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Date: Wed, 20 May 2009 14:58:32 -0400
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From: Gary Casey <casey.gary@yahoo.com>
Subject: Re: Engine Out Practise(sic)
In-Reply-To: <list-3647876@logan.com>
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Grayhawk,=0AToo late {:-) I already looked at it.  I compared it to the rou=
gh formula I have always used - % power =3D RPM/RPMmax * MAP/MAPmax  - and =
it comes within 1% of it.  I stopped there and didn't try to "reverse engin=
eer" the formula, except I have hunch it has to do with assuming the max RP=
M is 2500 (the 2.5) and the max MAP is 35 inches (the 3.5).  Regardless, th=
e formulas are not quite right.  As rpm increases, engine friction increase=
s and volumetric efficiency decreases so the increase in power won't go up =
directly with RPM.  The opposite is true of MAP.  It takes a certain MAP ju=
st to power the engine (overcome friction) so power will increase more than=
 the increase in MAP.  How much are these effects?  Without real data it wo=
uld be hard to say, but in the RPM and MAP ranges that are useful in aircra=
ft engines the factor for rpm might be something like 0.8. For MAP there is=
 a subtractive term of about 10 inches.  The equation would then look like =
 %power
 =3D (RPM/RPMmax*0.8) * (MAP-10)/MAPmax-10).  Gets hard to do in your head =
(well, mine anyway), so I just remember that a 10% reduction in RPM will dr=
op the power less than 10%, but a 10% drop in MAP will drop the power by mo=
re than 10%.  Then there is the effect of altitude..=0AGary=0A=0A=0A=0A____=
____________________________=0AFrom: Lancair Mailing List <lml@lancaironlin=
e.net>=0ASe=0A=0AIgnore the formula.  I got it from a very old email and it=
 is=0Awrong.  Perhaps Walter will correct it.=0A =0AGrayhawk the brainless=
=0A =0AIn a message dated 5/18/2009 7:39:13 P.M. Central Daylight Time,=0AS=
ky2high@aol.com writes:=0A =0AFinally, thanks to Walter at GAMI, =0A =0A%HP=
 =3D 100-((max RPM/100-RPM)*2.5+(Max MP-MP)*3.5)=0A=0A=0A      
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<html><head><style type=3D"text/css"><!-- DIV {margin:0px;} --></style></he=
ad><body><div style=3D"font-family:'times new roman', 'new york', times, se=
rif;font-size:12pt"><div>Grayhawk,</div><div>Too late {:-) I already looked=
 at it. =A0I compared it to the rough formula I have always used - % power =
=3D RPM/RPMmax * MAP/MAPmax =A0- and it comes within 1% of it. =A0I stopped=
 there and didn't try to "reverse engineer" the formula, except I have hunc=
h it has to do with assuming the max RPM is 2500 (the 2.5) and the max MAP =
is 35 inches (the 3.5). =A0Regardless, the formulas are not quite right. =
=A0As rpm increases, engine friction increases and volumetric efficiency de=
creases so the increase in power won't go up directly with RPM. =A0The oppo=
site is true of MAP. =A0It takes a certain MAP just to power the engine (ov=
ercome friction) so power will increase more than the increase in MAP. =A0H=
ow much are these effects? =A0Without real data it would be hard to say, bu=
t in the
 RPM and MAP ranges that are useful in aircraft engines the factor for rpm =
might be something like 0.8. For MAP there is a subtractive term of about 1=
0 inches. =A0The equation would then look like =A0%power =3D (RPM/RPMmax*0.=
8) * (MAP-10)/MAPmax-10). =A0Gets hard to do in your head (well, mine anywa=
y), so I just remember that a 10% reduction in RPM will drop the power less=
 than 10%, but a 10% drop in MAP will drop the power by more than 10%. =A0T=
hen there is the effect of altitude..</div><div>Gary</div><div style=3D"fon=
t-family:times new roman, new york, times, serif;font-size:12pt"><br><div s=
tyle=3D"font-family:times new roman, new york, times, serif;font-size:12pt"=
><font size=3D"2" face=3D"Tahoma"><hr size=3D"1"><b><span style=3D"font-wei=
ght: bold;">From:</span></b> Lancair Mailing List &lt;lml@lancaironline.net=
><br><b><span style=3D"font-weight: bold;">Se</span></b></font><br><br>=0A=
=0A =0A =0A<font id=3D"role_document" color=3D"#000000" size=3D"2" face=3D"=
Arial">=0A<div>Ignore the formula.=A0 I got it from a very old email and it=
 is=0Awrong.=A0 Perhaps Walter will correct it.</div>=0A<div>=A0</div>=0A<d=
iv>Grayhawk the brainless</div>=0A<div>=A0</div>=0A<div>=0A<div>In a messag=
e dated 5/18/2009 7:39:13 P.M. Central Daylight Time,=0ASky2high@aol.com wr=
ites:</div>=0A<blockquote style=3D"BORDER-LEFT:blue 2px solid;PADDING-LEFT:=
5px;MARGIN-LEFT:5px;"><font style=3D"BACKGROUND-COLOR:transparent;" color=
=3D"#000000" size=3D"2" face=3D"Arial">=0A  <div>=A0</div>=0A  <div>Finally=
, thanks to Walter at GAMI, </div>=0A  <div>=A0</div>=0A  <div>%HP =3D 100-=
((max RPM/100-RPM)*2.5+(Max MP-MP)*3.5)</div>=0A  <div>=A0</div></font></bl=
ockquote></div></font><div class=3D"aol_ad_footer" id=3D"ddbdc047eab2770a32=
5596731be3b6e"><br></div></div></div><div style=3D"position:fixed"></div></=
div><br>=0A=0A=0A=0A      </body></html>
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