Alternator (primary) sizing

OK, I’m about to step in it here. I’ve already reached out to someone I trust regarding such application so now I’m throwing it out to the world. Always worried about the alligator I don't see.

Background:
Electrically dependent aircraft
Dual battery, Alt and BU gen, redundant bus architecture.

Issue:
Had to raise top of airbox to allow straight alignment with TB inlet. A very short “S” turn of SCAT at those flow velocities is less than ideal to me.
Now there is interference between Alternator and airbox.

Question (simplified):
What size Alternator is “acceptable”?

Factors (slightly conservative):
9.13 amps = Full Avionics nominal load (no radio xmitting, AP actuators not cycling continuously, etc)
3 amps = Conservative Alt field load
1,2 amps – total relay draw
7 amps = FI/EI load
10 amps ~ pitot heat load
8 amp ~ luxury load/not required for heated seats

So
20.3 amps to fly the aircraft without shedding anything (assuming no scope creep)
30.3 if pitot heat is required
38.3 if seat heaters are on (installation is not a must have)

Would 40 amp primary alt be “safe”/manageable in an off-nominal situation? The slightly smaller diameter will definitely help.
The old 80% capacity rule for alts seems a bit outdated in dual bus/battery/source configurations.
This isn’t about trying to save money or weight. I want a 60 amp alt but clearances are making this a bigtime challenge. Attached pix are NOT with the cowl fully in position. There will be more, additional fiberglass work as a minimum and possibly a custom alt boss bracket needed.

Thoughts are appreciated. I'd like to order one before the weekend

Why don't you start by installing the shortest belt possible on that existing 60A alternator you already have? Then you'll get a much better idea of clearance required. Belts come in roughly 1/2" length increments.

Plus, after you fix the alternator in place, you can easily modify the fiberglass to create more clearance if needed.

hgerhardt said:

Why don't you start by installing the shortest belt possible on that existing 60A alternator you already have? Then you'll get a much better idea of clearance required. Belts come in roughly 1/2" length increments.

Plus, after you fix the alternator in place, you can easily modify the fiberglass to create more clearance if needed.

Click to expand...

The alternator used for fit-up is a dead Plane Power 60 amp. I've used cord to draw between the flywheel and pulley until the alt contacts the oil return line. I can increase that clearance with the B&C boss mount which is 1" longer increase the but that will increase the airbox interference. Pix are without the cowling all the way in position and pre-engine sag. Would be beneficial if I can leave enough flat area on top of the box for an alt-air door; hence the question.

You've not mentioned lights in your calcs. Add those in and it might look too marginal, especially if it's an IFR aircraft.

On the other hand, how big is the back up generator and can it charge the battery in normal operations?

Planning a 30amp MW back-up gen which has it's own sensing/activation/annunciation; so yes it can charge a battery or two. Obviously if that is active, you're shedding load and possibly looking to land. Will probably be using FlyLEDs. Lights, heaters, etc. on a non-essential bus. It just keeps snowballing.

40 amp main alternator

I actually started with a 60 amp and after 75 hrs thought I had an alternator issue. I ordered another alternator thinking that even if it was not the alternator good to have a back-up. ACS sent me a 40 amp and when I called made me a smoking good deal so I kept it and installed it. Turns out was not the alternator and have been flying with the 40 amp ever since. When I start it up occasionally goes up to 45 amp draw (not sure how a 40 amp does that) and eventually settle down to 20 to 25 amps. My back-up is on the essential bus and draws in the neighborhood 8 to 10 amps. Pitot heat is unregulated and draws 5 to 7 amps. The weak link is the main alternator and if I loose that time to find a place to land. Won’t matter 40 or 60 amps. PM sent.

Re-clocking

Until recently I did not know this was possible but if one has a fitment issue B&C (and maybe others) can re-clock your alternator if that would help.

Data

Just another data point.

I am running a dual battery, dual alternator, split bus. I have a 60 A B&C primary and the 20-40 A secondary.

RV-10 with EFII.

Essential bus runs between 7 and 11 amps continuously. Only thing on it is the EFII equipment, the secondary alternator, and an EarthX ETX900 battery.

Primary carries the rest of the loads. Usually around 20 - 25 amps with spikes from the LED strobes. If ALL of the lights are on, I can see almost 40 amps...but I have more than the average lights externally.

Recheck the alternator, something is not right there.

Something does not look right about that alternator. I have attached my EI60PP photo. The clocking of the pivot and the adjustment is maybe 20 deg for the adjustment with the pivot at 180. It is biased toward the engine. The B&C pivot and adjustment is 180 apart. The PP operates within 2 mm of the prop oil pressure line. It works fine if tweaked lightly.

I think you would have room for the 60A PP and if you dont, a smaller frame size is rare. Even that 40 amp is the same stator OD as the PP 60 amp.

I am with Dmatt.
I have a 60 amp B&C and wish I had used a 40 amp.
I assume most of us have way too large of a alt.
Mine has only run over 40 a couple of times after an issue and only for a minute of two.

Mine is not electrical dependent but i believe 40 is good in most cases.

70%

Everyone will have a different opinion on what size alternator.

I like the alternator to be able to carry 70% of full load.

In your case, 38.3 amps would make we want at least a 55 amp alternator.

The 20% overhead is a reasonable rule to cope with battery recharging. From my perspective the issue is what happens when the battery is significantly drained when the engine takes ages to start - for whatever reason. It doesn't happen often but it will happen at some point. Perhaps you can live without the seat heaters and the pitot heat for an hour? Perhaps you should install an ammeter on the alternator output wire? Most conventional RVs I have flown run less than 10amps continuous, with an electric engine I would have thought a 40A alternator would cope most of the time. Depends on your attitude to risk and weight. If you are comfortable with the potential compromise install a 40A.

Kinda sorta saved for the moment

Well if I had to re-do fiberglass, might as well do it for the higher capacity alt. Ordered the B&C 60 amp. I knew it was slightly smaller dimensionally but the much longer bracket would more than eat up that clearance margin so I thought. No interference and one or two size smaller belt will help out.

LX60 has a slightly smaller stator/rotor but the pulley is a bit smaller to make up the difference with increased RPM. Lighter too of course. At least for this issue, I finally got lucky. Important point for reference. The initial pix with interference were with cowl not completely up in place. More clearance with oil return line as well. If the reliability matches the build quality, I’ll be in good shape. Some pix attached for reference: B&C LX60 and PP AL12.