X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 06 Jun 2011 12:50:48 -0400 Message-ID: X-Original-Return-Path: Received: from nm23-vm3.bullet.mail.ne1.yahoo.com ([98.138.91.153] verified) by logan.com (CommuniGate Pro SMTP 5.4c3j) with SMTP id 5004089 for lml@lancaironline.net; Sun, 05 Jun 2011 06:56:04 -0400 Received-SPF: none receiver=logan.com; client-ip=98.138.91.153; envelope-from=casey.gary@yahoo.com Received: from [98.138.90.50] by nm23.bullet.mail.ne1.yahoo.com with NNFMP; 05 Jun 2011 10:55:29 -0000 Received: from [98.138.86.156] by tm3.bullet.mail.ne1.yahoo.com with NNFMP; 05 Jun 2011 10:55:28 -0000 Received: from [127.0.0.1] by omp1014.mail.ne1.yahoo.com with NNFMP; 05 Jun 2011 10:55:28 -0000 X-Yahoo-Newman-Property: ymail-3 X-Yahoo-Newman-Id: 979424.92886.bm@omp1014.mail.ne1.yahoo.com Received: (qmail 63627 invoked by uid 60001); 5 Jun 2011 10:55:28 -0000 DomainKey-Signature:a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=Message-ID:X-YMail-OSG:Received:X-Mailer:References:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type; b=u/7abC87NrkRRsO4CJ3AeH2FdAJFWegV8QjcSCvspDTTJnGkj5Rv22UYl2daCvKRvLAs35u5bVzFNSqhvQ0GlY3ddiI4tErDO+PpNlZhLQLdH6zNMwhzynuwEHL3wHiNtGtqy0LAJh4TGYdZW1IT0EiEWZvtoxhGFfXNvqpB5Cc=; X-Original-Message-ID: <805073.63386.qm@web125605.mail.ne1.yahoo.com> X-YMail-OSG: ADV8HjUVM1nRTHaR46enTfw5ANgKc35pGhp5k.hAZzG4Mao A10fTY7tmtjTqS.EgKIgEg_towYCqOe_N535XF_D5PsoJ8TMGaQNQCn54PzY OhciqZxlebDdU.rRH4At1ZNsn_CRPQdhOq1_E3ZTazBQ2kE0v9zCKCjyIdIY HE3CsHrWtQ7OEW_4_XnGs_o2r.X0fpkJ83ia5RX.aaU1Z8Le8sHdil.te26D zOvzKVDTwPZm_GT.3BygBPktHlNecotKTQK0aQUxEubtCkEgLxgCTreKR3t0 ciEeSeGptghlnMtc8LGeg5Jkqlpau0ZK30fSdP4TV8yM_C4pNTSw- Received: from [97.122.154.57] by web125605.mail.ne1.yahoo.com via HTTP; Sun, 05 Jun 2011 03:55:28 PDT X-Mailer: YahooMailRC/570 YahooMailWebService/0.8.111.303096 References: X-Original-Date: Sun, 5 Jun 2011 03:55:28 -0700 (PDT) From: Gary Casey Subject: Re: L-IV Choice of Engine X-Original-To: Lancair Mailing List In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="0-335084561-1307271328=:63386" --0-335084561-1307271328=:63386 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable More interesting comments below. Yes, the choice of prop certainly ha an e= ffect =0Aon the durability of the engine/prop combination. I'm never sure = what is meant =0Aby "harmonics", but the important factor is the torsional = resonant frequency and =0Ahow it relates to the frequency of the torsional = impulses produced by the engine =0A(not the prop). The torsional impulse i= s radically reduced by increasing the =0Anumber of cylinders, as the impuls= es then overlap to a degree. I've seen =0Anumbers (which I've forgotten), = but I'll bet that a 6-cylinder engine has an =0Aimpulse amplitude that is l= ess than half that of a 4-cylinder. And an 8 =0Acylinder engine is probabl= y much better than a 6. So I would not be too worried =0Aabout a prop dire= ctly connected to V8 crankshaft (the bending moment from =0Agyroscopic prec= ession loading is another matter). The big problem comes from =0Aadding a = speed reduction unit between the engine an prop, as that first reduces =0At= he resonant frequency of the system and is often too fragile to tolerate th= e =0Atorsional loads. Yes, the blades themselves have a resonant frequency= and that =0Acan cause problems. Props with a larger number of blades tend= to be designed =0Awith smaller, lighter blades and therefore exhibit highe= r resonant frequencies.=0A=0AAnother factor is noise. Props tend to produc= e cabin noise via the pressure =0Apulses that strike some portion of the ca= bin - like the windshield. These =0Apulses, one per blade passing, can coi= ncide with exhaust pulses, amplifying the =0Aperceived noise. So, the "bad= " blade numbers are 2 for a 4-cylinder engine, 3 =0Afor a 6 and 4 for an 8.= Guess which are the most common blade numbers :-)=0A=0AA complex subject,= to be sure.=0AGary Casey=0AES #157, 6-cylinder engine, 3-blade prop=0A=0AI= have read some discussion on the effects of different props, reducing harm= onic =0Avibration by changing the number of blades (4 blades on a 6-cylinde= r engine for =0Aexample, vs. 2 or 3 blades which have lower harmonics) and = this might be =0Abeneficial. That thought process might lead one to a 3 bl= ade or 5 blade on an =0A8-cylinder engine.=0A =0AAnother concept which mig= ht be considered is the prop material itself, with =0Acomposite props like = the MT damping out the combustion shock more than an =0Aaluminium prop.=0A = =0AGuess that=E2=80=99s what makes these planes =E2=80=9Cexperimental=E2=80= =9D.=0A =0ARobert M. Simon=0AES-P N301ES=0A =0AFrom: Jack Morgan [mailto:jm= organ1023@comcast.net] =0ASent: Thursday, June 02, 2011 8:29 AM=0ASubject: = Re: [LML] Re: L-IV Choice of Engine=0AThe most important issue being missed= is the torsional resonance the auto =0Acrankshaft receives from the propel= lor. The application of viscous couplings is =0Aoften tried to isolate the = auto crank from the propellor with some success but =0Athe weakness still r= emains. . . Jack Morgan --0-335084561-1307271328=:63386 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: quoted-printable
More interesting comments below.  Yes, the= choice of prop certainly ha an effect on the durability of the engine/prop= combination.  I'm never sure what is meant by "harmonics", but the im= portant factor is the torsional resonant frequency and how it relates to th= e frequency of the torsional impulses produced by the engine (not the prop)= .  The torsional impulse is radically reduced by increasing the number= of cylinders, as the impulses then overlap to a degree.  I've seen nu= mbers (which I've forgotten), but I'll bet that a 6-cylinder engine has an impulse amplitude= that is less than half that of a 4-cylinder.  And an 8 cylinder engin= e is probably much better than a 6.  So I would not be too worried abo= ut a prop directly connected to V8 crankshaft (the bending moment from gyro= scopic precession loading is another matter).  The big problem comes f= rom adding a speed reduction unit between the engine an prop, as that first= reduces the resonant frequency of the system and is often too fragile to t= olerate the torsional loads.  Yes, the blades themselves have a resona= nt frequency and that can cause problems.  Props with a larger number = of blades tend to be designed with smaller, lighter blades and therefore ex= hibit higher resonant frequencies.

