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From: "Gary Casey" <glcasey@adelphia.net>
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Subject: Re: Electrical ignition and airspeed
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<<My calculator gives slightly different results:

I get a change in air speed of 1.58% rather than 1.95%. I think you may have
used IAS rather than TAS.

Also, correcting for OAT and MAP gives 1.60% predicted change in air speed

If you are going to correct for accessory losses, then you should probably
also be correcting for the throttling loss. This power is the throttle
depression (inHg), 1" in this case, multiplied by RPM x displacement (in^3)x
volumetric efficiency(VE) divided by 122150 to get horsepower. I would guess
that VE for a naturally aspirated engine is about .92, somebody out there
may know the real value for a specific engine. VE will also change slightly
with RPM.

For a 1" throttle depression and a 360 cu in engine with .92 VE the
throttling loss is 6.78 HP.

Rob>>

You are right on all counts, Rob.  I was being a little loose with my
calculations.  First, I should have used TAS.  The correction for pumping
loss is probably appropriate as what we are trying to simulate is how much
relative power is produced at different altitudes, all at WOT.  To simulate
that by testing at partial throttle introduces the error due to pumping
loss.  In other words the engine will produce more power at the same MAP
when running at higher altitude than at lower altitude.

<<That means low electrical use and non-mag ignition systems will give you a
HP boost!  Turn off the lights!  Get rid of those mags!  Remove your vacuum
pump when you race!  Keep the oil pump!
 Grayhawk>>

Also exactly correct.  How many times have we all heard "no reason to mess
with that - the improvement is not measureable."  The winners - in more than
racing - do 100 things better, each of which produces an immeasurable
improvement.  For instance, electrical load is real.  A 100W landing light
consumes about 150 watts, or 0.2 hp.  Will that 1/5 hp make the plane go
faster?  Absolutely, without a doubt.  One could take the belt off the
alternator to eliminate that loss.  And remove the vacuum pump.  Each will
produce an immeasurable improvement.

<<In reviewing the WOT performance curves for O-360-A, I note that they
converge to 0 power at 59,000 ft. This would equate to around 0.24 inHg at 0
C.>>

An interesting observation.  However, one problem that I have observed in
looking at POH performance is that in some cases it looks like the data was
excessively extrapolated.  Like they made a couple of measurements and then
just filled out the rest of the chart.  In the case you bring up the offset
in manifold pressure readings is only 0.24 inches.  Sounds more like someone
was extrapolating to zero rather than giving actual data, as I don't see how
the friction horsepower could be so low as to account for only .24 inches of
manifold pressure.  I previously guessed 4, which is probably on the high
side.  2 or 3 might be more reasonable.

Gary Casey