I am doing a panel upgrade to make my RV-8 IFR capable. It will include a G3X, GPS 175, and remote transponder. Back up instruments will be conventional airspeed and altimeter, plus an AV-30 for back-up attitude information (and more). I have two VHF radios and an audio control panel. I have three busses: Battery, Radio, and Instrument, and have the opportunity to configure the avionics and systems to provide the best systems for each bus, operationally the best systems for each bus considering how they are used before engine start and engine running, and then considering power management with specific electrical failures. I will have a back-up 3 amp-hour battery for the new avionics. Given those considerations, I am looking for opinions on the best way to configure the electrical system. Thanks!
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Buss Architechture
- Thread starter Terry Lutz
- Start date
Carl Froehlich
Well Known Member
As a rough guess, I?d say your avionics will draw about 8-10amps. For a 3amp-hr battery that translates to perhaps 15 minutes. It is your airplane, but I would consider that insufficient. I suggest taking a look at your total system and strive for at least an hour of battery capacity when the normal stuff goes south. I offer that just adding a backup battery to the mix tends to fall short of what is possible for a given weight add.
Carl
Carl
Ed_Wischmeyer
Well Known Member
My RV-9A has the Garmin G3X system, and my backup is a G5 and an iPhone for navigation. Here’s my IFR backup strategy…
* Following the lead of a test pilot friend, my enroute minimums are a thousand foot overcast. If the engine goes poop when I’m in or above the clouds, I’ll have a screaming chance of finding a survivable place to put it down.
* If I lose electrical power, I can maintain course, attitude and altitude with the G5. I’ve practiced this. If I still had the fixed pitch prop, I could use engine sounds for approximate power. Can’t do that with the constant speed prop, but I can use throttle position and airspeed. I’ve felt no need for steam gauge backup.
* The G5 and the G3X use different sensors and different software algorithms, so this mitigates common mode failure, even though both systems are from the same vendor. The upside is that if the normal AHRS fails, the G5 can serve as a backup AHRS to the G3X. The failure mode means no OAT, hence, no TAS or winds, and no heading – but so what, course is what you really want, heading just being a way to iteratively determine your course.
* If I lose electrical power, ATC can still see me as a primary target. I’ll let them worry about traffic separation, and I will no longer care about precisely executing the flight plan.
* Given that I should – should – have a thousand feet of clear air below me, I don’t care much about being able to fly an instrument approach after an electrical failure. (This also answers the personal minimums question.)
* In the -9A, I practice flying with and without the autopilot. The workload and style of flying are different, and the autopilot does not necessarily make every aspect of flying easier.
* Years ago, a friend of mine used to fly IFR in his short wing RV. His comment was that IFR was doable as long as you didn’t have to do any paperwork, including charts. The RV-9A is challenging to hand fly inside a cumulus cloud. I have no illusion that after a power failure my navigation will be more than select a heading and re-check position every few minutes.
* When I had an RV-8 not many years back with its nice sport plane stability and handling, I wasn’t sure that I would want to fly it IFR, although I knew that I could if I got caught and had to. Everybody will have different abilities and risk mitigation strategies, of course.
* The GSU 25 AHRS regularly drops off during engine start. Garmin has a power management product that prevents this, and I wish I had it, but rewiring the plane for it is too big a job. The G3X system otherwise stays alive during engine start unless the battery is way down.
If you ever make it to Savannah, you can fly the -9A with me and see how much this IFR strategy works for you… or doesn’t.
Ed
* Following the lead of a test pilot friend, my enroute minimums are a thousand foot overcast. If the engine goes poop when I’m in or above the clouds, I’ll have a screaming chance of finding a survivable place to put it down.
* If I lose electrical power, I can maintain course, attitude and altitude with the G5. I’ve practiced this. If I still had the fixed pitch prop, I could use engine sounds for approximate power. Can’t do that with the constant speed prop, but I can use throttle position and airspeed. I’ve felt no need for steam gauge backup.
* The G5 and the G3X use different sensors and different software algorithms, so this mitigates common mode failure, even though both systems are from the same vendor. The upside is that if the normal AHRS fails, the G5 can serve as a backup AHRS to the G3X. The failure mode means no OAT, hence, no TAS or winds, and no heading – but so what, course is what you really want, heading just being a way to iteratively determine your course.
* If I lose electrical power, ATC can still see me as a primary target. I’ll let them worry about traffic separation, and I will no longer care about precisely executing the flight plan.
* Given that I should – should – have a thousand feet of clear air below me, I don’t care much about being able to fly an instrument approach after an electrical failure. (This also answers the personal minimums question.)
* In the -9A, I practice flying with and without the autopilot. The workload and style of flying are different, and the autopilot does not necessarily make every aspect of flying easier.
* Years ago, a friend of mine used to fly IFR in his short wing RV. His comment was that IFR was doable as long as you didn’t have to do any paperwork, including charts. The RV-9A is challenging to hand fly inside a cumulus cloud. I have no illusion that after a power failure my navigation will be more than select a heading and re-check position every few minutes.
* When I had an RV-8 not many years back with its nice sport plane stability and handling, I wasn’t sure that I would want to fly it IFR, although I knew that I could if I got caught and had to. Everybody will have different abilities and risk mitigation strategies, of course.
* The GSU 25 AHRS regularly drops off during engine start. Garmin has a power management product that prevents this, and I wish I had it, but rewiring the plane for it is too big a job. The G3X system otherwise stays alive during engine start unless the battery is way down.
If you ever make it to Savannah, you can fly the -9A with me and see how much this IFR strategy works for you… or doesn’t.
Ed
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Perhaps instead, a tool for considering configuration?
http://www.vansairforce.com/community/showpost.php?p=1423589&postcount=22
The above example was an attempt at optimizing for CPi EI, which has its own internal backup battery system. Another recent thread considered a three-bus dual alternator setup. Again, blocking it out is a whole lot easier for development and analysis, as compared to working with conventional diagrams, which come later.
http://www.vansairforce.com/community/showpost.php?p=1427638&postcount=26
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