). Inline connectors allow better service and prevent the wrong wire from reconnecting to the wrong terminal. Connectors won't loosen up as easily as a screw terminal over time. CPC Series I and Series II connectors are a great economical solution for inline disconnects.
4) Grounding
It is critical to have the battery negative and engine block ground electrically bonded to the airframe, not just to each other. If the engine block 2AWG ground wire and battery ground wire meet at an airframe point, make sure they attach to bare metal. The airframe, engine and battery ground should be the same ground potential to avoid problems. For example, many solenoids and contactors rely on the mounting bracket as the circuit ground for the activation coil. (Having any problems with your starter?). A number of other components should also make chassis bare metal contact using at least one mounting screw....dimmer power supplies, strobe power supplies, servos, fuel pumps, or anything else with an electric motor or oscillator. To prevent corrosion of a bare metal ground contact - I use DC5 (DC4 also works - both have the look and consistency of protoplasm jelly). A piloted circular bonding brush works well. One special exception to grounding is audio circuits. Audio lo wires (phone lo or mic lo) should return back to the audio panel and stay off airframe ground. Audio panels have special ways of grounding audio circuits internally that help reduce noise. To accomplish this I recommend isolation washers on phone and mic jacks. (Have audio noise? try isolation washers at all jack locations)
5) Shielding
There are a number of circuits that should be shielded and use shielded wire. Higher power circuits (minus battery and charging system) such as landing/taxi lights, pitot heat, p-leads, fuel pumps, nav lights (if not LED) and strobe lights (if not LED). These circuits tend to transmit noise and shielding helps prevent this. Other circuits are susceptible to receiving noise and should be shielded, such as audio wires and digital wires (RS232, RS485, ARINC429). The shield of a shielded wire should only be used as a shield and grounded at one end of the run. Some manufacturer diagrams show using a shield as a ground wire for a circuit...this is bad practice in my opinion. A shield is a shield and should not be part of the circuit.
6) Circuit Breaker mounting
Circuit Breaker spacing should take terminals into account when deciding the layout. For instance if you're using 7277-2-X... the spacing can be tighter side to side, but top/bottom space should leave plenty of clearance, so that a bus screw and neighboring ring terminal screws aren't temped to touch each other with panel flex.
7) Antennas
Ground Plane - most antennas (such as a regular bent whip COM antenna) require a good ground plane. In fact half the working antenna is the ground plane. Discard the cork or rubber gaskets they come with, strip the skin to metal for the antenna footprint, and make sure the base of the antenna makes good metal to metal contact when installed. Then edge seal the antenna to the aircraft with proseal. It is always good to make the base of the antenna the same ground potential as the airframe. Before final assembly - it might be worth it to have a local avionics shop connect a SWR meter directly to your radio rack connector to test your antenna installation.
Water protection - some upward facing antenna (such as GPS WAAS GA37) are supposed to have silicone filling the screw holes after installation. I usually use white RTV and carefully screed to match the surface of the antenna. If not done... water will sit in the cavity and cause hidden corrosion of the corresponding nutplate. I've seen this type of corrosion fail GPS antennas more than once. Silicone breaks up/removes easily when removal of the antenna is needed.... don't use proseal in the screw holes.
8) Radio Racks
No matter which type of vertical strap or structure is used to hold up radio racks near the back plates...take a few extra minutes to use nut plates instead of the screw nut combination. Small thing I know, but it makes a big difference. You should be able to release a back plate, unscrew the radio rack screws, and pull the rack aft through the instrument panel all while working inside the radio rack. The hanging back plate should have enough service loop to pull foreword and modify/service if needed. The time it takes to install the radio rack without behind the instrument panel access is the same amount of time it will take to service that radio connector in the finished aircraft.
9) Control Grips
I've run into a few loose or improperly installed stick grips. Clamping pressure alone should not be the method of holding the grip onto the stick. There should always be some type of through bolt, pin or sleeve that captures the grip to the tube. I know it is a pain to run wire around something going through the middle of the tube, but it is too important not to. For a number of reasons....I usually ditch the supplied grip wires and reterminate/solder my own. Using 22AWG tefzel, I usually solder a wire to every post on every switch...no jumpers at the switch. I then sleeve the bundle in XPF-3/8 snakeskin. Finally, I'll route the sleeved bundle through the tube and terminate the end of each wire to a HD Dconnector (pins) about 6 in away from the base of the grip movement. Any splices, jumpers or changes that need to be done can all be done on the aircraft side mating connector to the grip. This configuration has a few advantages. The bundle is smaller and slicker than most grip provided wires (easier routing and assembly), it is easier to troubleshoot a possible bad switch at the disconnect (no chasing your tail since all jumpers and grounds are left with the airframe disconnect), it is easier to replace a single switch and pull temporary slack back and forth for soldering. If a grip circuit is more than 3 amps...I'll use a relay to avoid using larger gauge wires in the grip.
