Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Tue, 22 Mar 2005 22:57:29 -0500 Message-ID: X-Original-Return-Path: Received: from [68.89.254.181] (HELO sdf1.mail.taturbo.com) by logan.com (CommuniGate Pro SMTP 4.3c3) with ESMTP id 815760 for lml@lancaironline.net; Tue, 22 Mar 2005 13:19:06 -0500 Received-SPF: none receiver=logan.com; client-ip=68.89.254.181; envelope-from=gwbraly@gami.com content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C52F0B.8A228FD6" X-MimeOLE: Produced By Microsoft Exchange V6.0.6487.1 Subject: RE: [LML] Re: superchargers X-Original-Date: Tue, 22 Mar 2005 12:18:25 -0600 X-Original-Message-ID: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [LML] Re: superchargers Thread-Index: AcUvAhk2RtfP07C9R3q/OWmrM6EjlAAAjtuA From: "George Braly" X-Original-To: "Lancair Mailing List" This is a multi-part message in MIME format. ------_=_NextPart_001_01C52F0B.8A228FD6 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable =20 From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of = REHBINC@aol.com Sent: Tuesday, March 22, 2005 11:11 AM To: Lancair Mailing List Subject: [LML] Re: superchargers =20 George, I have compressor maps for lots of aircraft centrifugal compressors - = - and only one of them gets to 80% at any point on the map. =20 It is unlikely that the fixed rpm ratio version of that unit would = operate=20 at that optimum efficiency for more than a small portion of its operating = life. You've got me there. I don't have any maps for "aircraft compressors". I = do however have many for general applications and it is not uncommon for = them to peak in the upper 80s to 90% efficiency. I only assume that = aircraft would use equipment with similar efficiency. =20 Why wouldn't an aircraft compressor operate at or near its optimum = efficiency nearly all of its life? Airplanes are routinely flown at = roughly the same altitudes and power settings for the majority of there = lives.<< =20 =20 Well, as it happens, I have on my desk the compressor map for what = is probably the best compressor Garrett has ever made for small general = aviation piston engines. The sweet spot for a mass flow of about 30 lb/min and a pressure ration = of 2.0 to 2.2 has an efficiency of 76%. It rapidly deteriorates down to 60% when you get off of that design = point. =20 =20 Thus, at 5000 feet MSL, at a pressure ratio of ~ 1.25 (36-38=94 Hg MP = or there about) the efficiency would be down around 60 to 65%. Airplanes, unlike cars, operate from a baseline 30=94 Hg ambient at = sea level - - to a baseline ambient of only 12-13=94 Hg , with = pressure ratios varying from 1.2 up to 2.8 to 3.0. Thus, at constant RPM, the compressor would spend most of its life = well off of any =93optimum=94 pre-determined design point. =20 The variable speed turbo-supercharger compressor helps to mitigate this = problem to a substantial degree. =20 =20 =20 >>If all you want to do is get away from detonation, cut the compression = ratio by a quarter point. If you want to make more power, ad an = intercooler.<< =20 Rob, I think that is not the optimal approach. However, I think that = is close to what a lot of automotive background people would suggest = when first considering the issues. =20 =20 Take another look at it. =20 Assuming the engine is already operating at or near the engine=92s = detonation limited maximum BMEP - - (many aircraft engines are) - - = then in order to improve the detonation margin, without reducing power = (reduction in CR goes the wrong direction on the power and the = efficiency issues) or in order to increase the BMEP - - and remain = free of detonation, one will have to do one or more of the following: =20 1) Reduce the CHTs;=20 2) Increase the fuel octane; 3) Reduce the induction air temperature. =20 =20 Cars with liquid cooled engines can=92t readily reduce the CHTs a lot. =20 Air cooled aircraft engines can improve the engine cooling with some = amazingly simple revisions to the baffling systems. That helps a lot. = You can =93buy=94 about 5 octane points of detonation protection by = dropping the CHTs by 50d F. =20 Fuel octane is difficult to mess around with for aircraft. =20 That leaves - - - intercoolers. =20 =20 Reductions in IAT from the intercooler will buy one LARGE improvements = in the detonation limited BMEP (ie, more horsepower becomes usefully = available). =20 =20 >>However, when you make the blanket statement that a mechanically = driven supercharger will result in a slower airplane, we have to part = company.<< =20 I didn=92t say that. I said the data I had seen revealed that one = particular aircraft using one particular configuration of belt driven = compressor ended up, based on actual test, going no faster with the = belt driven compressor than it did without.=20 =20 Beech 18s have mechanically driven compressors and they do go faster = with those than the would without. They go even faster at altitude = with higher gear ratios on the compressors when at altitude, but at the = penalty of substantial losses in low level single engine performance. =20 =20 Regards, George --- Incoming mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.859 / Virus Database: 585 - Release Date: 2/14/2005 --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.859 / Virus Database: 585 - Release Date: 2/14/2005 =20 ------_=_NextPart_001_01C52F0B.8A228FD6 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable

 

From: = Lancair Mailing List = [mailto:lml@lancaironline.net] On Behalf Of REHBINC@aol.com
Sent: Tuesday, March 22, = 2005 11:11 AM
To: Lancair Mailing List
Subject: [LML] Re: = superchargers

 

George,

  I have compressor maps for lots of aircraft centrifugal compressors - - and
only one of them gets to 80% at any point on the map.
 
  It is unlikely that the fixed rpm ratio version of that unit = would operate
at
that optimum efficiency for more than a small portion of its operating = life.

You've got me there. I don't have any maps for "aircraft compressors". I do however have many for general applications and it is not uncommon for them to peak in the upper 80s to = 90% efficiency. I only assume that aircraft would use equipment with similar efficiency.

 

Why wouldn't an aircraft compressor operate at or = near its optimum efficiency nearly all of its life? Airplanes are routinely flown = at roughly the same altitudes and power settings for the majority of there = lives.<<

 

 

Well,  as it = happens,   I have on my desk  the compressor map for what is probably  the best compressor Garrett has ever made for small general aviation piston = engines.

The sweet spot  for a mass = flow of about 30 lb/min and a pressure ration of 2.0 to 2.2  has an = efficiency of  76%.

It rapidly deteriorates down to 60% = when you get off of that design point.     =

 

Thus, at 5000 feet MSL,  at a pressure ratio of  ~ 1.25 (36-38” Hg MP or there about) the efficiency would be down around 60 to 65%.

Airplanes,  unlike cars,  operate from a baseline 30” Hg ambient at sea level - - to a = baseline ambient of only 12-13” Hg ,    with pressure ratios = varying from  1.2  up to 2.8 to 3.0.

Thus, at constant RPM,  the compressor would spend most of its life  well off  of any = “optimum”  pre-determined design point.

 

The variable speed = turbo-supercharger compressor helps to mitigate this problem to a substantial = degree.

 

 

 

>>If all = you want to do is get away from detonation, cut the compression ratio by a quarter = point. If you want to make more power, ad an intercooler.<<

 

Rob,  I think that is not the = optimal approach.  However,  I think that is close to  what =  a lot of  automotive background people would suggest when first = considering the issues. 

 

Take another look at = it.

 

Assuming the  engine is = already operating  at or near the engine’s  detonation = limited  maximum BMEP - -  (many aircraft engines are) - - then in order to = improve the detonation margin, without reducing power (reduction in CR goes the = wrong direction on the power and the efficiency issues)  or in order to = increase the  BMEP - - and remain free of detonation,  one will have to = do one or more of the following:

 

1)       = Reduce the CHTs;

2)       = Increase the fuel octane;

3)       = Reduce the induction air temperature.  =  

 

Cars with liquid cooled engines = can’t readily reduce the CHTs a lot.

 

Air cooled aircraft engines can = improve the engine cooling with some amazingly simple revisions to the baffling systems.  That helps a lot.  You can “buy”  = about 5 octane points of detonation protection by dropping the CHTs by 50d = F.

 

Fuel octane is difficult to mess = around with for aircraft.

 

That leaves - - - intercoolers.  

 

Reductions in IAT from the = intercooler will buy one  LARGE improvements in the  detonation limited = BMEP (ie, more horsepower becomes usefully = available).

 

 

>>However, = when you make the blanket statement that a mechanically driven supercharger will = result in a slower airplane, we have to part company.<<

 

I didn’t say that.  I = said the data I had seen revealed that one particular aircraft using one = particular configuration of belt driven compressor ended up, based on actual = test,  going no faster with the belt driven compressor than it did without. =

 

Beech 18s have mechanically driven compressors and they do go faster with those than the would = without.  They go even faster at altitude  with higher gear ratios on the = compressors when at altitude,  but at the penalty of substantial  losses = in low level single engine performance.

 

 

Regards,  = George


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