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  #21  
Old 11-11-2012, 11:21 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by Bill.Peyton View Post
Dan, Did you scarf the inner tube? I see it is mounted straight in on the exhaust tube. Bill
Cut the end at an angle? Yes, but let's consider why.

There's a tendency to think the slash cut end somehow generates low pressure. It does, but not much more, if any more, than any other shape which avoids capturing dynamic pressure.

Just for fun, I drilled a 5/8" hole in the wall of my big leaf blower's outlet tube, then made up a 5/8" tube with a pressure measurement connection on one end. The blower stream is not the same velocity, temperature, or density as an exhaust stream, but no matter. This is simply a comparison of pressure tap end shapes in a flow.



Note the pressure for the 1/2' insertion of the square end, as compared to the other two ends. It doesn't make much difference what sort of end you put on your pressure tap, as long as it is not faced into the airstream. If you reverse the airflow so that the moving air impacts the open end of the slash-cut or tipped tubes, you will capture some dynamic pressure, i.e. convert dynamic to static like a pitot tube. Instead of a vacuum you would probably measure a positive pressure, not at all what we want here.

The real reason to slash cut the tube is to simply avoid dynamic pressure. The negative pressure we wish to tap doesn't stem from airflow, but rather from the positive and negative wave pressures due to cylinder events and pipe tuning.

Take a look at Figure 1 in the CAFE report titled "Aircraft Exhaust Systems IV linked fro this page:

http://cafefoundation.org/v2/research_reports.php

Note there are three points in 720 degrees of crank rotation in which a single cylinder's exhaust pipe is subject to significant negative pressure....about 10" Hg in this graph. This is the real source of negative pressure for our evacuator. The reed valve blocks the positive and we keep the negative.

BTW, my install looks like the one on the right, above
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Last edited by DanH : 11-22-2012 at 06:20 AM.
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  #22  
Old 11-11-2012, 11:28 AM
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Thanks for the explanation Dan. The link requires permissions that prohibited access.
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  #23  
Old 11-11-2012, 05:23 PM
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Link fixed above, but here's Fig 1

Credit CAFE Foundation.


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Last edited by DanH : 11-22-2012 at 06:22 AM.
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  #24  
Old 12-01-2012, 09:00 PM
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Post RV10 Oil Vent Tube leaks

Quote:
Originally Posted by Bill.Peyton View Post
Now that I have around 45 hours on the engine I still find that I am consuming around 1 qt every 7 hours. I have been topping the oil off at 10 quarts. I am thinking that I may be running too high an oil level.
I ordered one of the air/oil separators, but I am now thinking that I don't want to install it until I find the root cause of the problem.
I am also concerned with the angle of the breather which is pointing forward per the plans.
What experience is out there regarding oil level and consumption and breather tube installation on the 540 D4A5?
We recently purchased an RV10, IO540, with about 300 hours. We have also been concerned with oil coming out of the vent tube after flight, and dripping on the hangar floor. We were told by a current builder of a RV10, who has built and been flying a RV7 for some years, that he removed his oil separator on the RV7 for the same reason. The consensus seems to be that we need to find the correct oil level. We are now operating at about 9 Qts, and it seems to be better so far. The forward facing opening of the tube could be an issue according to the Cessna service center mechanic we used for our Cessna 400. We would rather find someone with an RV10 that has solved this issue before we make changes.
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  #25  
Old 12-01-2012, 10:20 PM
N15JB N15JB is offline
 
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My first suggestion would be to fill to no more than 8 qts. and see what happens to your breather drip. There are many posts in the archives regarding this. No need to reinvent the wheel.

I fill to 8 qts. at each oil change, and by 25 hours I am down to 6.5 qts. The belly stays clean, and all temps & pressures are normal summer and winter.

Jim Berry
RV-10
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  #26  
Old 12-02-2012, 07:09 AM
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I allowed my oil to drop to 9 qt then monitored it. After 6 hours it was down to 8. I removed the mineral break in oil, changed the oil to Aeroshell 15W-50 since by 55 hours it should now be broken in, put in 11 qts total, one for the filter. After run up the level stabilized at 10.5. I am going to pull and inspect all the lower plugs this week to make certain that oil is not getting by the rings, then check the compression. Assuming everything is normal, I intend to allow it to decrease to 7 qts while monitoring the consumption one qt at a time. Ill report back
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  #27  
Old 12-02-2012, 04:05 PM
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Bill, While there are no hard and fast rules here, it will be most likely that the first 3qts will disappear overboard pretty soon.

We have 730 hours of tach time, and reliably add a Qt every 10 or so hours. We now fill to 7.5 when we do a change, and that means a full amount of 7.

When it gets to 6 or close enough to there in goes one more.

You may settle on 7-8, but I would be very surprised if you get much more.
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  #28  
Old 12-02-2012, 05:10 PM
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That's the plan. Wait and see where it settles out....
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  #29  
Old 12-02-2012, 07:56 PM
douglassmt douglassmt is offline
 
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I put in 9 at oil change, end up with 8-8.5 or so. Seems to work well, I add a quart at 7 or so depending on how far I'm flying. I've recently plotted my oil consumption and will post any useful info.
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  #30  
Old 12-03-2012, 06:02 AM
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DanH DanH is offline
 
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An aside......owners report a significant variation in oil usage rates, but it's not all due to consumption via the combustion chamber. I've never measured, but I'd bet you can note a big difference in breather oil loss merely by changing the hose routing. Quite a few are rigged with the hose coming off the breather fitting and immediately turning downward. Suspended oil droplets have a chance to go back to the case if the hose is routed up and back across the top of the firewall before turning down.

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