Air Filter Area ratio to inlet area?

scott.hibbs · Post by **scott.hibbs** » Fri Feb 17, 2012 10:42 pm

As some of you know, I am putting a LOM in my Rebel. I have decided I have to
use a NACA duct for intake air with an air filter just aft of the intake.
Does anyone know a recommended ratio of intake area to the air filter? I want
to keep the pressure as high as possible while passing through the filter. I
will be using a Bracket type air filter.

Scott

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Rickhm at home · Post by **Rickhm at home** » Fri Feb 17, 2012 10:42 pm

While the pressure is important, it is the volume/unit time that dictates
performance. I don't know the LOM engine, but for the sake of discussion
assume it is a 2.5L engine. Then it will require (at full throttle) 2.5
L/min of air! I think there are places on the web that will tell you the
airflow a given sized NACA will provide. Assume that a given setup will
only provide 2.0 L/min at full throttle. Then the hp will be reduced by
25%, so it the volume/unit time can have a significant impact.

Further thoughts.... Engine power is rated at sea level. If our
discussion engine generates 100 hp at full power as it goes up in altitude
it will loose power. The atmospheric pressure at 10,000 ft is 20.58 Vs
29.92 at sea level. The performance of the engine is
(20.58/29.92)100 = 69 hp. You can have a similar effect with the pressure
drop across your air filter. If the drop across your air filter is 5 mm
Hg, then at sea level you will see a reduction of 5/29.92 =17%. This is
why the air intake is up front in most applications. With it being up
front you get a ram air effect and the pressure is greater than 29.92
before the air filter. In most applications effort is put into reducing
the number of turns the air has to take because for every turn there is a
further reduction in pressure. This also helps us understand why
superchargers and turbo chargers are so effective. They actively pump the
air and pressurize the system to combat these effects. Just like a
reduction of 5 mm Hg causes a 15& reduction in performance, a 5 mm Hg
increase will improve performance by 17%.

Hope this helps.

Rick Muller
SR70

-----Original Message-----
From: scott.hibbs@dcsol.com [SMTP:scott.hibbs@dcsol.com]
Sent: Saturday, January 11, 2003 11:25 PM
To: murphy-rebel@dcsol.com
Subject: Air Filter Area ratio to inlet area?

As some of you know, I am putting a LOM in my Rebel. I have decided I have
to
use a NACA duct for intake air with an air filter just aft of the intake.
Does anyone know a recommended ratio of intake area to the air filter? I
want
to keep the pressure as high as possible while passing through the filter.
I
will be using a Bracket type air filter.

Scott

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Drew Dalgleish · Post by **Drew Dalgleish** » Fri Feb 17, 2012 10:42 pm

Rick I think you're a little short on your air requirements. How about 2.5l
needed every revolution? So it would need it's displacement times the
maximum RPM every minute.
Scott I just bought an air filter for a cessna 172 and built my air box to
fit it. My filter hangs out in the breeze like a super cubs so I get a
little ram air induction. If you can copy another instalation on an engine
with similar horsepower you should have something that'll work just fine.
Drew

At 10:30 AM 1/12/2003 -0700, you wrote:

While the pressure is important, it is the volume/unit time that dictates
performance. I don't know the LOM engine, but for the sake of discussion
assume it is a 2.5L engine. Then it will require (at full throttle) 2.5
L/min of air! I think there are places on the web that will tell you the
airflow a given sized NACA will provide. Assume that a given setup will
only provide 2.0 L/min at full throttle. Then the hp will be reduced by
25%, so it the volume/unit time can have a significant impact.

Further thoughts.... Engine power is rated at sea level. If our
discussion engine generates 100 hp at full power as it goes up in altitude
it will loose power. The atmospheric pressure at 10,000 ft is 20.58 Vs
29.92 at sea level. The performance of the engine is
(20.58/29.92)100 = 69 hp. You can have a similar effect with the pressure
drop across your air filter. If the drop across your air filter is 5 mm
Hg, then at sea level you will see a reduction of 5/29.92 =17%. This is
why the air intake is up front in most applications. With it being up
front you get a ram air effect and the pressure is greater than 29.92
before the air filter. In most applications effort is put into reducing
the number of turns the air has to take because for every turn there is a
further reduction in pressure. This also helps us understand why
superchargers and turbo chargers are so effective. They actively pump the
air and pressurize the system to combat these effects. Just like a
reduction of 5 mm Hg causes a 15& reduction in performance, a 5 mm Hg
increase will improve performance by 17%.

