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alpinelakespilot2000

Well Known Member
I just replaced my Plane Power with one of Vans traditional automotive alternators, the 35A 14184. Have been flying successfully with it for about 5-6 hours and under normal circumstances and under full load, it provides a steady14.3-14.4V to the bus.

However, I've noticed one scenario under which it can't hold that voltage. If I try to run everything electrically, including BOTH of my 100W landing and taxi lights and allow the rpms to drop below 750, my voltage drops to about 13.3-13.4V. If I turn off just one of the landing lights, OR if I leave both lights on but increase my rpm to >750, my voltage returns to the normal 14.3-14.4V.

Put differently, if I keep the rpms above 750, I have no problems at all, no matter how much load is on the system. If I allow the rpms to drop below 750, I must turn off one landing light in order to maintain the 14.3-14.4V. If I don't it will drop to about 13.3-13.4V.

FWIW, with everything on + one 100W landing light I seem to run about 20-21A. With everything running + both 100W landing lights, I seem to run about 27-28A.

Is this to be expected from my setup, is this just the "real" capacity of a 35A alternator coming into play, or should I be looking further for a problem?

Thanks.

Edit: See post #7 for solution.
 
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The "real" capacity of the alternator is going to be at normal rpm's, like 10,000 or so. At some speed under that it's going to drop off and I think you found that point. The alternator is working pretty hard with little cooling air at those speeds and no reserve to charge the battery. Assuming the same size pulleys, the PP alternator either was able to function better at these speeds or more likely it had greater capacity.
Your options are reduce load until takeoff, increase rpm, increase alternator capacity, or... What size pulley is on the engine? There are two different size "flywheel" pulleys, so you may be able to swap into the larger pulley and speed it it some.
Just my opinion, but expecting 28 amps continuously from a 35 amp alternator is pushing it pretty hard, I like to keep it under 60% or so.
Tim Andres
 
I have a b&c in my other (non RV) plane and I have to be over 1200 rpm for the alt to produce significant power. 750 is very low.
 
It sounds normal to me. At 13.3 volts, the alternator is putting out the maximum current that it can at that RPM. As long as the voltage does not drop below 13 volts, the battery is not being discharged, although it is not being charged either. The temperature of the alternator is a concern if heavily loaded. Many builders direct an air blast tube at the alternator to help keep it cool.
 
Cooling plenum for alternator

Mich48041 said:
It sounds normal to me. At 13.3 volts, the alternator is putting out the maximum current that it can at that RPM. As long as the voltage does not drop below 13 volts, the battery is not being discharged, although it is not being charged either. The temperature of the alternator is a concern if heavily loaded. Many builders direct an air blast tube at the alternator to help keep it cool.
Click to expand...

Agree with blast tube. I went beyond to make a plenum and feed with 1 inch branch off 2 inch heat scat, also have digital temp sensor. Runs about 115 f in flight and rises to 145 during taxi. Got 200 during taxi before plenum. 200 is OK, but I like my plenum and lower temps.
 
Ok. Sounds like it's about what I should be seeing. Good to get that reassurance.

I do have a blast tube aimed right at the back... Especially important I assume since Vans used these without the external fans, and I followed suit by taking my fan off to provide greater cowl clearance. Will see how long it lasts.

Thanks.

Update: replaced my Cessna split master bat/alt fld master switch with a regular switch from B&C and this eliminated a lot of resistance in my alt fld breaker circuit. Now I have no issues under any load conditions. Can get a rock solid 14.0 on Dynon/14.4V at main bus regardless of what's on.
 
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