Electrical system decision

[airlincoln]

Okay, so let's say you're building an RV-7A with one Slick magneto and one Lightspeed CDI (1.2A draw in cruise) for ignition. You're going to certify it day/night VFR (one comm radio [.2A] & xpndr [1.5A]) but hope to someday save enough money to equip it with a TruTrak pictorial pilot (1.3A), Garmin GNS 430 (1.5A) and Dynon D-180 (1.5A) and will pre-install the antennas/wiring/fuse blocks to do that. Which electric system would you use? For those AeroElectric fans out there, the choices are:

option #1 --> Z-11 (single-battery, single alternator)
option #2 --> Z-12 (single-battery, dual alternator [60A & 20A])
option #3 --> Z-13/8 (single-battery, dual alternator [60A & SD-8 [8A] backup)
option #4 --> None of the above OR Modification of one of the above

In the immortal words of the robot in Short Circuit: "Need More Input!"

Thanks in advance and feel free to shoot as many holes as possible in my plan.

Lincoln Keill
RV-7A (QB wings/fuselage on the way)

Bonus question: Which items would go on your E-bus if you had a 4A budget?

[Norman CYYJ]

I think you need to revisit the total current consumption, esp the radios. They draw more on transmit then mentioned. The current that should be plugged into your calculations should always be the maximum not the minimum draw on any piece of equipment.

[Mike S]

You state night VFR, but did not add the lighting load.

It appears you are reading Nuckols book, he has good info on system choices.

First step is to determine your flight mission, and to decide if you want to continue that mission on backup, or ??? Are you flying for fun, or do you need to get there???

For a mission as described above, IMHO, simplicity should figure into your design-------single alt single batt is simplest, and the engine will continue to run on the mag.

Do you have backup flight instruments that dont need electricity???

Oh, yes, welcome to the site, I see it is your first post.

Bonus question answer-----what ever would do the most to keep me alive.

P.S., what airline do you fly for??

[Neal@F14]

I'm doing the wiring for three different RVs at once right now. (4, 8 & 10)

A single battery, single alternator is plenty adequate if you're not intending to spend a great deal of time flying hard IFR and need to power pitot heat and redundant gyro instruments, etc. An Odyssey PC925MJ battery and a Plane-Power 60 or 70 amp alternator is what I'd use if I were building an RV-7.

Your GNS430 (12v) can draw as much as ~ 7 amps when transmitting, most people use a 10 amp breaker with them, but a 7.5 amp *might* get you by without tripping.

I may be a bit old fashioned, but I prefer good old fashioned individual circuit breakers for each device, wherever practical, with toggle breakers for autopilot, alt field, fuel pump, avionic master, nav lights, strobes, landing light, etc., and a push-pull breaker for each radio or instrument on the avionics bus which is also fed from the avionics master toggle breaker, and push-pull breakers that can be pulled to disable items fed from the main master bus like the flap motor and trim motor, and in your case, also the electronic ignition. This makes for an expensive shopping list of circuit breakers but I think it's worth it.

[az_gila]

Originally Posted by airlincoln

Okay, so let's say you're building an RV-7A with one Slick magneto and one Lightspeed CDI (1.2A draw in cruise) for ignition. You're going to certify it day/night VFR (one comm radio [.2A] & xpndr [1.5A]) but hope to someday save enough money to equip it with a TruTrak pictorial pilot (1.3A), Garmin GNS 430 (1.5A) and Dynon D-180 (1.5A) and will pre-install the antennas/wiring/fuse blocks to do that. Which electric system would you use? For those AeroElectric fans out there, the choices are:

option #1 --> Z-11 (single-battery, single alternator)
option #2 --> Z-12 (single-battery, dual alternator [60A & 20A])
option #3 --> Z-13/8 (single-battery, dual alternator [60A & SD-8 [8A] backup)
option #4 --> None of the above OR Modification of one of the above

In the immortal words of the robot in Short Circuit: "Need More Input!"

Thanks in advance and feel free to shoot as many holes as possible in my plan.

Lincoln Keill
RV-7A (QB wings/fuselage on the way)

Bonus question: Which items would go on your E-bus if you had a 4A budget?

Calculate the costs carefully for not putting in the D-180 now... if you have gyros and a set of engine instruments with full 4-cyl monitoring (recommended) the $ delta may not be too great...