Hope this helps.

Rick Muller
SR70

-----Original Message-----
From: scott.hibbs@dcsol.com [SMTP:scott.hibbs@dcsol.com]
Sent: Saturday, January 11, 2003 11:25 PM
To: murphy-rebel@dcsol.com
Subject: Air Filter Area ratio to inlet area?

As some of you know, I am putting a LOM in my Rebel. I have decided I have
to
use a NACA duct for intake air with an air filter just aft of the intake.
Does anyone know a recommended ratio of intake area to the air filter? I
want
to keep the pressure as high as possible while passing through the filter.
I
will be using a Bracket type air filter.

Scott

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scott.hibbs · Post by **scott.hibbs** » Fri Feb 17, 2012 10:42 pm

Thanks Rick & Drew,
I am familiar with the concept of the lost pressure across the filter as well
as turns in the plenum. I wanted to find out how much is lost at the filter
without having to buy several filters and testing. My plan at this point is
to double the filter area seen on a Super Cub since our hp's are roughly the
same. The plenum losses are the reason I am going to a side mounted NACA
duct. The path from the front cowl to the intake (clear back at the firewall)
is contorted. BTW, the LOM has a supercharger so it will be interesting to
see how well it works at differing density altitudes. I may also add a lip
to "help" the NACA duct out.

Scott

-> Wrom: WLSZLKBRNVWWCUFPEGAUTFJMVRESKPNKMBIPBA
-> While the pressure is important, it is the volume/unit time that dictates
-> performance. I don't know the LOM engine, but for the sake of discussion
-> assume it is a 2.5L engine. Then it will require (at full throttle) 2.5
-> L/min of air! I think there are places on the web that will tell you the
-> airflow a given sized NACA will provide. Assume that a given setup will
-> only provide 2.0 L/min at full throttle. Then the hp will be reduced by
-> 25%, so it the volume/unit time can have a significant impact.
Snip

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Brian Cross · Post by **Brian Cross** » Fri Feb 17, 2012 10:42 pm

Hi

Being a four stroke engine, would not the air requirements be 2.5l every
other revolution?

Brian #328R

At 01:11 PM 01/12/2003 -0500, you wrote:

Rick I think you're a little short on your air requirements. How about 2.5l
needed every revolution? So it would need it's displacement times the
maximum RPM every minute.
Scott I just bought an air filter for a cessna 172 and built my air box to
fit it. My filter hangs out in the breeze like a super cubs so I get a
little ram air induction. If you can copy another instalation on an engine
with similar horsepower you should have something that'll work just fine.
Drew

At 10:30 AM 1/12/2003 -0700, you wrote:
While the pressure is important, it is the volume/unit time that dictates
performance. I don't know the LOM engine, but for the sake of discussion
assume it is a 2.5L engine. Then it will require (at full throttle) 2.5
L/min of air! I think there are places on the web that will tell you the
airflow a given sized NACA will provide. Assume that a given setup will
only provide 2.0 L/min at full throttle. Then the hp will be reduced by
25%, so it the volume/unit time can have a significant impact.

Further thoughts.... Engine power is rated at sea level. If our
discussion engine generates 100 hp at full power as it goes up in altitude
it will loose power. The atmospheric pressure at 10,000 ft is 20.58 Vs
29.92 at sea level. The performance of the engine is
(20.58/29.92)100 = 69 hp. You can have a similar effect with the pressure
drop across your air filter. If the drop across your air filter is 5 mm
Hg, then at sea level you will see a reduction of 5/29.92 =17%. This is
why the air intake is up front in most applications. With it being up
front you get a ram air effect and the pressure is greater than 29.92
before the air filter. In most applications effort is put into reducing
the number of turns the air has to take because for every turn there is a
further reduction in pressure. This also helps us understand why
superchargers and turbo chargers are so effective. They actively pump the
air and pressurize the system to combat these effects. Just like a
reduction of 5 mm Hg causes a 15& reduction in performance, a 5 mm Hg
increase will improve performance by 17%.

Hope this helps.

Rick Muller
SR70

-----Original Message-----
From: scott.hibbs@dcsol.com [SMTP:scott.hibbs@dcsol.com]
Sent: Saturday, January 11, 2003 11:25 PM
To: murphy-rebel@dcsol.com
Subject: Air Filter Area ratio to inlet area?

As some of you know, I am putting a LOM in my Rebel. I have decided I have
to
use a NACA duct for intake air with an air filter just aft of the intake.
Does anyone know a recommended ratio of intake area to the air filter? I
want
to keep the pressure as high as possible while passing through the filter.
I
will be using a Bracket type air filter.

