Bill, Lorn,
In a private message, Lorn Olsen added
the following to the question that you have. He said,
>> . . . Can you tell
us how much the thetaPP is retarded based on an RPM reduction. That is, Flying
at 9,500 ft at 2,500 RPM vs 2,400, 2,300 2,200 and 2,100. If you don't know the
answer, although it doesn't seem to be in your nature, can you venture a guess?
I have been flying at 15,500
ft at full throttle indicating about 130 KTS. I have then reduced the RPM to
2,300 waited 15 minutes and saw no decrease in airspeed. Could this be from effectively
advancing the spark by reducing the RPM and therefore keeping the HP the same?
If Bill and I are not
thinking correctly, please let us know.<<
As Yogi said… in theory…
and in practice.
When we do this exercise (changing RPM
from 2500 to 2100 , for example, to see the effects on peak pressures) on
the test stand, we don’t see much change in
theta(p-p). Why? Probably because the higher RPM
results in some increased “turbulent mixing” and a slightly faster
burn rate as a result as compared to the slower RPM and lower “filling
speeds” of the air coming into the cylinder. So there are
some offsetting dynamic issues.
I found the results to be mildly
surprising, although I am learning to get used to that as we take harder
and harder looks at the real time internal cylinder combustion data.
As to the results that Lorn observed at
15,500 feet, with minimal speed changes going from 2500 to 2300 RPM - - -
I suspect strongly that “prop efficiency”
changes as a function of RPM are playing a significant role in what you
observed. Normally, the lower RPM will put the prop into a
slightly more efficient point on the prop efficiency curve, thus, mostly making
up for the Hp change with the lower RPM. Also, there may be some
slight F/A ratio changes with the RPM change and that can change the Hp around
a bit in a non-linear manner with the RPM change.
Regards, George
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf
Of Bill&Sue
Sent: Thursday, December 29, 2005
9:01 PM
To: Lancair Mailing List
Subject: [LML] Where has all the
power gone?
Dumb question for you smart guys.
I've been reading all of the spark timing information with
great interest. We have a while before we have to settle on an ignition system
for our non TC 550 and, of course, would like to make the most informed
decision. Soooo, George, Scott, Hamid, Paul, Rick, et al, how about this:
Seems to me that rpm changes effectively adjust
ignition timing. For example, if you're running at 2500 rpm and the spark
occurs at 25 btc, the flame propagates at a rate that produces max pressure at
some point after tdc, lets just say 15 degrees. If you reduced the rpm to 2200
and the spark occurred at 25 btc the flame would propagate at the same rate as
before but the piston is moving slower. Seems to me that the max pressure would
occur at somewhere less than 15 after tdc, effectively advancing the ignition.
Most agree that the majority of the advantage of adjusting
spark timing happens at lower power settings. To me, it looks like the
advantage is better fuel economy as opposed to more power. Couldn't one simply
pull the prop control back to gain this same advantage?
Probably lots of flaws in my thinking, but I'm not smart
enough to see 'em. Help me out.