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From: "Al Gietzen" <ALVentures@cox.net>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: Air Flow Question
Date: Wed, 23 Dec 2009 08:20:37 -0800
Message-ID: <90EA3D29E8584A35A91FAB668D262B71@BigAl>
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Tracy wrote:
My 5" round inlet for the radiator looks ridiculously small compared to
yours but so far it is cooling the 20B OK. =20
=20
Now that sort of boggles my mind as it seems to violate the laws of =
physics.
Let's just take a modest climb power of, say; 225 hp. At that power, =
the
energy going into the coolant is about 6000 Btu/min. In order to remove
that amount of heat, at a typical air temp increase of 75 degrees; takes
about 4000 cfm air flow. A 5" dia inlet is 0.14 sq feet, meaning an =
average
inlet velocity about 29,000 ft/min, or 330 mph. Even at 100 air temp
increase (unlikely on a 90F day) it's 250 mph. And I'm guessing your =
climb
speed is half that. Similar math suggests you'd be limited to a steady
state (cruise) power of about 50%.
=20
Of course, being a pusher driver, I think of inlet air speeds in terms =
of
the speed of the airplane. So does the fact that the inlet is behind =
prop
give a much higher effective inlet velocity? I've been thinking that =
the
turbulence in the prop wash would negate a good portion of the extra =
mean
air velocity because of reduced inlet effectiveness.
=20
It will be interesting to know how it works out on a hot day.=20
=20
Great that you have your 20B in the air and working well.
=20
Merry Christmas and Happy New Year to all.
=20
Al G
=20
=20
=20
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<p class=3DMsoNormal style=3D'margin-bottom:12.0pt'><font size=3D2 =
color=3Dblue
face=3DVerdana><span =
style=3D'font-size:11.0pt;font-family:Verdana;color:blue'>Tracy</span></f=
ont><font
size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:11.0pt;font-family:Verdana;
color:blue'> wrote:</span></font></p>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<p class=3DMsoNormal style=3D'margin-left:.5in'><font size=3D3 =
face=3D"Times New Roman"><span
style=3D'font-size:12.0pt'>My 5" round inlet for the radiator looks
ridiculously small compared to yours but so far it is cooling the 20B =
OK.
</span></font></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'>Now that sort of boggles my mind =
as it
seems to violate the laws of physics. Let’s just take a modest =
climb
power of, say; 225 hp. At that power, the energy going into the =
coolant
is about 6000 Btu/min. In order to remove that amount of heat, at =
a
typical air temp increase of 75 degrees; takes about 4000 cfm air =
flow. A
5” dia inlet is 0.14 sq feet, meaning an average inlet velocity =
about 29,000
ft/min, or 330 mph. Even at 100 air temp increase (unlikely on a =
90F day)
it’s 250 mph. And I’m guessing your climb speed is =
half
that. Similar math suggests you’d be limited to a steady =
state (cruise)
power of about 50%.</span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'>Of course, being a pusher driver, =
I
think of inlet air speeds in terms of the speed of the airplane. =
So does
the fact that the inlet is behind prop give a much higher effective =
inlet
velocity? I’ve been thinking that the turbulence in the prop =
wash would
negate a good portion of the extra mean air velocity because of reduced =
inlet
effectiveness.</span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'>It will be interesting to know =
how it
works out on a hot day. </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'>Great that you have your 20B in =
the air
and working well.</span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'>Merry Christmas and Happy New =
Year to
all.</span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
style=3D'font-size:
11.0pt;font-family:Verdana;color:blue'> </span></font></p>
<p class=3DMsoNormal><font size=3D2 color=3Dblue face=3DVerdana><span =
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11.0pt;font-family:Verdana;color:blue'>Al G</span></font></p>
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