11) Wire Harness Routing
Separation - When running fore and aft under the floor - High power wires should run on one side (i.e. RH) and low power signal and coax wires would run opposite (i.e. LH), especially if the battery is relocated to the tail.
Tail Frames - I've seen lots of large non-factory holes cut in tail frames that don't need to be. Wires can run over the frames instead of through them using lighting hole cable tie bases. If holes are cut, don't encroach the structural grooves.
Tie Bases - I'm not a big fan of the adhesive cable tie bases, but I really don't like the adels either. Cable tie bases that use a screw or rivet to hold in place are the best IMHO. A better hold than adhesive but better serviceability than an adel.
I hope this information is helpful to anyone. It takes a lot of hours of focus, dedication and passion to build an airplane. I felt compelled to contribute to the tribal knowledge out of mutual respect. Thanks Doug for hosting these forums.
After completing, modifying, repairing and maintaining a number of experimentals, I've come up with a few observations that may or may not be useful. Just preferences and observations I've collected.
1) GPU port
Every aircraft should have one IMHO. As soon as you need to spend time updating software, databases, troubleshoot systems, configure....a need for more than battery power arises. A GPU port doesn't necessarily have to be an exterior door. Makes for an easier jump if necessary.
2) 28V vs 12V
More than once, I've come across an electrical installation that reaches maximum capacity and the owner wishes to add more... especially air conditioning. A problem arises because the charging system, wiring, breakers and components have all been embedded with 12VDC items. The avoidance of more expensive refit/rework usually ends up in a hybrid system with two alternators...something that makes me sad every time I see it. Design for a 28VDC system. Before integration/completion...it doesn't add that much more in cost.
2) Terminations - Prefab vs In aircraft
Personal belief .... I believe routing, combing, anchoring and terminating harnesses in the aircraft is a much better than prefab bench work that gets placed into the aircraft. Don't get me wrong - some things such as control sticks and component disconnects should be done on the bench. If the majority of the termination is done in the aircraft - a natural service loop and serviceability will occur with fewer unnecessary disconnects. Placement of components, harness and assembly are naturally done better. Think about it...how you figure out to terminate and assemble in the aircraft is going to be very similar to how disassembly and maintenance will occur.
3) Terminal strips
Too many times I've seen poor wire protection and poor bussing due to terminal strips. Terminal strips should not be used for bus distribution. Circuit Breakers should be located and grouped strategically. The bussing/distribution should occur via bus bars directly on the circuit breakers. Remember circuit protection is mainly to save the wire....not the components; to prevent wires and wire bundles from melting into a useless mess. Be mindful whenever the gauge of a wire changes protection should be there. Using remote terminal strips away from the breakers for bussing is dangerous if the smaller gauge wires are not protected. It is too easy to feed a larger gauge wire to a terminal strip, leaving the smaller gauge distribution wires unprotected. I suggest not using terminal strips at all, but if you do ...the gauge of the wires on each side of a junction should all be the same. Terminal strips also make a poor disconnect solution (i.e. panel or component disconnect). Inline connectors allow better service and prevent the wrong wire from reconnecting to the wrong terminal. Connectors won't loosen up as easily as a screw terminal over time. CPC Series I and Series II connectors are a great economical solution for inline disconnects.
4) Grounding
It is critical to have the battery negative and engine block ground electrically bonded to the airframe, not just to each other. If the engine block 2AWG ground wire and battery ground wire meet at an airframe point, make sure they attach to bare metal. The airframe, engine and battery ground should be the same ground potential to avoid problems. For example, many solenoids and contactors rely on the mounting bracket as the circuit ground for the activation coil. (Having any problems with your starter?). A number of other components should also make chassis bare metal contact using at least one mounting screw....dimmer power supplies, strobe power supplies, servos, fuel pumps, or anything else with an electric motor or oscillator. To prevent corrosion of a bare metal ground contact - I use DC5 (DC4 also works - both have the look and consistency of protoplasm jelly). A piloted circular bonding brush works well. One special exception to grounding is audio circuits. Audio lo wires (phone lo or mic lo) should return back to the audio panel and stay off airframe ground. Audio panels have special ways of grounding audio circuits internally that help reduce noise. To accomplish this I recommend isolation washers on phone and mic jacks. (Have audio noise? try isolation washers at all jack locations)
5) Shielding
There are a number of circuits that should be shielded and use shielded wire. Higher power circuits (minus battery and charging system) such as landing/taxi lights, pitot heat, p-leads, fuel pumps, nav lights (if not LED) and strobe lights (if not LED). These circuits tend to transmit noise and shielding helps prevent this. Other circuits are susceptible to receiving noise and should be shielded, such as audio wires and digital wires (RS232, RS485, ARINC429). The shield of a shielded wire should only be used as a shield and grounded at one end of the run. Some manufacturer diagrams show using a shield as a ground wire for a circuit...this is bad practice in my opinion. A shield is a shield and should not be part of the circuit.
