Today's Message Index:
----------------------
1. 02:11 AM - Re: cooling air requirement (James R. Cunningham)
2. 05:21 AM - (Gary Casey)
3. 08:49 AM - Re: cooling air requirement (James R. Cunningham)
4. 08:52 AM - Re: (James R. Cunningham)
Message 1
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| Subject: | Re: cooling air requirement |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
Gary, you made a good post with this info. Thanks. Some comments are
inserted below.
Gary Casey wrote:
> The calculations for cooling air requirements aren't too difficult if you're
> only looking for a rough order of magnitude answer.
And not too difficult if you are looking for a pretty close estimate on
an engine-specific answer.
> A 100-hp engine will dissipate about 25hp through cooling.
The specific engine that I used to make my calculations was rejecting
about 45% of power as waste heat for the two input conditions that I set
up.
> Some goes into the oil, so let's say
> the cylinder cooling requirement is 20 hp.
Don't hold me to this, but I think Lycoming uses a maximum of 15% of the
heat into the oil. I can look it up if anyone is interested. In the
meantime, your number is 20%, which should be slightly conservative for
a Lycoming and perhaps more accurate for some other engines.
> You could probably assume the
> air temperature rise across the engine is 30 degrees:
This you can't do. You need to calculate the temperature rise across
the face for the specific flight conditions. It will typically range
from 30 to about 150 degrees F (Lycoming uses a constant 150 degrees).
It is usually well over 30 and usually well under 150.
> To absorb 850 BTU/min of heat you need 1,650 CFM air
> flow (850 X 14 / 7.2). Problem is it takes a significant pressure to force
> that much air around the cylinders - a few inches of water maybe.
Pressure drop is specific to each engine and varies substantially with
cylinder size and fin design. For the specific engine and input
conditions I used in my example, the pressure drop across the face
ranged from 25 to 43 psf. Well less than an inch of water. Your mileage
may vary with other engines.
> Normal ventilating fans are rated at zero pressure drop (blowing into free air)
and
> may not be able to flow nearly their rated flow with a significant pressure
> drop.
Well said.
> The best suggestion I have heard is simply to build a reasonably
> large plenum that will capture propeller blast and use that, watching the
> cylinder head temperatures.
It might be worth computing the flow available before trying that.
> A very effective cooling method would be to add
> water mist to the air, but the steam cooling that will result is probably
> very uneven and that would worry me.
Me too.
JimC
Message 2
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--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
Previously I said:
<<A 100-hp engine will
dissipate about 25hp through cooling. Some goes into the oil, so let's say
the cylinder cooling requirement is 20 hp.>>
I was in error as I was using the rule of thumb of 20% of the power goes
into cooling. Actually, it is more like 20% of the INPUT power. Of the
heat energy from the fuel a very rough estimate is that 50% goes out the
exhaust, 25% into useful work (hp out the crank) and 20% out the cooling
system. The remaining 5% into the oil.
Gary Casey
Message 3
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| Subject: | Re: cooling air requirement |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
James R. Cunningham wrote:
Gary said:
> > Problem is it takes a significant pressure to force
> > that much air around the cylinders - a few inches of water maybe.
I said:
> Pressure drop is specific to each engine and varies substantially with
> cylinder size and fin design. For the specific engine and input
> conditions I used in my example, the pressure drop across the face
> ranged from 25 to 43 psf. Well less than an inch of water. Your mileage may
vary with other engines.
Oops. Wish I'd said more so I could take that back too. When I
converted to inches of water, I omitted the multiplication by 12. For
the engine and conditions I used, the pressure drop ranged from about
3.5 to about 8 inches. Gary, you were spot on when you said, "a few
inches of water".
All the best,
Jim
Message 4
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| Subject: | Re: ngines-List: |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
Welcome to the club. And I was in error when I converted psf to inches
of water. You just made me feel better, knowing I'm not alone. I'd buy
the estimate of 5% into the oil (but I'd design the oil cooling system
to allow for more, and would include a cowl flap on the oil cooler
outlet).
Jim
Gary Casey wrote:
>
> --> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
>
> Previously I said:
> <<A 100-hp engine will
> dissipate about 25hp through cooling. Some goes into the oil, so let's say
> the cylinder cooling requirement is 20 hp.>>
>
> I was in error as I was using the rule of thumb of 20% of the power goes
> into cooling. Actually, it is more like 20% of the INPUT power. Of the
> heat energy from the fuel a very rough estimate is that 50% goes out the
> exhaust, 25% into useful work (hp out the crank) and 20% out the cooling
> system. The remaining 5% into the oil.
>
> Gary Casey
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