Paul wrote:

The range of prop RPM should be the same in both a geared
Mazda and a direct drive AC engine. Unless of course you are cruising
the Mazda at way less than 50% power. I think something else was going
on with your AC pump failure situation.

Russell Kent wrote:

Eh?  What's prop RPM got to do with it?  The speed of the pump is directly
proportional to the speed of the engine.  I think that Ed just couldn't find the
proper pulleys to get the proper ratio; he needed something between the "too fast
at max" and the "too slow at idle" pulleys.

Russell Kent

Russell, "range of prop RPM has everything to do with it :-) I know
you are very very bright so think about it and re-read Ed's message :-)
No offense intended. Ed also did not mention a pressure regulator.

Russell Kent wrote:

Thanks (I think :-)

My point was you introduced PROP RPM when what is really at work is ENGINE RPM.  Since
the PSRU is fixed-ratio, they boil down to the same thing, but why confuse the issue
when the Mazda ain't gonna drive the pump from the prop side of the PSRU?

Your statement that the Mazda-driven prop and the LycoContiLohmFrank-driven prop both
have the same range is incorrect.  Assuming a 2.17:1 drive ratio, the Mazda-driven prop
has max power at ~2600 RPM (5600 engine RPM), cruises at ~2400 RPM (5200 engine RPM),
and idles at ~500 RPM (1100 engine RPM), or maybe even ~400 RPM (900 engine RPM).
You'll notice that this is a dynamic range of about O6 (larger value is about 6x
smaller value).

Assume that Ed's aircraft vacuum pump is rated for 700-2700 RPM range; below 700 RPM
and there isn't sufficient vacuum; above 2700 and the pump begins to disintegrate.  On
an aviation engine, this is perfect (1:1 drive ratio).  Notice that this is a dynamic
range of about O3.8 (larger value is about 3.8x smaller value).

No matter what pulley Ed selects for the pump, his engine's RPM range exceeds the
pump's.  He either has to idle higher to get vacuum at idle while keeping the pump RPM
reasonable at max power, or he has to burn up pumps (by over-reving them at max power)
so that he has vacuum at idle.  He cannot win.

Russell Kent

Not true. The pump has the same dynamic range as it is geared from the Lyc or Cont.
The lower potential rotary idle has nothing to do with it. You make an approach 
at the same prop RPM. Anyway I think my Continental will idle at 500 RPM :-).

BTW the Ross box Ed is using shutters badly at anything less than 1500 rotary engine
RPM anyway.

Paul Lamar

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