X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Fri, 30 Dec 2005 15:39:35 -0500 Message-ID: X-Original-Return-Path: Received: from sfa.gami.com ([68.89.254.162] verified) by logan.com (CommuniGate Pro SMTP 5.0.5) with ESMTP id 906339 for lml@lancaironline.net; Fri, 30 Dec 2005 13:03:17 -0500 Received-SPF: none receiver=logan.com; client-ip=68.89.254.162; envelope-from=gwbraly@gami.com Received: from localhost (localhost.localdomain [127.0.0.1]) by sfa.gami.com (Postfix) with ESMTP id 5304629C070 for ; Fri, 30 Dec 2005 12:01:54 -0600 (CST) Received: from sdf1.mail.taturbo.com (unknown [10.10.10.173]) by sfa.gami.com (Postfix) with ESMTP id 03CFE29C06E for ; Fri, 30 Dec 2005 12:01:53 -0600 (CST) X-MimeOLE: Produced By Microsoft Exchange V6.0.6603.0 content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C60D6B.C504378C" Subject: RE: [LML] Where has all the power gone? X-Original-Date: Fri, 30 Dec 2005 12:06:34 -0600 X-Original-Message-ID: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [LML] Where has all the power gone? Thread-Index: AcYM7fhFCIiNej5dRW+G1L4b+NCAsQAem26w From: "George Braly" X-Original-To: "Lancair Mailing List" X-Virus-Scanned: by amavisd-new-2.3.2 (20050629) (Debian) at gami.com This is a multi-part message in MIME format. ------_=_NextPart_001_01C60D6B.C504378C Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Bill, Lorn, =20 =20 =20 In a private message, Lorn Olsen added the following to the question that you have. He said,=20 =20 >> . . . 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? =20 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? =20 If Bill and I are not thinking correctly, please let us know.<< =20 =20 As Yogi said... in theory... and in practice. =20 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. =20 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. =20 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. =20 Regards, George=20 =20 =20 =20 =20 =20 _____ =20 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? =20 Dumb question for you smart guys. =20 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: =20 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.=20 =20 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? =20 Probably lots of flaws in my thinking, but I'm not smart enough to see 'em. Help me out. =20 Bill Harrelson 5zq@cox.net N5ZQ 320 1,100+ hrs N6ZQ IV 4.239% =20 =20 =20 ------_=_NextPart_001_01C60D6B.C504378C Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

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.

 

Bill Harrelson

N5ZQ 320 1,100+ hrs

N6ZQ  IV  = 4.239%

 

 

 

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