Scott

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Drew Dalgleish · Post by **Drew Dalgleish** » Fri Feb 17, 2012 10:42 pm

Whoops I guess I didn't think that all the way through. I just knew it had
to be more than 2.5l/min.
Drew

At 05:41 PM 1/12/2003 -0500, you wrote:

Hi

Being a four stroke engine, would not the air requirements be 2.5l every
other revolution?

Brian #328R

At 01:11 PM 01/12/2003 -0500, you wrote:
Rick I think you're a little short on your air requirements. How about 2.5l
needed every revolution? So it would need it's displacement times the
maximum RPM every minute.
Scott I just bought an air filter for a cessna 172 and built my air box to
fit it. My filter hangs out in the breeze like a super cubs so I get a
little ram air induction. If you can copy another instalation on an engine
with similar horsepower you should have something that'll work just fine.
Drew

At 10:30 AM 1/12/2003 -0700, you wrote:
While the pressure is important, it is the volume/unit time that dictates
performance. I don't know the LOM engine, but for the sake of discussion
assume it is a 2.5L engine. Then it will require (at full throttle) 2.5
L/min of air! I think there are places on the web that will tell you the
airflow a given sized NACA will provide. Assume that a given setup will
only provide 2.0 L/min at full throttle. Then the hp will be reduced by
25%, so it the volume/unit time can have a significant impact.

Further thoughts.... Engine power is rated at sea level. If our
discussion engine generates 100 hp at full power as it goes up in altitude
it will loose power. The atmospheric pressure at 10,000 ft is 20.58 Vs
29.92 at sea level. The performance of the engine is
(20.58/29.92)100 = 69 hp. You can have a similar effect with the pressure
drop across your air filter. If the drop across your air filter is 5 mm
Hg, then at sea level you will see a reduction of 5/29.92 =17%. This is
why the air intake is up front in most applications. With it being up
front you get a ram air effect and the pressure is greater than 29.92
before the air filter. In most applications effort is put into reducing
the number of turns the air has to take because for every turn there is a
further reduction in pressure. This also helps us understand why
superchargers and turbo chargers are so effective. They actively pump the
air and pressurize the system to combat these effects. Just like a
reduction of 5 mm Hg causes a 15& reduction in performance, a 5 mm Hg
increase will improve performance by 17%.

Hope this helps.

Rick Muller
SR70

-----Original Message-----
From: scott.hibbs@dcsol.com [SMTP:scott.hibbs@dcsol.com]
Sent: Saturday, January 11, 2003 11:25 PM
To: murphy-rebel@dcsol.com
Subject: Air Filter Area ratio to inlet area?

As some of you know, I am putting a LOM in my Rebel. I have decided I

have

to
use a NACA duct for intake air with an air filter just aft of the intake.
Does anyone know a recommended ratio of intake area to the air filter? I
want
to keep the pressure as high as possible while passing through the filter.
I
will be using a Bracket type air filter.

Scott

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Brian Cross · Post by **Brian Cross** » Fri Feb 17, 2012 10:42 pm

Hi Drew

I wasn't trying to get picky on details to try to make you feel bad or
anything.

The 1.25 L/rev. would only of course be at full power & assuming no loss in
the induction circuit. The ram air pressure at cruise speed would add a
little to the induction inlet pressure. This addition would be about 0.74
inches of mercury. This is not too much when you can only get 29.92 inches
of mercury maximum with no losses of any kind.

Just food for thought.

Cheers

Brian #328R

At 07:10 PM 01/12/2003 -0500, you wrote:

Whoops I guess I didn't think that all the way through. I just knew it had
to be more than 2.5l/min.
Drew

At 05:41 PM 1/12/2003 -0500, you wrote:
Hi

Being a four stroke engine, would not the air requirements be 2.5l every
other revolution?

Brian #328R

At 01:11 PM 01/12/2003 -0500, you wrote:
Rick I think you're a little short on your air requirements. How about 2.5l
needed every revolution? So it would need it's displacement times the
maximum RPM every minute.
Scott I just bought an air filter for a cessna 172 and built my air box to
fit it. My filter hangs out in the breeze like a super cubs so I get a
little ram air induction. If you can copy another instalation on an engine
with similar horsepower you should have something that'll work just fine.
Drew

At 10:30 AM 1/12/2003 -0700, you wrote:
altitude
pressure
pump the
have
filter.
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