Another factor is noise. &= nbsp;Props tend to produce cabin noise via the pressure pulses that strike = some portion of the cabin - like the windshield.  These pulses, one pe= r blade passing, can coincide with exhaust pulses, amplifying the perceived= noise.  So, the "bad" blade numbers are 2 for a 4-cylinder engine, 3 = for a 6 and 4 for an 8.  Guess which are the most common blade numbers= :-)

A complex sub= ject, to be sure.
Gary Casey
ES #157, 6-cylinder engine, 3-blade prop

=

I have read some discussion on th= e effects of different props, reducing harmonic vibration by changing the n= umber of blades (4 blades on a 6-cylinder engine for example, vs. 2 or 3 bl= ades which have lower harmonics) and this might be beneficial.  That thought process = might lead one to a 3 blade or 5 blade on an 8-cylinder engine.

<= p class=3D"MsoNormal" style=3D"margin-top: 0in; margin-right: 0in; margin-b= ottom: 0.0001pt; margin-left: 0in; font-size: 12pt; font-family: serif; col= or: black; "> 

Another  = ;concept which might be considered is the prop material itself, with compos= ite props like the MT damping out the combustion shock more than an alumini= um prop.

 

Guess that=E2=80=99s what makes these planes =E2=80=9Cexperi= mental=E2=80=9D.

 

Robert M. Simon<= /span>

ES-P N301ES

=  

From: Jack Morgan [mailto:jmorgan1023@c= omcast.net] 
Sent: Thursday, June 02, 2011 8:29 AM
<= b>Subject: Re: [LML] Re: L-IV Choice of Engine

The most important issue being missed is th= e torsional resonance the auto crankshaft receives from the propellor. The application of viscous couplings is often tried to isolate the auto crank = from the propellor with some success but the weakness still remains. <= span style=3D"color: rgb(31, 73, 125); ">. . Jack Morgan

=0A=0A=0A
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