The Dynon (and other similar system) advantage is that it can come with an extra battery, so even if your main electrical system fails, you can still get EFIS and EMS readings. It also makes the panel much simpler...

Go for the Z-11 and down load the electrical load per phase of flight spreadsheet from AeroElectric Bobs web site and calculate the alternator output you need... for VFR and a small, modern radio stack, it might only be 40 Amps...

gil A - just got my D-180...

[Ron Lee]

With one Lightspeed ignition, GNS 430, transponder, Trio autopilot, XM radio, intercom, RMI engine monitor and strobes I am using about 5 amps not transmitting.

[f1rocket]

I have Option 2. If I were to do it over again, I'd go with option 5. (Dual battery and a single alternator).

[gmcjetpilot]

Option #1 (has served 100's of thousands of general aviation aircraft for a half of century or more.)

At big aircraft companies each department wants to put more (or less) of one thing or another to favor their "system". Structural guys want more beef in the airframe and electrical system guys want more wires. Than there are practical issues the head design chief must consider: payload, range, performance, cost, weight............................... Stop the madness, KISS... The chief electrical guy will always want more stuff.

Early RV's weighed routinely around 900 lbs, plus or minus. Now all the flying pigs come in at 1100 lbs nominally. To be fair Van grew the RV-7 and RV-8, but still where did that extra 200 lbs of weight come from? That is almost another engine's worth of weight! Well some comes from dual electric everything. Aeroelectric never saw a cool electical gadget or extra cuircuit, battery or alternator that was not considered better.

My self I kind of have Option 1.5 - one alt, one main battery and one small aux battery for dual electronic ignition (not self powered magneto or P-mag). If I had a magneto or P-mag I would go option 1. My handheld GPS has its own internal battery as does the Dynon EFIS. The main battery should drive your min devices for at least as long as you fuel endurance anyway. Why have dual batteries or alternators. I suppose for a long extended cross country (really across the country) I might cary some spare parts, may be a spare alternator and tools to swap it out? That would solve that issue, if the alternator failed, land and replace it.

Try and resist making a super deluxe heavy electrical system unless that effort, cost and weight will really makes sense. VFR it does not, in my opinion..

Change of the alternator going bye-bye is reasonable to expect at some point; however there are ways to assure the alternator is unlikely to ever fail maintence and design. **

** Alternators fail from being run too hot (no blast air). They fail from too fast RPM (pulley dia. mismatched, does not hurt within reason, about 10,000 rpm max, just that it wears brushes faster). They also fail from too much load (alternator undersized for job, 60%-70% max of rated, again heat related). Regular planned replacement of alternator brushes, tear down/inspection and even proactive rebuilds assure reliable service. Whether at 500, 1000 or 2000 hours, the alternator should be torn down before failure. In new cars the last 10-15 years we can get 10 years with out a problem. In the 1970's you where lucky to get two years out of an alternator. Airplane engines shaking, hot cowl/exhaust, undersized power capacity (for weight) and the dry conditions we have (flying at altitude), shortens the longevity. However not overloading it, keeping it as cool as possible and maintaining it, can give you high reliability. The bottom line, keep the alternator from failing and have a main battery (in good condition) as standby. The alternator needs maintenance as much as the engine. Option 1 will serve most VFR RV'ers.

DON'T HAVE ELECTRICAL ENVY, BUILD IT LIGHT AND SIMPLE

[f1rocket]

So George, after all those words, you have a single alternator, dual battery system. Got it.

[Jekyll]

Night VFR with 1 mag and a lightspeed - carry a flashlight and you have all the redundancy you need for safe flight and landing. The rest is wieght, cost and additional failure modes. Z-11 gives this abilty.

If you want to continue flying with your systems fully operating, then you might consider more extravagant solutions to keep the amps flowing to your systems.

Other archetectures will help in the case of a master contactor failure which, in the Z-11 stops all your power.

A simple, yet very capable addition to the panel for a Z-11 is a TrueTrak ADI with a backup battery, The batter adds $100 and several ounces. With this in your Z-11, you have 1 mag and a lit, battery powered ADI to get back on terra firma (the more firma, the less terra). A functioning engine and ADI are usually good things to have at night.

Jekyll