On May 26, 4:52 pm, Dallas <Cybnorm@[EMAIL PROTECTED]
> wrote:
> In calm air, run an engine up to 1700 RPM and measure the airflow behind
> the prop.
>
> Then set up a fan and blow 40 mph of wind into a prop turning 1700 RPM.
>
> Seems to me the airflow would be the same, the difference would be the
less
> strain on the engine to turn 1700 RPM.
1700 isn't an arbitrary figure. Blowing 40 mph of wind
through the prop would reduce the load on the engine at 1700,
defeating the main purpose for selecting that particular RPM.
The engine manufacturer works with the airframe
manufacturer to determine the best runup setting. In your case, 1700
will give the highest cylinder compression pressures, and checking the
mags at that RPM will give them the most op****tunity to fail to spark,
which is what a mag check is all about. Air is a dielectric, an
insulator, and higher cylinder pressures pack more of it between the
electrodes and eventually a bad plug (or bad mag) can't arc through
it.
Higher than 1700 results in more induction system drag,
less volumetric efficiency, and less air entering the cylinder,
lowering its ultimate compression pressure. Lower than 1700 will have
the throttle closed more, again reducing airflow into the cylinder.
We have a Citabria 7ECA that has the O-235 in it. Its
runup RPM is 1800. My old A-65 gets its highest pressures around 1600.
1700 was picked for your engine, and running up into a really strong
wind could reduce the load enough that you'd have the throttle closed
a bit, but POHs don't mention that. I guess they figure that you won't
be flying in any wind strong enough to make a difference.
Dan
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