6) Circuit Breaker mounting
Circuit Breaker spacing should take terminals into account when deciding the layout. For instance if you're using 7277-2-X... the spacing can be tighter side to side, but top/bottom space should leave plenty of clearance, so that a bus screw and neighboring ring terminal screws aren't temped to touch each other with panel flex.
7) Antennas
Ground Plane - most antennas (such as a regular bent whip COM antenna) require a good ground plane. In fact half the working antenna is the ground plane. Discard the cork or rubber gaskets they come with, strip the skin to metal for the antenna footprint, and make sure the base of the antenna makes good metal to metal contact when installed. Then edge seal the antenna to the aircraft with proseal. It is always good to make the base of the antenna the same ground potential as the airframe. Before final assembly - it might be worth it to have a local avionics shop connect a SWR meter directly to your radio rack connector to test your antenna installation.
Water protection - some upward facing antenna (such as GPS WAAS GA37) are supposed to have silicone filling the screw holes after installation. I usually use white RTV and carefully screed to match the surface of the antenna. If not done... water will sit in the cavity and cause hidden corrosion of the corresponding nutplate. I've seen this type of corrosion fail GPS antennas more than once. Silicone breaks up/removes easily when removal of the antenna is needed.... don't use proseal in the screw holes.
8) Radio Racks
No matter which type of vertical strap or structure is used to hold up radio racks near the back plates...take a few extra minutes to use nut plates instead of the screw nut combination. Small thing I know, but it makes a big difference. You should be able to release a back plate, unscrew the radio rack screws, and pull the rack aft through the instrument panel all while working inside the radio rack. The hanging back plate should have enough service loop to pull foreword and modify/service if needed. The time it takes to install the radio rack without behind the instrument panel access is the same amount of time it will take to service that radio connector in the finished aircraft.
9) Control Grips
I've run into a few loose or improperly installed stick grips. Clamping pressure alone should not be the method of holding the grip onto the stick. There should always be some type of through bolt, pin or sleeve that captures the grip to the tube. I know it is a pain to run wire around something going through the middle of the tube, but it is too important not to. For a number of reasons....I usually ditch the supplied grip wires and reterminate/solder my own. Using 22AWG tefzel, I usually solder a wire to every post on every switch...no jumpers at the switch. I then sleeve the bundle in XPF-3/8 snakeskin. Finally, I'll route the sleeved bundle through the tube and terminate the end of each wire to a HD Dconnector (pins) about 6 in away from the base of the grip movement. Any splices, jumpers or changes that need to be done can all be done on the aircraft side mating connector to the grip. This configuration has a few advantages. The bundle is smaller and slicker than most grip provided wires (easier routing and assembly), it is easier to troubleshoot a possible bad switch at the disconnect (no chasing your tail since all jumpers and grounds are left with the airframe disconnect), it is easier to replace a single switch and pull temporary slack back and forth for soldering. If a grip circuit is more than 3 amps...I'll use a relay to avoid using larger gauge wires in the grip.
11) Wire Harness Routing
Separation - When running fore and aft under the floor - High power wires should run on one side (i.e. RH) and low power signal and coax wires would run opposite (i.e. LH), especially if the battery is relocated to the tail.
Tail Frames - I've seen lots of large non-factory holes cut in tail frames that don't need to be. Wires can run over the frames instead of through them using lighting hole cable tie bases. If holes are cut, don't encroach the structural grooves.
Tie Bases - I'm not a big fan of the adhesive cable tie bases, but I really don't like the adels either. Cable tie bases that use a screw or rivet to hold in place are the best IMHO. A better hold than adhesive but better serviceability than an adel.
I hope this information is helpful to anyone. It takes a lot of hours of focus, dedication and passion to build an airplane. I felt compelled to contribute to the tribal knowledge out of mutual respect. Thanks Doug for hosting these forums.
