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The alternator speed isn't a problem, it doesn't care how fast you spin it (unless you have an alternator that is way out of balance). On my RV-4 with Lycoming engine the alternator is over-driven about 4:1, I have the 2-1/2 inch stock alternator pulley, the crank/ring gear pulley is about 10" diameter so the alternator will see 10,000 at cruise. This is typical of most Lycoming installations on experimentals and is not a problem on thousands of planes.
My e-shaft pulley is 4-1/2 inches dia. I think it is a stock pulley as the timing pointer is at the edge of the pulley. That makes the alternator over-driven at 1.8:1. Using a 6000 rpm cruise with my RD-1C that makes the alternator spin at 10,800. Close enough.
There are enough Lyc drivers that were uncomfortable with this that someone makes a 4" pulley for the alternator. Aircraft Spruce 02-03 catalog page 245, p/n 07-06828 $48.95. (A waste o' money except for the feel good factor. If you'll accept MY explanation I'll accept your 48.95 and SAVE YOU shipping costs!
P-)
Regarding water pump speed, I've been working on this for a few weeks with my 13B dynafocal adaptor. I've mostly come to the conclusion (yet open to more data or ideas) that the factory pump and pulley are sized to provide the needed flow capacity at any given engine speed at the horsepower levels we are running at. So, to make it easy I'm sticking with the stock pump at the stock pulley ratio.
Mike
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
At 12:06 2004-05-05, you wrote:
Well, that's interesting. This reminds me of a nagging question I have had, but haven't asked.
I was curious about the need to go to a smaller crank pulley to slow down the water pump and alternators.
From memory, the crank pulley is about 5-1/2", the alternator pulleys are about 2-1/2", and the water pump
pulley is about 4" diameter.
So, running an RD-2C (2.85 ratio), the alternators will be turning about 20k during climb-out and the wp will be turning
about 10K rpm. That seems too fast to me. I could probably find larger pulleys for the alternators, but
then there's the water pump pulley. Your unplanned experiment would suggest that we can run the wp slower
and still cool adequately.
This brings me to a second option. I think a simpler approach would be to reduce the size of the crank pulley.
The problem is that I haven't found a multi-V reducing pulley at any of the after-market suppliers. I really don't
want to change over all the multi-V pulleys to single V pulleys, but that is one option.
Another solution might be to find a smaller pulley from another vehicle and adapt it to fit the rotary. If that doesn't
work, I could have one made.
So, am I worrying about nothing, or is this a real problem?
Mark
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